Neal R Armstrong

Interests

Research

Interface Science of Materials for Energy Conversion, Light Emission and Sensing; Surface Photoemission Spectroscopies (UPS, XPS), Scanning Probe Microscopies, Electrochemistry, New Approaches to Development and Characterization of Energy Conversion Materials

Teaching

Measurement science/Analytical chemistry

Courses

Scholarly Contributions

Books

• Armstrong, N. R. (2012). Surface intiated atom transfer radical polymerizations from indium tin oxide electrodes: Electrochemistry of polymer brushes.
• Armstrong, N. R. (2001). Light-emitting electrochemical processes.
• Armstrong, N. R. (1993). Polarization-dependent femtosecond dynamics of MBE-grown phthalocyanine organic thin films.

Chapters

• Armstrong, N. R. (2017). Liquid-crystal approaches to organic photovoltaics. In Organic Photovoltaics: Mechanisms, Materials, and Devices.
• Armstrong, N. R. (2017). Spectroelectrochemistry of monolayer and submonolayer films using an electroactive integrated optical waveguide. In Interfacial Electrochemistry: Theory: Experiment, and Applications.

Journals/Publications

• Armstrong, N. R. (2019). Impact of Titanium Dioxide Surface Defects on the Interfacial Composition and Energetics of Evaporated Perovskite Active Layers. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2019). Zinc Phthalocyanine-Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics. Journal of Physical Chemistry C.
• Armstrong, N. R. (2017). Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide: Structure, Energetics, and Charge Transfer Properties. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2017). Critical Interface States Controlling Rectification of Ultrathin NiO-ZnO p-n Heterojunctions. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2017). Determining Band-Edge Energies and Morphology-Dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy. Journal of the American Chemical Society.
• Ehamparam, R., Oquendo, L. E., Liao, M. W., Brynnel, A. K., Ou, K., Armstrong, N. R., McGrath, D., & Saavedra, S. S. (2017). Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide: Structure, Energetics and Charge Transfer Properties. ACS Advanced Materials and Interfaces, 9, 29213-29223. doi:10.1021/acsami.7b07394
• Ou, K., Ehamparam, R., MacDonald, G., Stubhan, T., Wu, X., Shallcross, R. C., Richards, R., Brabec, C. J., Saavedra, S. S., & Armstrong, N. R. (2017). Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin Film Morphology and Device Performance. ACS Applied Materials and Interfaces, 8, 19787-19798. doi:10.1021/acsami.6b02792
• Shallcross, R. C., Zheng, Y., Saavedra, S. S., & Armstrong, N. R. (2017). Determining Band-edge Energies and Morphology-dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy. Journal of the American Chemical Society. doi:DOI: 10.1021/jacs.7b00516.
• Steirer, K. X., Ou, K. L., Armstrong, N. R., & Ratcliff, E. L. (2017). Critical Interface States Controlling Rectification of Ultrathin NiO- ZnO p-n Heterojunctions. ACS APPLIED MATERIALS & INTERFACES, 9(36), 31111-31118.
• Armstrong, N. R. (2016). Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2016). Introduction: Electronic Materials. Chemical Reviews.
• Armstrong, N. R. (2016). Overcoming Electrode-Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers. Advanced Energy Materials.
• Armstrong, N. R. (2016). Phosphonic Acids for Interfacial Engineering of Transparent Conductive Oxides. Chemical Reviews.
• Armstrong, N. R. (2016). Surface modification of indium-tin oxide with functionalized perylene diimides: Characterization of orientation, electron-transfer kinetics and electronic structure. Journal of Physical Chemistry C.
• Paniagua, S. A., Giordano, A. J., Smith, O. L., Barlow, S., Li, H., Armstrong, N. R., Pemberton, J. E., Bredas, J., Ginger, D., & Marder, S. R. (2016). Phosphonic acids for interfacial engineering of transparent conductive oxides. Chemical Reviews, 116, 7117–7158. doi:10.1021/acs.chemrev.6b00061
• Ratcliff, E. L., Shallcross, R. C., & Armstrong, N. R. (2016). Introduction: Electronic Materials. CHEMICAL REVIEWS, 116(21), 12821-12822.
• Zhang, H., Shallcross, R. C., Li, N., Stubhan, T., Hou, Y., Chen, W., Ameri, T., Turbiez, M., Armstrong, N. R., & Brabec, C. J. (2016). Overcoming Electrode Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers. Advanced Energy Materials, 1502195. doi:10.1002/aenm.201502195
• Zheng, Y., Giordano, A. J., Shallcross, R. C., Fleming, S. R., Barlow, S., Armstrong, N. R., Marder, S. R., & Saavedra, S. S. (2016). Surface modification of indium-tin oxide with functionalized perylene diimides: Characterization of orientation, electron-ˇ‐transfer kinetics and electronic structure. Journal of Physical Chemistry C, 120, 20040–20048. doi:10.1021/acs.jpcc.6b06812
• Armstrong, N. R. (2015). Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers. ACS Nano.
• Armstrong, N. R. (2015). Influence of molecular orientation on charge-transfer processes at phthalocyanine/metal oxide interfaces and relationship to organic photovoltaic performance. Journal of Physical Chemistry C.
• Armstrong, N. R. (2015). Investigation of ultra-thin titania films as hole-blocking contacts for organic photovoltaics. Journal of Materials Chemistry A.
• Armstrong, N. R. (2015). Nickel oxide interlayer films from nickel formate-ethylenediamine precursor: Influence of annealing on thin film properties and photovoltaic device performance. Journal of Materials Chemistry A.
• Armstrong, N. R. (2015). Quantifying the extent of contact doping at the interface between high work function electrical contacts and poly(3-hexylthiophene) (P3HT). Journal of Physical Chemistry Letters.
• Armstrong, N. R. (2015). Solution Processed Titanyl Phthalocyanines as Donors in Solar Cells: Photoresponse to 1000 nm. ACS Applied Materials and Interfaces.
• Ehamparam, R., Pavlopoulos, N., Liao, M. W., Hill, L. J., Armstrong, N. R., Pyun, J., & Saavedra, S. S. (2015). Characterization of Band Edge Energetics and Rates of Charge Injection Processes in Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers. ACS Nano, 9, 8786–8800. doi:10.1021/acsnano.5b01720
• Lin, H., MacDonald, G. A., Shi, Y., Polaske, N. W., McGrath, D. V., Marder, S. R., Armstrong, N. R., Ratcliff, E. L., & Saavedra, S. S. (2016). Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal-Oxide Interfaces and Relationship to Organic Photovoltaic Performance. Journal of Physical Chemistry C, 119, 10304-10313. doi:10.1021/acs.jpcc.5b02971
• Mayukh, M., Macech, M. R., Placencia, D., Cao, Y., Armstrong, N. R., & McGrath, D. V. (2014). Solution Processed Titanyl Phthalocyanines as Donors in Solar Cells: Photoresponse to 1000 nm. ACS Applied Materials and Interfaces, 9, 8786-8800. doi:10.1021/acsami.5b05900
• Shallcross, R. C., Stubhan, T., Ratcliff, E. L., Kahn, A., Brabec, C. J., & Armstrong, N. R. (2015). Quantification of Contact Doping Due to Fermi-Level Pinning at the Interface Between High Work Function Electrical Contacts and P3HT. Journal of Physical Chemistry Letters, 6, 1303-1309. doi:10.1021/acs.jpclett.5b00444
• Steirer, K. X., Richards, R. E., Sigdel, A. K., Garcia, A., Ndione, P. F., Hammond, S., Baker, D., Ratcliff, E. L., Curtis, C., Ginley, D. S., Olson, D. C., Armstrong, N. R., & Berry, J. J. (2015). Nickel oxide interlayer films from nickel formate-ethylenediamine precursor: Influence of annealing on thin film properties and photovoltaic device performance. Journal of Materials Chemistry A, 3, 10949–10958. doi:10.1039/c5ta01379h
• Armstrong, N. R. (2014). Pentafluorophenoxy boron subphthalocyanine (F5BsubPc) as a multifunctional material for organic photovoltaics. ACS Applied Materials and Interfaces.
• Morse, G. E., Gantz, J. L., Steirer, K. X., Armstrong, N. R., & Bender, T. P. (2014). Pentafluorophenoxy boron subphthalocyanine (F₅BsubPc) as a multifunctional material for organic photovoltaics. ACS Applied Materials and Interfaces, 6(3), 1515-1524.
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  Abstract: We have demonstrated that pentafluoro phenoxy boron subphthalocyanine (F5BsubPc) can function as either an electron donor or an electron acceptor layer in a planar heterojunction organic photovolatic (PHJ OPV) cell. F5BsubPc was incorporated into devices with the configurations ITO/MoO3/F5BsubPc/C60/BCP/Al (F 5BsubPc used as an electron-donor/hole-transport layer) and ITO/MoO3/Cl-BsubPc/F5BsubPc/BCP/Al (F5BsubPc used as an electron-acceptor/electron-transport layer). Each unoptimized device displayed open-circuit photopotentials (Voc) close to or in excess of 1 V and respectrable power conversion efficiencies. Ultraviolet photoelectron spectroscopy (UPS) was used to characterize the band-edge offset energies at the donor/acceptor junctions. HOMO and LUMO energy level offsets for the F 5BsubPc/C60 heterojunction were determined to be ca. 0.6 eV and ca. 0.7 eV, respectively. Such offsets are clearly large enough to produce rectifying J/V responses, efficient exciton dissociation, and photocurrent production at the interface. For the Cl-BsubPc/F5BsubPc heterojunction, the estimated offset energies were found to be ca. 0.1 eV. However, reasonable photovoltaic activity was observed, with photocurrent production coming from both BsubPc species layers. Incident and absorbed photon power conversion efficiencies (IPCE and APCE) showed that photocurrent production qualitatively tracked the absorbance spectra of the donor/acceptor heterojunctions, with some additional photocurrent activity on the low energy side of the absorbance band. We suggest that photocurrent production at higher wavelengths may be a result of charge-transfer species at the donor/acceptor interface. Cascade photovoltaics were also fabricated to expand on the understanding of the role of F5BsubPc in such device architectures. © 2013 American Chemical Society.
• Armstrong, N. (2013). Critical interfaces in emerging thin film photovoltaic technologies: From basic science to policy and economic issues. Optics InfoBase Conference Papers.
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  Abstract: The widespread adoption of photovoltaic energy conversion technologies, especially in the U.S., is determined by a combination of scientific challenges, economics and energy policy. The Department of Energy has set as a goal the creation of PV platforms that efficiently produce electrical power at a cost of well under $1 per watt installed, which creates breathtaking technical challenges to overcome. This talk will focus on some of the scientific/technical challenges we face in creating 'scalable' (solar cells that can be printed at 100's of meters per day), where materials costs are extremely low, accompanied by low (but increasing) efficiencies. We will review studies underway that allow us to understand and control the critical interfaces in these PV platforms, that currently limit both efficiency an lifetime. We view all of this basic science in the context of a constantly changing political and economic environment for PV-based electricity, and an exciting future for these new PV technologies. Renewable Energy and the Environment Congress. © 2013.
• Armstrong, N. R. (2013). Energy level alignment and morphology of Ag and Au nanoparticle recombination contacts in tandem planar heterojunction solar cells. Journal of Physical Chemistry C.
• Armstrong, N. R. (2013). Influence of electrode surface composition and energetics on small-molecule organic solar cell performance: Polar versus nonpolar donors on indium tin oxide contacts. Journal of Physical Chemistry C.
• Armstrong, N. R. (2013). Organic solar cells - At the interface. Journal of Physical Chemistry Letters.
• Armstrong, N. R. (2013). Structure-processing-property correlations in solution-processed, small-molecule, organic solar cells. Journal of Materials Chemistry C.
• Armstrong, N. R. (2013). Titanium dioxide electron-selective interlayers created by chemical vapor deposition for inverted configuration organic solar cells. Journal of Materials Chemistry A.
• Gantz, J., Placencia, D., Giordano, A., Marder, S. R., & Armstrong, N. R. (2013). Influence of electrode surface composition and energetics on small-molecule organic solar cell performance: Polar versus nonpolar donors on indium tin oxide contacts. Journal of Physical Chemistry C, 117(3), 1205-1216.
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  Abstract: We present a systematic study of the band-edge energy offsets and shifts in local vacuum levels for indium tin oxide (ITO)/donor heterojunctions, using vacuum-deposited chloroindium phthalocyanine (ClInPc), titanyl phthalocyanine (TiOPc), pentacene (PEN), and copper phthalocyanine (CuPc) donor layers. We include a comparison of the performance of ITO/donor/C60 planar heterojunction (PHJ) organic solar cells (OPVs) as a function of activation and modification of the ITO contact. UV photoemission spectroscopy (UPS) was used to characterize the interfacial region between these donors and ITO to infer the ordering of these first deposited 1-2 monolayers as a function of ITO activation and modification. For the polar donors ClInPc and TiOPc, deposited on air plasma (AP) treated, high work function ITO (φeff ≈ 5.1-5.2 eV), shifts in local vacuum level observed during deposition of the first two monolayers of donor molecules suggest that the halo-metal or oxo-metal bond points (on average) toward the oxide surface in the first monolayer. Inversion of this orientation is inferred during formation of the second monolayer. Thin-film absorbance data for the Q-band spectra of ClInPc or TiOPc 1-2 monolayers on AP-ITO confirmed the organization of the first deposited layers. Use of nonactivated ITO contacts, or contacts modified with pentafluorobenzylphosphonic acid (F5BPA), disrupts this order in the first two monolayers as revealed in the UPS and absorbance data, leading to decreases in OPV open-circuit photopotential (VOC), fill factor (FF), and efficiency (η). For nonpolar donors like PEN and CuPc, shifts in local vacuum level as a function of donor coverage were less sensitive to ITO surface composition, and PEN-based OPVs show relatively invariant VOC, J SC, FF, and η values regardless of ITO pretreatment or F 5BPA modification.The interaction of ClInPc donor layers with AP-ITO is strong enough to generate photovoltaic activity in the absence of a C 60 acceptor layer (ITO/ClInPc/Al device configurations), where photocurrent appears to be generated at the ITO/ClInPc interface. Such an effect is not seen for ITO/ClInPc/Al devices on nonactivated ITO substrates, highlighting the importance of the interactions between the donor layer and the hole-harvesting interface in PHJ OPVs. These results suggest that the organization of a dipolar donor layer at a hole-harvesting oxide contact can affect OPV performance where charge harvesting efficiency is sensitive to molecular organization in the interfacial region near the contact. © 2012 American Chemical Society.
• Ou, K., Tadytin, D., Steirer, K. X., Placencia, D., Nguyen, M., Lee, P., & Armstrong, N. R. (2013). Titanium dioxide electron-selective interlayers created by chemical vapor deposition for inverted configuration organic solar cells. Journal of Materials Chemistry A, 1(23), 6794-6803.
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  Abstract: We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12-36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P3HT/PC61BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large areas, and may be a viable non-contact alternative to electron-selective interlayer formation. Oxide-based electron-selective interlayers, such as TiO2, need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N2 gas stream, the CVD process provides nanometer control of film thickness to produce 12-36 nm thickness device-quality films. The best performing CVD films, processed at substrate temperatures of ca. 210°C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO2. Solution electrochemistries (voltammetry) of probe molecules were shown to be easily accessible indicators of film porosity and are predictive for electron harvesting selectivity (and hole-blocking) in an inverted configuration OPV platform. Small molecules whose redox potentials placed them energetically above the conduction band edge energy (ECB) were reduced/oxidized at nearly the same rates as for bare ITO. Probe molecules whose redox potentials place them energetically within the band gap region, below ECB, show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO 2 films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (R P) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO 2/P3HT:PC61BM/MoO3/Ag, using TiO2 films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (JSC), open-circuit photopotential (VOC), and fill-factor (FF), versus bare ITO. OPVs using amorphous, conformal 24 nm TiO2 interlayers showed the highest fill factors, lowest R S, highest RP and power conversion efficiencies of ca. 3.7%. © 2013 The Royal Society of Chemistry.
• Steirer, K. X., Macdonald, G. A., Olthof, S., Gantz, J., Ratcliff, E. L., Kahn, A., & Armstrong, N. R. (2013). Energy level alignment and morphology of Ag and Au nanoparticle recombination contacts in tandem planar heterojunction solar cells. Journal of Physical Chemistry C, 117(43), 22331-22340.
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  Abstract: We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doubling occurs using these two metal recombination contacts (RC) in TOPVs (ITO/CuPc/C 60/(Ag,Au)/CuPc/C60/BCP/Al) and correlate the differences with energetic dissimilarities revealed by UV-photoemission (UPS) and inverse photoemission (IPES) spectroscopies, and morphology as revealed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). Ag interlayer RCs produce the expected voltage doubling in the open-circuit voltage (VOC) for the TOPV, whereas Au RCs showed poor voltage addition. Significant shifts in ionization potential and electron affinity and shifts in local work function were observed for C60/metal heterojunctions and for heterojunctions based on C60/metal/C60 and for C 60/metal/CuPc, with clear evidence for partial charge redistribution between C60 and Ag nanoparticles. AFM and FE-SEM images revealed discrete Ag nanoparticles at the C60 interface, whereas Au/C 60 heterojunctions consisted of more uniform Au thin films that wet the C60 surface and penetrated below the surface. These studies point to the need for careful control of both electronic and morphological properties of thin RCs in emerging tandem organic solar cell technologies. © 2013 American Chemical Society.
• Wunsch, B. H., Rumi, M., Tummala, N. R., Risko, C., Kang, D., Steirer, K. X., Gantz, J., Said, M., Armstrong, N. R., Brédas, J., Bucknall, D., & Marder, S. R. (2013). Structure-processing-property correlations in solution-processed, small-molecule, organic solar cells. Journal of Materials Chemistry C, 1(34), 5250-5260.
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  Abstract: Alkyl chains are often attached to the periphery of semiconductor molecules to impart solubility and they represent a pervasive structural element in solution processable, organic photovoltaics (OPV). It is important to understand the effects of such substitutions on the morphology and performance of organic solar cells. This investigation focuses on determining structure-property correlations in OPV devices constructed with small-molecule, solution processable electron donors based on benzothiadiazole-dithienopyrrole, mixed with the electron acceptor PCBM. Two donor molecules with the same opto-electronic molecular properties but differing alkyl substituents-without (BD) or with (BD6) hexyl side chains-are studied. The resulting device data for fabricated solar cells, across a range of processing conditions, is compared to thin-film morphology, spectroscopy, thermal analysis, and molecular dynamics simulations. Two device states of higher and lower performance, depending on the casting solvent, are obtained for the molecule without the side chains (BD); both states have amorphous mesoscale structure, but show subtle differences in the nanoscale phase separation. In contrast, for the molecule with side chains (BD6) devices have highly variable reproducibility and middling efficiency and photocurrent. The BD6 donor exhibits lower miscibility with PCBM, which correlates with the formation of a donor-enriched layer on the surface of the solar cell. © 2013 The Royal Society of Chemistry.
• Zacher, B., Gantz, J. L., Richards, R. E., & Armstrong, N. R. (2013). Organic solar cells - At the interface. Journal of Physical Chemistry Letters, 4(11), 1949-1952.
• Armstrong, N. R. (2012). Built-in potential in conjugated polymer diodes with changing anode work function: Interfacial states and deviation from the schottky-mott limit. Journal of Physical Chemistry Letters.
• Armstrong, N. R. (2012). Electrical property heterogeneity at transparent conductive oxide/organic semiconductor interfaces: Mapping contact ohmicity using conducting-tip atomic force microscopy. ACS Nano.
• Armstrong, N. R. (2012). Electron-transfer processes in zinc phthalocyanine-phosphonic acid monolayers on ITO: Characterization of orientation and charge-transfer kinetics by waveguide spectroelectrochemistry. Journal of Physical Chemistry Letters.
• Armstrong, N. R. (2012). Energy level alignment in PCDTBT:PC70BM solar cells: Solution processed NiOx for improved hole collection and efficiency. Organic Electronics.
• Armstrong, N. R. (2012). Surface composition, work function, and electrochemical characteristics of gallium-doped zinc oxide. Thin Solid Films.
• Armstrong, N. R. (2012). Surface-initiated synthesis of poly(3-methylthiophene) from indium tin oxide and its electrochemical properties. Langmuir.
• Armstrong, N. R. (2012). The modification of indium tin oxide with phosphonic acids: Mechanism of binding, tuning of surface properties, and potential for use in organic electronic applications. Accounts of Chemical Research.
• Doubina, N., Jenkins, J. L., Paniagua, S. A., Mazzio, K. A., MacDonald, G. A., Jen, A. K., Armstrong, N. R., Marder, S. R., & Luscombe, C. K. (2012). Surface-initiated synthesis of poly(3-methylthiophene) from indium tin oxide and its electrochemical properties. Langmuir, 28(3), 1900-1908.
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  PMID: 22149001;Abstract: Poly(3-methylthiophene) (P3MT) was synthesized directly from indium tin oxide (ITO) electrodes modified with a phosphonic acid initiator, using Kumada catalyst transfer polymerization (KCTP). This work represents the first time that polymer thickness has been controlled in a surface initiated KCTP reaction, highlighting the utility of KCTP in achieving controlled polymerizations. Polymer film thicknesses were regulated by the variation of the solution monomer concentration and ranged from 30 to 265 nm. Electrochemical oxidative doping of these films was used to manipulate their near surface composition and effective work function. Doped states of the P3MT film are maintained even after the sample is removed from solution and potential control confirming the robustness of the films. Such materials with controllable thicknesses and electronic properties have the potential to be useful as interlayer materials for organic electronic applications. © 2011 American Chemical Society.
• Hotchkiss, P. J., Jones, S. C., Paniagua, S. A., Sharma, A., Kippelen, B., Armstrong, N. R., & Marder, S. R. (2012). The modification of indium tin oxide with phosphonic acids: Mechanism of binding, tuning of surface properties, and potential for use in organic electronic applications. Accounts of Chemical Research, 45(3), 337-346.
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  PMID: 22011002;Abstract: Transparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in next-generation organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITO-organic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ.This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or hole-harvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties. © 2011 American Chemical Society.
• Lin, H., Polaske, N. W., Oquendo, L. E., Gliboff, M., Knesting, K. M., Nordlund, D., Ginger, D. S., Ratcliff, E. L., Beam, B. M., Armstrong, N. R., McGrath, D. V., & Saavedra, S. S. (2012). Electron-transfer processes in zinc phthalocyanine-phosphonic acid monolayers on ITO: Characterization of orientation and charge-transfer kinetics by waveguide spectroelectrochemistry. Journal of Physical Chemistry Letters, 3(9), 1154-1158.
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  Abstract: Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with k s,app = 2 × 10 2 s -1, independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, k s,app = 2 × 10 3 s -1, whereas for upright Pc aggregates, k s,app = 7 × 10 2 s -1. The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells. © 2012 American Chemical Society.
• MacDonald, G. A., Veneman, P. A., Placencia, D., & Armstrong, N. R. (2012). Electrical property heterogeneity at transparent conductive oxide/organic semiconductor interfaces: Mapping contact ohmicity using conducting-tip atomic force microscopy. ACS Nano, 6(11), 9623-9636.
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  PMID: 23030667;Abstract: We demonstrate mapping of electrical properties of heterojunctions of a molecular semiconductor (copper phthalocyanine, CuPc) and a transparent conducting oxide (indium-tin oxide, ITO), on 20-500 nm length scales, using a conductive-probe atomic force microscopy technique, scanning current spectroscopy (SCS). SCS maps are generated for CuPc/ITO heterojunctions as a function of ITO activation procedures and modification with variable chain length alkyl-phosphonic acids (PAs). We correlate differences in small length scale electrical properties with the performance of organic photovoltaic cells (OPVs) based on CuPc/C60 heterojunctions, built on these same ITO substrates. SCS maps the "ohmicity" of ITO/CuPc heterojunctions, creating arrays of spatially resolved current-voltage (J-V) curves. Each J-V curve is fit with modified Mott-Gurney expressions, mapping a fitted exponent (γ), where deviations from γ = 2.0 suggest nonohmic behavior. ITO/CuPc/C60/BCP/Al OPVs built on nonactivated ITO show mainly nonohmic SCS maps and dark J-V curves with increased series resistance (R S), lowered fill-factors (FF), and diminished device performance, especially near the open-circuit voltage. Nearly optimal behavior is seen for OPVs built on oxygen-plasma-treated ITO contacts, which showed SCS maps comparable to heterojunctions of CuPc on clean Au. For ITO electrodes modified with PAs there is a strong correlation between PA chain length and the degree of ohmicity and uniformity of electrical response in ITO/CuPc heterojunctions. ITO electrodes modified with 6-8 carbon alkyl-PAs show uniform and nearly ohmic SCS maps, coupled with acceptable CuPc/C60OPV performance. ITO modified with C14 and C18 alkyl-PAs shows dramatic decreases in FF, increases in R S, and greatly enhanced recombination losses. © 2012 American Chemical Society.
• MacLeod, B. A., Horwitz, N. E., Ratcliff, E. L., Jenkins, J. L., Armstrong, N. R., Giordano, A. J., Hotchkiss, P. J., Marder, S. R., Campbell, C. T., & Ginger, D. S. (2012). Built-in potential in conjugated polymer diodes with changing anode work function: Interfacial states and deviation from the schottky-mott limit. Journal of Physical Chemistry Letters, 3(9), 1202-1207.
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  Abstract: We use electroabsorption spectroscopy to measure the change in built-in potential (V BI) across the polymer photoactive layer in diodes where indium tin oxide electrodes are systematically modified using dipolar phosphonic acid self-assembled monolayers (SAMs) with various dipole moments. We find that V BI scales linearly with the work function (φ) of the SAM-modified electrode over a wide range when using a solution-coated poly(p-phenylenevinylene) derivative as the active layer. However, we measure an interfacial parameter of S = eδV BI/δφ < 1, suggesting that these ITO/SAM/polymer interfaces deviate from the Schottky-Mott limit, in contrast to what has previously been reported for a number of ambient-processed organic-on-electrode systems. Our results suggest that the energetics at these ITO/SAM/polymer interfaces behave more like metal/organic interfaces previously studied in UHV despite being processed from solution. © 2012 American Chemical Society.
• Ratcliff, E. L., Garcia, A., Cowan, S. R., Meyer, J., Steirer, K. X., Paniagua, S. A., Giordano, A., Marder, S., Armstrong, N. R., Kahn, A., Ginley, D., & Olson, D. C. (2012). Understanding energy level alignment in PCDTBT:PC₇₀BM solar cells. Conference Record of the IEEE Photovoltaic Specialists Conference, 3377-3379.
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  Abstract: Blended heterojunctions continue to improve in overall efficiency, with current power conversion efficiencies (PCE) at >7%, with interfaces being a dominating factor in improving PCE. Charge separation is dictated by energetic alignment between the donor and acceptor. Selective interlayers are used to preferentially harvest one charge via either thermodynamic or kinetic pathways. Energetic level alignment between the blend and the charge collection interfaces can impact the open circuit voltage (VOC) through both loss mechanisms and poor choice of interlayers can result in less than ideal charge collection efficiencies. Here we report on the surface work function characterization of interlayers and phosphonic acid modifiers using ultraviolet photoelectron spectroscopy and the measured energetic level alignment between the interlayers and PCDTBT:PC70BM BHJs. We will address the overall charge extraction from the bulk heterojunction of PCDTBT:PC70BM and subsequent impact on VOC, JSC, and PCE. © 2012 IEEE.
• Ratcliff, E. L., Meyer, J., Steirer, K. X., Armstrong, N. R., Olson, D., & Kahn, A. (2012). Energy level alignment in PCDTBT:PC ₇₀BM solar cells: Solution processed NiO _x for improved hole collection and efficiency. Organic Electronics: physics, materials, applications, 13(5), 744-749.
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  Abstract: Solution-based NiO x outperforms PEDOT:PSS in device performance and stability when used as a hole-collection layer in bulk-heterojunction (BHJ) solar cells formed with poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5- (4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and PC 70BM. The origin of the enhancement is clarified by studying the interfacial energy level alignment between PCDTBT or the 1:4 blended heterojunctions and PEDOT:PSS or NiO x using ultraviolet and inverse photoemission spectroscopies. The 1.6 eV electronic gap of PEDOT:PSS and energy level alignment with the BHJ result in poor hole selectivity of PEDOT:PSS and allows electron recombination at the PEDOT:PSS/BHJ interface. Conversely, the large band gap (3.7 eV) of NiO x and interfacial dipole (≥0.6 eV) with the organic active layer leads to a hole-selective interface. This interfacial dipole yields enhanced electron blocking properties by increasing the barrier to electron injection. The presence of such a strong dipole is predicted to further promote hole collection from the organic layer into the oxide, resulting in increased fill factor and short circuit current. An overall decrease in recombination is manifested in an increase in open circuit voltage and power conversion efficiency of the device on NiO x versus PEDOT:PSS interlayers. © 2012 Elsevier B.V. All rights reserved.
• Ratcliff, E. L., Sigdel, A. K., MacEch, M. R., Nebesny, K., Lee, P. A., Ginley, D. S., Armstrong, N. R., & Berry, J. J. (2012). Surface composition, work function, and electrochemical characteristics of gallium-doped zinc oxide. Thin Solid Films, 520(17), 5652-5663.
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  Abstract: Gallium-doped zinc oxide (GZO) possesses the electric conductivity, thermal stability, and earth abundance to be a promising transparent conductive oxide replacement for indium tin oxide electrodes in a number of molecular electronic devices, including organic solar cells and organic light emitting diodes. The surface chemistry of GZO is complex and dominated by the hydrolysis chemistry of ZnO, which influences the work function via charge transfer and band bending caused by adsorbates. A comprehensive characterization of the surface chemical composition and electrochemical properties of GZO electrodes is presented, using both solution and surface adsorbed redox probe molecules. The GZO surface is characterized using monochromatic X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy after the following pretreatments: (i) hydriodic acid etch, (ii) potassium hydroxide etch, (iii) RF oxygen plasma etching, and (iv) high-vacuum argon-ion sputtering. The O 1s spectra for the GZO electrodes have contributions from the stoichiometric oxide lattice, defects within the lattice, hydroxylated species, and carbonaceous impurities, with relative near-surface compositions varying with pretreatment. Solution etching procedures result in an increase of the work function and ionization potential of the GZO electrode, but yield different near surface Zn:Ga atomic ratios, which significantly influence charge transfer rates for a chemisorbed probe molecule. The near surface chemical composition is shown to be the dominant factor in controlling surface work function and significantly influences the rate of electron transfer to both solution and tethered probe molecules. © 2012 Elsevier B.V.
• Wang, W., & Armstrong, N. (2012). Doping effect on chloroindium phthalocyanine (ClInPc)/C60 solar cells. Materials Research Society Symposium Proceedings, 1390, 9-14.
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  Abstract: For inorganic semiconductor solar cells, controlled doping is important because it can cause Fermi level shift of the inorganic semiconductor and achieve ohmic contact at the metal-semiconductor interface. In this paper we show that doping can also be used to shift Fermi level in organic semiconductors and cause changes in solar cell performance. We have made chloroindium phthalocyanine (ClInPc)/C60 heterojunction solar cells, where tetrafluoroteracyano-quinodimethane (F4-TCNQ) is used to dope ClInPc layer. Ultraviolet photoemission spectroscopy (UPS) is used to investigate the ITO/ClInPc interfaces. The result shows that doping causes a Fermi level shift at the ITO/ClInPc interface as it does for inorganic semiconductors. As the doping increases, dark saturation current J 0 of the solar cell increases, while open-circuit voltage Voc, short-circuit current Jsc and fill factor decreases. As a result, the efficiency of the solar cell decreases as doping increases. More UPS studies on ClInPc (doped with F4TCNQJ/C60 junction are needed to correlate the energy band diagram of the whole solar cell structure with the J-V characteristics. © 2012 Materials Research Society.
• Armstrong, N. R. (2011). Characterization of phosphonic acid binding to zinc oxide. Journal of Materials Chemistry.
• Armstrong, N. R. (2011). Colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles into Pt-Co3O4 nanowires. Chemistry of Materials.
• Armstrong, N. R. (2011). Enhanced efficiency in plastic solar cells via energy matched solution processed NiO x interlayers. Advanced Energy Materials.
• Armstrong, N. R. (2011). Evidence for near-surface NiOOH species in solution-processed NiO x selective interlayer materials: Impact on energetics and the performance of polymer bulk heterojunction photovoltaics. Chemistry of Materials.
• Armstrong, N. R. (2011). Highly photoactive titanyl phthalocyanine polymorphs as textured donor layers in organic solar cells. Journal of Physical Chemistry C.
• Armstrong, N. R. (2011). Phosphonic acid functionalized asymmetric phthalocyanines: Synthesis, modification of indium tin oxide, and charge transfer. Langmuir.
• Armstrong, N. R. (2011). Planar and textured heterojunction organic photovoltaics based on chloroindium phthalocyanine (ClInPc) versus titanyl phthalocyanine (TiOPc) donor layers. Organic Electronics.
• Armstrong, N. R. (2011). Poly(aniline) nanowires in sol-gel coated ITO: A pH-responsive substrate for planar supported lipid bilayers. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2011). Selective interlayers and contacts in organic photovoltaic cells. Journal of Physical Chemistry Letters.
• Armstrong, N. R., & Saavedra, S. S. (2011). Interface science of emerging organic solar cell platforms: Electrochemical and spectroelectrochemical approaches to the characterization of interfacial charge transfer processes. ACS National Meeting Book of Abstracts.
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  Abstract: Organic solar cell (OPV) energy conversion platforms have been identified as one possible pathway photovoltaic platforms with a cost of less than $1/W p installed, and have recently shown impressive increases in efficiency. OPVs are created from inexpensive, earth-abundant, complex material sets with a multitude of oxide/organic, metal/organic and organic/organic' interfaces, whose composition and energetics can control device efficiency. This talk will focus on our recent efforts to bring electrochemical models and electrochemical/spectroelectrochemical characterization to study critical interfaces in OPV materials and platforms, demonstrating especially the enhanced sensitivity that ATR spectroelectrochemical studies can bring to the understanding of electron transfer to monolayers of redox active molecules which are components of OPV platforms.
• Chenhao, G. e., Orosz, K. S., Armstrong, N. R., & Saavedra, S. S. (2011). Poly(aniline) nanowires in sol-gel coated ITO: A pH-responsive substrate for planar supported lipid bilayers. ACS Applied Materials and Interfaces, 3(7), 2677-2685.
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  PMID: 21707069;PMCID: PMC3145051;Abstract: Facilitated ion transport across an artificial lipid bilayer coupled to a solid substrate is a function common to several types of bioelectronic devices based on supported membranes, including biomimetic fuel cells and ion channel biosensors. Described here is fabrication of a pH-sensitive transducer composed of a porous sol-gel layer derivatized with poly(aniline) (PANI) nanowires grown from an underlying planar indium-tin oxide (ITO) electrode. The upper sol-gel surface is hydrophilic, smooth, and compatible with deposition of a planar supported lipid bilayer (PSLB) formed via vesicle fusion. Conducting tip AFM was used to show that the PANI wires are connected to the ITO, which convert this electrode into a potentiometric pH sensor. The response to changes in the pH of the buffer contacting the PANI nanowire/sol-gel/ITO electrode is blocked by the very low ion permeability of the overlying fluid PSLB. The feasibility of using this assembly to monitor facilitated proton transport across the PSLB was demonstrated by doping the membrane with lipophilic ionophores that respond to a transmembrane pH gradient, which produced an apparent proton permeability several orders of magnitude greater than values measured for undoped lipid bilayers. © 2011 American Chemical Society.
• Hotchkiss, P. J., Malicki, M., Giordano, A. J., Armstrong, N. R., & Marder, S. R. (2011). Characterization of phosphonic acid binding to zinc oxide. Journal of Materials Chemistry, 21(9), 3107-3112.
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  Abstract: Radio Frequency (RF) sputter-deposited zinc oxide (ZnO) films have been modified with alkylphosphonic acids in order to study both the binding of the phosphonic acid (PA) group to the ZnO surface and the packing of the alkyl chain. The characterization of these PA-modified ZnO substrates by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), atomic force microscopy (AFM) and contact angle measurements is presented herein. The surface modification procedure is straightforward and was adapted from earlier work. XPS analysis shows that oxygen plasma (OP) treatment creates reactive oxygen species on the surface of ZnO, allowing for a more robust binding of PAs to the ZnO surface. IRRAS analysis indicates that octadecylphosphonic acid binds to the ZnO surface in a predominantly tridentate fashion, forming dense, well-packed monolayers with alkyl chains in a fully anti-conformation. AFM and contact angle measurements indicate good surface coverage of the PAs with little to no multilayer formation. © The Royal Society of Chemistry 2011.
• Keng, P. Y., Bull, M. M., Shim, I., Nebesny, K. G., Armstrong, N. R., Sung, Y., Char, K., & Pyun, J. (2011). Colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles into Pt-Co₃O₄ nanowires. Chemistry of Materials, 23(5), 1120-1129.
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  Abstract: In this report, functional one-dimensional (1-D) Pt-Co3O 4 heterostruc-tures with enhanced electrochemical properties were synthesized via colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Colloidal polymerization of dipolar nanoparticles into hollow metal-semiconductor nanowires was achieved via a consecutive galvanic replacement reaction between Co0 and Pt2+ precursors, followed by a nanoscale Kirkendall oxidation reaction and a calcination treatment. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark field scanning TEM (HAADF-STEM), and field-emission scanning electron microscopy (FESEM) revealed the structural and morphological evolution of the hollow cobalt oxide nanowires (D = 40 nm) with platinum nanoparticles (PtNPs; D ∼ 2 nm) entrapped within the growing oxide shell. Various calcination conditions were investigated via X-ray photoelectron spectroscopy (XPS) to obtain the optimal surface composition of the metallic Pt and semiconducting Co3O4 phases. Cyclic voltammetry of the 1-D Pt-Co3O4 heterostructures demonstrated a sevenfold enhancement in specific capacitance in comparison to the pristine Co 3O4 nanowires. Preliminary results also showed that the calcined 1-D Pt-Co3O4 heterostructures catalytically hydrogenate methyl orange, and the rates of the hydrogenation were dependent on surface composition.(Figure Presented) © 2011 American Chemical Society.
• Placencia, D., Wang, W., Gantz, J., Jenkins, J. L., & Armstrong, N. R. (2011). Highly photoactive titanyl phthalocyanine polymorphs as textured donor layers in organic solar cells. Journal of Physical Chemistry C, 115(38), 18873-18884.
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  Abstract: We present a comparison of the photovoltaic activity of organic solar cells (OPVs) based on vacuum-deposited and solvent-annealed titanyl phthalocyanine (TiOPc) donor layers with C60 as the electron acceptor, where the TiOPc donor layer exists in three different polymorphic forms: TiOPc included the "as-deposited" form, with a Q-band absorbance spectrum reminiscent of the phase I polymorph, and films subjected to solvent annealing which systematically increased the fraction of either the phase II (α-phase) or the phase III (γ-TiOPc) polymorphs. As-deposited TiOPc/C60 heterojunctions showed open-circuit photopotentials (VOC) of ca. 0.65 V, with estimated AM 1.5G efficiencies of ca. 1.4%. Partial conversion of these thin films to their phase II or phase III polymorphs significantly enhanced the short-circuit photocurrent (JSC), as a result of (i) texturing of the TiOPc layers (ca. 100 nm length scales) and (ii) enhancements in near-IR absorptivity/photoelectrical activity. All TiOPc/C60 heterojunctions, characterized by UV-photoelectron spectroscopy (UPS), showed that estimated EHOMOPc - ELUMOC60 energy differences, which set the upper limit for VOC, are nearly identical for each TiOPc polymorph. Incident and absorbed photon current efficiency measurements (IPCE, APCE) are consistent with previous studies that showed a majority of the photoactivity in these higher order polymorphs deriving from excitonic states created at λmax ≈ 680 and 844 nm for both the phase II and the phase III polymorphs. The near-IR absorbance features (844 nm) in these Pc polymorphs are believed to have substantial charge-transfer (CT) character, leading to enhanced probabilities for photoinduced electron transfer (PIET). APCE measurements of TiOPc/C60 OPVs, however, show that higher photocurrent yields per absorbed photon arise from the higher energy (680 nm) excitonic state. When C70 is used as the electron acceptor, with its higher electron affinity, APCE was increased throughout the visible and near-IR region, now showing a nearly constant APCE across the entire Q-band spectrum of the TiOPc polymorphs, consistent with increased driving force (ELUMOPc - ELUMOC60) for PIET involving the lowest energy CT excitonic state. The highest estimated AM 1.5G efficiencies for these textured TiOPc/C60 heterojunctions, with the TiOPc film partially converted to phase II or phase III polymorphs, are 4.5% and 3.5%, respectively. © 2011 American Chemical Society.
• Polaske, N. W., Lin, H., Tang, A., Mayukh, M., Oquendo, L. E., Green, J. T., Ratcliff, E. L., Armstrong, N. R., Saavedra, S. S., & McGrath, D. V. (2011). Phosphonic acid functionalized asymmetric phthalocyanines: Synthesis, modification of indium tin oxide, and charge transfer. Langmuir, 27(24), 14900-14909.
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  PMID: 22047210;Abstract: Metalated and free-base A 3B-type asymmetric phthalocyanines (Pcs) bearing, in the asymmetric quadrant, a flexible alkyl linker of varying chain lengths terminating in a phosphonic acid (PA) group have been synthesized. Two parallel series of asymmetric Pc derivatives bearing aryloxy and arylthio substituents are reported, and their synthesis and characterization through NMR, combustion analysis, and MALDI-MS are described. We also demonstrate the modification of indium tin oxide (ITO) substrates using the PA functionalized asymmetric Pc derivatives and monitoring their electrochemistry. The PA functionalized asymmetric Pcs were anchored to the ITO surface through chemisorption and their electrochemical properties characterized using cyclic voltammetry to investigate the effects of PA structure on the thermodynamics and kinetics of charge transfer. Ionization energies of the modified ITO surfaces were measured using ultraviolet photoemission spectroscopy. © 2011 American Chemical Society.
• Ratcliff, E. L., Meyer, J., Steirer, K. X., Garcia, A., Berry, J. J., Ginley, D. S., Olson, D. C., Kahn, A., & Armstrong, N. R. (2011). Evidence for near-surface NiOOH species in solution-processed NiO _x selective interlayer materials: Impact on energetics and the performance of polymer bulk heterojunction photovoltaics. Chemistry of Materials, 23(22), 4988-5000.
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  Abstract: The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiOx) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O2-plasma treatment of the as-deposited NiO x films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O2-plasma treated NiOx films are compared using both electron-only and BHJ devices. O2-plasma-treated NiOx interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiOx interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiOx films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime. © 2011 American Chemical Society.
• Ratcliff, E. L., Zacher, B., & Armstrong, N. R. (2011). Selective interlayers and contacts in organic photovoltaic cells. Journal of Physical Chemistry Letters, 2(11), 1337-1350.
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  Abstract: Organic photovoltaic cells (OPVs) are promising solar electric energy conversion systems with impressive recent optimization of active layers. OPV optimization must now be accompanied by the development of new charge-selective contacts and interlayers. This Perspective considers the role of interface science in energy harvesting using OPVs, looking back at early photoelectrochemical (photogalvanic) energy conversion platforms, which suffered from a lack of charge carrier selectivity. We then examine recent platforms and the fundamental aspects of selective harvesting of holes and electrons at opposite contacts. For blended heterojunction OPVs, contact/interlayer design is especially critical because charge harvesting competes with recombination at these same contacts. New interlayer materials can modify contacts to both control work function and introduce selectivity and chemical compatibility with nonpolar active layers and add thermodynamic and kinetic selectivity to charge harvesting. We briefly discuss the surface and interface science required for the development of new interlayer materials and take a look ahead at the challenges yet to be faced in their optimization. © 2011 American Chemical Society.
• Ratcliff, E., Zacher, B., Steirer, X., Gantz, J., MacDonald, G., Macech, M., Tadytin, D., Ou, K., & Armstrong, N. R. (2011). The interface science of interlayer materials and contacts in organic solar cells. Conference Record of the IEEE Photovoltaic Specialists Conference, 003472-003476.
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  Abstract: Organic solar cells (OPVs) continue to see striking increases in efficiencies, with some research cells exceeding 8% power conversion efficiency, and module efficiencies approaching or exceeding ca. 4%.[1-3] These efficiencies are not yet adequate to create a PV technology which hits the DOE target of $1 per watt (peak), but there is reason for optimism that the low materials costs of this technology will drive it to this goal, provided that both efficiencies and lifetime can be improved. Contact and interlayer materials are a critical feature of OPVs that require significant optimization for both single-junction and for tandem OPV platforms. This paper provides a brief overview of new interlayer materials, how they function, their limitations, and surface and interface science approaches to their optimization. © 2011 IEEE.
• Steirer, K. X., Ndione, P. F., Widjonarko, N. E., Lloyd, M. T., Meyer, J., Ratcliff, E. L., Kahn, A., Armstrong, N. R., Curtis, C. J., Ginley, D. S., Berry, J. J., & Olson, D. C. (2011). Enhanced efficiency in plastic solar cells via energy matched solution processed NiO _x interlayers. Advanced Energy Materials, 1(5), 813-820.
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  Abstract: We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work-function of the hole collecting indium-tin oxide (ITO) contact, modifi ed with a solution-processed nickel oxide (NiO x) hole-transport layer (HTL), is matched to the ionization potential of the donor material in a bulk-heterojunction solar cell. Addition of the NiO x HTL to the hole collecting contact results in a power conversion Efficiency ( PCE ) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiO x films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiO x HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiO x HTL is shown to provide superior contact properties by increasing the ITO/NiO x contact work-function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiO x interface with the poly(N-9′-heptadecanyl-2,7-carbazole-alt- 5,5-(4′,7′-di- 2-thienyl-2′,1′,3′-benzothidiazole) (PCDTBT):[6,6]-phenyl-C61 butyric acid methyl ester PCBM and [6,6]-phenyl-C71 butyric acid methyl ester (PC 70 BM) active layer. A high work-function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Wang, W., Placencia, D., & Armstrong, N. R. (2011). Planar and textured heterojunction organic photovoltaics based on chloroindium phthalocyanine (ClInPc) versus titanyl phthalocyanine (TiOPc) donor layers. Organic Electronics: physics, materials, applications, 12(2), 383-393.
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  Abstract: Planar and textured heterojunction solar cells (OPVs) are reported for vacuum deposited chloroindium phthalocyanine (ClInPc)/C60 heterojunctions, and their response compared to previously explored OPVs based on titanyl phthalocyanine (TiOPc)/C60 heterojunctions. As for TiOPc/C60 OPVs, the photoelectrical activity of ClInPc/C60 OPVs extends well into the near infrared, with good activity out to ca. 900 nm. As-deposited ClInPc films (Phase I) produce open-circuit photopotentials, VOC as high as ca. 0.8 V (Phase I form of ClInPc), ca. 0.15 V larger than previously observed for Phase I TiOPc/C60 OPVs. The offsets in frontier orbital energies (EHOMOPc-ELUMOC60) revealed by UV-photoemission studies (UPS) are slightly smaller for ClInPc/C60 versus TiOPc/C 60 heterojunctions, and the interface dipole contribution (shift in local vacuum level) to these offsets is in the opposite direction for ClInPc/C60 versus TiOPc/C60 heterojunctions, or missing altogether, suggesting differences in molecular interaction at the Pc/C 60 interface. Higher VOC values are correlated with lower reverse saturation currents, Jo, for ClInPc/C60, versus other Pc or pentacene/C60 heterojunctions, suggesting weak intermolecular interactions at the ClInPc/C60 interface and large barriers to dark charge injection. Solvent annealing of the ClInPc films enhances the near-IR response, and textures the Pc film, enhancing the Pc/C 60 interfacial contact area and the short-circuit photocurrent, JSC. JSC under AM 1.5 illumination conditions was estimated by integration of the incident photon current efficiency (IPCE) response, to compare relative power conversion efficiencies for the two different device types. The estimated efficiency of Phase I ClInPc/C 60 OPVs is ca. 2.6%. The estimated AM 1.5 efficiency of ClInPc/C 60 OPVs with solvent annealed Pc layers is estimated to be ca. 3.3%, arising from the extensive texturing achieved of the Pc layer, which nearly doubles JSC for the Phase II versus Phase I Pc films. © 2010 Elsevier B.V. All rights reserved.
• Zacher, B., & Armstrong, N. R. (2011). Modeling the effects of molecular length scale electrode heterogeneity in organic solar cells. Journal of Physical Chemistry C, 115(51), 25496-25507.
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  Abstract: Kinetic Monte Carlo simulations of planar heterojunction (PHJ) organic solar cells (OPVs) constructed with electronically heterogeneous electrodes are presented which correlate the extent and length scale of electrode heterogeneity with their capacity for collecting photogenerated charge carriers. The PHJ OPV is modeled as an ensemble of discrete 1 nm 3 molecular sites and 1 nm 2 electrode sites for which we individually assign various effective activation energies for charge hopping. Utilizing Marcus theory to describe charge transfer reactions within the energetically disordered lattice, described by a Gaussian distribution of discrete site energies, we demonstrate the sensitivity of solar cell device performance to nanometer length scale heterogeneity at the electrode-active layer contact. Such sensitivity is reflected in the steady-state charge density profiles in the vicinity of the electrode-active material interface, charge collection efficiencies, and rates of recombination at the donor-acceptor (D/A) interface. Additionally, we demonstrate how implementation of idealized interlayers placed between the electrode and active layer functions to mitigate the negative effects of electrode heterogeneity. © 2011 American Chemical Society.
• Araci, Z. O., Shallcross, C. R., Armstrong, N. R., & Saavedra, S. S. (2010). Potential-modulated attenuated total reflectance characterization of charge injection processes in monolayer-tethered CdSe nanocrystals. Journal of Physical Chemistry Letters, 1(12), 1900-1905.
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  Abstract: Reversible electron injection into pyridine-capped CdSe nanocrystals (pyr-CdSe NCs), tethered to indium-tin oxide (ITO) substrates using mercaptoalkylcarboxylic acids, is characterized using attenuated total reflectance (ATR) spectroelectrochemistry on a planar waveguide. The sensitivity of this technique provides for characterization of redox processes in submonolayer films of pyr-CdSe NCs. Optically determined onset potentials for electron injection, measured as bleaching/recovery of the exciton absorption band, provide estimates for the conduction band edge (ECB). Potential-modulated attenuated total reflectance (PM-ATR), in which the in-phase and out-of-phase reflectance response is measured as a function of modulation frequency, provides estimates for rates of electron injection. These apparent rate constants are found to be nearly independent of tether chain length, suggesting that communication between tethered NCs and electrochemically less active (i.e., less conductive) regions on the ITO surface is rate-limiting. © 2010 American Chemical Society.
• Armstrong, N. R. (2010). A planar, chip-based, dual-beam refractometer using an integrated organic light-emitting diode (OLED) light source and organic photovoltaic (OPV) detectors. Analytical Chemistry.
• Armstrong, N. R. (2010). Evaluation of transparent carbon nanotube networks of homogeneous electronic type. ACS Nano.
• Armstrong, N. R. (2010). Ferrocene functional polymer brushes on indium tin oxide via surface-initiated atom transfer radical polymerization. Langmuir.
• Armstrong, N. R. (2010). Photoelectrochemical processes in polymer-tethered CdSe nanocrystals. Journal of the American Chemical Society.
• Armstrong, N. R. (2010). Photoemission spectroscopy of tethered CdSe nanocrystals: Shifts in ionization potential and local vacuum level as a function of nanocrystal capping ligand. ACS Applied Materials and Interfaces.
• Armstrong, N. R. (2010). Potential-modulated attenuated total reflectance characterization of charge injection processes in monolayer-tethered CdSe nanocrystals. Journal of Physical Chemistry Letters.
• Armstrong, N. R. (2010). Self-organized thin films of hydrogen-bonded phthalocyanines: Characterization of structure and electrical properties on nanometer length scales. Chemistry of Materials.
• Armstrong, N. R. (2010). Synthesis and colloidal polymerization of ferromagnetic Au-Co nanoparticles into Au-Co3O4 nanowires. Journal of the American Chemical Society.
• Armstrong, N. R. (2010). Work function control of hole-selective polymer/ITO anode contacts: An electrochemical doping study. Journal of Materials Chemistry.
• Armstrong, N. R., Placencia, D., Manglesdorf, D., & Wang, W. (2010). Small molecule organic solar cells with enhanced near-IR photoactivity: The role of texturing and molecular architecture of the active layers on solar cell performance. Conference Record of the IEEE Photovoltaic Specialists Conference, 503-507.
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  Abstract: Small molecule organic solar cells (OPV), using donor layers based on tetravalent and trivalent metal phthalocyanines (e.g. oxo-titanium TiOPc, or chloroindium CllnPc), are discussed here. Asdeposited thin films of these Pcs undergo phase changes to form polymorphs with excellent near-IR photoactivity, accompanied by nano-texturing of the donor layers. In OPVs based on Pc/C 60heterojunctions, this results in a factor of up to 5x increase in short-circuit photocurrents (Jsc) and power conversion efficiencies. Fully optimized single junction OPVs based on these materials may ultimately approach 6% efficiencies, however their biggest impact is in providing guidance for the formation of new, solution processable forms of these materials, which retain this near-IR sensitivity and high power conversion efficiencies. © 2010 IEEE.
• Hotchkiss, P. J., Sharma, A., Haldi, A., Potscavage Jr., W. J., Hong, L. i., Paramonov, P. B., Paniagua, S. A., Jones, S. C., Kippelen, B., Armstrong, N. R., Brödas, J., Barlow, S., Kim, P., Perry, J. W., Marder, S. R., Hotchkiss, P. J., Sharma, A., Haldi, A., Potscavage Jr., W. J., , Hong, L. i., et al. (2010). Polymers and composites for efficient production storage and utilization of energy. ACS National Meeting Book of Abstracts.
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  Abstract: Polymers can play an important role in the production, storage, and efficient utilization of energy. In this talk I will provide an overview of recent results on the development of polymer/nanoparticle composites for capacitor applications, and polymers for use in organic photovoltaics and organic light-emitting diodes.
• Jackson, R. K., Munro, A., Nebesny, K., Armstrong, N., & Graham, S. (2010). Evaluation of transparent carbon nanotube networks of homogeneous electronic type. ACS Nano, 4(3), 1377-1384.
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  PMID: 20201542;Abstract: In this report, we present a description of the optical and electronic properties of as-deposited, annealed, and chemically treated single-walled carbon nanotube (SWNT) films showing metallic or semiconducting behavior. As-deposited and annealed semiconducting SWNT films were significantly less conductive than metallic SWNT films; however, chemical treatment of semiconducting SWNT films resulted in sheet resistance values as low as 60 Ω · sq-1 in comparison to 76 Ω · sq -1 for similarly processed metallic SWNT films. We conclude that the greater improvement of electrical conductivity observed in the semiconducting SWNT film results from the difference in the density of available electronic states between metallic and semiconducting SWNTs. A corroborative investigation of the change in surface work function and the chemical composition of SWNT films, as revealed by X-ray photoelectron spectroscopy, is provided to support these conclusions and to give new perspective to the formation of electronically homogeneous SWNT networks. © 2010 American Chemical Society.
• Jenkins, J. L., Araci, Z. O., Shallcross, R. C., Ratcliff, E. L., Saavedra, S. S., Armstrong, N. R., Jenkins, J. L., Araci, Z. O., Shallcross, R. C., Ratcliff, E. L., Saavedra, S. S., & Armstrong, N. R. (2010). Advances in potential-modulated attenuated total reflectance (PM-ATM) spectroscopy for characterization of transparent metal oxide/semiconductor interfaces relevant to solar energy conversion. ACS National Meeting Book of Abstracts.
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  Abstract: The atomic- and molecular-level behavior of materials at transparent metal oxide/semiconductor interfaces significantly affects the overall photoconversion efficiency of photovoltaics. However, it is often difficult to isolate and characterize this critical interface. This talk will describe recent advances in the application of potential-modulated attenuated total reflectance (PM-ATR) spectroscopy to probe indium tin oxide (ITO)/CdSe nanocrystal interfaces. The ITO/CdSe interface serves as a chemically modified working electrode in a traditional 3-electrode electrochemical cell. Potential-dependant absorbance changes in the nanocrystals are monitored as the electrode potential is modulated at a range of frequencies. The phase shift of the optical change relative to the potential modulation frequency reflects interfacial properties that may be relevant to charge transfer dynamics. Waveguide geometry affords the sensitivity to examine sub-monolayer coverages of nanocrystals. By combining optical and electrochemical data, PM-ATR gives insight into the fundamental chemical properties governing the behavior of this interface while also enabling more strategic design of future energy conversion platforms.
• Kim, B. Y., Armstrong, N. R., Pyun, J., Kim, B. Y., Armstrong, N. R., & Pyun, J. (2010). Colloidal polymerization: A route to heterostructured semiconductor nanowires for energy. ACS National Meeting Book of Abstracts.
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  Abstract: We will discuss our efforts in the developmentof a new synthetic methodology, termed, "Colloidal Polymerization", where polymer coated ferromagnetic nanoparticles are used as "colloidal monomers" to form fused 1-D nanomaterials of cobalt oxide. The synthesis and electrochemical characterization of these materials will be discussed. These materials are currently being evaluated as nanostructured electrode materials of Li-batteries and supercapacitors.
• Kim, B. Y., Shim, I., Araci, Z. O., Saavedra, S. S., L., O., Armstrong, N. R., Sahoo, R., Srivastava, D. N., & Pyun, J. (2010). Synthesis and colloidal polymerization of ferromagnetic Au-Co nanoparticles into Au-Co₃O₄ nanowires. Journal of the American Chemical Society, 132(10), 3234-3235.
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  PMID: 20163141;Abstract: (Figure Presented) The preparation of cobalt oxide nanowires with gold nanoparticle (AuNP) inclusions (Au-Co3O4 nanowires) via colloidal polymerization of dipolar core-shell NPs is reported. Polystyrene-coated ferromagnetic NPs composed of a dipolar metallic cobalt shell and a gold NP core (PS-AuCoNPs) were synthesized by thermolysis of octacarbonyldicobalt [Co2(CO)8] in the presence of AuNP seeds and polymeric ligands. The colloidal polymerization process of these dipolar PS-AuCoNPs comprises dipolar nanoparticle assembly and solution oxidation of preorganized NPs to form interconnected cobalt oxide nanowires via the nanoscale Kirkendall effect, with AuNP inclusions in every repeating unit of the one-dimensional mesostructure. Calcination of the polymer-coated nanowires afforded polycrystalline Au-Co3O4 nanowires that were determined to be electroactive. Nanocomposite materials were characterized by transmission electron microscopy, field-emission scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and cyclic voltammetry. We demonstrate that the optical and electrochemical properties of Au-Co 3O4 nanowires are significantly enhanced in comparison with hollow Co3O4 nanowires prepared via colloidal polymerization. Copyright © 2010 American Chemical Society.
• Kim, B., Ratcliff, E. L., Armstrong, N. R., Kowalewski, T., & Pyun, J. (2010). Ferrocene functional polymer brushes on indium tin oxide via surface-initiated atom transfer radical polymerization. Langmuir, 26(3), 2083-2092.
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  PMID: 19968255;Abstract: The synthesis and electrochemical characterization of ferrocene functional polymethacrylate brushes on indium tin oxide (ITO) electrodes using surface-initiated atom transfer radical polymerization (SI-ATRP) is reported. SI-ATRP of ferrocene-containing methacrylate (FcMA) monomers from a phosphonic acid initiator-modified ITO substrate yielded well-defined, homo- and block (co)polymer brushes of varying molar mass (4,000 to 37,000 g/mol). Correlation of both electrochemical properties and brush thicknesses confirmed, controlled. SI-ATRP from modified ITO surfaces. The preparation of block copolymer brushes with varying sequences of FcMA. segments was conducted to interrogate the effects of spacing from the ITO electrode surface on the electrochemical properties of a tethered electroactive film. © 2009 American Chemical Society.
• Kumaran, N., Veneman, P. A., Minch, B. A., Mudalige, A., Pemberton, J. E., O'Brien, D. F., & Armstrong, N. R. (2010). Self-organized thin films of hydrogen-bonded phthalocyanines: Characterization of structure and electrical properties on nanometer length scales. Chemistry of Materials, 22(8), 2491-2501.
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  Abstract: We present the structural and electrical property characterization of solution-deposited thin films of a self-organizing phthalocyanine 2,3,9,10,16,17,23,24-octa(2-(4-octylbenzamide ethyl-sulfanyl) copper(II) phthalocyanine, Pc (1). Self-organization into columnar aggregates occurs mainly as a result of hydrogen bonding interactions between the benzamide groups in the side chains. These H-bonding interactions lead to ordered films of Pc (1), with the plane of the Pc parallel to the substrate plane, on gold surfaces modified with amide or amine functionalities, and on freshly cleaved highly order pyrolitic graphite (HOPG). Atomic force microscopy (AFM) confirms layer-by-layer growth for these Pc films up to coverages of ∼10 monolayers, when deposition occurs from dilute solutions of the Pc. Conductive-tip AFM (C-AFM) measurements of these ordered Pc films, on modified Au and HOPG substrates, confirm the layered nature of these Pc films and show that the conductance decreases incrementally with the number of Pc monolayers being probed. On HOPG substrates, these measurements lead to an estimation of the conductance per Pc monolayer, and the Pc molecule-HOPG contact resistance of 8 Mω, a value which is comparable with previously explored organic semiconductor materials. © 2010 American Chemical Society.
• Munro, A. M., Zacher, B., Graham, A., & Armstrong, N. R. (2010). Photoemission spectroscopy of tethered CdSe nanocrystals: Shifts in ionization potential and local vacuum level as a function of nanocrystal capping ligand. ACS Applied Materials and Interfaces, 2(3), 863-869.
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  PMID: 20356292;Abstract: We report the characterization of the frontier orbital energies and interface dipole effects for bare and ligand-capped 3.6 and 6.0 nm diameter CdSe nanocrystals (NC) tethered to smooth gold substrates, using He(I) and He(II) UV photoemission spectroscopy. Changes in the ionization potential (IP) of the NCs and local effective work function of the films were explored as a function of the dipolar nature of the NC capping ligands. The addition of thiol-capping ligands 1-hexanethiol, 1-benzenethiol, and 4-fluorothiophenol to both sizes of NCs produces negligible shifts in energy offset between the high kinetic energy edge of the CdSe NCs and the gold substrate Fermi energy. However, the local vacuum level and IP of the nanocrystal layer are altered by as much as 0.3 eV. We demonstrate the importance of determining both the local vacuum level and the high kinetic energy edge of a tethered NC sample. These studies demonstrate a method that can be used in the future to characterize the frontier orbital energy offsets for modified or unmodified nanocrystalline films, in which the NCs are incorporated into host materials, for applications ranging from photovoltaics to light-emitting diodes. © 2010 American Chemical Society.
• Ratcliff, E. L., Lee, P. A., & Armstrong, N. R. (2010). Work function control of hole-selective polymer/ITO anode contacts: An electrochemical doping study. Journal of Materials Chemistry, 20(13), 2672-2679.
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  Abstract: We present a novel method for electrodeposition of ultra-thin films of poly-3-hexylthiophene (e-P3HT) on chemically modified indium-tin oxide (ITO) electrodes, to produce a hole-selective contact with an easily tuned work function (Φ), as demonstrated by a combination of spectroelectrochemistry and ultraviolet photoemission spectroscopy (UPS). Selective contacts for optimized charge injection have become essential components for both thin film organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Electrochemically doped e-P3HT thin films, using counter ions such as PF 6- do not suffer from stability issues associated with more "acidic" polymer layers (e.g. PEDOT:PSS). By controlling the oxidation state of the e-P3HT film via electrochemical doping we control the charge density within the film, resulting in an increase in work function with an increase in degree of oxidation. The method of electrochemical formation and doping of the e-P3HT film, using either constant potential step (CA) versus pulsed-potential step (PPS) protocols, has a significant secondary impact on the work function, as a result of the interface dipole effects from entrapment of these counter ions in the near-surface region of the polymer film. These results have significance for the performance of both OLEDs and OPVs built on these doped e-P3HT layers. © 2010 The Royal Society of Chemistry.
• Ratcliff, E. L., Shallcross, R. C., Lee, P. A., Armstrong, N. R., Ratcliff, E. L., Shallcross, R. C., Lee, P. A., Armstrong, N. R., Ratcliff, E. L., Shallcross, R. C., Lee, P. A., Armstrong, N. R., Ratcliff, E. L., Shallcross, R. C., Lee, P. A., & Armstrong, N. R. (2010). Electrodeposited poly(thiophene) thin film contacts for organic photovoltaics and organic light emitting diodes. ACS National Meeting Book of Abstracts.
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  Abstract: We discuss a unique approach to the formation of hole-selective contacts on transparent conducting oxides, impacting both organic solar cell (OPV) and organic light emitting diode (OLED) technologies. We use electrodeposition, on activated and chemically modified indium-tin oxide (ITO) electrodes to grow poly(thiophene) (e.g. e-P3HT) films whose electrical properties and texturing at nanometer length scales is controlled by the electrodeposition protocol. Prior studies of OPVs created from these materials have been described in Chem. Mater. 2008, 20, 5796-5806. This talk will focus on the control of effective work function in e-P3HT films by control of counter ion placement and degree of formation of polaronic or bi-polaronic states, using photoemission spectroscopies. Changes in doping in these films has a profound effect on both turn-on voltages in OLEDs and fill-factors and series resistance in OPVs.
• Ratcliff, E. L., Veneman, P. A., Simmonds, A., Zacher, B., Huebner, D., Saavedra, S., & Armstrong, N. R. (2010). A planar, chip-based, dual-beam refractometer using an integrated organic light-emitting diode (OLED) light source and organic photovoltaic (OPV) detectors. Analytical Chemistry, 82(7), 2734-2742.
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  PMID: 20218580;PMCID: PMC2956611;Abstract: We present a simple chip-based refractometer with a central organic light-emitting diode (OLED) light source and two opposed organic photovoltaic (OPV) detectors on an internal reflection element (IRE) substrate, creating a true dual-beam sensor platform. For first-generation platforms, we demonstrate the use of a single heterojunction OLED based on electroluminescence from an Alq3/TPD heterojunction (tris-(8-hydroxyquinoline)aluminum/N,N'- bis(3-methylphenyl)-N,N'-diphenylbenzidine) and light detection with planar heterojunction pentacene/C60 OPVs. The sensor utilizes the considerable fraction of emitted light from conventional thin-film OLEDs that is coupled into guided modes in the IRE, instead of into the forward (display) direction. A ray-optics description is used to describe light throughput and efficiency-limiting factors for light coupling from the OLED into the substrate modes, light traversing through the IRE substrate, and light coupling into the OPV detectors. The arrangement of the OLED at the center of the chip provides for two sensing regions: a "sample" channel and a " reference" channel, with detection of light by independent OPV detectors. This configuration allows for normalization of the sensor response against fluctuations in OLED light output, stability, and local fluctuations (temperature) that might influence sensor response. The dual-beam configuration permits significantly enhanced sensitivity to refractive index changes, relative to singlebeam protocols, and is easily integrated into a fieldportable instrumentation package. Changes in refractive index (ΔRI) between 10 -2 and 10-3 RI units could be detected for single beam operation, with sensitivity increased to ΔRI ≈ 10-4 RI units when the dualbeam configuration is employed. © 2010 American Chemical Society.
• Shallcross, R. C., D'Ambruoso, G. D., Pyun, J., & Armstrong, N. R. (2010). Photoelectrochemical processes in polymer-tethered CdSe nanocrystals. Journal of the American Chemical Society, 132(8), 2622-2632.
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  PMID: 20136125;Abstract: We demonstrate the electrochemical capture of CdSe semiconductor nanocrystals (NCs), with thiophene-terminated carboxylic acid capping ligands, at the surfaces of electrodeposited poly(thiophene) films (i) poly((diethyl)propylenedixoythiophene), P(Et)2ProDOT; (ii) poly(propylenedioxythiophene), PProDOT; and (iii) poly(ethylenedioxythiophene), PEDOT, coupled with the exploration of their photoelectrochemical properties. Host polymer films were created using a kinetically controlled electrodeposition protocol on activated indium-tin oxide electrodes (ITO), producing conformai films that facilitate high rates of electron transfer. ProDOT-terminated, ligand-capped CdSe-NCs were captured at the outer surface of the host polymer films using a unique pulse-potential step electrodeposition protocol, providing for nearly closepacked monolayers of the NCs at the host polymer/solution interface. These polymer-confined CdSe NCs were used as sensitizers in the photoelectrochemical reduction of methyl viologen (MV+2). High internal quantum efficiencies (IQEs) are estimated for photoelectrochemical sensitized MV+2 reduction using CdSe NCs ranging from 3.1 to 7.0 nm diameters. Cathodic photocurrent at high MV+2 concentrations are limited by the rate of hole-capture by the host polymer from photoexcited NCs. The rate of this hole-capture process is determined by (a) the onset potential for reductive dedoping of the host polymer film; (b) the concentration ratio of neutral to oxidized forms of the host polymer ([P(n)]/[P(ox)]); and (c) the NC diameter, which controls its valence band energy, EVB. These relationships are consistent with control of photoinduced electron transfer by Marcus-like excess free energy relationships. Our electrochemical assembly methods provide an enabling route to the capture of functional NCs in conducting polymer hosts in both photoelectrochemical and photovoltaic energy conversion systems. © 2010 American Chemical Society.
• Alloway, D. M., & Armstrong, N. R. (2009). Organic heterojunctions of layered perylene and phthalocyanine dyes: Characterization with UV-photoelectron spectroscopy and luminescence quenching. Applied Physics A: Materials Science and Processing, 95(1), 209-218.
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  Abstract: We present here the characterization of organic/organic′ heterojunctions created from either of two perylene dyes, perylenetetracarboxylicdianhydride (PTCDA) or the bisimide derivative perylenetetracarboxylicdianhydride-N,N′-bis (butyl)imide (C4-PTCDI), and two chloro-metallated donor phthalocyanines (ClAlPc or ClInPc). The perylene dyes were selected to create thin films with the core of the perylene dye parallel to the substrate plane (PTCDA) or nearly vertical to the substrate plane, with layer planes defined by the butyl substituents (C4-PTCDI). We compare the frontier orbital offsets revealed by UV-photoelectron spectroscopy, and quenching of luminescence of the perylene dyes, as a function of Pc coverage. The ionization potentials (IPs) of the Pc layers, the degree to which interface dipoles are formed at the Pc/perylene dye interface, and the degree of quenching of the perylene luminescence are affected by the structure of the Pc/perylene interface. Pc/PTCDA heterojunctions show significant interface dipoles and higher IPs for the first-deposited Pc layers compared to Pc/C4-PTCDI heterojunctions, which show negligible interface dipoles and lower overall IP values for initial Pc layers. Luminescence of the selectively excited perylene layers is quenched by the addition of even submonolayer coverages of Pc. This quenching process occurs as a result of both energy transfer (perylene to Pc) and charge transfer (Pc to perylene). Luminescence from monomeric and aggregated ClAlPc and ClInPc monolayers is seen on C4-PTCDI films, whereas only luminescence from the aggregated forms of these Pcs is seen on PTCDA films. These studies reveal aspects of organic heterojunction energetics which may have important implications for organic solar cell design. © 2009 Springer-Verlag.
• Alloway, D. M., Graham, A. L., Yang, X., Mudalige, A., Colorado Jr., R., Wysocki, V. H., Pemberton, J. E., Lee, T. R., Wysocki, R. J., & Armstrong, N. R. (2009). Tuning the effective work function of gold and silver using ω-functionalized alkanethiols: Varying surface composition through dilution and choice of terminal groups. Journal of Physical Chemistry C, 113(47), 20328-20334.
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  Abstract: Thiol-based self-assembled monolayers (SAMs) have been used to tune the effective work function of gold over a range of ca. 1.8 eV via two strategies: (i) the use of ω-functionalized alkanethiols where the tail groups have widely varying electronegativity or (ii) by the creation of two-component SAMs from selected mixtures of methyl-terminated alkanethiols (C16) and alkanethiols fluorinated at the two terminal carbon atoms (C16F2). UV-photoelectron spectroscopy (UPS) was used to monitor changes in effective work function, using shifts in the low kinetic energy edge of these photoemission spectra to quantify the shift in local vacuum level resulting from the interface dipole effect created by the surface modifier. Tail groups on alkanethiol chains varied from -CH3, to -phenyl, -Cl, -Br, and -CF3 or -CF 2CF3, which provided a shift in local vacuum level that varied linearly with the calculated molecular dipole moment of the individual modifiers, as observed previously for a more limited range of alkanethiols (J. Phys. Chem. B 2003, 107, 11690). The studies presented here confirm that the intrinsic dipole in the gold-thiolate bond is small (less than 100 meV), whereas the silver-thiolate bond appears to have a strongly polar character, in the direction Ag+-S- (ca. 900 meV). The use of a simple point dipole model to rationalize these apparent shifts in vacuum level was further explored using SAMs derived from various mixtures of C16 and C16F2. The low kinetic energy edge in the UV-photoemission spectra and the effective work function are observed to increase monotonically in energy with increasing C16F2 coverage, confirming that little surface segregation occurs in these selfassembled monolayers over a wide concentration range. © 2009 American Chemical Society.
• Armstrong, N. R. (2009). An electroactive fiber optic chip for spectroelectrochemical characterization of ultra-thin redox-active films. Analyst.
• Armstrong, N. R. (2009). Colloidal polymerization of polymer-coated ferromagnetic nanoparticles into cobalt oxide nanowires. ACS Nano.
• Armstrong, N. R. (2009). Efficient CdSe nanocrystal diffraction gratings prepared by microcontact molding. ACS Nano.
• Armstrong, N. R. (2009). Modification of the surface properties of indium tin oxide with benzylphosphonic acids: A joint experimental and theoretical study. Advanced Materials.
• Armstrong, N. R. (2009). Organic heterojunctions of layered perylene and phthalocyanine dyes: Characterization with UV-photoelectron spectroscopy and luminescence quenching. Applied Physics A: Materials Science and Processing.
• Armstrong, N. R. (2009). Organic photovoltaic cells based on solvent-annealed, textured titanyl phthalocyanine/c60 heterojunctions. Advanced Functional Materials.
• Armstrong, N. R. (2009). Organic/organic' heterojunctions: Organic light emitting diodes and organic photovoltaic devices. Macromolecular Rapid Communications.
• Armstrong, N. R. (2009). Oxide contacts in organic photovoltaics: Characterization and control of near-surface composition in indium-tin oxide (ITO) electrodes. Accounts of Chemical Research.
• Armstrong, N. R. (2009). Site-isolated, intermolecularly photocrosslinkable and patternable dendritic quinacridones. Chemical Communications.
• Armstrong, N. R. (2009). Tuning the effective work function of gold and silver using ω-functionalized alkanethiols: Varying surface composition through dilution and choice of terminal groups. Journal of Physical Chemistry C.
• Armstrong, N. R., Veneman, P. A., Ratcliff, E., Placencia, D., & Brumbach, M. (2009). Oxide contacts in organic photovoltaics: Characterization and control of near-surface composition in indium-tin oxide (ITO) electrodes. Accounts of Chemical Research, 42(11), 1748-1757.
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  PMID: 19728725;Abstract: (Figure presented) The recent improvements in the power conversion efficiencies of organic photovoltaic devices (OPVs) promise to make these technologies increasingly attractive alternatives to more established photovoltaic technologies. OPVs typically consist of photoactive layers 20-100 nm thick sandwiched between both transparent oxide and metallic electrical contacts. Ideal OPVs rely on ohmic top and bottom contacts to harvest photogenerated charges without compromising the power conversion efficiency of the OPV. Unfortunately, the electrical contact materials (metals and metal oxides) and the active organic layers in OPVs are often incompatible and may be poorly optimized for harvesting photogenerated charges. Therefore, further optimization of the chemical and physical stabilities of these metal oxide materials with organic materials will be an essential component of the development of OPV technologies. The energetic and kinetic barriers to charge injection/collection must be minimized to maximize OPV power conversion efficiencies. In this Account, we review recent studies of one of the most common transparent conducting oxides (TCOs), indium-tin oxide (ITO), which is the transparent bottom contact in many OPV technologies. These studies of the surface chemistry and surface modification of ITO are also applicable to other TCO materials. Clean, freshly deposited ITO is intrinsically reactive toward H2O, CO, CO2, etc. and is often chemically and electrically heterogeneous in the near-surface region. Conductive-tip atomic force microscopy (C-AFM) studies reveal significant spatial variability in electrical properties. We describe the use of acid activation, small-molecule chemisorption, and electrodeposition of conducting polymer films to tune the surface free energy, the effective work function, and electrochemical reactivity of ITO surfaces. Certain electrodeposited poly(thiophenes) show their own photovoltaic activity or can be used as electronically tunable substrates for other photoactive layers. For certain photoactive donor layers (phthalocyanines), we have used the polarity of the oxide surface to accelerate dewetting and "nanotexturing" of the donor layer to enhance OPV performance. These complex surface chemistries will make oxide/organic interfaces one of the key focal points for research in new OPV technologies. © 2009 American Chemical Society.
• Armstrong, N. R., Wang, W., Alloway, D. M., Placencia, D., Ratcliff, E., & Brumbach, M. (2009). Organic/organic' heterojunctions: Organic light emitting diodes and organic photovoltaic devices. Macromolecular Rapid Communications, 30(9-10), 717-731.
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  PMID: 21706658;Abstract: Heterojunctions created from thin films of two dissimilar organic semiconductor materials [organic/organic' (O/O') heterojunctions] are an essential component of organic light emitting diode displays and lighting systems (OLEDs, PLEDs) and small molecule or polymer-based organic photovoltaic (solar cell) technologies (OPVs). O/O' heterojunctions are the site for exciton formation in OLEDs, and the site for exciton dissociation and photocurrent production in OPVs. Frontier orbital energy offsets in O/O' heterojunctions establish the excess free energy controlling rates of charge recombination and formation of emissive states in OLEDs and PLEDs. These energy offsets also establish the excess free energy which controls charge separation and the shortcircuit photocurrent (JSc) in OPVs, and set the upper limit for the open-circuit photopotential (Vbc). We review here how these frontier orbital energy offsets are determined using photoemission spectroscopies, how these energies change as a function of molecular environment, and the influence of interface dipoles on these frontier orbital energies. Recent examples of hetero-junctions based on small molecule materials are shown, emphasizing those heterojunctions which are of interest for photovoltaic applications. These include heterojunctions of perylenebisimide dyes with trivalent metal phthalocyanines, and heterojunctions of titanyl phthalocyanine with C 6o, and with pentacene. Organic solar cells comprised of donor/ ¡ acceptor pairs of each of these last three materials confirm that the Vbc scales with the energy offsets between the HOMO of the donor and LUMO of the acceptor (EUOMOD - ELVMQA). © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.
• Beam, B. M., Armstrong, N. R., & Mendes, S. B. (2009). An electroactive fiber optic chip for spectroelectrochemical characterization of ultra-thin redox-active films. Analyst, 134(3), 454-459.
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  PMID: 19238279;Abstract: The first, fully integrated, planar fiber optic platform with spectroelectrochemical capabilities, termed the electroactive fiber optic chip (EA-FOC) is presented here. Spectroelectrochemical techniques provide complementary optical and electrochemical data which are important for applications ranging from thin film characterization to advanced sensor design. To create the EA-FOC a side-polished fiber optic is coated with a thin film of indium-tin oxide (ITO) as the working electrode and used to probe electrochemically-driven changes in absorbance for surface-confined redox species. A sensitivity enhancement of ∼40 times higher than a transmission measurement is demonstrated for this first-generation EA-FOC, using the methylene blue (MB) redox couple, cycling between the visibly colored, oxidized form of MB, and its leuco (transparent) reduced form. Additionally, the EA-FOC is used to probe the redox spectroelectrochemistry of an electrodeposited thin film, about 0.3% of a monolayer, of the conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT). Unlike other waveguide formats, the EA-FOC offers an ease of use due to its ability to simply couple to light sources and detectors through standard fiber connectors to create a sensitive planar waveguide spectroelectrochemical platform. © The Royal Society of Chemistry.
• Beam, B. M., Simmonds, A., Veneman, P. A., Ratcliff, E., Mendes, S. B., Saavedra, S. S., & Armstrong, N. R. (2009). Waveguide-based chemical and spectroelectrochemical sensor platforms. ECS Transactions, 19(6), 109-117.
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  Abstract: Here we review the progress in the development of waveguide-based spectroelectrochemical platforms, with special emphasis on the most recent technologies: the electroactive fiber-optic chip (EA-FOC) and chip-like spectroelectrochemical platforms using organic light emitting diode (OLED) light sources coupled with organic photovoltaic photodetectors (OPV-PD). Both technologies simplify spectroelectrochemical data collection through eliminating the need for free-space optics and benefit from the increased sensitivity of waveguide based devices producing new chemical sensor platforms. © The Electrochemical Society.
• D'Ambruoso, G. D., Ross, E. E., Armstrong, N. R., & McGrath, D. V. (2009). Site-isolated, intermolecularly photocrosslinkable and patternable dendritic quinacridones. Chemical Communications, 3222-3224.
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  PMID: 19587920;Abstract: Quinacridone-cored dendrimers with photocrosslinkable cinnamate moieties on the periphery can be patterned down to 5 micron features while retaining luminescence. © 2009 The Royal Society of Chemistry.
• Hotchkiss, P. J., Hong, L. i., Paramonov, P. B., Paniagua, S. A., Jones, S. C., Armstrong, N. R., Brédas, J., & Marder, S. R. (2009). Modification of the surface properties of indium tin oxide with benzylphosphonic acids: A joint experimental and theoretical study. Advanced Materials, 21(44), 4496-4501.
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  Abstract: (Figure Presented) Benzylphosphonic acids with various fluorine substitutions are designed and synthesized. They are used to modify ITO such that the work function can be tuned over a range of 1.2 eV while keeping the surface energy relatively constant. The experimentally measured work function changes are also compared to and agree well with those estimated from DFT calculations. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
• Keng, P. Y., Kim, B. Y., Ratcliff, E. L., Armstrong, N. R., & Pyun, J. (2009). Colloidal polymerization of polymer coated ferromagnetic nanoparticles: Chemistry on the mesoscale for energy storage and conversion. ACS National Meeting Book of Abstracts.
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  Abstract: The preparation of nanostructured semiconductor metal oxide materials has generated considerable interest as enhanced electrode materials for applications in energy storage and conversion. We will describe a facile and versatile method to prepare one-dimensional semiconducting metal oxide nanomaterials via the dipolar assembly and chemical transformation of metallic cobalt nanoparticles into cobalt oxide materials. We describe this process as a "Colloidal Polymerization" as dipolar nanoparticles are analogous to A-B monomers commonly used in step-growth polymerizations and donor-acceptor monomers utilized for supramolecular polymerizations. Synthetic aspects to control cobalt oxide nanowire composition, chain length and architecture will be discussed, along with electrical, spectroscopic and electrochemical characterization of these novel materials. The potential of these materials as nanostructured electrode materials for batteries, supercapacaitors and water-splitting will also be discussed.
• Keng, P. Y., Kim, B. Y., Shim, I., Sahoo, R., Veneman, P. E., Armstrong, N. R., Yoo, H., Pemberton, J. E., Bull, M. M., Griebel, J. J., Ratcliff, E. L., Nebesny, K. G., & Pyun, J. (2009). Colloidal polymerization of polymer-coated ferromagnetic nanoparticles into cobalt oxide nanowires. ACS Nano, 3(10), 3143-3157.
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  PMID: 19799415;Abstract: The preparation of polystyrene-coated cobalt oxide nanowires is reported via the colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Using a combination of dipolar nanoparticle assembly and a solution oxidation of preorganized metallic colloids, interconnected nanoparticles of cobalt oxide spanning micrometers in length were prepared. The colloidal polymerization of PS-CoNPs into cobalt oxide (CoO and Co 3O4) nanowires was achieved by bubbling O2 into PS-CoNP dispersions in 1,2-dichlorobenzene at 175 °C. Calcination of thin films of PS-coated cobalt oxide nanowires afforded Co3O4 metal oxide materials. Transmission electron microscopy (TEM) revealed the formation of interconnected nanoparticles of cobalt oxide with hollow inclusions, arising from a combination of dipolar assembly of PS-CoNPs and the nanoscale Kirkendall effect in the oxidation reaction. Using a wide range of spectroscopic and electrochemical characterization techniques, we demonstrate that cobalt oxide nanowires prepared via this novel methodology were electroactive with potential applications as nanostructured electrodes for energy storage. © 2009 American Chemical Society.
• Keng, P. Y., Ratcliff, E. L., Armstrong, N. R., & Pyun, J. (2009). Electroactive metal oxide nanowires via magnetic assembly and oxidation of polymer coated ferromagnetic nanoparticles. ACS National Meeting Book of Abstracts.
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  Abstract: We report a facile method to prepare polymer coated cobalt oxide nanowires via the solution dipolar assembly and oxidation of ferromagnetic cobalt nanoparticles. This phenomenon is an example of utilizing the magnetic assembly of polymer coated ferromagnetic cobalt nanoparticles and the nanoscale Kirkendall effect to prepare continuous nanowires of cobalt oxide. An overview of synthetic methods, morphological imaging via electron microscopy (TEM, FE-SEM) and electrochemical characterization of nanowire thin films on indium tin oxide and platinum electrodes will be discussed. The potential of these materials for nanostructured electrodes for batteries and water-splitting will also be discussed and explored more extensively in future research.
• Placencia, D., Wang, W., Shallcross, R. C., Nebesny, K. W., Brumbach, M., & Armstrong, N. R. (2009). Organic photovoltaic cells based on solvent-annealed, textured titanyl phthalocyanine/c60 heterojunctions. Advanced Functional Materials, 19(12), 1913-1921.
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  Abstract: Organic photovoltaic cells (OPV) with good near-IR photoactivity are created from highly textured titanyl phthalocyanine (TiOPc)/C60 heterojunctions. Vacuum deposited TiOPc thin films are converted to the near-IR absorbing "Phase II" polymorph using post-deposition solvent annealing. The Phase IρPhase II transition broadens the absorbance spectrum of the Pc film producing absorptivities (α=105 cm-1) from 600-900 nm, along with substantial texturing of the Pc layer. Atomic force microscopy and fieldemission scanning electron microscopy of the solvent annealed films show that the surface roughness of the Pc layers is increased by a factor of greater than 2× as a result of the phase transformation. Current-voltage ( J-V ) responses for white light illumination of ITO (100 nm)/TiOPc (20 nm)/C60 (40 nm)/BCP (10 nm)/Al (100 nm) OPVs show a near doubling of the shortcircuit photocurrent ( JSC), with only a small decrease in open-circuit photopotential (Voc), and a concomitant increase in power conversion efficiency. Incident photon current efficiency (IPCE) plots confirmed the enhanced near-IR OPV activity, with maximum IPCE values of ca. 30% for devices using Phase II-only TiOPc films. UV-photoelectron spectroscopy (UPS) of TiOPc/C60 heterojunctions, for both Phase I and Phase II TiOPc films, suggest that the Phase II polymorph has nearly the same HOMO energy as seen in the Phase I polymorph, and similar frontier orbital energy offsets, EHOMOPc-ELUMOC60, leading to comparable open-circuit photovoltages. These studies suggest new strategies for the formation of higher efficiency OPVs using processing conditions which lead to enhance near-IR absorptivities, and extensive texturing of crystalline donor or acceptor films. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.
• Shallcross, R. C., Chawla, G. S., Marikkar, F. S., Tolbert, S., Pyun, J., & Armstrong, N. R. (2009). Efficient CdSe nanocrystal diffraction gratings prepared by microcontact molding. ACS Nano, 3(11), 3629-3637.
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  PMID: 19803496;Abstract: We describe the formation of efficient transmission diffraction gratings created from patterned high quality ligand-capped CdSe nanocrystals (NCs), using a facile microcontact molding procedure. Soft polymer replicas of commercially available master gratings were "inked" with solvated NCs and the resulting pattern transferred to a variety of substrates after drying. Large-area (>0.5 cm2), defect free diffraction gratings were prepared with a variety of submicrometer line spacings and feature sizes down to ca. 160 nm. The morphology of the resulting pattern was tuned by controlling the concentration of the NC-based ink. Optimized gratings (1200 g/ mm) showed an increase in transmission diffraction efficiency (DE) with increasing nanocrystal diameter. DE = ca. 15% (488 nm) for 2.5 nm diameter NCs versus DE = ca.25 -30% (488 nm) for 7.3 nm nanocrystals. These increases in DE are ascribed to changes in both the real (n) and imaginary (k) components of the complex index of refraction as NC diameter increases. We demonstrate the ability to in- and out-couple incident laser radiation into internal reflection elements using these stamped NC gratings, including single-mode waveguides, offering a novel application of ordered nanocrystal thin films. © 2009 American Chemical Society.
• Armstrong, N. R. (2008). Characterization of the angular orientation distribution of discotic molecules in thin-film assemblies: Combinations of polarized transmission and reflection - absorption infrared spectroscopies. Journal of Physical Chemistry C.
• Armstrong, N. R. (2008). Electrodeposited, "textured" poly(3-hexyl-thiophene) (e-P3HT) films for photovoltaic applications. Chemistry of Materials.
• Armstrong, N. R. (2008). Phosphonic acid modification of indium-tin oxide electrodes: Combined XPS/UPS/ contact angle studies. Journal of Physical Chemistry C.
• Armstrong, N. R. (2008). Theoretical characterization of the indium tin oxide surface and of its binding sites for adsorption of phosphonic acid monolayers. Chemistry of Materials.
• Armstrong, N. R. (2008). Titanyl phthalocyanine/C60 heterojunctions: Band-edge offsets and photovoltaic device performance. Journal of Physical Chemistry C.
• Armstrong, N. R., Alloway, D., Graham, A., Shallcross, C., Brumbach, M., Veneman, P. A., Placencia, D., & Wang, W. (2008). Critical interfaces in new solar cell materials: Organic heterojunctions and heterojunctions involving semiconductor nanoparticles. Proceedings of SPIE - The International Society for Optical Engineering, 7034.
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  Abstract: The heterojunctions formed between different organic dyes (O/O′ heterojunctions), organic dyes with contacting oxide or metal electrodes (O/I heterojunctions), and semiconductor nanoparticles with organic host polymers and ligands (SC-NP/ O heterojunctions) must be understood and optimized in order to enhance the energy conversion efficiencies of photovoltaics using these materials as their active components. We have used combinations of UV-photoelectron spectroscopy, and X-ray photoelectron spectroscopy (UPS/XPS) in the characterization of representative heterojunctions, and extrapolate these studies to the optimization of new photovoltaic and photoelectrochemical energy conversion devices.
• Brumbach, M., Placencia, D., & Armstrong, N. R. (2008). Titanyl phthalocyanine/C₆₀ heterojunctions: Band-edge offsets and photovoltaic device performance. Journal of Physical Chemistry C, 112(8), 3142-3151.
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  Abstract: Planar heterojunction organic photovoltaic devices have been created using oxo-titanium phthalocyanine (TiOPc) as the donor layer and fullerene (C 60) as the acceptor layer, with comparisons to devices based on copper phthalocyanine (CuPc) as the donor. TiOPc/C60 and CuPc/C 60 heterojunctions were first characterized by a combination of UV-photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to estimate the frontier orbital energy offset (EHOMOD -ELUMoA), which is related to the open-circuit photopotential (VOC). A small interface dipole effect was seen at the TíOPC/C60 interface (eD ≈ 0.02 eV), whereas a significant interface dipóle was observed for the CuPc/C60 interface (eD ≈ 0.3 eV). On the basis of the work presented here and previously reported electrochemical and UPS/XPS studies, we estimate an E HOMOD - ELUMOA energy offset of ca. 1.1 eV for the TiOPc/C60 heteroj unction and 0.7 eV for the CuPc/C60 heteroj unction. Maximum Voc values observed at room temperature for corresponding planar heterojunction photovoltaic devices were 0.3-0.4 V lower than the energy offset potentials, even at high light intensities, where the maximum VOC, at room temperature, was achieved. TiOPc/C60 heterojunctions offer higher VOC values than CuPc/C60 heterojunctions, but with a lower intrinsic driving force for exciton dissociation (photoinduced charge transfer). © 2008 American Chemical Society.
• Jenkins, J. L., Ratcliff, E. L., Shallcross, R. C., Head, J. L., & Armstrong, N. R. (2008). Photovoltaic devices created from electrodeposited "nano- textured" poly(thiophene) films. Proceedings of SPIE - The International Society for Optical Engineering, 7047.
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  Abstract: Polythiophene films can be electrodeposited on modified ITO substrates, textured to increase their active surface area, doped to enhance charge transport, and then interfaced with C60 thin films to create "planar heterojunction" photovoltaic devices with power conversion efficiencies up to 1%. Preliminary results indicate that these electrodeposited films (e-P3HT) modified with appropriate ligands can serve as hosts for semi-conducting nanoparticles (CdSe NPs). These NPs may ultimately extend the device spectral sensitivity into the red and near-IR spectral regions.
• Kumaran, N., Donley, C. L., Mendes, S. B., & Armstrong, N. R. (2008). Characterization of the angular orientation distribution of discotic molecules in thin-film assemblies: Combinations of polarized transmission and reflection - absorption infrared spectroscopies. Journal of Physical Chemistry C, 112(13), 4971-4977.
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  Abstract: We present a protocol to determine the angular orientation of surface-confined discotic molecules in ordered thin-film assemblies through a combination of polarized reflection absorption infrared spectroscopy (RAIRS) and polarized transmission infrared (IR) spectroscopy. We focus here on the determination of the orientation of side-chain-modified disk-like phthalocyanine molecules that self-organize to form parallel columnar aggregates in Langmuir - Blodgett (LB) films. This work complements and follows on work published previously (Langmuir 2005,21,360-368) where we used the visible absorbance dichroism in similar assemblies to determine the tilt angles of discotic molecules. The approach described here is applicable to all disk-like molecules that possess distinct in-plane or out-of-plane IR transitions. Molecular orientation is determined by comparison of absorbance intensities that are measured using (i) a RAIRS spectrum of the thin-film material on a gold surface, (ii) transmission spectra taken at two different polarizations of the thin-film material on an IR-transparent silicon substrate, and (iii) a transmission IR spectrum of an isotropic sample. We chose two distinct IR transitions of this molecule (a C-O-C "in-plane" stretch, υC-O-C, and a C-H out-of-plane bend, δC-H), which we initially assume are orthogonal to each other. The υC-O-C transition is modeled as an in-plane circular dipole, and the δC-H transition is modeled as a linear out-of-plane dipole. Formalisms are described that allow the determination of two independent values for order parameters within the thin film using the dichroism in these transitions in all three IR spectra, assuming that the microstructure of these thin films is the same on both the gold and silicon substrates. The spectra from the δC-H transition appear to best describe the orientation of these molecules within the molecular assembly, these IR results agreeing best with earlier visible absorbance and X-ray diffraction studies. The preferential alignment of the δC-H transition is parallel to the substrate, arising from Pc molecules that are nearly upright, edge-on to the substrate plane. The approach described here provides a means of complete description of the tilt angles of the monomer building blocks in any discotic assembly in thin-film formats, where distinct in-plane and out-of-plane optical transitions are available. © 2008 American Chemical Society.
• Paniagua, S. A., Hotchkiss, P. J., Jones, S. C., Marder, S. R., Mudalige, A., Marrikar, F. S., Pemberton, J. E., & Armstrong, N. R. (2008). Phosphonic acid modification of indium-tin oxide electrodes: Combined XPS/UPS/ contact angle studies. Journal of Physical Chemistry C, 112(21), 7809-7817.
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  Abstract: Indium-tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecaf-luorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance. © 2008 American Chemical Society.
• Paramonov, P. B., Paniagua, S. A., Hotchkiss, P. J., Jones, S. C., Armstrong, N. R., Marder, S. R., & Bredas, J. (2008). Theoretical characterization of the indium tin oxide surface and of its binding sites for adsorption of phosphonic acid monolayers. Chemistry of Materials, 20(16), 5131-5133.
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  Abstract: A study was carried out to present the first theoretical characterization of a model indium tin oxide (ITO) surface and of the nature of the binding sites that determine the adsorption of phosphonic acid (PA) monolayers. The advantage of the approach is to provide from the theory side core-level energies calculated at the quantum-mechanical level for specific atom types, which leads on the experimental side to unambiguous assignments of the various peak components in the XPS spectra. The capability of the methodology to describe the PA binding was demonstrated through a comparison with experimental XPS spectra. The results of the study confirm that, under the typical reaction conditions, PA adsorption on ITO takes place predominantly through bidentate/tridentate binding, which involves P-O-In bonds. First-principles calculations also were shown to be underway to characterize the work function modifications induced by self-assembly of various PA molecules on ITO.
• Ratcliff, E. L., Jenkins, J. L., Nebesny, K., & Armstrong, N. R. (2008). Electrodeposited, "textured" poly(3-hexyl-thiophene) (e-P3HT) films for photovoltaic applications. Chemistry of Materials, 20(18), 5796-5806.
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  Abstract: Organic photovoltaic devices have been created on activated and modified ITO electrodes from electrodeposited poly(3-hexylthiophene) (e-P3HT) donor layers, using pulsed-potential-step (PPS) electrodeposition protocols. PPS electrodeposition uses a series of potential steps of diffusion-controlled e-P3HT deposition, alternated with rest periods where no deposition occurs and the diffusion layer region near the electrode/solution interface refills with thiophene monomer. To create the most photoactive e-P3HT films, a "carpet" layer of polymer was first deposited using dual step chronoamperometry, to create a smooth, pinhole-free film on the ITO electrode. PPS electrodeposition was subsequently used to electrodeposit additional polymer and texture the e-P3HT surface, as revealed by both AFM and SEM. The extent of doping of the polymer film was controlled by the last applied rest potential and monitored by anion incorporation into the e-P3HT film using X-ray photoelectron spectroscopy (XPS). Textured and electrochemically doped e-P3HT films were used as the donor layer in photovoltaic devices, using vacuum deposited C 60 as the electron acceptor/electron transport layer: (ITO/e-P3HT/C60/BCP/Al). The performance of these ultrathin OPVs was markedly dependent upon the degree of electrochemical doping of the P3HT layers. The best OPV performance was obtained for e-P3HT films with an average doping level (ratio of oxidized to reduced thiophene units) of approximately 35%, as estimated by XPS. At 100 mW/cm2 white light illumination, optimized devices give a VOC ∼ 0.5 V and a maximum JSC ∼ 3 mA/cm2, with series resistance (RS) below 1 Ω·cm2, shunt resistance (RP) in excess of 160 kΩ·cm2, fill-factors (FF) of approximately 0.65, and an overall power conversion efficiency of approximately 1%. These results demonstrate the promise of electrochemical protocols for the creation of a variety of hybrid energy conversion materials. © 2008 American Chemical Society.
• Shallcross, R. C., Graham, A. L., D'Ambruoso, G. D., Korth, B. D., Hall Jr., H. K., Zheng, Z., Pyun, J., & Armstrong, N. R. (2008). Electrochemically formed semiconductor nanoparticle/polymer composite thin films. ACS National Meeting Book of Abstracts.
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  Abstract: Semiconductor nanoparticles (SC-NP), such as those created from CdSe, have great potential as the primary light absorber in both solar electric (photovoltaic) and solar hydrogen energy conversion technologies. The primary steps in solar energy conversion include light absorption (where these SC-NPs excel); exciton dissociation into mobile charge carriers; charge transport; and iv) charge collection (and/or production of a fuel such as hydrogen) can be difficult to control for materials systems in which the SC-NP is only dispersed in a polymer host. We have recently shown that electroactive ligands (based on electron-rich thiophenes) can be used to protect the SC-NP, and that these ligands then allow for electrochemical cross-linking of the SC-NP into a conducting or semiconducting polymer host. This talk will focus on formation and characterization of our most recent SC-NP/polythiophene composite materials, and the energetics of photoinduced electron transfer in these materials.
• Veneman, A., Zacher, B., Huebner, D., Simmonds, A., & Armstrong, N. R. (2008). A planar integrated photometer/refractometer using an organic light emitting diode light source and an organic photovoltaic detector. Proceedings of SPIE - The International Society for Optical Engineering, 7054.
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  Abstract: A simple refractometer/photometer is described which uses a vacuum-deposited multilayer organic light emitting diode (OLED) light source and a vacuum-deposited planar heterojunction organic photovoltaic (OPV) detector, separated from each other on a thin glass attenuated total reflectance (ATR) element by 1-2 cm. A portion of the light output from the OLED light source is internally reflected in the ATR element, and the evanescent field from this internally reflected light interacts with solutions of variable refractive index in the region between the OLED and OPV. We document here the simple construction principles for devices of this type, and the characterization of the operation of this first-generation device in terms of i) photocurrent in the OPV detector versus light output from the OLED and ii) the response of the device to solutions of differing refractive index. In these first-generation devices we estimate a sensitivity to changes in refractive index of +/- of 10-3 units.
• Armstrong, N. R. (2007). Conducting polymer diffraction gratings on gold surfaces created by microcontact printing and electropolymerization at submicron length scales. Langmuir.
• Armstrong, N. R. (2007). Modification of BaTiO3 thin films: Adjustment of the effective surface work function. Journal of Materials Chemistry.
• Armstrong, N. R. (2007). Modification of indium - Tin oxide electrodes with thiophene copolymer thin films: Optimizing electron transfer to solution probe molecules. Langmuir.
• Armstrong, N. R. (2007). Planar fiber-optic chips for broadband spectroscopic interrogation of thin films. Applied Spectroscopy.
• Armstrong, N. R. (2007). Poly(3,4-ethylenedioxythiophene)-semiconductor nanoparticle composite thin films tethered to indium tin oxide substrates via electropolymerization. Journal of the American Chemical Society.
• Armstrong, N. R. (2007). Surface composition and electrical and electrochemical properties of freshly deposited and acid-etched indium tin oxide electrodes. Langmuir.
• Armstrong, N. R. (2007). pH-sensing properties of poly(aniline) ultrathin films self-assembled on indium-tin oxide. Analytical Chemistry.
• Beam, B. M., Shallcross, R. C., Jang, J., Armstrong, N. R., & Mendes, S. B. (2007). Planar fiber-optic chips for broadband spectroscopic interrogation of thin films. Applied Spectroscopy, 61(6), 585-592.
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  PMID: 17650368;Abstract: A planar fiber-optic chip (FOC) has been developed using side-polished optical fibers and characterized for broadband absorbance and fluorescence detection of molecular films. FOC technology combines the sensitivity of an attenuated total reflection (ATR) element with the ease of use of fiber-optic-based spectrometers and light sources to create an improved platform for spectroscopic analysis of molecular adsorbates. A multi-mode optical fiber (core diameter = 50 μm, numerical aperture = 0.22, stepped refractive index profile) mounted in a glass V-groove block was side-polished to create a planar platform that allows access to the evanescent field escaping from the fiber core. For this generation of FOC technology, the exposed evanescent field has an interaction length of approximately 17.2 mm. The FOC platform was independently characterized through measurements of thin-film and bulk absorbing samples. The device performance was compared to the existing ATR technology and methods for increasing sensitivity of the FOC were investigated and validated. Additionally, we have demonstrated the ability of the FOC to both evanescently excite and collect fluorescence through guided modes of the optical fiber for a surface-confined luminescent semiconductor nanoparticle film (4 nm diameter, ligand capped, CdSe core). The FOC described here with a supported planar interface can facilitate the use of conventional planar deposition technologies and provide a robust planar platform that is amenable for incorporation into various sensor technologies. © 2007 Society for Applied Spectroscopy.
• Brumbach, M., Veneman, P. A., Matrikar, F. S., Schulmeyer, T., Simmonds, A., Xia, W., Lee, P., & Armstrong, N. R. (2007). Surface composition and electrical and electrochemical properties of freshly deposited and acid-etched indium tin oxide electrodes. Langmuir, 23(22), 11089-11099.
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  PMID: 17880253;Abstract: We compare the near-surface composition and electroactivity of commercial indium tin oxide (ITO) thin films, activated by plasma cleaning or etching with strong haloacids, with ITO films that have been freshly deposited in high vacuum, before and after exposure to the atmosphere or water vapor. Conductive-tip AFM, X-ray photoelectron spectroscopy (XPS), and the electrochemistry of probe molecules in solution were used to compare the relative degrees of electroactivity and the near-surface composition of these materials. Brief etching of commercial ITO samples with concentrated HCl or HI significantly enhances the electrical activity of these oxides as revealed by C-AFM. XPS was used to compare the composition of these activated surfaces, focusing on the intrinsically asymmetric O 1 s line shape. Energy-loss processes associated with photoemission from the tin-doped, oxygen-deficient oxides complicate the interpretation of the O Is spectra. O Is spectra from the stoichiometric indium oxide lattice are accompanied by higher-binding-energy peaks associated with hydroxylated forms of the oxide (and in some cases carbonaceous impurities) and overlapping photoemission associated with energy-loss processes. Characterization of freshly sputter-deposited indium oxide (10) and ITO films, transferred under high vacuum to the surface analysis environment, allowed us to differentiate the contributions of tin doping and oxygen-vacancy doping to the O Is line shape, relative to higher-binding-energy O 1 s components associated with hydroxyl species and carbonaceous impurities. Using these approaches, we determined that acid activation and O 2 plasma etching create an ITO surface that is still covered with an average of one to two monolayers of hydroxide. Both of these activation treatments lead to significantly higher rates of electron transfer to solution probe molecules, such as dimethyferrocene in acetonitrile. Solution electron-transfer events appear to occur at no more than 4 × 10 7 electroactive sites per cm 2 (each with diameters of ca. 50-200 nm) (i.e., a small fraction of the geometric area of the electrode). Electron-transfer rates correlate with the near-surface tin dopant concentration, suggesting that these electroactive sites arise from near-surface tin enrichment. © 2007 American Chemical Society.
• Chenhao, G. e., Armstrong, N. R., & Saavedra, S. S. (2007). pH-sensing properties of poly(aniline) ultrathin films self-assembled on indium-tin oxide. Analytical Chemistry, 79(4), 1401-1410.
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  PMID: 17297939;Abstract: The structural and functional properties of ultrathin (
• Marikkar, F. S., Carter, C., Kieltyka, K., Robertson, J. W., Williamson, C., Simmonds, A., Zangmeister, R., Fritz, T., & Armstrong, N. R. (2007). Conducting polymer diffraction gratings on gold surfaces created by microcontact printing and electropolymerization at submicron length scales. Langmuir, 23(20), 10395-10402.
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  PMID: 17722941;Abstract: Conducting polymer diffraction gratings on Au substrates have been created using microcontact printing of C 18-alkanethiols, followed by electropolymerization of either poly(aniline) (PANI) or poly(3,4- ethylenedioxythiophene) (PEDOT). Soft-polymer replicas of simple diffraction grating masters (1200 lines/mm) were used to define the alkanethiol template for polymer growth. Growth of PANI and PEDOT diffraction gratings was followed in real time, through in situ tapping-mode atomic force microscopy, and by monitoring diffraction efficiency (DE) as a function of grating depth. DE increased as grating depth increased, up to a limiting efficiency (13-26%, with white light illumination), defined by the combined optical properties of the grating and the Au substrate, and ultimately limited by the loss of resolution due to coalescence of the polymer films. Grating efficiency is strongly dependent upon the grating depth and the refractive index contrast between the grating material and the surrounding solutions. Both PEDOT and PANI gratings show refractive index changes as a function of applied potential, consistent with changes in refractive index brought about by the doping/dedoping of the conducting polymer. The DE of PANI gratings are strongly dependent on the pH of the superstrate solution; the maximum sensitivity (ΔDE/ΔpH) is achieved with PANI gratings held at +0.4 V versus Ag/AgCl, where the redox chemistry is dominated by the acid-base equilibrium between the protonated (emeraldine salt) and deprotonated (emeraldine base) forms of PANI. Simulations of DE were conducted for various combinations of conducting polymer refractive index and grating depth, to compute sensitivity parameters, which are maximized when the grating depth is ca. 50% of its maximum obtainable depth. © 2007 American Chemical Society.
• Marrikar, F. S., Brumbach, M., Evans, D. H., Lebrón-Paler, A., Pemberton, J. E., Wysocki, R. J., & Armstrong, N. R. (2007). Modification of indium - Tin oxide electrodes with thiophene copolymer thin films: Optimizing electron transfer to solution probe molecules. Langmuir, 23(3), 1530-1542.
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  PMID: 17241084;Abstract: We describe the modification of indium-tin oxide (ITO) electrodes via the chemisorption and electropolymerization of 6-{2,3-dihydrothieno[3,4-b]-1.4- dioxyn-2-yl methoxy}hexanoic acid (EDOTCA) and the electrochemical co-polymerization of 3,4-ethylenedioxythiophene (EDOT) and EDOTCA to form ultrathin films that optimize electron-transfer rates to solution probe molecules. ITO electrodes were first activated using brief exposure to strong haloacids, to remove the top ̃8 nm of the electrode surface, followed by immediate immersion into a 50:50 EDOT/EDOTCA comonomer solution. Potential step electrodeposition for brief deposition times was used to grow copolymer films of thickness 10-100 nm. The composition of these copolymer films was characterized by solution depletion studies of the monomers and atomic force microscopy (AFM), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy (reflection-absorption infrared spectroscopy (RAIRS)) of the product films. The spectroscopic data suggest that the composition of the copolymer approaches 80% EDOTCA when electropolymerization occurs from concentrated (10 mM) solutions. AFM characterization shows that electrodeposited poly(EDOT)/poly(EDOTCA) (PEDOT/PEDOTCA) films are quite smooth, with texturing on the nanometer scale. RAIRS studies indicate that these films consist of a combination of EDOTCA units with noninteracting -COOH groups and adjacent hydrogen-bonded -COOH groups. The EDOTCA-containing polymer chains appear to grow as columnar clusters from specific regions, oriented nearly vertically to the substrate plane. As they grow, these columnar clusters overlap to form a nearly continuous redox active polymer film. ITO activation and formation of these copolymer films enhances the electroactive fraction of the electrode surface relative to a nonactivated, unmodified "blocked" ITO electrode. Outer-sphere solution redox probes (dimethylferrocene) give standard rate coefficients, ks ≥ 0.4 cm·-1, at 10 nm thick copolymer films of PEDOT/PEDOTCA, which is 3 orders of magnitude greater than that on the unmodified ITO surface and approaches the values for k s seen on clean gold surfaces. © 2007 American Chemical Society.
• Schulmeyer, T., Paniagua, S. A., Veneman, P. A., Jones, S. C., Hotchkiss, P. J., Mudalige, A., Pemberton, J. E., Marder, S. R., & Armstrong, N. R. (2007). Modification of BaTiO₃ thin films: Adjustment of the effective surface work function. Journal of Materials Chemistry, 17(43), 4563-4570.
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  Abstract: Sputter-deposited BaTiO3 thin films have been modified with an alkylphosphonic acid and a partially-fluorinated alkylphosphonic acid in order to model the surface composition of similarly modified BaTiO3 nanoparticles. We present here the surface characterization of these modified films by a combination of X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). BaTiO3 layers of average thicknesses ca. 2 nm were prepared by radio frequency (rf) magnetron sputter deposition on Ag films, to avoid charging effects during XPS/UPS characterization. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8- tridecafluorooctyl phosphonic acid (perfluorohexyloctyl phosphonic acid, FHOPA), molecules with quite different molecular dipole moments, were chemisorbed from solution to the BaTiO3 surface. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) of the modified BaTiO3 films indicated bidentate bonding of the alkylphosphonic acid to the oxide. Modification of the BaTiO3 surface with the partially-fluorinated alkylphosphonic acid (versus the normal alkylphosphonic acid) significantly changes the BaTiO3 interface dipole as revealed by UPS/XPS measurements, which, in turn, changes the frontier orbital offsets between the oxide and the organic modifier. The Royal Society of Chemistry.
• Shallcross, R. C., D'Ambruoso, G. D., Korth, B. D., Hall Jr., H. K., Zheng, Z., Pyun, J., & Armstrong, N. R. (2007). Poly(3,4-ethylenedioxythiophene)-semiconductor nanoparticle composite thin films tethered to indium tin oxide substrates via electropolymerization. Journal of the American Chemical Society, 129(37), 11310-11311.
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  PMID: 17718491;Abstract: Nanocomposite thin films are described which have been created from electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) and new forms of ligand-capped CdSe nanoparticles (NP), using carboxylate ligands with 3,4-propylenedioxythiophene (ProDOT) capping groups (ProDOT-CA). A thin PEDOT film is first tethered to an indium tin oxide electrode, followed by electrochemical cross-linking of the ligand-capped nanoparticle with the PEDOT film. The resultant thin film material photoelectrochemically reduces solution C60 at potentials positive of its E°, where the PEDOT film is reduced (de-doped) and capable of hole capture from the photoexcited NP. This approach is distinctive from solution cast polymer/CdSe hybrid materials in that it enables direct wiring of the nanoparticle to the donor polymer and hole collecting electrode. Copyright © 2007 American Chemical Society.
• Armstrong, N. R. (2006). Characterization of proton transport across a waveguide-supported lipid bilayer. Journal of the American Chemical Society.
• Armstrong, N. R. (2006). Dendritic arrays of [Re6(μ3-Se)8] ²⁺ core-containing clusters: Exploratory synthesis and electrochemical studies. Journal of the American Chemical Society.
• Armstrong, N. R. (2006). Electrochemical copolymerization and spectroelectrochemical characterization of 3,4-ethylenedioxythiophene and 3,4-ethylenedioxythiophene-methanol copolymers on indium-tin oxide. Macromolecules.
• Armstrong, N. R. (2006). Potential-modulated, attenuated total reflectance spectroscopy of poly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedioxythiophene methanol) copolymer films on indium-tin oxide. Journal of Physical Chemistry B.
• Armstrong, N. R. (2006). Small molecule chemisorption on indium - Tin oxide surfaces: Enhancing probe molecule electron-transfer rates and the performance of organic light-emitting diodes. Journal of Physical Chemistry B.
• Armstrong, N. R., Xia, W., Minch, B., Simmonds, A., Carter, C., Donley, C. L., Zangmeister, R. A., Drager, A., Cherian, S. K., LaRussa, L., Kippelen, B., Yoo, S., Domercq, B., Mathine, D. L., & O'Brien, D. F. (2006). Discotic mesophase materials: Their use in organic field-effect transistors and organic photovoltaics. Proceedings - Electrochemical Society, PV 2004-22, 376-384.
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  Abstract: New discotic mesophase materials, especially those based on phthalocyanines, have become increasingly interesting as charge transport and photoactive materials for new molecular electronic technologies (organic field-effect transistors (OFETs) and organic photovoltaics (OPVs)). Side chains added to the periphery of these molecules render them solution processable and enhance the coherence in their rod-like aggregates. It has been hypothesized that these columnar aggregates will provide for charge mobilities competitive with amorphous silicon (ca. 1 cm 2/volt-sec), when the coherence length exceeds ca. 100 nm, and the processing conditions lead to thin films with low charge trap densities, and good electrical contacts (source and drain electrodes in OFETs; anodes/cathodes In OPVs). Our work with octa-substituted phthalocyanines, where the alkoxy- or thio-ether-linked side chains are terminated with benzyl groups, has shown that it is possible to achieve the desired coherence and electrical properties on sub-micron length scales (measured by conductive-tip AFM), but that achieving the desired device properties still requires optimization to achieve i) low contact resistances, and ii) the proper column orientation for both OFET and OPVs.
• Carter, C., Brumbach, M., Donley, C., Hreha, R. D., Marder, S. R., Domercq, B., Yoo, S., Kippelen, B., & Armstrong, N. R. (2006). Small molecule chemisorption on indium - Tin oxide surfaces: Enhancing probe molecule electron-transfer rates and the performance of organic light-emitting diodes. Journal of Physical Chemistry B, 110(50), 25191-25202.
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  PMID: 17165963;Abstract: Indium - tin oxide (ITO) surfaces have been modified by chemisorption of carboxylic acid functionalized small molecules: ferrocene dicarboxylic acid (1), 3-thiophene acetic acid (2), and 6-{4-[{4′-[[4-(5-carboxy-pentyloxy)- phenyl]-(4-methoxy-phenyl)-amino]-biphenyl-4-yl}-(4-methoxy-phenyl)-amino] -phenoxy}-hexanoic acid (p-OMe)2-TPD-(C5-COOH) 2) (3). Voltammetrically determined surface coverages of 1-3 increased in two stages, the first stage completing in minutes, the latter stage taking several hours. Electron-transfer rate coefficients, kS, for the probe molecule ferrocene in acetonitrile likewise increased in two stages with increasing surface coverages of 1, 2, and 3. Fourier transform infrared spectroscopy of In2O3 powders, exposed for long periods to ethanol solutions of each modifier, confirmed the formation of indium oxalate-like surface species. X-ray photoelectron spectroscopy of carboxy-terminated alkanethiol-modified gold surfaces, exposed to these same In2O3(powder)/small molecule modifier solutions, showed the capture of trace levels of indium as a result of the chemisorption of these small molecules, suggesting that slow etching of the ITO surface also occurs during the chemisorption event. Conventional aluminum quinolate/bis-triarylamine organic light-emitting diodes (OLEDs) created on ITO surfaces modified with 1, 2, and 3, with and without an overlayer of PEDOT:PSS (a poly(thiophene)/ poly(stryenesulfonate) ITO modifier), showed leakage currents lowered by several orders of magnitude and an increase in OLED device efficiency. © 2006 American Chemical Society.
• J., W., Wysocki Jr., R. J., Armstrong, N. R., & Saavedra, S. S. (2006). Potential-modulated, attenuated total reflectance spectroscopy of poly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedioxythiophene methanol) copolymer films on indium-tin oxide. Journal of Physical Chemistry B, 110(10), 4900-4907.
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  PMID: 16526729;Abstract: We report the first application of a potential-modulated spectroelectrochemical ATR (PM-ATR) instrument utilizing multiple internal reflections at an optically transparent electrode to study the charge-transfer kinetics and electrochromic response of adsorbed films. A sinusoidally modulated potential waveform was applied to an indium-tin oxide (ITO) electrode while simultaneously monitoring the optical reflectivity of thin (2-6 equivalent monolayers) copolymer films of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4ethylenedioxythiophene methanol) (PEDTM), previously characterized in our laboratory.1 At high modulation frequencies the measured response of the polymer film is selective toward the fastest electrochromic processes in the film, presumably those occurring within the first adsorbed monolayer. Quantitative determination of the electrochromic switching rate, derived from the frequency response of the attenuated reflectivity, shows a linear decrease in the rate, from 11 × 10 3 s -1 to × 103 s -1, with increasing proportions of PEDTM in the copolymer, suggesting that interactions between the methanol substituent on EDTM and the ITO surface slow the switching process by limiting the rate of conformational change in the polymer film. © 2006 American Chemical Society.
• J., W., Wysocki, R. J., Armstrong, N. R., & Saavedra, S. S. (2006). Electrochemical copolymerization and spectroelectrochemical characterization of 3,4-ethylenedioxythiophene and 3,4-ethylenedioxythiophene-methanol copolymers on indium-tin oxide. Macromolecules, 39(13), 4418-4424.
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  Abstract: This work describes the electrochemical copolymerization and spectroelectrochemical characterization of 3,4-ethylenedioxythiophene (EDOT) with a commonly used EDOT derivative: 2,3-dihydrothieno[3,4b]-l,4-dioxyn-2-yl methanol (EDTM), on indium-tin oxide (ITO) electrodes, as a function of the EDTM/EDOT comonomer feed ratio. The potential of initial polymerization and the degree of optical contrast between reduced and oxidized states increased steadily with increasing proportions of EDTM. Reactivity ratios were determined by spectroscopic characterization of the copolymer film and by monitoring the depletion of monomer from the starting solution by liquid chromatography, following the formation of relatively thick PEDOT/PEDTM films. Average reactivity ratios of 1.5 ±0.2 and 0.4 ±0.3 were obtained for EDOT and EDTM, respectively, demonstrating preferential deposition of EDOT on ITO electrode surfaces. Significant differences were noted at low and high degrees of conversion, indicating changes in copolymer composition with film thickness. These results have real significance for the characterization of electron-transfer rates for the first monolayer of PEDOT/ PEDTM on ITO, determined by a new mode of potential-modulated attenuated total reflectance spectroelectrochemistry. © 2006 American Chemical Society.
• McBee, T. W., Wang, L., Chenhao, G. e., Beam, B. M., Moore, A. L., Gust, D., Moore, T. A., Armstrong, N. R., & Saavedra, S. S. (2006). Characterization of proton transport across a waveguide-supported lipid bilayer. Journal of the American Chemical Society, 128(7), 2184-2185.
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  PMID: 16478151;Abstract: Cellular energy transduction processes are often driven by transmembrane ion gradients, and numerous artificial biomembrane systems have been developed that allow for chemically or light-induced charge transport into/out of liposomes. Liposomal architectures, however, are not readily interfaced to a solid-state transducer. Formation of an ion gradient across a planar-supported membrane, "wired" to a substrate electrode, may ultimately allow utilization of the potential energy to drive other electrochemical processes. Described here is a novel conductive polymer/planar waveguide assembly that provides for highly sensitive transduction of proton transport across a planar-supported lipid bilayer (PSLB). A quinone proton shuttle is embedded in the PSLB, which is coupled to the planar optical waveguide electrode through a pH-sensitive, self-assembled conductive polymer film. Interfacial potential and absorbance changes in the conductive polymer film provide for sensitive characterization of transmembrane proton transport. The general and flexible nature of this architecture makes it adaptable to many different types of transmembrane transport chemistries, particularly light-activated systems. Copyright © 2006 American Chemical Society.
• Roland, B. K., Flora, W. H., Selby, H. D., Armstrong, N. R., & Zheng, Z. (2006). Dendritic arrays of [Re₆(μ₃-Se)₈] ²⁺ core-containing clusters: Exploratory synthesis and electrochemical studies. Journal of the American Chemical Society, 128(20), 6620-6625.
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  PMID: 16704262;Abstract: The reaction between the previously reported site-differentiated cluster solvate [Re6(μ3-Se)8-(PEt3) 5(MeCN)](SbF6)2 (1) with pyridyl-based ditopic ligands 4,4′-trimethylenedipyridine (2), 1,2-bis(4-pyridyl)-ethane (3), and (E)-1,2-bis(4-pyridyl)ethene (4) afforded cluster complexes of the general formula [Re6(μ3-Se)8(PEt3) 5(L)](SbF6)2 (5-7), where L represents one of the pyridyl-based ligands. Reacting these cluster complex-based ligands with the fully solvated cluster complex [Re6(μ3-Se) 8(MeCN)6](SbF6)2 (8) produced dendritic arrays of the general formula {Re6(μ3-Se) 8[Re6(μ3-Se)8(PEt 3)5(L)]6}(SbF6)14 (9-11), each featuring six circumjacent [Re6(μ3-Se) 8(PEt3)5]2- units bridged to a [Re6(μ3-Se)8]2-core cluster by the pyridyl-based ligands. Electrochemical studies using a thin-layer electrochemical cell revealed cluster-based redox events in these cluster arrays. For 9 (L = 2), one reversible oxidation event corresponding to the removal of 7 electrons was observed, indicating noninteraction or extremely weak interactions between the clusters. For 10 (L = 3), two poorly resolved oxidation waves were found. For 11 (L = 4), two reversible oxidation events, corresponding respectively to the removal of 1 and 6 electrons, were observed with the 1-electron oxidation event occurring at a potential 150 mV more positive than the 6-electron oxidation. These electrochemical studies suggest intercluster coupling in 11 via through-bond electronic delocalization, which is consistent with electronic spectroscopic studies of this same molecule. © 2006 American Chemical Society.
• Armstrong, N. R. (2005). Conducting polymer growth in porous sol-gel thin films: Formation of nanoelectrode arrays and mediated electron transfer to sequestered macromolecules. Chemistry of Materials.
• Armstrong, N. R. (2005). Covalently bound hole-injecting nanostructures. Systematics of molecular architecture, thickness, saturation, and electron-blocking characteristics on organic light-emitting diode luminance, turn-on voltage, and quantum efficiency. Journal of the American Chemical Society.
• Armstrong, N. R. (2005). Dendritic incorporation of quinacridone: Solubility, aggregation, electrochemistry, and solid-state luminescence. Chemical Communications.
• Armstrong, N. R. (2005). Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy. Langmuir.
• Armstrong, N. R. (2005). Electrochemically tunable surface-plasmon-enhanced diffraction gratings and their (bio-) sensing applications. Langmuir.
• Armstrong, N. R. (2005). High-performance hole-transport layers for polymer light-emitting diodes. Implementation of organosiloxane cross-linking chemistry in polymeric electroluminescent devices. Journal of the American Chemical Society.
• Armstrong, N. R. (2005). Molecular ordering in monolayers of an alkyl-Substituted perylene-bisimide dye by attenuated total reflectance ultraviolet-visible spectroscopy. Applied Spectroscopy.
• Armstrong, N. R. (2005). Octakis(2-benzyloxyethylsulfanyl) copper (II) phthalocyanine: A new liquid crystalline discotic material with benzyl-terminated, thioether-linked side chains. Chemistry of Materials.
• Armstrong, N. R. (2005). Tetrameric arrays of the [Re6(μ3-Se) 8]²⁺ clusters supported by a porphyrin core: Synthesis, characterization, and electrochemical studies. Comptes Rendus Chimie.
• Armstrong, N. R. (2005). Voltammetric and waveguide spectroelectrochemical characterization of ultrathin poly(aniline)/poly(acrylic acid) films self-assembled on indium-tin oxide. Talanta.
• Chenhao, G. e., J., W., Mendes, S. B., Armstrong, N. R., & Saavedra, S. S. (2005). Voltammetric and waveguide spectroelectrochemical characterization of ultrathin poly(aniline)/poly(acrylic acid) films self-assembled on indium-tin oxide. Talanta, 65(5), 1126-1131.
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  PMID: 18969922;Abstract: We report on the spectroelectrochemical characterization of conducting polymer (CP) films, composed of alternating layers of poly(aniline) (PANI) and poly(acrylic acid) (PAA), deposited on ITO-coated, planar glass substrates using layer-by-layer self-assembly. Absorbance changes associated with voltammetrically induced redox changes in ultrathin films composed of only two bilayers (ITO/PANI/PAA/PANI/PAA) were monitored in real time using a unique multiple reflection, broadband attenuated total reflection (ATR) spectrometer. CP films in contact with pH 7 buffer undergo a single oxidation/reduction process, with ca. 12.5% of the aniline centers in the film being oxidized and reduced. The ATR spectra indicate that during an anodic sweep, the leucoemeraldine form of PANI in these films is oxidized to generate both the emeraldine and pernigraniline forms simultaneously. A comparison of the behavior observed during anodic and cathodic sweeps suggests that the rate of oxidation is limited by structural changes in the polymer film originating in electrostatic repulsion between positively charged PANI chains. © 2004 Elsevier B.V. All rights reserved.
• Flora, W. H., Mendes, S. B., J., W., Saavedra, S. S., & Armstrong, N. R. (2005). Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy. Langmuir, 21(1), 360-368.
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  PMID: 15620325;Abstract: We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV-visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.
• Huang, Q., Evmenenko, G. A., Dutta, P., Lee, P., Armstrong, N. R., & Marks, T. J. (2005). Covalently bound hole-injecting nanostructures. Systematics of molecular architecture, thickness, saturation, and electron-blocking characteristics on organic light-emitting diode luminance, turn-on voltage, and quantum efficiency. Journal of the American Chemical Society, 127(29), 10227-10242.
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  PMID: 16028934;Abstract: Hole transporting materials are widely used in multilayer organic and polymer light-emitting diodes (OLEDs, PLEDs, respectively) and are indispensable if device electroluminescent response and durability are to be truly optimized. This contribution analyzes the relative effects of tin-doped indium oxide (ITO) anode-hole transporting layer (HTL) contact versus the intrinsic HTL materials properties on OLED performance. Two siloxane-based HTL materials, N,N-bis(p-trichlorosilylpropyl)-naphthalen-1-yl)-N,N-diphenyl-biphenyl-4, 4′-diamine (NPB-Si2) and 4,4′-bis[(p- trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si2), are designed and synthesized. They have the same hole transporting triarylamine cores as conventional HTL materials such as 1,4-bis(1-naphthylphenylamino) biphenyl (NPB) and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl)-4,4- diamine (TPD), respectively. However, they covalently bind to the ITO anode, forming anode-HTL contacts that are intrinsically different from those of the anode to TPD and NPB. Applied to archetypical tris(8-hydroxyquinolato) aluminum(III) (Alq)-based OLEDs as (1) the sole HTLs or (2) anode-NPB HTL interlayers, NPB-Si2 and TPD-Si2 enhance device electroluminescent response significantly versus comparable devices based on NPB alone. In the first case, OLEDs with 36 000 cd/m2 luminance, 1.6% forward external quantum efficiency (ηext), and 5 V turn-on voltages are achieved, affording a 250% increase in luminance and ∼50% reduction in turn-on voltage, as compared to NPB-based devices. In the second case, even more dramatic enhancement is observed (64 000 cd/m2 luminance; 2.3% ηext; turn-on voltages as low as 3.5 V). The importance of the anode-HTL material contact is further explored by replacing NPB with saturated hydrocarbon siloxane monolayers that covalently bind to the anode, without sacrificing device performance (30 000 cd/m2 luminance; 2.0% ηext; 4.0 V turn-on voltage). These results suggest new strategies for developing OLED hole transporting structures. © 2005 American Chemical Society.
• J., W., Armstrong, N. R., & Saavedra, S. (2005). Erratum: Conducting polymer growth in porous sol-gel thin films: Formation of Nanoelectrode arrays and mediated electron transfer to sequestered macromolecules (Chemistry of Materials (2005) 17 (3652-3660)). Chemistry of Materials, 17(26), 6842-.
• J., W., Armstrong, N. R., & Saavedra, S. S. (2005). Conducting polymer growth in porous sol-gel thin films: Formation of nanoelectrode arrays and mediated electron transfer to sequestered macromolecules. Chemistry of Materials, 17(14), 3652-3660.
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  Abstract: The templated electrochemical growth of poly(3,4-ethylenedioxythiophene) (PEDOT) into porous sol-gel (PSG) films and PEDOT-mediated electron transfer to ferrocene-modified dendrimers encapsulated within these sol-gel matrices, were analyzed. The conditions needed to optimize PEDOT electropolymerization within the PSG films such that barely emergent PEDOT features protrude from a PSG thin film surface, were also described. It was observed that oxidation/reduction of the encapsulated Fc-PAMAM units could be voltammetrically detected after PEDOT growth into the sol-gel film. The results show that up to ca. 20% of these Fc-PAMAM units became electrochemically active, with high rates of electron transfer, when EDTM was conjugated to the Fc-PAMAM dendrimer.
• J., W., Simmonds, A. G., Mendes, S. B., Armstrong, N. R., & Saavedra, S. S. (2005). Molecular ordering in monolayers of an alkyl-Substituted perylene-bisimide dye by attenuated total reflectance ultraviolet-visible spectroscopy. Applied Spectroscopy, 59(10), 1248-1256.
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  PMID: 18028621;Abstract: Surface-relative orientational parameters were determined for monolayer films of N, N'-ditridecylperylenetetracarboxylic dianhydridediimide (C 13-PTCDI) in terms of the relative electronic transition dipole strengths, providing a three-dimensional view of the absorption dipole distribution. In order to obtain a macroscopically ordered film, C 13-PTCDI was deposited by (1) horizontal Langmuir-Blodgett (LB) transfer onto methyl- and phenyl-silanized glass, and (2) vapor deposition onto oriented films of poly(tetrafluoroethylene) (PTFE) on glass. Films of LB-deposited C 13-PTCDI were found to be completely Isotropic prior to annealing. After annealing, these films remained isotropic in the plane of the substrate while the out-of-plane anisotropy was significantly enhanced. In contrast, films of C 13-PTCDI vapor deposited onto oriented poly(tetrafluoroethylene) (PTFE)-modified substrates yielded films with a high degree of both in- and out-of-plane anisotropy. Atomic force microscopy (AFM) images of both the LB- and vapor-deposited films show substantial differences in film morphology and long-range order. These results demonstrate that molecular orientation in C 13-PTCDI films can be controlled by varying substrate surface chemistry and post-deposition processing. © 2005 Society for Applied Spectroscopy.
• Minch, B. A., Xia, W., Donley, C. L., Hernandez, R. M., Carter, C., Carducci, M. D., Dawson, A., O'Brien, D. F., & Armstrong, N. R. (2005). Octakis(2-benzyloxyethylsulfanyl) copper (II) phthalocyanine: A new liquid crystalline discotic material with benzyl-terminated, thioether-linked side chains. Chemistry of Materials, 17(7), 1618-1627.
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  Abstract: The synthesis and materials properties of a new liquid crystalline phthalocyanine, octakis (2-benzyloxyethylsulfanyl) copper (II) phthalocyanine, 2, are reported. This phthalocyanine possesses eight benzyl-terminated side chains with thioether links to the Pc core, which promote greater interaction between the discotic mesogens than has been seen in previous Pcs reported from this group which had benzylterminated alkoxy-based side chains. The use of thioether links to these side chains promotes a red-shift in the Q-band absorbance spectrum, small decreases in first oxidation potential, increased solution aggregation constants, and significantly higher K→LC transition temperatures than seen in conventional alkoxy-based side-chain-modified Pcs. Powders and single crystals of this material were characterized by X-ray diffraction and found to represent two closely related polymorphs. Room-temperature XRD of powders of 2 showed a monoclinic unit cell: a = 22.81(1) Å, b = 9.780(5) Å, c = 19.314(6) Å, α = 90°, β= 106.918(14)°, γ = 90°, V = 4472 Å 3, two molecules per unit cell, with a helical twist of the Pcs along the b-axis. The packing architecture in these powders showed a transition to an oblique (distorted rectangular, Cold) phase at temperatures in the LC regime, which was reversible upon cooling. The single-crystal diffraction data showed a monoclinic unit cell: a = 23.911 (18) Å, b = 5.224(3) Å, c = 39.17(3) Å (c ≈ √ 3a); α = 90°, β= 96.971(14)°, γ = 90°, volume = 4903(8) Å 3, and AFM characterization of the plane of these single crystals clearly showed evidence for layered growth in this material. Langmuir-Blodgett films of 2, characterized by AFM, X-ray reflectivity, and X-ray diffraction, showed hexagonal close packing of parallel Pc columns in the as-deposited film, and upon annealing, showed formation of at least two different domains, with column-column spacings of ca. 21 A and 26 Å, corresponding to the and faces of a 2D lattice, closely related to the unit cell found for the polycrystalline material. © 2005 American Chemical Society.
• Ortiz, A., Flora, W. H., D'Ambruoso, G. D., Armstrong, N. R., & McGrath, D. V. (2005). Dendritic incorporation of quinacridone: Solubility, aggregation, electrochemistry, and solid-state luminescence. Chemical Communications, 444-446.
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  PMID: 15654364;Abstract: The first incorporation of quinacridone, a technologically important organic electroluminescent emitter, into dendrimers increases solubility, decreases aggregation, retards heterogeneous electron transfer, and enhances luminescence in condensed phases (powders and thin films).
• Roland, B. K., Flora, W. H., Armstrong, N. R., & Zheng, Z. (2005). Tetrameric arrays of the [Re₆(μ₃-Se) ₈]²⁺ clusters supported by a porphyrin core: Synthesis, characterization, and electrochemical studies. Comptes Rendus Chimie, 8(11-12), 1798-1807.
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  Abstract: A tetracluster array {[Re6(μ3-Se)8 (PEt3)5]4 [5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine]}(SbF6) 8 (R4) featuring a central porphyrin core and four circumjacent [Re6(μ3-Se)8] 2+ units was synthesized by reacting the previously reported cluster solvate [Re6(μ3-Se)8 (PEt3)5(MeCN)](SbF6)2 (R) with 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine. Metallation of R4 with metal (Co2+, Ni2+, Cu2+, Zn2+) salts afforded the corresponding cluster-studded metalloporphyrins (R4-Co, R4-Ni, R4-Cu, and R4-Zn). All compounds were characterized by 1H and 31P NMR spectroscopy and elemental analysis (CHN). Electrochemical studies revealed one chemically reversible oxidation event attributable to the simultaneous removal of four electrons, one from each of the four cluster units. The uncoupled redox event suggests minimal electrical communication between the cluster sites. The cluster arrays were also studied by UV-vis spectroscopy. The electronic spectra of R4-Co, R4 -Ni, and R4-Cu each showed the absorptions of the metalloporphyrins and the cluster complex. For R4-Zn, solvatochromism was observed. Its electronic absorptions in a variety of solvents of different dielectric constants were studied. © 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.
• Tian, S., Armstrong, N. R., & Knoll, W. (2005). Electrochemically tunable surface-plasmon-enhanced diffraction gratings and their (bio-) sensing applications. Langmuir, 21(10), 4656-4660.
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  PMID: 16032885;Abstract: Electrochemistry was combined with surface-plasmon-enhanced diffraction (ESPD) to investigate a redox-switchable polymer grating and its (bio-)sensing applications. Patterned arrays of polyaniline (PANI)/poly(styrenesulfonate) (PSS) were fabricated by the combination of electropolymerization and micromolding in capillaries (MIMIC) and were used as an optical grating for surface-plasmon-enhanced diffraction experiments. The diffraction efficiency (DE) could be tuned by changes in the applied potential, and by changes in the pH of the surrounding solution (dielectric medium). The response of the DE to the pH depends strongly on the redox state of the PANI/PSS grating. If the polymer grating is mainly in its reduced state, the DE shows a linear dependence on the pH. The DE of the PANI/PSS grating can also be modulated by an electrocatalytic event: by keeping PANI/PSS in its oxidized form, the addition of β-nicotinamide adenine dinucleotide (NADH) increases the DE with the increase of NADH concentration, which points to the possibility of the use of ESPD technologies for biosensing. © 2005 American Chemical Society.
• Yan, H., Lee, P., Armstrong, N. R., Graham, A., Evmenenko, G. A., Dutta, P., & Marks, T. J. (2005). High-performance hole-transport layers for polymer light-emitting diodes. Implementation of organosiloxane cross-linking chemistry in polymeric electroluminescent devices. Journal of the American Chemical Society, 127(9), 3172-3183.
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  PMID: 15740157;Abstract: This contribution describes an organosiloxane cross-linking approach to robust, efficient, adherent hole-transport layers (HTLs) for polymer light-emitting diodes (PLEDs). An example is 4,4′-bis[(p- trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2), which combines the hole-transporting efficiency of N,N-diphenyl-N,N-bis(3- methylphenyl)-1,1-biphenyl)-4,4-diamine) (TPD, prototypical small-molecule HTL material) and the strong cross-linking/densification tendencies of organosilanol groups. Covalent chemical bonding of TPDSi2 to PLED anodes (e.g., indium tin oxide, ITO) and its self-cross-linking enable fabrication of three generations of insoluble PLED HTLs: (1) self-assembled monolayers (SAMs) of TPDSi2 on ITO; (2) cross-linked blend networks consisting of TPDSi2 + a hole transporting polymer (e.g., poly(9,9-dioctylfluorene- co-N-(4-(3-methylpropyl))diphenylamine), TFB) on ITO; (3) TPDSi2 + TFB blends on ITO substrates precoated with a conventional PLED HTL, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS). PLED devices fabricated using these new HTLs exhibit comparable or superior performance vs comparable devices based on PEDOT-PSS alone. With these new HTLs, current efficiencies as high as ∼17 cd/A and luminances as high as ∼140,000 cd/m2 have been achieved. Further experiments demonstrate that not only do these HTLs enhance PLED anode hole injection but they also exhibit significantly greater electron-blocking capacity than PEDOT-PSS. The present organosiloxane HTL approach offers many other attractions such as convenience of fabrication, flexibility in choosing HTL components, and reduced HTL-induced luminescence quenching, and can be applied as a general strategy to enhance PLED performance. © 2005 American Chemical Society.
• Armstrong, N. R. (2004). Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities. Journal of Materials Research.
• Armstrong, N. R. (2004). Effects of field dependent mobility and contact barriers on liquid crystalline phthalocyanine organic transistors. Journal of Applied Physics.
• Armstrong, N. R. (2004). LB films of rodlike phthalocyanine aggregates: Specular X-ray reflectivity studies of the effect of interface modification on coherence and microstructure. Langmuir.
• Cherian, S., Donley, C., Mathine, D., LaRussa, L., Xia, W., & Armstrong, N. (2004). Effects of field dependent mobility and contact barriers on liquid crystalline phthalocyanine organic transistors. Journal of Applied Physics, 96(10), 5638-5643.
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  Abstract: Copper phthalocyanine (CuPc) transistors were fabricated using the Langmuir-Blodgett technique to produce bottom contact organic field effect transistors (OFETs) on silicon substrates. The resultant devices were measured and the performance was analyzed using a two-dimensional numerical simulation of the device structure. A hole barrier at the Au/phthalocyanine source and drain contacts was seen from the experimental data. The numerical simulations were used to extract a barrier height of 0.415 eV at the Au/phthalocyanine contacts. Also, a Frenkel-Poole mobility model was used to account for the drain current in the transistors and a high field mobility of 0.018 cm 2/V sec was extracted from the experimental data. The resultant device parameters were compared to simple analytical results and the benefits of enhanced two-dimensional modeling of OFETs are shown. © 2004 American Institute of Physics.
• Donley, C. L., Zangmeister, R. A., Xia, W., Minch, B., Drager, A., Cherian, S. K., LaRussa, L., Kippelen, B., Domercq, B., Mathine, D. L., O'Brien, D. F., & Armstrong, N. R. (2004). Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities. Journal of Materials Research, 19(7), 2087-2099.
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  Abstract: The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir-Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10-6 cm2·V-1·s-1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10-4 cm2·V-1·s-1. Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad. © 2004 Materials Research Society.
• Jones-Willy, S., Xia, W., Saavedra, S. S., & Armstrong, N. R. (2004). Patterned deposition of tobacco mosaic virus on mica surfaces. Materials Research Society Symposium Proceedings, EXS(1), 327-329.
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  Abstract: We show the formation of aligned tobacco mosaic virus (TMV) particles on mica surfaces through a process of micro-contact printing, providing parallel arrays of TMV with sub-micron features sizes. Key features of this process include dilution of the TMV solutions with bovine serum albumin (BSA) to prevent undo aggregation, formation of extremely TMV solutions, and allowing the "TMV-inked" stamp to dry in a humid environment, prior to TMV deposition. © 2004 Materials Research Society.
• Xia, W., Minch, B. A., Carducci, M. D., & Armstrong, N. R. (2004). LB films of rodlike phthalocyanine aggregates: Specular X-ray reflectivity studies of the effect of interface modification on coherence and microstructure. Langmuir, 20(19), 7998-8005.
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  PMID: 15350064;Abstract: We present here X-ray specular reflectivity (XRR) characterization of the ordering of Langmuir-Blodgett films of the liquid crystalline phthalocyanine (Pc) 2, 3, 9, 10, 16, 17, 23, 34-octakis(2-benzyloxyethoxy)copper-(II) phthalocyanine, 1, on Si(100) wafers with a native oxide layer and these same substrates modified with monolayers of varying percentages of methyl- and phenyl-terminated silanes. The central copper atom in these Pc's provides for high contrast in X-ray reflectivity for single-bilayer films of 1. The XRR data are modeled as arising from multiple layers of organic material, above and below the rows of copper atoms in the aligned Pc cores; variations in the total thickness of these films, and the spacing between the rows of copper atoms, are attributed to changes in the interaction with the substrate, and changes in the Pc orientation and side chain packing. The most pronounced effect on these parameters comes from variations in the ratio of the phenyl-silane versus methyl-silane content of the substrate modifiers and annealing of these films past their crystalline (K) → liquid crystalline (LC) mesophase transition temperature. Transfer of multiple bilayers of this Pc leads to additional changes in the thickness of each layer, eventually forming a hexagonal close-packed array, reminiscent of bulk fibers of this material, as revealed by X-ray diffraction (XRD).
• Yoo, S., Domercq, B., Donley, C. L., Carter, C., Xia, W., Minch, B. A., O'Brien, D. F., Armstrong, N. R., & Kippelen, B. (2004). Organic photovoltaic cells containing discotic liquid crystalline phthalocyanines. Proceedings of SPIE - The International Society for Optical Engineering, 5215, 71-78.
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  Abstract: We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C60. These improvements are explained by change in structure and morphology upon thermal annealing.
• Yoo, S., Domercq, B., Marder, S. R., Armstrong, N. R., & Kippelen, B. (2004). Modeling of organic photovoltaic cells with large fill factor and high efficiency. Proceedings of SPIE - The International Society for Optical Engineering, 5520, 110-117.
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  Abstract: Organic photovoltaic cells exhibiting an ideal diode behavior with large fill factor (FF) are presented. It is demonstrated that the current-voltage characteristics can be well described using the equivalent circuit model that is also used for inorganic solar cells. Resistances associated with the cells and other diode parameters are extracted by fitting the experimental electrical characteristics using the equivalent circuit model. The effects of these parameters on FF are quantitatively described. Changes in these parameters under different illumination conditions are presented and compared to those occurring in inorganic pn-junction solar cells.
• Alloway, D. M., Hofmann, M., Smith, D. L., Gruhn, N. E., Graham, A. L., Colorado Jr., R., Wysocki, V. H., Lee, T. R., Lee, P. A., & Armstrong, N. R. (2003). Interface dipoles arising from self-assembled monolayers on gold: UV-photoemission studies of alkanethiols and partially fluorinated alkanethiols. Journal of Physical Chemistry B, 107(42), 11690-11699.
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  Abstract: Gold surfaces modified with C3-C18-alkanethiols (CH3(CH2)x-1SH; HXSH; x = 3, 8, 12, 16, 18) and C16-alkanethiols, fluorinated at the outer 1, 2, 4, and 10 methylene positions (CF3(CF2)Y-1(CH2)XSH; FyHxSH; y = 1, x = 15; y = 2, x = 14; y = 4, x = 12; y = 10, x = 6) were characterized by He(I) UV-photoelectron spectroscopy (UPS). (Detailed X-ray photoelectron spectroscopic characterization of the partially fluorinated thin films is given in the Supporting Information). Long incubation times of the gold surface with the alkanethiol solutions lead to compact monolayer films for all of the alkanethiols, as indicated by the exponential decrease in emission intensity versus alkyl chain length for both the gold Fermi edge (UPS data), and by a parallel decrease in Au(4f) photoemission intensity using X-ray photoelectron spectroscopy. Changes in the effective work function of these surfaces due to the presence of significant interfacial dipoles are observed (i) as alkyl chain length is increased, and (ii) as the fraction of fluorinated methylene groups is increased in a constant length alkyl chain. Negative shifts of the low kinetic energy photoemission edge with increasing alkyl chain length in the HxSH series are consistent with the presence of a large positive interface dipole. The largest part of this shift (ca. 1.0 eV) appears between the C3- and C8-alkyl chain lengths. Adding -CFx groups to the outer end of the C16-alkyl chain positively shifts the low-kinetic-energy photoemission edge, consistent with the presence of a large negative interface dipole that completely compensates for the positive dipole from the alkyl portion of the chain. Examining C13-C16 alkyl chains fluorinated at only the outer methyl group shows that this negative dipole depends on the orientation of the -CF3 group (i.e., "odd-even" effects in the effective work function are observed). Comparison of the shifts in gold/SAM vacuum level (changes in effective work function) as a function of the apparent dipole moment of the molecule provides an estimate of the band-edge offsets for these molecules on the gold surface, an estimate of the intrinsic shift in a vacuum level at zero dipole moment of the adsorbate, and an estimate of the intrinsic dipole moment for the gold-thiolate bond.
• Armstrong, N. R. (2003). An inorganic-organic hybrid composite featuring metal-chalcogenide clusters. Journal of Cluster Science.
• Armstrong, N. R. (2003). Broadband coupling into a single-mode, electroactive integrated optical waveguide for spectroelectrochemical analysis of surface-confined redox couples. Analytical Chemistry.
• Armstrong, N. R. (2003). Guest emission processes in doped organic light-emitting diodes: Use of phthalocyanine and naphthalocyanine near-IR dopants. Journal of Physical Chemistry B.
• Armstrong, N. R. (2003). Interface dipoles arising from self-assembled monolayers on gold: UV-photoemission studies of alkanethiols and partially fluorinated alkanethiols. Journal of Physical Chemistry B.
• Armstrong, N. R. (2003). Interface modification of ITO thin films: Organic photovoltaic cells. Thin Solid Films.
• Armstrong, N. R. (2003). Thin films of polymerized rodlike phthalocyanine aggregates. Langmuir.
• Armstrong, N. R., Carter, C., Donley, C., Simmonds, A., Lee, P., Brumbach, M., Kippelen, B., Domercq, B., & Yoo, S. (2003). Interface modification of ITO thin films: Organic photovoltaic cells. Thin Solid Films, 445(2), 342-352.
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  Abstract: In this paper we review our recent studies of the surface characterization of commercially available indium-tin-oxide (ITO) thin films, using photoelectron spectroscopies (XPS and UPS) and electrochemistry of chemisorbed probe molecules such as ferrocene dicarboxylic acid (Fc(COOH)2). The modification of these ITO films through chemisorption of carboxylic acid-substituted small molecules, such as Fc(COOH)2, 3-thiophene acetic acid (3-TAA), and the subsequent modification of these interfaces with electrochemically grown conducting polymer (CP) films is also introduced. We report preliminary results of our studies changes in performance of vacuum deposited organic photovoltaic (PV) cells as a result of these ITO substrate modification steps. The surfaces of as-received ITO films, and those cleaned by various solution and plasma-etching processes, are unavoidably hydrolyzed to In(OH)3-like and InOOH-like surface species, which leaves the ITO surface with at most 40-50% of the electronically active sites available for electron transfer reactions. Modification of the ITO surface with electroactive small molecules such as Fc(COOH)2 and 3-TAA provides for better wettability of organic layers to the polar ITO surface and enhanced electrical contact (lower series resistance, RS) between the ITO anode, spin-cast or electrodeposited PEDOT:PSS layers and copper phthalocyanine (CuPc) layers in multilayer (CuPc/C60/BCP) excitonic PV cells. Improvements in PV J/V (current/voltage) responses are noted mainly through increases in short-circuit photocurrent and lowered series resistances (RS) when electroactive small molecules are chemisorbed to the ITO surface, prior to spin-casting of conducting polymer, PEDOT:PSS, layers. © 2003 Elsevier B.V. All rights reserved.
• Bradshaw, J. T., Mendes, S. B., Armstrong, N. R., & Saavedra, S. S. (2003). Broadband coupling into a single-mode, electroactive integrated optical waveguide for spectroelectrochemical analysis of surface-confined redox couples. Analytical Chemistry, 75(5), 1080-1088.
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  PMID: 12641226;Abstract: A single-mode, electroactive waveguiding platform capable of measuring spectroelectrochemical responses of surfaceadsorbed redox-active molecules over a broad spectral bandwidth has been created. This new planar waveguide spectrometer is a combination of the previously developed electroactive integrated optical waveguide (EAIOW; Dunphy, D. R.; Mendes, S. B.; Saavedra, S. S.; Armstrong, N. R. Anal. Chem. 1997, 69, 3086-3094) with a recently reported simplified approach to broadband coupling (Bradshaw, J. T.; Mendes, S. B.; Saavedra, S. S. Anal. Chem. 2002, 74, 1751-1759). With the use of a commercially available prism as an incoupling element, the EA-IOW can now guide visible light from at least 500 to 700 nm, improving upon its previously demonstrated monochromatic nature. Coupling profiles of various laser lines along with transmission spectra of narrow band-pass filters at various potentials are used to demonstrate the optical characteristics of this broadband EA-IOW and to compare its response to that of a conventional transmission instrument. Assessment of spectral resolution, performed by measuring the fwhm of various laser lines, ranges from 0.6 to 0.8. To demonstrate the capabilities of this technology, we show the acquisition of absorbance spectra of two different adsorbates, cytochrome c and ferrocenedicarboxylic acid, as a function of applied potential. Subtleties in the redox chemistries of adsorbed molecules, which were difficult to monitor with a monochromatic waveguide, are readily apparent when using the broadband coupling scheme.
• Donley, C. L., Xia, W., Minch, B. A., Zangmeister, R. A., Drager, A. S., Nebesny, K., O'Brien, D. F., & Armstrong, N. R. (2003). Thin films of polymerized rodlike phthalocyanine aggregates. Langmuir, 19(16), 6512-6522.
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  Abstract: We report here the synthesis, characterization, and thin film formation of a polymerizable octa-substituted phthalocyanine (Pc) with styryl-terminated side chains, CuPc(OCH2CH2OCH2CH=CHPh) 8, 2,3,9,10,16,17,23,24-octakis(2-cinnamyloxyethoxy) phthalocyaninato copper(II) (1). We compare this Pc with a previously discussed phthalocyanine, also possessing styryl groups at the termini of the side chains, but with one alkoxy group in the side chain removed, CuPc(OCH 2CH2CH=CHPh)8 (2) (J. Am. Chem. Soc. 2001, 123, 3595). Both 1 and 2 are related to the octa-substituted phthalocyanine CuPc(OCH2CH2OBz)8, 2,3,9,10,16,17,23, 24-octakis (2-benzyloxyethoxy) phthalocyaninato copper(II) (3), which has been shown to form highly coherent rodlike aggregates in Langmuir-Blodgett (LB) films, with excellent control of rod orientation (J. Am. Chem. Soc. 1999, 121, 8628; Langmuir 2001, 17, 7071). Irradiation of the styryl π-π* absorbance bands (254 nm) for horizontally transferred LB films of 1 and 2 results in stabilization of their rodlike aggregates, through formation of cyclobutane links between adjacent side chains. Compound 1 shows a maximum 75% conversion of styryl groups versus ca. 55% conversion in 2. Polymerized thin films of 1 are insoluble in common solvents, and "ribbonlike" features can be lithographically produced with widths of 8 microns and heights of ca. 20 nm, maintaining control over the orientation of the Pc rods in the patterned features. Long-range order in both the as-deposited, annealed, and polymerized thin films was confirmed by atomic force microscopy and X-ray reflectometry, and small differences in the orientation of individual Pc's, between films of 1 and 3, were determined by transmission and reflectance Fourier transform infrared spectroscopy. Higher dark and photoconductivities and higher electrical anisotropies were observed in films of 1, after annealing and polymerization, versus those seen for films of 3.
• Flora, W. H., Hall, H. K., & Armstrong, N. R. (2003). Guest emission processes in doped organic light-emitting diodes: Use of phthalocyanine and naphthalocyanine near-IR dopants. Journal of Physical Chemistry B, 107(5), 1142-1150.
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  Abstract: A series of near-IR emitting organic light-emitting diodes (OLEDs) have been prepared using poly(vinyl-carbazole) (PVK) as the hole transport polymer, aluminum quinolate (Alq3) or a sulfonamide derivative (Al(qs)3) as the host dye (H), and phthalocyanine (Pc) or naphthalocyanine (NPc) dopants as guest dyes (G). The separation of the emission spectrum of the guest dye from that of the host dye allows for independent assessment of their relative efficiencies and correlation of these efficiencies with their tendencies toward charge capture and recombination reminiscent of solution electrogenerated chemiluminescent reactions. Although the absorbance spectra of the guest dyes are significantly separated from the emission spectra of the host dyes, the high molar absorptivities of the Pc and NPc dopants lead to Förster radii and relative energy-transfer (ET) efficiencies comparable to values previously obtained for quinacridone or rubrene dopants, which have been used to create intense green or yellow emission in Alq3-based OLEDs. We show that hole capture by the guest dye from oxidized PVK, PVK+•/G → PVK/G+•, followed by cross-reactions, Alq3-•(Al(qs)3-•)/G+ • → Alq3(Al(qs)3)/G*, can be an important charge-trapping (CT), light-producing pathway at sufficiently high concentrations of the guest dye. Relative efficiencies of Pc* or NPc* emission in these single-layer devices increased exponentially with the electrochemically predicted excess free energy for the oxidation of G to form G+• by PVK+•.
• Roland, B. K., Flora, W. H., Carducci, M. D., Armstrong, N. R., & Zheng, Z. (2003). An inorganic-organic hybrid composite featuring metal-chalcogenide clusters. Journal of Cluster Science, 14(4), 449-458.
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  Abstract: The cluster complex, [Re6(μ3-Se) 8(PEt3)5(4-vinylpyridine)](SbF 6)2 (1), featuring the face-capped octahedral [Re 6(μ3-Se)8]2+ cluster core and site-differentiating triethylphosphine and 4-vinylpyridine ligands has been synthesized and structurally characterized. The complex crystallized in monoclinic space group P21 with a = 11.748(1) Å, b = 15.1212(2) Å, c = 19.941(2) Å, α = 90°, β = 94.411(3)°, γ = 90°, V = 3531.8(7) Å3, Z = 2, R1 = 0.0531, wR2 = 0.0774. Copolymerization of styrene with 1 via the polymerizable 4-vinylpyridine ligand afforded a novel inorganic-organic hybrid composite of high molecular weight and a low polydispersity index. Structural integrity of the cluster building block is maintained upon hybrid formation.
• Armstrong, N. R. (2002). Broadband spectroelectrochemical attenuated total reflectance instrument for molecular adlayer studies. Applied Spectroscopy.
• Armstrong, N. R. (2002). Characterization of indium-tin oxide interfaces using X-ray photoelectron spectroscopy and redox processes of a chemisorbed probe molecule: Effect of surface pretreatment conditions. Langmuir.
• Armstrong, N. R. (2002). Electrogenerated chemiluminescence from phosphorescent molecules used in organic light-emitting diodes. Journal of the Electrochemical Society.
• Armstrong, N. R. (2002). Electronic energy levels in individual molecules, thin films, and organic heterojunctions of substituted phthalocyanines. Journal of Physical Chemistry B.
• Armstrong, N. R. (2002). Interfacial trap states in junctions of molecular semiconductors. Chemical Physics.
• Armstrong, N. R. (2002). Nanometer-scale dielectric self-assembly process for anode modification in organic light-emitting diodes. Consequences for charge injection and enhanced luminous efficiency. Chemistry of Materials.
• Armstrong, N. R. (2002). Planar integrated optical waveguide sensor for isopropyl alcohol in aqueous media. Journal of Sol-Gel Science and Technology.
• Armstrong, N. R. (2002). Progress in high work function TCO OLED anode alternatives and OLED nanopixelation. Synthetic Metals.
• Armstrong, N. R. (2002). Selective deposition of rod-like phthalocyanine aggregates on Au surfaces patterned with a combination of microcontact printing and electropolymerization. Advanced Functional Materials.
• Armstrong, N. R. (2002). Starch-iodine films respond to water vapor. Analytica Chimica Acta.
• Donley, C., Dunphy, D., Paine, D., Carter, C., Nebesny, K., Lee, P., Alloway, D., & Armstrong, N. R. (2002). Characterization of indium-tin oxide interfaces using X-ray photoelectron spectroscopy and redox processes of a chemisorbed probe molecule: Effect of surface pretreatment conditions. Langmuir, 18(2), 450-457.
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  Abstract: Surface characterization of indium-tin oxide (ITO) thin films has been carried out with monochromatic X-ray photoelectron spectroscopy (XPS) following various solution pretreatments, RF air plasma etching or high-vacuum argon-ion sputtering. Commercially available ITO films show high concentrations of hydrolyzed oxides and oxy-hydroxides in the near-surface region, along with stoichiometric oxide (In2O3, SnO2) and variable levels of oxygen defect sites. XPS revealed that solution and vacuum treatments changed both the relative surface coverage of the hydroxides and, to a lesser extent, the concentration of oxide defect sites in the near-surface region. These pretreatments have a significant effect on both the coverage and electron-transfer rates for chemisorbed ferrocene dicarboxylic acid (Fc(COOH)2), with the air-plasma-etched ITO showing the highest surface coverage of Fc(COOH)2 and an RCA treatment showing the highest electron-transfer rates.
• Gross, E. M., Armstrong, N. R., & Wightman, R. M. (2002). Electrogenerated chemiluminescence from phosphorescent molecules used in organic light-emitting diodes. Journal of the Electrochemical Society, 149(5), E137-E142.
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  Abstract: The role of triplet states in the electrogenerated chemiluminescence (ECL) of organic light-emitting diode (OLED) materials was investigated. The spectroscopic, electrochemical, and electrochemiluminescent properties of two phosphorescent materials, platinum(II) octaethylporphyrin (PtOEP) and tris(2-phenylpyridine)iridium(II) [Ir(ppy)3] are described. Radical cations and anions of PtOEP were generated in solution and recombined to produce ECL. The ECL from PtOEP was less than a 9,10-diphenylanthracene (DPA) standard. Ir(ppy)3 could be electro-oxidized and reduced in acetonitrile to produced ECL that was brighter than DPA. The light from both of these compounds was stable, and their emission spectra were the same as those reported for OLEDs based on these compounds. © 2002 The Electrochemical Society. All rights reserved.
• J., W., Donley, C. L., Armstrong, N. R., & Saavedra, S. S. (2002). Broadband spectroelectrochemical attenuated total reflectance instrument for molecular adlayer studies. Applied Spectroscopy, 56(7), 920-927.
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  Abstract: The study of thin film molecular architectures is an increasingly important endeavor with respect to the development and characterization of materials ranging from liquid crystalline displays to receptor-based biosensors. Here we describe an apparatus capable of simultaneously acquiring broadband spectroscopic and electrochemical information on molecularly thin films deposited on a transparent electrode surface through a multiple internal reflection geometry. To demonstrate the capabilities of this system, the spectroelectrochemical behavior of a single, neutral copper phthalocyanine bilayer was evaluated.
• Malinsky, J. E., G., J., Jabbour, G. E., Shaheen, S. E., Anderson, J. D., Lee, P., Richter, A. G., Burin, A. L., Ratner, M. A., Marks, T. J., Armstrong, N. R., Kippelen, B., Dutta, P., & Peyghambarian, N. (2002). Nanometer-scale dielectric self-assembly process for anode modification in organic light-emitting diodes. Consequences for charge injection and enhanced luminous efficiency. Chemistry of Materials, 14(7), 3054-3065.
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  Abstract: Layer-by-layer, self-limiting chemisorptive siloxane self-assembly using Si3O2Cl8 as the precursor affords thin, conformal, relatively dense, largely pinhole-free dielectric films that can be deposited on oxide surfaces with sub-nanometer control of film thickness (8.3(1) Å/layer). Deposition chemistry, microstructure, and hole injection/work function modification properties of these (SiO2)x-like films on single-crystal Si(111) and polycrystalline indium tin oxide (ITO) substrates have been characterized by synchrotron specular X-ray reflectivity, cyclic voltammetry, X-ray and UV photoelectron spectroscopy, and atomic force microscopy. Chemisorption of these (SiO2)x films onto the ITO anodes of three-layer, vapor-deposited organic electroluminescent devices (ITO/(SiO2)x/TPD/Alq/Al) nearly triples the external quantum and luminous efficiencies. The efficiency enhancement is attributed to hole and electron injection fluence balance caused by modification of the effective voltage profile brought about by the assembly of well-ordered siloxane layers. Interestingly, as a function of increasing (SiO2)x layer thickness, device turn-on voltage first increases (x = 0 → 1), progressively decreases (x = 1 → 2 → 3), and then increases (x = 3 → 4). A theoretical model based upon computation at the ab initio level is proposed in which the self-assembled dielectric layers induce an additional, thickness-dependent "built-in" electric field across the organic transport layers, thereby simultaneously enhancing electron injection from the cathode (increasing luminescence efficiency) and decreasing the efficiency of hole injection (changing the turn-on voltage).
• Marks, T. J., Veinot, J. G., Cui, J., Yan, H., Wang, A., Edleman, N. L., Ni, J., Huang, Q., Lee, P., & Armstrong, N. R. (2002). Progress in high work function TCO OLED anode alternatives and OLED nanopixelation. Synthetic Metals, 127(1-3), 29-35.
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  Abstract: The progress in high work function transparent conducting oxide (TCO) organic light-emitting diodes (OLED) anode alternatives and OLED nanopixelation was presented. Two complementary areas of interest in OLED science and engineering, development and application of new transparent conducting oxide (TCO) materials and OLED anodes, and effective patterning strategies for nanofabrication were addressed. Results showed that the Alq-based OLED heterostructures exhibited no apparent influence of quantum confinement or accompanying degradation of either rectification of emissive efficiency.
• Schlettwein, D., Hesse, K., Gruhn, N. E., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (2002). Electronic energy levels in individual molecules, thin films, and organic heterojunctions of substituted phthalocyanines. Journal of Physical Chemistry B, 105(21), 4791-4800.
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  Abstract: Ultraviolet photoelectron spectroscopy was performed on phthalocyanine samples in the gas phase and as thin vapor-deposited solid films. The results are compared to those of molecular orbital calculations and electrochemical redox experiments in solutions and as solid films. Electron-withdrawing groups or substituents are introduced to the phthalocyanine system to increase the ionization potential and electron affinity. An almost parallel shift of photoelectron spectra is obtained. Unsubstituted phthalocyaninatozinc (PcZn), complexes of tetrapyridotetraazaporphyrins (TPyTAPZn, TPyTAPVO) and hexadecafluorophthalocyanines (F16PcZn, F16PcVO, F16PcAlF) have been studied. The alignment of energy levels in organic heterojunctions prepared from the substituted phthalocyanines and the unsubstituted zinc complex is discussed in detail. The offset between the highest occupied electronic levels, the establishment of a surface dipole, and the redistribution of charge in the heterointerface region beyond molecular contact is shown.
• Schlettwein, D., Oekermann, T., Jaeger, N., Armstrong, N. R., & Wöhrle, D. (2002). Interfacial trap states in junctions of molecular semiconductors. Chemical Physics, 285(1), 103-112.
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  Abstract: Interfacial states that were established in contacts of molecular semiconductors with aqueous electrolytes or in contacts with another organic semiconductor as a solid film were analyzed by photoelectrochemical experiments and by photoelectron spectroscopy. A crucial role of such states was indicated in the interfacial charge transfer and recombination kinetics of light-induced charge carriers and also in the energetic alignment in the solid contacts. Unsubstituted zinc-phthalocyanine (PcZn) served as model compound. The role of chemical interactions in the establishment of these interfacial states was investigated by use of different reaction partners, i.e., different redox couples in the electrolyte contacts and molecular semiconductors of different ionization potential in the solid contacts. Implications of these results for the use of organic semiconductor thin films in devices of molecular electronics and of dye molecules in dyesensitized solar cells were also discussed. © 2002 Elsevier Science B.V. All rights reserved.
• Skrdla, P. J., Armstrong, N. R., & Saavedra, S. (2002). Starch-iodine films respond to water vapor. Analytica Chimica Acta, 455(1), 49-52.
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  Abstract: Starch-iodine indicator films were found to have useful spectroscopic properties for the detection of water vapor. The large colorimetric response of these easily prepared films was easily detected by the absorption of 632.8nm HeNe laser light, using a planar integrated optical waveguide (IOW) platform. The detection limit of a prototype sensor was found to be below 5% relative humidity (RH), with response times of the order of seconds. © 2002 Elsevier Science B.V. All rights reserved.
• Skrdla, P. J., Mendes, S. B., Armstrong, N. R., & Saavedra, S. S. (2002). Planar integrated optical waveguide sensor for isopropyl alcohol in aqueous media. Journal of Sol-Gel Science and Technology, 24(2), 167-173.
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  Abstract: An attenuated total reflection (ATR) sensor for water-miscible organic solvents was constructed using a combination of sol-gel processing and integrated optical waveguide (IOW) technologies. The sensor consisted of single-mode, sol-gel based planar waveguide coated with a 40 nm thick, porous sol-gel indicator layer prepared from methyltriethoxysilane and doped with methyl red. The response of the senor to aqueous isopropyl alcohol (IPA) was investigated. Solvation of the indicator dye by IPA causes the absorbance spectrum to undergo a blue shift coupled with an increase in molar absorptivity. IPA was detected by measuring changes in ATR of the guided mode at 488 nm. A response curve extending from 1 to 100% (v/v) IPA in water was constructed for the sensor, from which a detection limit of 0.7% (v/v) IPA/water was estimated. Response and reversal times were typically less than one minute, making this sensor potentially attractive for on-line monitoring applications. The rapid response characteristics are attributable to relatively weak, reversible interactions between the indicator and analyte.
• Zangmeister, R. A., O'Brien, D., & Armstrong, N. R. (2002). Selective deposition of rod-like phthalocyanine aggregates on Au surfaces patterned with a combination of microcontact printing and electropolymerization. Advanced Functional Materials, 12(3), 179-186.
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  Abstract: Patterned thin films of a unique phthalocyanine (Pc), (2,3,9,10,16,17,23,24-oktakis((2-benzyloxy)ethoxy)phthalocyaninato) copper (1), are created by means of capillary flow of chloroform solutions into micrometer-dimension hydrophobic/hydrophilic channels. These channels are created by a combination of microcontact printing of octadecylmercaptan (C18-SH) layers on gold, creating a hydrophobic channel bottom, and oxidative electropolymerization of m-aminophenol (at pH 4), creating hydrophilic channel walls (microcontact printing/electropolymerization, μCP-EP). The polyphenoxide channel walls grow perpendicular to the substrate plane, with excellent retention of structure to thicknesses (ca. 30 nm) far exceeding the height of the C18-self-assembled monolayer (SAM). Doping of the C18-SAM layers with small percentages of 1-phenyldodecane provides for efficient wetting of the channel bottoms with solutions of 1, consistent with recent studies, which have shown the need for phenyl-termination of substrates surfaces to create coherent rod-like aggregate structures. These optimized channels provide for excellent capillary flow conditions for chloroform solutions of 1, yielding 10 μm width Pc films with thicknesses exceeding the height of the polyphenoxide channel walls by up to 10×. We also present evidence for formation of birefringent (ordered) material near the hydrophilic walls.
• Armstrong, N. R. (2001). Indium tin oxide alternatives - High work function transparent conducting oxides as anodes for organic light-emitting diodes. Advanced Materials.
• Armstrong, N. R. (2001). Interface electronic structure of organic semiconductors with controlled doping levels. Organic Electronics.
• Armstrong, N. R. (2001). Joint experimental and theoretical characterization of the electronic structure of 4,4′-Bis(N-m-tolyl-N-phenylamino)biphenyl (TPD) and substituted derivatives. Journal of Physical Chemistry A.
• Armstrong, N. R. (2001). One-dimensional polymers of octasubstituted phthalocyanines. Journal of the American Chemical Society.
• Armstrong, N. R. (2001). Single reactor route to polar superlattices. Layer-by-layer self-assembly of large-response molecular electrooptic materials by protection-deprotection. Chemistry of Materials.
• Armstrong, N. R. (2001). Transfer of rodlike aggregate phthalocyanines to hydrophobized gold and silicon surfaces: Effect of phenyl-terminated surface modifiers on thin film transfer efficiency and molecular orientation. Langmuir.
• Armstrong, N. R., Wightman, R. M., & Gross, E. M. (2001). Light-emitting electrochemical processes. Annual Review of Physical Chemistry, 52, 391-422.
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  PMID: 11326070;Abstract: Electrochemical processes leading to light emission are reviewed, with emphasis on aspects of this subject relevant to the understanding and optimization of electrogenerated luminescence (EL) in organic thin-film materials. The basic energetic requirements of light emission from electrochemically initiated solution redox reactions [electrogenerated chemiluminescence (ECL)] are reviewed first. This review is followed by a discussion of light-emitting electrochemical processes that have been observed in hybrids of ionically conducting polymers and electronically conducting polymers. Finally, the features of EL in insulating polymers and molecular thin films are reviewed, along with recent electrochemical and ECL studies of the small-molecule components of certain organic light-emitting diodes. These studies provide a conceptual framework for understanding and optimizing these materials and the EL process.
• Blochwitz, J., Fritz, T., Pfeiffer, M., Leo, K., Alloway, D. M., Lee, P. A., & Armstrong, N. R. (2001). Interface electronic structure of organic semiconductors with controlled doping levels. Organic Electronics: physics, materials, applications, 2(2), 97-104.
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  Abstract: We investigate the properties of inorganic - organic interfaces by ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) and transport experiments. In particular, we study the interface between inorganic conductive substrates and organic layers that are intentionally p-type doped by co-evaporation of a matrix material and acceptor molecules. The photoemission spectra clearly show that the Fermi levels shift due to the doping and that the space charge layer width changes with doping (high doping - small width). The changes in the electronic structure of the interface due to doping agree well with results of transport experiments. © 2001 Elsevier Science B.V. All rights reserved.
• Cornil, J., Gruhn, N. E., Santos, D. D., Malagoli, M., Lee, P. A., Barlow, S., Thayumanavan, S., Marder, S. R., Armstrong, N. R., & Brédas, J. (2001). Joint experimental and theoretical characterization of the electronic structure of 4,4′-Bis(N-m-tolyl-N-phenylamino)biphenyl (TPD) and substituted derivatives. Journal of Physical Chemistry A, 105(21), 5206-5211.
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  Abstract: We investigate by means of gas-phase ultraviolet photoelectron spectroscopy complemented by quantum-chemical calculations how the frontier levels of the 4,4′-bis(N-m-7tolyl-N-phenylamino)biphenyl (TPD) molecule are affected upon substitution of the terminal aryl rings with methoxy groups or fluorine atoms. These results provide strategies to modulate energy barriers at metal/organic or organic/organic interfaces involving TPD and its derivatives; it is shown that the change in the energy of the HOMO level of TPD upon derivatization is strongly affected by inductive effects taking place in the a skeleton.
• Cui, J., Wang, A., Edleman, N. L., Ni, J., Lee, P., Armstrong, N. R., & Marks, T. J. (2001). Indium tin oxide alternatives - High work function transparent conducting oxides as anodes for organic light-emitting diodes. Advanced Materials, 13(19), 1476-1480.
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  Abstract: The applicability of non-indium tin oxide (ITO) transparent conducting oxide materials as effective organic light-emitting diodes (OLED) was described. These materials exhibit high electrical conductivity, electroluminescence and optical transparency, and work function greater than that of commercial ITO. The anode work function was found to be an important factor along with OLED hole injection efficiency, chemical compatibility of the TCO surface with organic overlayer and microstructure dependent charge injection kinetics at the interface.
• Drager, A. S., Zangmeister, R. A., Armstrong, N. R., & O'Brien, D. (2001). One-dimensional polymers of octasubstituted phthalocyanines. Journal of the American Chemical Society, 123(15), 3595-3596.
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  PMID: 11472132;
• Van, M., Richter, A. G., Malinsky, J. E., Lee, P. A., Armstrong, N. R., Dutta, P., & Marks, T. J. (2001). Single reactor route to polar superlattices. Layer-by-layer self-assembly of large-response molecular electrooptic materials by protection-deprotection. Chemistry of Materials, 13(1), 15-17.
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  Abstract: Protection-deprotection of silyl-derivatized surface hydroxyl groups represents a new, "one-pot" strategy for the layer-by-layer construction of polar, covalently interlinked electrooptic molecular superlattices. These, siloxane-based multilayer structures are shown to be smooth, adherent, and structurally regular and to exhibit excellent electrooptic/nonlinear optic responses with χ(2)zzz∼220 pm/V and r33∼80 pm/V without electric field poling. This all "wet-chemical self-assembly approach should be readily amenable to automation.
• Zangmeister, R. A., Smolenyak, P. E., Drager, A. S., O'Brien, D. F., & Armstrong, N. R. (2001). Transfer of rodlike aggregate phthalocyanines to hydrophobized gold and silicon surfaces: Effect of phenyl-terminated surface modifiers on thin film transfer efficiency and molecular orientation. Langmuir, 17(22), 7071-7078.
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  Abstract: We present here a study of the effects of surface modification on the efficiency of transfer of ultrathin Langmuir-Blodgett films of rodlike aggregates of the phthalocyanine (Pc) (2,3,9,10,16,17,23,24-octakis-((2-benzyloxy)ethoxy)phthalocyaninato)copper (1) to gold and silicon substrates. These surface modifications pact on (i) the molecular orientation of the individual Pcs within the rodlike aggregates, (ii) the coherence of the aggregates (providing for coherence lengths of some Pc rods of over 100 nm), and (iii) the optical anisotropy and the overall uniformity and lack of pinholes in Pc bilayer films, on the 100 μm distance scale. Methyl-terminated and phenyl-terminated surface modifiers were added to both Au and Si surfaces through thiol and silane chemistries, respectively. The phenyl-terminated modifier for Au surfaces, benzyloxyethanethiol, 2, mimics the side chain composition of Pc 1. Both methyl- and phenyl-terminated modifiers produced hydrophobic surfaces, as revealed by high water contact angles. FT-IR spectroscopy and AFM characterization of horizontally transferred bilayer LB thin films of I consistently showed higher degrees of coverage on the surfaces with phenyl-terminated modifiers. Reflection-absorption FT-IR of 1-5 bilayer films on modified Au substrates showed that the average tilt angle (ψ) and rotation angle (θ) of the individual Pc chromophores within these films varied with increasing Pc film coverage but that changes to ψ were significantly less on Au surfaces modified with 2. Phenyl-phenyl intermolecular interactions have been hypothesized to lead to the unique formation of rodlike aggregates of 1 and are shown here to be critical in establishing coherent thin films of this material during the transfer of the first bilayer.
• Armstrong, N. R. (2000). Electrogenerated chemiluminescence from derivatives of aluminum quinolate and quinacridones: Cross-reactions with triarylamines lead to singlet emission through triplet-triplet annihilation pathways. Journal of the American Chemical Society.
• Armstrong, N. R. (2000). High Tg hole transport polymers for the fabrication of bright and efficient organic light-emitting devices with an air-stable cathode. IEEE Journal of Quantum Electronics.
• Armstrong, N. R. (2000). Phthalocyanines and porphyrins as materials. Journal of Porphyrins and Phthalocyanines, 4(4), 414-417.
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  Abstract: Several recent studies of phthalocyanines and porphyrins as materials in emerging technologies are reviewed here. Emphasis is placed on the use of these materials as components in building materials where the symmetry, optical and electrical properties of the molecule are important. Aggregates or polymers of these molecules have been known for some time to possess interesting electrical conductivities, and more recently interesting optical properties. Their optical properties as isolated species in condensed phases have also recently become interesting, and their ability to form new hybrid materials, by mixing or by thin film deposition, with other molecules with different electron affinities and ionization potentials, now appears to be extremely attractive. Device technologies in which we can anticipate these molecules appearing in the near future include organic light-emitting diodes, organic field effect transisitors, organic photovoltaics, optical limiters and optically based chemical sensors. Copyright (C) 2000 John Wiley and Sons, Ltd.
• Armstrong, N. R. (2000). Phthalocyanines and porphyrins as materials. Journal of Porphyrins and Phthalocyanines.
• Armstrong, N. R. (2000). Preparation of polyetherol-appended sulfur porphyrazines and investigations of peripheral metal ion binding in polar solvents. Inorganic Chemistry.
• Back, A., Alloway, D., Schlettwein, D., Schilling, B., Wang, J. -., Carducci, M., & Armstrong, N. R. (2000). Ordered thin films of perylenetetracarboxylicdianhydride-bisimide and bis-(N-alkyl)-quinacridone dyes. Materials Research Society Symposium - Proceedings, 620, M3.2.1-M3.2.12.
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  Abstract: We review here the recent characterization of vacuum deposited monolayer and multilayer thin films of two different perylenetetracarboxylic-dianydride-bisimides (Cn-PTCDI; n=4,5), quinacridone, and two new bis-(N-alkyl)-quinacridone dyes (DIQA and DEHQA) on single crystal alkali halides using a combination of in situ luminescence spectroscopies and ex situ tapping mode AFM. Flat lying monolayer structures are indicated for PTCDA on the (100) faces of NaCl, KCl and KBr, for C4-PTCDI on KCl, for C5-PTCDI on both KCl and KBr and for DIQA on both KCl and KBr. Coherent thin films, exhibiting layer-by-layer growth can be achieved for PTCDA on all substrates, for C4-PTCDI on KCl and for DIQA on both KBr and KCl. Both C4-PTCDI and DIQA appear to fulfill the requirements for dyes which exhibit layered growth with the molecular plane inclined at steep angles to the surface normal.
• Ehrlich, L. A., Skrdla, P. J., Jarrell, W. K., Sibert, J. W., Armstrong, N. R., Saavedra, S. S., G., A., & Hoffman, B. M. (2000). Preparation of polyetherol-appended sulfur porphyrazines and investigations of peripheral metal ion binding in polar solvents. Inorganic Chemistry, 39(18), 3963-3969.
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  PMID: 11198848;Abstract: We describe the complete synthesis and characterization of a new family of peripherally functionalized porphyrazines (pz's) with four, three, or two (in a trans conformation) bis[thioethoxy(ethoxy)ethanol] moieties appended at the pyrroles. These 'polyetherol' groups serve as weak exocyclic binding sites for a number of metal ions and also provide solubility of the pz's in low molecular-weight alcohols and water. Electronic spectra of the modified porphyrazines exhibit distinct changes in the visible region (both absorbance and fluorescence) in response to treatment with Ag+, Pb2+, Cd2+, Cs+, and Ni2+ in solution. Such properties make these compounds intriguing candidates for incorporation into the transducer layers in optically based chemical sensors.
• Gross, E. M., Anderson, J. D., Slaterbeck, A. F., Thayumanavan, S., Barlow, S., Zhang, Y., Marder, S. R., Hall, H. K., Nabor, M. F., Wang, J. -., Mash, E. A., Armstrong, N. R., & Wightman, R. M. (2000). Electrogenerated chemiluminescence from derivatives of aluminum quinolate and quinacridones: Cross-reactions with triarylamines lead to singlet emission through triplet-triplet annihilation pathways. Journal of the American Chemical Society, 122(20), 4972-4979.
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  Abstract: Solution electrogenerated chemiluminescence (ECL) was evaluated for molecules of interest for organic light-emitting diodes (OLEDs), using high- frequency voltage pulses at a microelectrode Radical cations of different energies were electrogenerated from a series of triarylamine hole-transport materials (x-TPD), in the presence of radical anions of a high electron affinity sulfonamide derivative of tris(8-hydroxyquinoline)aluminum (Al(qs)3), or a bis(isoamyl) derivative of quinacridone (DIQA). The resultant emission was from the excited singlet states 1Al(qs)3* or 1DIQA*, the same excited state produced in OLEDs based on these molecules. In solution, the majority of the reaction pairs had insufficient energy to populate 1Al(qs)3* or 1DIQA* directly, but could form the triplet states 3Al(qs)3* or 3DIQA*. The reaction order and the temporal response of the emission were consistent with subsequent formation of the excited singlet states via triplet-triplet annihilation (TTA). For reactions with a low excess Gibbs free energy to form the triplet state (Δ(T)G), the efficiency increased exponentially with an increase in driving force (increase in oxidation potential of x-TPD), then reached a plateau. At the maximum, the efficiencies for formation of 1Al(qs)3* or 1DIQA* via the TTA route reach as high as a few percent. The computed energetics of these reactions suggest that similar light-producing electroluminescent reactions, proceeding via triplet formation, could also occur in condensed phase organic thin films.
• Jabbour, G. E., Shaheen, S. E., Morrell, M. M., Anderson, J. D., Lee, P., Thayumanavan, S., Barlow, S., Bellmann, E., Grubbs, R. H., Kippelen, B., Marder, S., Armstrong, N. R., & Peyghambarian, N. (2000). High T_g hole transport polymers for the fabrication of bright and efficient organic light-emitting devices with an air-stable cathode. IEEE Journal of Quantum Electronics, 36(1), 12-17.
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  Abstract: An organic electroluminescent device with a luminous efficiency of 20 lm/W, at 14 cd/m2, and an external quantum efficiency of 4.6% has been fabricated using a high Tg hole transport polymer, a small molecule emission layer, and a LiF/Al cathode. The device quantum efficiency can be increased by tuning the ionization potential of the hole-transport moieties. When tested under pulsed voltage mode, in air at room temperature, and without any encapsulation, the device showed a high peak brightness of 4.4×106 cd/m2 at 100 A/cm2 and an efficiency of 4.4 cd/A.
• Malinsky, J. E., Li, W., Wang, Q., Cui, J., Chou, H., Marks, T. J., Jabbour, G. E., Shaheen, S. E., Anderson, J. D., Lee, P., Richter, A. G., Kippelen, B., Dutta, P., Peyghambarian, N., & Armstrong, N. (2000). Self-assembly processes for organic led transport layers and electrode passivation. Materials Research Society Symposium - Proceedings, 558, 459-467.
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  Abstract: This contribution describes the use of self-limiting siloxane chemisoroption processes to self-assemble building blocks for the modification of vacuum-deposited organic LED (OLED) devices. One approach consists of the use of self-assembling OLED hole transport materials for application in hybrid self-assembled + vapor deposited two-layer devices. Another approach involves the application of self-limiting, chemisorptive self-assembly techniques to introduce thin dielectric films between the anode and hole transport layer of a vapor deposited two-layer OLED device.
• Armstrong, N. R. (1999). Absence of final-state screening shifts in photoemission spectroscopy frontier orbital alignment measurements at organic/semiconductor interfaces. Surface Science.
• Armstrong, N. R. (1999). Covalently interlinked organic LED transport layers via spin-coating/siloxane condensation. Advanced Materials.
• Armstrong, N. R. (1999). Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study. Journal of Applied Physics.
• Armstrong, N. R. (1999). Formation and Spectroelectrochemical Characterization of Multilayer and Submonolayer Thin Films of 2,3,9,10,16,17,23,24-Octa(2-benzyloxy-ethoxy) Phthalocyninato Copper (CuPc(OC2OBz)8). Journal of Porphyrins and Phthalocyanines.
• Armstrong, N. R. (1999). HOMO/LUMO Alignment at PTCDA/ZnPc and PTCDA/ClInPc Heterointerfaces Determined by Combined UPS and XPS Measurements. Journal of Physical Chemistry B.
• Armstrong, N. R. (1999). Highly ordered thin films of octasubstituted phthalocyanines. Journal of the American Chemical Society.
• Armstrong, N. R. (1999). Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2. Journal of Applied Physics.
• Armstrong, N. R. (1999). Organic light emitting devices containing a highly substituted isoindole or polyisoindole. Thin Solid Films.
• Armstrong, N. R. (1999). Organic light-emitting diode with 20 Im/W efficiency using a triphenyldiamine side-group polymer as the hole transport layer. Applied Physics Letters.
• Armstrong, N. R. (1999). Oxadiazole metal complex for organic light-emitting diodes. Advanced Materials.
• Armstrong, N. R. (1999). Photoconductive properties of PVK-based photorefractive polymer composites doped with fluorinated styrene chromophores. Journal of Materials Chemistry.
• Armstrong, N. R. (1999). Photorefractive polymer composites with short response times. Journal of Imaging Science and Technology.
• Armstrong, N. R. (1999). Reduction of indicator leaching from doped sol-gels by attachment of macromolecular carriers. Applied Spectroscopy.
• Armstrong, N. R. (1999). Self-assembly processes for organic LED electrode passivation and charge injection balance. Advanced Materials.
• Armstrong, N. R. (1999). Sol-gel-based, planar waveguide sensor for water vapor. Analytical Chemistry.
• Armstrong, N. R. (1999). Structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride grown on reconstructed and unreconstructed Au(100). Surface Science.
• Dunphy, D. R., Mendes, S. B., Li, L., Burke, J. J., Lee, J. E., Armstrong, N. R., & Saavedra, S. (1999). New planar waveguide attenuated total reflectance techniques for organic thin film spectroscopy and chemical sensing. Proceedings of SPIE - The International Society for Optical Engineering, 3602, 140-148.
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  Abstract: The planar integrated optical waveguide (IOW) is an inherently sensitive geometry for attenuated total reflection (ATR) spectroscopy of interfacial samples. A major disadvantage that has limited its wider use is the difficulty of measuring broadband spectra. Due to the quantized nature of light propagation in a planar IOW, conventional grating and prism couplers are efficient only over a narrow (
• Gauvin, S., Santerre, F., Dodelet, J. P., Ding, Y., Hlil, A. R., Hay, A. S., Anderson, J., Armstrong, N. R., Gorjanc, T. C., & D'Iorio, M. (1999). Organic light emitting devices containing a highly substituted isoindole or polyisoindole. Thin Solid Films, 353(1), 218-222.
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  Abstract: A highly substituted and fluorescent Isoindole has been used in a bilayer type of organic electroluminescent device with either PVK or Alq3. The best results (maximum luminance > 6000 cd/m2, apparent quantum yield at max luminance of 1.1%, and a low threshold voltage of 4 V) are obtained for the combination: anode/Isoindole/Alq3/cathode. These devices are characterized by the Alq3 electroluminescent spectrum. The role of Isoindole is that of a hole transport material. This is in agreement with the reversible character of Isoindole electrochemical oxidation and the stability of the Isoindole cation radical in solution. Lower maximum luminances (approx. 30 cd/m2) are obtained when Isoindole is replaced with Polyisoindole, a polymer containing the Isoindole in its backbone.
• Hendrickx, E., Zhang, Y., Ferrio, K. B., Herlocker, J. A., Anderson, J., Armstrong, N. R., Mash, E. A., Persoons, A. P., Peyghambarian, N., & Kippelen, B. (1999). Photoconductive properties of PVK-based photorefractive polymer composites doped with fluorinated styrene chromophores. Journal of Materials Chemistry, 9(9), 2251-2258.
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  Abstract: We have synthesized nine anisotropic chromophores, with different degrees of fluorination, and studied the effect of the chromophore's ionization potential on charge-transfer complexation, photoconductivity, and response time in photorefractive polymer mixtures based on poly(vinylcarbazole). (2,4,7-Trinitrofluoren-9-ylidene)malononitrile (TNFDM) or C60 provided the sensitization. We have found evidence of strong complexation between TNFDM and the chromophore. At high electric fields, the photoconductivity decays during illumination and reaches a limiting value that correlates with the chromophore's ionization potential. A buildup of C60- radical anions is observed simultaneously. The strong decline in photoconductivity correlates with an increase in the photorefractive grating buildup time.
• Jabbour, G. E., Kippelen, B., Armstrong, N. R., & Peyghambarian, N. (1999). Organic electroluminescent devices: aluminum alkali-halide composite cathode for enhanced device performance. Proceedings of SPIE - The International Society for Optical Engineering, 3623, 58-63.
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  Abstract: Cathodes made with Al-LiF or Al-CsF composites are found to enhance the performance of organic light-emitting devices. With a composite cathode, devices based on an organic bilayer structure have shown lower operating voltage, higher efficiency and better forward light output than devices with LiF/Al, Mg or Al cathode. Unlike devices with Al and Li alloy cathode, OLEDs with composite cathode can be made with good reproducibility.
• Jabbour, G. E., Shaheen, S. E., Morrell, M. M., Anderson, J. D., Lee, P., Thayumanavan, S., Barlow, S., Marder, S., Bellmann, E., Grubbs, S. R., Kippelen, B., Armstrong, N. R., & Peyghambarian, N. (1999). Hybrid bilayer organic light-emitting devices based on high T_g hole transport polymers. Proceedings of SPIE - The International Society for Optical Engineering, 3623, 20-27.
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  Abstract: We report on organic electroluminescent devices based on Al cathode with luminous efficiency of 20 lm/W and external quantum efficiency of 4.6%. When pulsed in air at room temperature and without any encapsulation, high peak brightness of 4.4 × 106 cd/m2 and high efficiency of 4.4 cd/A are obtained. In contrast to current belief, we demonstrate that device quantum efficiency can be increased by tuning the ionization potential of the hole-transport moieties. The high efficiency and peak brightness reported here with Al cathode are encouraging for the manufacturing of stable devices and the development of electrically injected organic lasers.
• Jabbour, G. E., Shaheen, S. E., Morrell, M. M., Kippelen, B., Armstrong, N. R., & Peyghambarian, N. (1999). Aluminum composite cathodes: A new method for the fabrication of efficient and bright organic light-emitting devices. Optics and Photonics News, 10(4), 24-27.
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  Abstract: A composite cathode of aluminum-lithium fluoride (Al-LiF), based on 8-tris-hydroxy-quinoline aluminum (Alq3) as the light-emitting medium and N-N′-diphenyl-N-N′bis(3-methylphenyl)-[1-1′-biphenyl]- 4-4′-diamine (TPD) as the hole transport layer, showed an order to magnitude increase in performance compared to Al-only organic light emitting diodes (OLEDs). Using composite cathodes eliminates the need for a thin insulating layer to be deposited between the metal and the underlying organic layer, thus simplifying device geometry.
• Kippelen, B., Hendrickx, E., Ferrio, K. B., Herlocker, J., Zhang, Y., Peyghambarian, N., Marder, S. R., Anderson, J., Armstrong, N. R., & Mery, S. (1999). Photorefractive polymer composites with short response times. Journal of Imaging Science and Technology, 43(5), 405-412.
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  Abstract: We report on a photorefractive polymer composite that shows an initial rise time of 4 ms at a grating spacing of 3.1 μm. A systematic characterization of the ionization potential of a series of chromophores used in photorefractive applications is presented. No obvious correlation between material response time and ionization potential is found in PVK:ECZ:TNFDM composites doped with chromophores with an ionization potential higher or comparable to that of carbazole. All the chromophores under investigation are found to form a charge-transfer complex with the sensitizer TNFDM and a correlation between the spectral position of this band and the value of the ionization potential of the chromophore is demonstrated.
• Malinsky, J. E., Jabbour, G. E., Shaheen, S. E., Anderson, J. D., Richter, A. G., Marks, T. J., Armstrong, N. R., Kippelen, B., Dutta, P., & Peyghambarian, N. (1999). Self-assembly processes for organic LED electrode passivation and charge injection balance. Advanced Materials, 11(3), 227-231.
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  Abstract: The performance of organic LEDs can be improved by modifying the organic thin film-electrode interface. Here is reported the use of layer-by-layer, self-limiting chemisorptive siloxane self-assembly techniques to deposit layers of the capping agent octachlorotrisiloxane between an indium tin oxide anode layer and the hole transport layer. Such techniques can yield nearly pinhole-free dielectric layers with angstrom-level control over total dielectric layer thickness. Examination of the physical and electronic properties of the resulting devices shows that the external quantum and luminous efficiencies of the LED are significantly enhanced after the introduction of the dielectric layers.
• Malinsky, J. E., Li, W., Wang, Q., Gui, J., Chou, H., Marks, T. J., Jabbour, G. E., Shaheen, S. E., Anderson, J. D., Lee, P., Richter, A. G., Kippelen, B., Dutta, P., Peyghambarian, N., & Armstrong, N. (1999). Self-assembly processes for organic LED transport layers and electrode passivation. Materials Research Society Symposium - Proceedings, 561, 201-209.
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  Abstract: This contribution describes the use of self-limiting siloxane chemisorption processes to self-assemble building blocks for the modification of vacuum-deposited organic LED (OLED) devices. One approach consists of the use of self-assembling OLED hole transport materials for application in hybrid self-assembled + vapor deposited two-layer devices. Another approach involves the application of self-limiting, chemisorptive self-assembly techniques to introduce thin dielectric films between the anode and hole transport layer of a vapor deposited two-layer OLED device.
• Marks, T. J., Cui, J., Malinsky, J. E., Wang, Q., Jabbour, G. E., Shaheen, S. E., Anderson, J. D., Paul, A. L., Richter, A. G., Kippelen, B., Dutta, P., Peyghambarian, N., & Armstrong, N. R. (1999). Approaches to organic light-emitters via layer-by-layer self-assembly. American Chemical Society, Polymer Preprints, Division of Polymer Chemistry, 40(2), 1196-1197.
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  Abstract: Self-assembly provides a means with which it is possible to form robust, pinhole-free, adherent thin films with covalently interlinked, glassy structures of readily controllable thickness. The incorporation of a self-assembled hole transport layer structure into a conventional vapor deposited organic light emitting diode (OLED) yields a stable structure with low turn-on voltages, high light output, and good external quantum efficiency. A totally self-assembled system was fabricated, demonstrating the ability of the technique to form OLED devices.
• Schlaf, R., Crisafulli, L. A., Murata, H., Merritt, C. D., Kafafi, Z. H., Schroeder, P. G., Nelson, M. W., Parkinson, B. A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1999). Determination of the electronic structure of organic Schottky contacts by photoemission spectroscopy. Proceedings of SPIE - The International Society for Optical Engineering, 3797, 189-197.
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  Abstract: The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the organic luminescent semiconductor Gaq3 relative to the Fermi level of Au was determined by depositing a Gaq3 thin film in a multi-step growth procedure on an Au foil. Before growth and after each deposition step the sample was characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. Such measurements offer a direct way to determine the electronic structure of the interface. Our experiments demonstrate that this method allows distinguishing between band bending, charging and interface dipole related shifts in the UP-spectra. The additional XPS measurements allow the precise determination of the band bending occurring across the interface while comparison between XPS and UPS work function measurements allows one to pinpoint the organic film thickness dependent onset of charging phenomena. Our results show that the interface dipoles at Gaq3 Schottky contacts with Au, Pt and Ag amount to 0.6-0.7 eV. Our experiments also show that final state screening shifts can be dismissed as insignificant in such orbital line-up measurements. This was shown at the chloroindium phthalocyanine (ClInPc)/highly oriented pyrolytic graphite (HOPG) interface where no such shifts were observed.
• Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1999). Absence of final-state screening shifts in photoemission spectroscopy frontier orbital alignment measurements at organic/semiconductor interfaces. Surface Science, 420(1), L122-L129.
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  Abstract: Chloroindium phthalocyanine (ClInPc) thin films were grown by vapor deposition on highly oriented pyrolytic graphite (HOPG) substrates. The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the phthalocyanine layer relative to the Fermi level of the HOPG substrate was determined by combined X-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. The measurements revealed that the electronic structure of this interface is almost free of band bending, which results in only very weak shifts of the ClInPc-related XPS core-level peaks. Therefore, the influence of final-state screening shifts on orbital alignment measurements at organic/semiconductor interfaces with photoemission spectroscopy (PES) can be investigated in this system. The comparison of our results with PES measurements of monolayers and multilayers of xenon on palladium substrates revealed that final-state screening plays no or only an insignificant role within the accuracy that can be achieved in such measurements. © 1999 Elsevier Science B.V.
• Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1999). HOMO/LUMO Alignment at PTCDA/ZnPc and PTCDA/ClInPc Heterointerfaces Determined by Combined UPS and XPS Measurements. Journal of Physical Chemistry B, 103(15), 2984-2992.
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  Abstract: The offsets between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) at the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/chloroindium phthalocyanine (ClInPc) and PTCDA/zinc phthalocyanine (ZnPc) organic heterojunctions were estimated from a combination of X-ray and ultraviolet photoemission (XPS,UPS) measurements. This combined method allows an improved determination of the electronic structure of such organic/organic' interfaces due to the separate determination of the band bending (charge redistribution) following heterojunction formation. Both interfaces have large offsets in the onset for photoemission from their HOMO levels (PTCDA/ZnPc: 0.88 eV; PTCDA/CnlInPc: 0.93 eV). Using thin film absorbance data, the corresponding offsets in LUMO levels were estimated to be 0.66 eV (PTCDA/ZnPc) and 0.34 (PTCDA/ClInPc). The ZnPc/PTCDA interface showed a significant interface dipole (0.25 eV) while the ClInPc/PTCDA contact was essentially dipole free.
• Schlaf, R., Schroeder, P. G., Nelson, M. W., Parkinson, B. A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1999). Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS₂. Journal of Applied Physics, 86(3), 1499-1509.
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  Abstract: Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) allow the measurement of band bending with much higher accuracy than could be achieved in experiments carried out by UPS alone. These experiments also show that, due to the different depth sensitivity of high binding energy cutoff (secondary edge) and XPS core levels (or UPS valence bands), it is very important to grow relatively thick overlayers in order to measure orbital alignment and interface dipole correctly. © 1999 American Institute of Physics.
• Schmitz-Hübsch, T., Fritz, T., Staub, R., Back, A., Armstrong, N. R., & Leo, K. (1999). Structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride grown on reconstructed and unreconstructed Au(100). Surface Science, 437(1), 163-172.
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  Abstract: The growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) has been characterized on Au(100) from the submonolayer range to multilayer films by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) and under ambient conditions. Two different structures of PTCDA are found, one of which is the well-known herringbone structure, with the plane of the molecule flat-lying on the substrate surface. This structure is point-on-line coincident with the Au(100) lattice. The second structure which is predominant on this surface consists of closely spaced rods, each rod consisting of closely packed PTCDA molecules. We propose that this structure is associated with the (010) bulk plane of the α-polymorph of PTCDA. The reason for the different growth behavior compared to PTCDA on Au(111) at the very same growth conditions can be found in the nanoscopic structure of the reconstructed Au(100)hex surface. This reconstruction of the Au(100) surface provides trenches along the Au[011] azimuth which act as a specific adsorption site for PTCDA, leading to very low diffusion rates of this molecule following its deposition. The diffusion rate of first-deposited PTCDA was low enough to enable us to image single molecules at room temperature. These findings, in comparison with results on the reconstructed Au(111) surface, show that not only the substrate material and the symmetry, but also the surface structure on the nanoscopic scale can rule the growth of the organic overlayer.
• Shaheen, S. E., Jabbour, G. E., Kippelen, B., Peyghambarian, N., Anderson, J. D., Marder, S. R., Armstrong, N. R., Bellmann, E., & Grubbs, R. H. (1999). Organic light-emitting diode with 20 Im/W efficiency using a triphenyldiamine side-group polymer as the hole transport layer. Applied Physics Letters, 74(21), 3212-3214.
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  Abstract: We have used triphenyldiamine side-group polymers as hole transport layers in multilayer organic light-emitting diodes using 8-hydroxyquinoline aluminum (Alq3) as an emission layer. The device efficiency systematically increases as the ionization potential of the hole transport layer is shifted further from the work function of the indium-tin-oxide anode. We attribute this trend to better balance of hole and electron charges in the device. An optimized device consisting of a fluorinated version of the polymer as the hole transport layer, quinacridone doped Al as the emission layer, and a LiF/Al cathode results in a peak external luminous efficiency of 20 Im/W. © 1999 American Institute of Physics.
• Shaheen, S. E., Kippelen, B., Peyghambarian, N., Wang, J. -., Anderson, J. D., Mash, E. A., Lee, P. A., Armstrong, N. R., & Kawabe, Y. (1999). Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study. Journal of Applied Physics, 85(11), 7939-7945.
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  Abstract: The mechanics of Forster energy transfer and charge transfer in organic light emitting diodes (OLED) based on 8-hydroxyquinoline aluminum (Alq3) were studied. A soluble derivative of quinacridone, N-N′-di-isoamyl quinacridone (DIQA) was synthesized and was used with Alq3 to dope films of poly(9-vinylcarbazole) (PVK). Measurements of the photoluminescence (PL) and electroluminescence (EL) of the films indicated a more enhanced efficiency that PVK:Alq3 films. However, the optimal DIQA concentration was found to be lower for EL that for PL.
• Skrdla, P. J., Saavedra, S. S., & Armstrong, N. R. (1999). Reduction of indicator leaching from doped sol-gels by attachment of macromolecular carriers. Applied Spectroscopy, 53(7), 785-791.
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  Abstract: The extent of leaching of indicator dyes of varying molecular mass, following encapsulation into hydrated sol-gel monoliths prepared from tetramethylorthosilicate (TMOS), was investigated. Dye modification of a water-soluble phthalocyanine with poly(ethylene glycol) side chains (2000 MW) reduced leaching relative to the unmodified dye, but significant losses were still observed. Monoliths doped with fluorescein modified with dextrans of higher molecular weight were investigated in an attempt to determine the minimum size necessary for dye retention. In comparison to monoliths doped with unmodified fluorescein, attachment to a dextran carrier significantly decreased the extent of dye leaching. However, even a 70000 MW carrier was not sufficient to prevent some loss of dye. In contrast, no leaching of myoglobin (18000 MW) occurred, which shows that when the entrapped molecule has a organized tertiary structure, diffusion within the pores is eliminated. Malachite green and its dextran conjugate were used to examine the effect of electrostatic interactions on leaching. Minimal loss of both the dye and the conjugate was observed, which indicates that when a favorable interaction between the dye and sol-gel matrix exists, attachment to a macromolecular carrier may not be necessary to prevent leaching.
• Skrdla, P. J., Saavedra, S. S., Armstrong, N. R., Mendes, S. B., & Peyghambarian, N. (1999). Sol-gel-based, planar waveguide sensor for water vapor. Analytical Chemistry, 71(7), 1332-1337.
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  PMID: 21662954;Abstract: A water vapor sensor based on a combination of sol-gel processing and planar optical waveguide technologies has been developed. The indicator erythrosin B was entrapped in a thin sol-gel film (thickness ~100 nm) prepared from methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane. This dye exhibits an increase in absorbance in the presence of liquid or gaseous water. The sol-gel layer containing the dye was deposited onto a sol-gel-derived, single-mode planar waveguide. Outcoupled light intensity measurements (at 514.5 nm) over a range of water vapor concentrations (in a nitrogen gas stream) yielded a response over a wide range of relative humidity (
• Smolenyak, P. E., Peterson, R. A., Dunphy, D. R., Mendes, S., Nebesny, K. W., O'Brien, D., Saavedra, S. S., & Armstrong, N. R. (1999). Formation and Spectroelectrochemical Characterization of Multilayer and Submonolayer Thin Films of 2,3,9,10,16,17,23,24-Octa(2-benzyloxy-ethoxy) Phthalocyninato Copper (CuPc(OC₂OBz)₈). Journal of Porphyrins and Phthalocyanines, 3(6-7), 620-633.
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  Abstract: We review here the self-assembly, electrochemical and spectroelectrochemical properties of 2,3,9,10,16,17,23,24-octa(2-benzyloxyethoxy) phthalocyaninato copper (CuPc(OC2OBz)8), where terminal benzyl groups on the eight ethylene oxide side chains assist in forming unusually ordered, mechanically rigid thin films. New spectroscopic characterization of cast CuPc(OC2OBz)8 films is discussed in comparison with cast films of its metal-free analogue H2Pc(OC2OBz)8 and similar copper and dihydrogen phthalocyanines with benzyl - terminated triethylene oxide substituents, CuPc((OC2O)3Bz)8, and H2Pc((OC2O)3Bz)8, which do not demonstrate the same degree of ordering as CuPc(OC2OBz)8. AFM studies of horizontally transferred LB films of CuPc(OC2OBz)8 show column-column distances of ca 2.8 nm and confirm the high degree of ordering previously surmised from spectroscopic characterization of multilayer thin films. The oxidative electrochemistry of multilayer thin films prepared from these Pcs is strongly dependent on the chemical identity of the supporting electrolyte anion and on annealing of the thin films. Compliance of the films to counter-ion transport limits the extent of electrochemical doping. Preliminary studies of the oxidative electrochemistry of isolated CuPc(OC2OBz)8 aggregates (diluted into an electroinactive methyl arachidate thin film) on an electroactive, integrated optic waveguide (EA-IOW) are also presented. Monitoring the change in absorbance at 633 nm on the waveguide surface allows the determination of the onset potential for oxidation of the isolated aggregates, which appears to be less positive in potential versus that observed for multilayer Pc assemblies. Copyright © 1999 John Wiley & Sons, Ltd.
• Smolenyak, P., Peterson, R., Nebesny, K., Törker, M., O'Brien, D. F., & Armstrong, N. R. (1999). Highly ordered thin films of octasubstituted phthalocyanines. Journal of the American Chemical Society, 121(37), 8628-8636.
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  Abstract: Phthalocyanines (Pc) modified at eight positions with benzyloxyethoxy groups (CuPc(OC2OBz)8 and H2Pc(OC2OBz)8) form discotic mesophases in the bulk and self-assemble into cofacial rodlike aggregates in monolayers. These Pc aggregates form close packed arrays which are simple to process into highly coherent multilayer thin films. When these self-organizing molecular systems are compressed on the surface of an LB trough, the Pc columns align parallel to the compression barriers and form a rigid bilayer film (ca. 5.6 nm thick). Sections of these bilayer films can be mechanically removed from the trough surface with complete retention of integrity of the remaining film. This unprecedented behavior appears to arise from the multiple π-π interactions between adjacent Pcs, resulting in unusually strong noncovalent interactions. AFM and STM studies of monolayer and bilayer films show that they consist of Pc columns which are coherent over distances of 50-100 nm. Thin films consisting of up to 15 bilayers (ca. 84 nm) can be quickly built by a 'mechanical stamping' process, and small-angle X-ray scattering characterization of these film materials confirms their unusual coherence. Large electrical and optical anisotropies are observed in multilayer films of CuPc(OC2OBz)8 and H2Pc(OC2OBz)8 which are comparable to those previously reported for thin film assemblies based on side-chain-modified silicon phthalocyanine polymers.
• Wang, J. F., Jabbour, G. E., Mash, E. A., Anderson, J., Zhang, Y., Lee, P. A., Armstrong, N. R., Peyghambarian, N., & Kippelen, B. (1999). Oxadiazole metal complex for organic light-emitting diodes. Advanced Materials, 11(15), 1266-1269.
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  Abstract: An oxadiazole metal complex, tris(2-(5-phenyl-1,3,4-oxadiazol-y)phenonate) aluminum (AlPOP) is characterized for use as organic light emitting diodes (LED). Such oxadiazole complex is a good electron transporter with excellent shelf lifetime stability in organic LEDs. OLEDs based on this complex show a strong blue emission with high external efficiency and brightness.
• Weijin, L. i., Wang, Q., Cui, J., Chou, H., Shaheen, S. E., Jabbour, G. E., Anderson, J., Lee, P., Kippelen, B., Peyghambarian, N., Armstrong, N. R., & Marks, T. J. (1999). Covalently interlinked organic LED transport layers via spin-coating/siloxane condensation. Advanced Materials, 11(9), 730-734.
• Anderson, J. D., McDonald, E. M., Lee, P. A., Anderson, M. L., Ritchie, E. L., Hall, H. K., Hopkins, T., Mash, E. A., Wang, J., Padias, A., Thayumanavan, S., Barlow, S., Marder, S. R., Jabbour, G. E., Shaheen, S., Kippelen, B., Peyghambarian, N., Wightman, R. M., & Armstrong, N. R. (1998). Electrochemistry and electrogenerated chemiluminescence processes of the components of aluminum quinolate/triarylamine, and related organic light- emitting diodes. Journal of the American Chemical Society, 120(37), 9646-9655.
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  Abstract: Solution electrochemical studies have been conducted of the principal lumophores, dopants, and hole-transport agents of aluminum-quinolate (Alq3)- based organic light-emitting diodes (OLEDs) along with the characterization of their electrogenerated chemiluminescence (ECL). In acetonitrile/benzene solvent mixtures, Alq3 shows single one-electron reduction and oxidation processes, with a separation between the first oxidation and first reduction potentials, ΔE(electrochemical) = 3.03 V, close to the estimates of energy difference between HOMO and LUMO levels obtained from absorbance spectra of thin films of Alq3. ΔE(optical) = 3.17 eV. A new sulfonamide derivative of Alq3, (Al(qs)3), showed a positive shift (ca. 0.32 V) in the first reduction potential versus the parent molecule, and resolution of the overall reduction process into three successive, chemically reversible, one-electron reductions. Two successive one-electron oxidations are seen for 4,4'-bis-(m- tolyphenylamino)biphenyl (TPD), a hole-transporting material in many bilayer OLEDs, and for TPDF2, a fluorinated version of TPD, with TPDF2 oxidation occurring 0.1 V positive of that for TPD. Electrogenerated chemiluminescence reactions (Alq3-./TPD+. (or TPDF2+.) and Al(qs)3-./TPD+. (or TPDF2+.)) were found to produce emission spectra from Alq3(*s) or Al(qs)3(*s) states which nearly identical to those seen from OLEDs based upon these molecules. Emission intensities increased with the increasing potential difference between the relevant redox couples. The diisoamyl derivative of quinacridone (DIQA), a quinacridone dopant for certain Alq3- based OLEDs, undergoes two successive one-electron reductions and two successive one-electron oxidations. The ECL reactions DIQA-./DIQA+., DIQA+./Alq3-., DIQA+./Al(qs)3-./TPD+. and DIQA-./TPDF2+. all produce the same singlet emissive state, DIQA(*s), and the same emission spectral response seen in quinacridone and DIQA-doped OLEDs.
• Armstrong, N. R. (1998). Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices. Applied Physics Letters.
• Armstrong, N. R. (1998). Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode. Journal of Applied Physics.
• Armstrong, N. R. (1998). Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements. Applied Physics Letters.
• Armstrong, N. R. (1998). Electrochemistry and electrogenerated chemiluminescence processes of the components of aluminum quinolate/triarylamine, and related organic light- emitting diodes. Journal of the American Chemical Society.
• Armstrong, N. R. (1998). Exciplex electroluminescence from organic bilayer devices composed of triphenyldiamine and quinoxaline derivatives. Advanced Materials.
• Armstrong, N. R. (1998). Influence of surface reactions and ionization gradients on junction properties of F16PcZn. Journal of Materials Chemistry.
• Armstrong, N. R. (1998). Photoemission spectroscopy of LiF coated Al and Pt electrodes. Journal of Applied Physics.
• Armstrong, N. R. (1998). Ultrathin Films of Perylenedianhydride and Perylenebis(dicarboximide) Dyes on (001) Alkali Halide Surfaces. Chemistry of Materials.
• Armstrong, N. R., Anderson, J., Lee, P., McDonald, E., Wightman, R., Hall, H. K., Hopkins, T., Padias, A., Thayumanavan, S., Barlow, S., & Marder, S. (1998). Electrochemical models for the radical annihilation reactions in organic light-emitting diodes. Proceedings of SPIE - The International Society for Optical Engineering, 3476, 178-187.
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  Abstract: Bilayer organic light emitting diodes (OLED), based upon vacuum deposited molecules, or single layer OLEDs, based upon spin-cast polymeric materials, doped with these same molecules, produce light from emissive states of the lumophores which are created through annihilation reactions of radical species, which can be modeled through solution electrochemistry. Differences seen in solution reduction and oxidation potentials of molecular components of OLEDs are a lower limit estimate to the differences in energy of these same radical species in the condensed phase environment. The light emitted from an aluminum quinolate (Alq3)/triarylamine (TPD)-based OLED, or an Alq3/PVK single layer OLED, can be reproduced from solution cross reactions of Alq3/TPD.
• Armstrong, N. R., Dunphy, D., Smolenyak, P., Rengel, H., Mendes, S., Saavedra, S. S., O'Brien, D., & Wegner, G. (1998). Electrochemical processes of the polyphthalocyaninatosiloxanes and related cofacially aggregated phthalocyanine assemblies. American Chemical Society, Polymer Preprints, Division of Polymer Chemistry, 39(1), 723-724.
• Chakrabarti, A., Schmidt, A., Valencia, V., Fluegel, B., Mazumdar, S., Armstrong, N., & Peyghambarian, N. (1998). Evidence for exciton-exciton binding in a molecular aggregate. Physical Review B - Condensed Matter and Materials Physics, 57(8), R4206-R4209.
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  Abstract: We present evidence for a biexciton state in a molecular aggregate. Femtosecond pump-probe spectroscopy of the H aggregate of an epitaxially grown trivalent metal-halogen-phthalocyanine gives distinct photoinduced absorptions to a continuum of two-exciton states and to a bound biexciton state. No signature of the biexciton is found in the J aggregate of a very similar trivalent metal-halogen-phthalocyanine. The strong exciton-exciton interaction in the H-aggregate system has its origin in the unique cofacial stacking of the molecular components in this material.
• Hiller, S., Schlettwein, D., Armstrong, N. R., & Wöhrle, D. (1998). Influence of surface reactions and ionization gradients on junction properties of F₁₆PcZn. Journal of Materials Chemistry, 8(4), 945-954.
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  Abstract: Compared to unsubstituted phthalocyaninatozinc(II) (PcZn), electron withdrawing fluorine atoms in hexadecafluorophthalocyaninatozinc(II) (F16PcZn) cause a stabilization of the frontier orbitals of about 1.6 eV. This is concluded from photoelectron spectroscopy (UPS) at thin films on Au surfaces. From experiments at thin films [physical vapor deposition (PVD)] of PcZn deposited on top of F16PcZn under UHV conditions it is seen that a closed film of PcZn is formed at least within 5 nm average film thickness, that thermodynamic equilibrium between the films is achieved by charge transfer in redox reactions at the interface which, however, do not lead to a macroscopic space-charge layer. To study electrical device properties thin films of F16PcZn and PcZn were prepared in a range between 90 nm and 240 nm. Changes in electrical properties of ITO, Au|F16PcZn]metal (metal = In, Au) and ITO|F16PcZn|PcZn|Au devices have been studied in the dark and under illumination. Results of current-voltage characteristics and short-circuit photocurrent spectra of devices as prepared and measured under high vacuum (HV, 10-5-10-6 mbar) and after exposure to air are presented. In vacuum symmetrical I(U) characteristics were found for ITO|F16PcZn|Au devices. After exposure to air a decrease in dark conductivity, unsymmetrical I(U) characteristics and a considerable photovoltage (UOC) was measured under illumination. The magnitude of UOC as well as its direction can be clearly correlated with the exposure to atmosphere. This observation leads to a discussion based on a local asymmetry in O2 content as caused by slow diffusion into F16PcZn. O2 would lead to a decrease in the local majority carrier density as typically expected for organic n-type conductors. Rectification found in F16PcZn|In devices can be explained by a chemical reaction between the distinct electron acceptor F16PcZn and In as a donor. Photocurrent action spectra of devices with different thicknesses of F16PcZn layers in ITO|F16PcZn|PcZn|Au revealed detailed information about the site of charge carrier generation (photoactive area). The junction properties are discussed in detail based on the frontier orbital positions of PcZn and F16PcZn, and the work functions of the corresponding electrode materials.
• Jabbour, G. E., Kippelen, B., Armstrong, N. R., & Peyghambarian, N. (1998). Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices. Applied Physics Letters, 73(9), 1185-1187.
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  Abstract: Cathodes made with Al-LiF or Al-CsF composites are found to greatly enhance the performance of organic light-emitting devices (OLEDs). With a composite cathode, devices based on an organic bilayer structure have shown lower operating voltage, higher efficiency, and better forward light output than devices with LiF/Al, Mg, or Al cathode. Unlike devices with an Al and Li alloy cathode, OLEDs with a composite cathode can be made with good reproducibility. © 1998 American Institute of Physics.
• Jabbour, G. E., Schlaf, R., Armstrong, N. R., Kippelen, B., & Peyghambarian, N. (1998). Effects of insulating layers on the performance of organic electroluminescent devices. Proceedings of SPIE - The International Society for Optical Engineering, 3281, 182-190.
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  Abstract: In this paper, we demonstrate the enhancement in the performance of organic electroluminescent devices upon the insertion of an insulating layer or layers of LiF in the device structure. Highly efficient and bright organic light-emitting devices were fabricated with this approach. External quantum efficiencies approaching 3 % and light output exceeding 45,000 cd/m2 have been achieved for green light-emitting devices with Al cathode. This technique can be extended to fabricate efficient blue and sharp-red light-emitting devices. In this respect, using Al as the electron injecting electrode, blue light-emitting devices with external quantum efficiency of 1.4% and light output more than 4,000 cd/m2 have been achieved without the use of dopants. For sharp-red light-emitting devices, record efficiency and light output were obtained when LiF was used. Devices without the LiF layer showed light output levels lower than 5 cd/m2, whereas, with the insertion of LiF before the cathode, the external quantum efficiency exceeded 1% and light output was higher than 320 cd/m2. All of these devices had lower operational voltage than similar devices without the LiF layer. Preliminary UPS-XPS results revealed a sharp decrease in the work function of aluminum upon the deposition of sub-monolayer of LiF. Although, the use of the LiF layer on the indium-tin-oxide anode showed some enhancement in device performance, the contribution to device performance is lower than the case with the same insulator deposited at the cathode side, indicating that the cathode is more problematic than the hole injecting indium-tin-oxide electrode.
• Malinsky, J. E., Weijin, L. i., Chou, H., Wuping, M. a., Geng, L., Marks, T. J., Jabbour, G. E., Shaheen, S. E., Kippelen, B., Peyghambarian, N., Dutta, P., Richter, A. J., Armstrong, N. R., Lee, P. A., & Anderson, J. D. (1998). Molecular self-assembly approaches to multilayer organic light-emitting diode structures. Proceedings of SPIE - The International Society for Optical Engineering, 3281, 148-155.
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  Abstract: An attractive and challenging approach to the construction of robust, structurally precise thin film materials with large second-order optical nonlinearities or electroluminescent characteristics is the covalent self-assembly of arrays of tailored molecular building blocks. In this contribution, we discuss the implementation of self-limiting siloxane self-assembly processes to achieve the fabrication of structurally regular organic LED (OLED) devices. Areas surveyed include: i) the use of layer-by-layer self-assembly for ITO electrode modification/passivation/hole-electron carrier balancing in vapor depositied devices, ii) the synthesis of chlorosilane-functionalized precursor molecules for hole transport (HTL), emissive layer (EML), and electron transport layer (ETL) self-assembly, iii) the physicochemical and microstructural characterization of the HTL self-assembly process employing a triarylamine precursor, iv) the fabrication and characterization of a hybrid self-assembled + vapor deposited two-layer OLED, v) the fabrication and characterization of a fully self-assembled two-layer OLED.
• Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1998). Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements. Applied Physics Letters, 73(8), 1026-1028.
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  Abstract: The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N ′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be -0.13 eV from Alq3 to TPD. By including the known HOMO-lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be -0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3. © 1998 American Institute of Physics.
• Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W., Jabbour, G., Kippelen, B., Peyghambarian, N., & Armstrong, N. R. (1998). Photoemission spectroscopy of LiF coated Al and Pt electrodes. Journal of Applied Physics, 84(12), 6729-6736.
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  Abstract: Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most likely caused by a redistribution of charged Frenkel or Schottky type defects within the LiF layer. The work function of both samples after LiF deposition was dramatically lowered compared to the values obtained on the clean sputtered metal surfaces. © 1998 American Institute of Physics.
• Schlettwein, D., Back, A., Schilling, B., Fritz, T., & Armstrong, N. R. (1998). Ultrathin Films of Perylenedianhydride and Perylenebis(dicarboximide) Dyes on (001) Alkali Halide Surfaces. Chemistry of Materials, 10(2), 601-612.
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  Abstract: Ultrathin films of two perylene dyes, 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) and N,N′-di-n-butylperylene-3,4,9,10-bis(dicarboximide) (C4-PTCDI), have been grown on the (001) faces of freshly cleaved single crystal, NaCl, KCl, and KBr. Tapping mode AFM studies of these materials show that their crystalline motifs vary widely depending upon the substrate and growth conditions and that a form of layered growth is observable in the first few monolayers of deposition for both dyes. Nucleation of these crystalline deposits at edge sites on these substrates appears to be central to the growth of the first monolayers of these materials. Luminescence spectra for both PTCDA and C4-PTCDI, captured in situ during the growth of the first 1-4 monolayers of material, show the presence of a monomer-like entity during the initial growth stage. For PTCDA thin films this luminescence signal decays rapidly as nucleation of the crystalline film occurs. As film coverage is increased, luminescence of ultrathin films of both dyes is dominated by emission from one or more excimeric states. For C4-PTCDI/KC1 (001) the monomer-like emission does not completely disappear during the first monolayers of film growth, suggesting a degree of disorder sustained on a distance scale of a few molecular units at the grain boundaries and edges of the crystalline regions. The relative ratio of monomer/excimer emission of C4-PTCDI on KCl (001) is strongly temperature dependent and changes reversibly as the temperature is cycled between room temperature and 100°C. The luminescence intensity for thin films of both dyes on all of the substrates investigated is enhanced in the presence of atmospheric oxygen, consistent with previously reported declines in the dark conductivity of these materials as they are taken from UHV to atmospheric environments.
• Shaheen, S. E., Jabbour, G. E., Morrell, M. M., Kawabe, Y., Kippelen, B., Peyghambarian, N., Nabor, M. -., Schlaf, R., Mash, E. A., & Armstrong, N. R. (1998). Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode. Journal of Applied Physics, 84(4), 2324-2327.
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  Abstract: We report a two-layer, blue organic light-emitting diode with a 4,4′-bis(2,2-diphenylvinyl)-1,1′ -biphenyl emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of 3000 cd/m2. Insertion of the thin LiF layer results in a 50-fold increase in the device efficiency compared to a device with an aluminum only cathode, and eliminates the need for an electron-transporting layer, such as tris(8-hydroxyquinoline)aluminum. This results in a device with excellent color purity with an emission peak at 476 nm and a full width at half maximum of 78 nm. Using ultraviolet photoelectron spectroscopy, we find that the effective work-function of aluminum decreases dramatically with sub-monolayer amounts of LiF deposited on the surface. © 1998 American Institute of Physics.
• Wang, J., Kawabe, Y., Shaheen, S. E., Morrell, M. M., Jabbour, G. E., Lee, P. A., Anderson, J., Armstrong, N. R., Kippelen, B., Mash, E. A., & Peyghambarian, N. (1998). Exciplex electroluminescence from organic bilayer devices composed of triphenyldiamine and quinoxaline derivatives. Advanced Materials, 10(3), 230-233.
• Weijin, L. i., Malinsky, J. E., Chou, H., Wuping, M. a., Geng, L., Marks, T. J., Jabbour, G. E., Shaheen, S. E., Kippelen, B., Peyghambarian, N., Dutta, P., Richter, A. G., Armstrong, N. R., Lee, P. A., & Anderson, J. D. (1998). Molecular self-assembly routes to electroluminescent multilayer structures. American Chemical Society, Polymer Preprints, Division of Polymer Chemistry, 39(2), 1083-1084.
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  Abstract: Self-limiting siloxane chemisorptive self-assembly approaches have been employed to construct multilayer organic LED (OLED) structures. Extensive characterization of the deposition of a prototype triaryamine hole transport layer precursor on ITO using ellipsometry, X-ray reflectivity, AFM, XPS, and cyclic voltammetry reveal that smooth, contiguous, pinhole-free multilayer structures can be sequentially built up angstrom-level precision. Vapor deposition of a conventional emissive/electron transport layer and a metal electrode on the self-assembled HTL structure yields stable OLED structures with low turn-on voltages, high light output, and good external quantum efficiencies. Finally, self-assembly of an emissive layer on an ITO/HTL structure, followed by a LiF buffer layer and metal electrode, affords the first totally self-assembled two-layer OLED devices which emit continuous blue light.
• Armstrong, N. (1997). The electroactive integrated optical waveguide:  ultrasensitive spectroelectrochemistry of submonolayer adsorbates. Analytical chemistry, 69(15).
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  Highly sensitive spectroelectrochemistry of adsorbed films on ITO is demonstrated with the electroactive integrated optical waveguide (EA-IOW). The EA-IOW, a single-mode planar waveguide coated with an ITO layer, is ∼10(4)-fold more sensitive to changes in absorbance occurring during electrochemical events versus a single-pass transmission spectroelectrochemical experiment, as demonstrated by reduction of surface-adsorbed methylene blue. Furthermore, the EA-IOW is selective to near-surface events, as it is relatively insensitive to absorbance by solutions of dissolved chromophores at
• Armstrong, N. R. (1997). Experimental determination of quantum dipoles at semiconductor heterojunctions prepared by van der Waals epitaxy: Linear correction term for the electron affinity rule. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1997). Spectroscopic and electrochemical characterization of Langmuir-Blodgett films of (2,3,9,10,16,17,23,24-octakis-((2-benzyloxy)ethoxy)phthalocyaninato)copper and its metal-free analogue. Langmuir.
• Armstrong, N. R. (1997). The Electroactive Integrated Optical Waveguide: Ultrasensitive Spectroelectrochemistry of Submonolayer Adsorbates. Analytical Chemistry.
• Armstrong, N. R. (1997). Van der Waals epitaxy of the layered semiconductors SnSe2 and SnS2: Morphology and growth modes. Surface Science.
• Dunphy, D. R., Mendes, S. B., Saavedra, S. S., & Armstrong, N. R. (1997). The Electroactive Integrated Optical Waveguide: Ultrasensitive Spectroelectrochemistry of Submonolayer Adsorbates. Analytical Chemistry, 69(15), 3086-3094.
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  PMID: 21639329;Abstract: Highly sensitive spectroelectrochemistry of adsorbed films on ITO is demonstrated with the electroactive integrated optical waveguide (EA-IOW). The EA-IOW, a single-mode planar waveguide coated with an ITO layer, is ∼104-fold more sensitive to changes in absorbance occurring during electrochemical events versus a single-pass transmission spectroelectrochemical experiment, as demonstrated by reduction of surface-adsorbed methylene blue. Furthermore, the EA-IOW is selective to near-surface events, as it is relatively insensitive to absorbance by solutions of dissolved chromophores at
• Li, W., Malinsky, J. E., Chou, H., Ma, W., Geng, L., Marks, T. J., Jabbour, G. E., Shaheen, S. E., Kippelen, B., Peyghambarian, N., Anderson, J., Lee, P., & Armstrong, N. (1997). Molecular self-assembly routes to optically functional thin films: Electroluminescent multilayer structures. Materials Research Society Symposium - Proceedings, 488, 385-393.
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  Abstract: This contribution describes the use of layer-by-layer self-limiting siloxane chemisorption processes to self-assemble structurally regular multilayer organic LED (OLED) devices. Topics discussed include: 1) the synthesis of silyl-functionalized precursor molecules for hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL) self-assembly, 2) the use of layer-by-layer self-assembly for ITO electrode modification/passivation/hole-electron balancing in a vapor-deposited device, 3) the microstructure/chemical characterization of HTL self-assembly using a prototype triarylamine precursor, 4) fabrication and properties of a hybrid self-assembled+vapor deposited two-layer LED, 4) fabrication and properties of a fully self-assembled two-layer OLED.
• Mazumdar, S., Chandross, M., Shimoi, Y., Chakrabarti, A., Armstrong, N. R., & Peyghambarian, N. (1997). Biexcitons in mixed-stack charge-transfer solids, conjugated polymers, and molecular aggregates. Proceedings of SPIE - The International Society for Optical Engineering, 3145, 69-80.
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  Abstract: Ultrafast spectroscopic studies of organic solids reveal features unexpected within simple noninteracting models for these systems. We consider: (a) organic mixed-stack charge-transfer solids, (b) conjugated polymers, and (c) aggregates of metal-halogen phthalocyanines, and show that in all cases the photophysics is dominated by excitons and bound multiexciton states. Theoretical modeling is simplest for charge-transfer solids, where stable multiexcitons are verified by femtosecond pump-probe spectroscopy as well as two-photon absorption. In conjugated polymers, pump-probe spectroscopy reveals features due to biexcitons as well as a low energy charge-transfer exciton. In both charge-transfer solids and conjugated polymers the biexcitons are bound by electron-electron Coulomb interactions. In contrast, the exciton-exciton binding in molecular aggregates can originate from various sources and the magnitude of the biexciton binding energy in these systems is considerably smaller. No evidence for a biexciton in molecular aggregates have found to date. We present the first experimental evidence for a biexciton in the H-aggregate of a metal-halogen-phthalocyanine. Biexcitons, well-established in conventional semiconductors, are therefore characteristic elementary excitations of several different classes of organic solids.
• Schlaf, R., Armstrong, N. R., Parkinson, B. A., Pettenkofer, C., & Jaegermann, W. (1997). Van der Waals epitaxy of the layered semiconductors SnSe₂ and SnS₂: Morphology and growth modes. Surface Science, 385(1), 1-14.
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  Abstract: The recent development of so-called van der Waals epitaxy offers a whole new variety of material combinations for semiconductor heterojunctions. In this paper we investigate the morphology and the growth modes of the layered semiconductors SnSe2 (Eg = 1.03 eV) and SnS2 (Eg = 2.18 eV) on a variety of layered single crystalline substrate materials (highly oriented pyrolytic graphite, MoTe2, MoS2, WSe2, GaSe, SnS2 and SnSe2). The growth modes were investigated by low energy electron diffraction, photoemission spectroscopy and scanning tunnelling microscopy. We found that both of the materials grow in a two-dimensional layer-by-layer fashion with three-dimensional nucleation. However, despite the strong similarities of both of the materials the results indicate that SnS2 grows in a more ideal layer-by-layer mode than SnSe2. This is attributed to different surface diffusion rates due to different growth temperatures. The difference in empirically determined optimized growth temperatures are explained by the different thermal stabilities of the materials. © 1997 Elsevier Science B.V.
• Schlaf, R., Lang, O., Pettenkofer, C., Jaegermann, W., & Armstrong, N. R. (1997). Experimental determination of quantum dipoles at semiconductor heterojunctions prepared by van der Waals epitaxy: Linear correction term for the electron affinity rule. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 15(3), 1365-1370.
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  Abstract: We determined the band offset of eight layered heterocontacts consisting of combinations between SnS2, SnSe2, WSe2, MoS2, MoTe2, InSe, and GaSe. The comparison of offsets predicted by the electron affinity rule (EAR) revealed a systematic deviation. Due to the absence of structural dipoles in layered materials, this deviation corresponds to the magnitude of the quantum dipoles at the interface which allows the development of a quantum dipole correction term for the EAR. The corrected EAR is still a linear rule. The error margin of the corrected EAR lies well within the experimental error of photoemission spectroscopy experiments, thus proving the general applicability of linear rules for the determination of the band offset. © 1997 American Vacuum Society.
• Smolenyak, P. E., Osburn, E. J., Chen, S. -., Chau, L. -., O'Brien, D., & Armstrong, N. R. (1997). Spectroscopic and electrochemical characterization of Langmuir-Blodgett films of (2,3,9,10,16,17,23,24-octakis-((2-benzyloxy)ethoxy)phthalocyaninato)copper and its metal-free analogue. Langmuir, 13(24), 6568-6576.
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  Abstract: Spectroscopic, electrochemical and spectroelectrochemical, and scanning probe microscopic characterization of thin films of (2,3,9,10,16,17,23,24-Octakis((2-benzyloxy)ethoxy)phthalocyaninato)copper, and its dihydrogen analogue, in contact with aqueous electrolytes is described. These materials form highly ordered Langmuir-Blodgett (LB) thin films, where the phthalocyanine (Pc) rings orient with the plane of the ring nearly perpendicular to the compression direction on the LB trough. The resultant thin films show remarkable mechanical stability on the trough, and atomic force microscopy studies show that the transferred thin films, in their fully compressed states, consist of stable bilayers of the Pc. X-ray diffraction studies indicate partial overlap of the benzyl-terminated side chains in adjacent Pc columns. While the DMF solution electrochemical behavior of these molecules showed a single quasi-reversible oxidation process, LB thin films showed two poorly resolved oxidation processes. Two electrochemically distinct domains in these thin films are indicated, which are a function of the aggregation behavior of these materials, and result from electrochemically driven changes in film morphology.
• Yang, L., Huskey, M. J., Armstrong, N. R., & Saavedra, S. S. (1997). Chemical and biochemical sensors based on sol-gel derived, laminate planar waveguide structures. Polymeric Materials Science and Engineering, Proceedings of the ACS Division of Polymeric Materials Science and Engineering, 76, 453-.
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  Abstract: The sol-gel method is being investigated as a generalized strategy for fabricating planar waveguide chemical sensors and biosensors. Recently, a technically simple sol-gel dip-coating method has been developed to fabricate high quality glass integrated optical waveguides (TOWs) that support a single guided mode at visible wavelengths and that have propagation losses of less than 0.2 dB/cm. Subsequently, these waveguides have been used as a basis for development of a novel chemical sensing platform designed for optical detection analytes that form a light-absorbing complex within the evanescent field of the planar IOW in contact with a sample medium.
• Armstrong, N. (1996). Sol-gel-based, planar waveguide sensor for gaseous iodine. Analytical chemistry, 68(11).
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  A novel sensor for gaseous iodine has been developed using a combination of sol-gel processing and planar integrated optical waveguiding technologies. The sensing principle is based on the detection of a charge transfer complex formed between iodine and phenyl groups that have been incorporated into a porous, methylated glass film. The glass film was prepared from siloxane precursors by the sol-gel method. Sensors were fabricated by coating the film over a single-mode planar waveguide. Light was coupled into and out of the laminate structure using integral grating couplers, and formation of the charge transfer complex was monitored as attenuated total reflection of the guided wave. The sensor exhibits a linear response to I(2) in the range of 100 ppb to 15 ppm with response and recovery times less than 15 s. The response is selective to 4 ppm iodine in the presence of 10 ppm chlorine and is stable for at least 3 months.
• Armstrong, N. R. (1996). Dye-sensitizing effect of TiOPc thin film on n-TiO2 (001) surface. Journal of Physical Chemistry.
• Armstrong, N. R. (1996). Novel amphiphilic phthalocyanines: Formation of Langmuir-Blodgett and cast thin films. Langmuir.
• Armstrong, N. R. (1996). Photoelectrochemical effects and (photo)conductivity of "n-type" phthalocyanines. Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals.
• Armstrong, N. R. (1996). Substituted aluminum and zinc quinolates with blue-shifted absorbance/luminescence bands: Synthesis and spectroscopic, photoluminescence, and electroluminescence characterization. Chemistry of Materials.
• Armstrong, N. R. (1996). Supramolecular fibers from a liquid crystalline octa-substituted copper phthalocyanine. Advanced Materials.
• Hopkins, T. A., Meerholz, K., Shaheen, S., Anderson, M. L., Schmidt, A., Kippelen, B., Padias, A. B., Hall Jr., H. K., Peyghambarian, N., & Armstrong, N. R. (1996). Substituted aluminum and zinc quinolates with blue-shifted absorbance/luminescence bands: Synthesis and spectroscopic, photoluminescence, and electroluminescence characterization. Chemistry of Materials, 8(2), 344-351.
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  Abstract: Two novel lumophores based on aluminum and zinc metallo-8-hydroxyquinolates have been prepared as electroluminescent materials, and their absorbance, photoluminescence, and electroluminescence properties compared with unsubstituted versions of these same complexes. 8-Hydroxy-5-piperidinylquinolinesulfonamide (1) was synthesized in order to add an electron-withdrawing substituent at the 5-position in 8-hydroxyquinoline, increasing the solubility of the corresponding metal quinolate complexes in nonpolar solvents, and producing a blue-shift in the emission wavelength maximum, relative to complexes formed from the unsubstituted compound. The aluminum complex (Al(QS)3) and the zinc complex (Zn(QS)2) of 1 were compared with the aluminum and zinc complexes of unsubstituted 8-hydroxyquinoline (AlQ3 and ZnQ2), both as solutions and as pure thin films, or as poly-(N-vinylcarbazole) (PVK) thin films doped with the metal quinolates. Ultraviolet photoelectron spectroscopy data are presented to assist in estimating the energies of the highest occupied molecular orbitals (HOMO) of AlQ3, ZnQ2, Al(QS)3, and Zn(QS)2. Electroluminescence data shows that ITO/Al(QS)3-PVK/aluminum and ITO/Zn(QS)2-PVK/aluminum devices exhibit good diode-like electrical behavior. Electroluminescence spectra mimic the photoluminescence spectra for all complexes.
• Karmann, E., Meyer, J. -., Schlettwein, D., Jaeger, N. I., Anderson, M., Schmidt, A., & Armstrong, N. R. (1996). Photoelectrochemical effects and (photo)conductivity of "n-type" phthalocyanines. Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals, 283, 283-291.
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  Abstract: In vacuo thin films of the phthalocyanine derivatives TPyTAPZn and F16PcZn show n-type behavior in thermopower and conductivity measurements. After exposure to air the properties are changed characteristically. Photoelectrochemical experiments show that while TPyTAPZn still behaves like an n-type semiconductor, F16PcZn exhibits the properties of a photoconductor. Time-dependent measurements give further insight into the differences between the two materials in their interaction with gaseous dopants. The results are discussed in terms of different positions of the HOMO- and LUMO-orbitals of TPyTAPZn and F16PcZn taken from UPS-measurements.
• Osburn, E. J., Chau, L. -., Chen, S. -., Collins, N., O'Brien, D., & Armstrong, N. R. (1996). Novel amphiphilic phthalocyanines: Formation of Langmuir-Blodgett and cast thin films. Langmuir, 12(20), 4784-4796.
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  Abstract: The synthesis and solution and thin film characterization of four octasubstituted phthalocyanines (Pcs) are reported. [2,3,9,10,16,17,23,24-octakis(dodecylcarboxylato)phthalocyaninato]copper, [2,3,9,10,16,17,-23,24-octakis(N-dodecylamide)phthalocyaninato]copper, and [2,3,9,10,16,17,23,24-octakis(2-(benzyloxy)-ethoxy)phthalocyaninato]copper, and the corresponding zinc phthalocyanine analog, were prepared and their aggregation properties characterized in chloroform solutions. The octa-ester-modified Pes showed the greatest tendency toward solution aggregation (Kagg ≥ 4.8 × 106), and the octakis((benzyloxy)ethoxy)-modified Pcs showed the least tendency toward aggregation (290 ≤ Kagg ≤ 1300). Both cast and Langmuir-Blodgett (LB) thin films of these compounds showed the tendency toward formation of columnar aggregates, as evidenced by their blue-shifted Q-band absorbance spectra. Fully compressed thin films of these Pcs show the rings tilted by ca. 50-60° with respect to the surface normal, with essentially cylindrical symmetry of the side chains about each aggregate. The octaester Pcs, which showed the strongest solution aggregation tendencies, proved to form the most highly ordered LB thin films, as evidenced by the dichroism in several key vibrational bands. Molecular dynamics simulations are presented which show hypothetical structures for single molecules of each of the above compounds, as well as pentamer linear cofacial aggregates which demonstrated stability in these calculations and which are structures consistent with the spectroscopic characterizations of these materials.
• Yanagi, H., Chen, S., Lee, P. A., Nebesny, K. W., Armstrong, N. R., & Fujishima, A. (1996). Dye-sensitizing effect of TiOPc thin film on n-TiO₂ (001) surface. Journal of Physical Chemistry, 100(13), 5447-5451.
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  Abstract: Dye-sensitization processes for the (001) surface of n-type titanium oxide (n-TiO2) single crystal coated with vacuum-deposited titanylphthalocyanine (TiOPc) were investigated by means of photoelectrochemical measurements and X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). The electrode of n-TiO2 coated with TiOPc exhibited two photooxidation current bands in an aqueous electrolyte including hydroquinone (H2Q), due to direct excitation of n-TiO2 below 420 nm and electron transfer from TiOPc to n-TiO2 under the Q-band absorption of TiOPc at 600-800 nm, respectively. The former photocurrent process due to n-TiO2 excitation was remarkably decreased by presence of surface states, which originated from defects of oxygen induced by Ar ion bombardment of the n-TiO2 surface. The photooxidation process sensitized by the latter was evaluated from the determined energy levels of electronic bands for n-TiO2 and TiOPc. The LUMO level of TiOPc was located 0.3 eV above that of n-TiO2, which enabled electrons to transfer from TiOPc to n-TiO2. The excited state of TiOPc was not quenched by the surface states of n-TiO2 but was efficiently transferred to n-TiO2 at the n-TiO2/TiOPc interface. © 1996 American Chemical Society.
• Yang, L., Saavedra, S. S., & Armstrong, N. R. (1996). Sol-gel-based, planar waveguide sensor for gaseous iodine. Analytical Chemistry, 68(11), 1834-1841.
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  PMID: 21619094;Abstract: A novel sensor for gaseous iodine has been developed using a combination of sol-gel processing and planar integrated optical waveguiding technologies. The sensing principle is based on the detection of a charge transfer complex formed between iodine and phenyl groups that have been incorporated into a porous, methylated glass film. The glass film was prepared from siloxane precursors by the sol-gel method. Sensors were fabricated by coating the film over a single-mode planar waveguide, light was coupled into and out of the laminate structure using integral grating couplers, and formation of the charge transfer complex was monitored as attenuated total reflection of the guided wave. The sensor exhibits a linear response to I2 in the range of 100 ppb to 15 ppm with response and recovery times less than 15 s. The response is selective to 4 ppm iodine in the presence of 10 ppm chlorine and is stable for at least 3 months.
• Armstrong, N. R. (1995). Electronic states of vapor deposited electron and hole transport agents and luminescent materials for light-emitting diodes. Journal of Applied Physics.
• Armstrong, N. R. (1995). Epitaxial Growth of the Ionic Polymer Fluoroaluminum Phthalocyanine on the Basal Plane of Single Crystal Tin Disulfide. Chemistry of Materials.
• Armstrong, N. R. (1995). Growth of layered semiconductors by molecular-beam epitaxy: Formation and characterization of GaSe, MoSe2, and phthalocyanine ultrathin films on sulfur-passivated GaP(111). Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1995). Large molecule epitaxy on single crystal metals, insulators and single crystal and mbe-grown layered semiconductors. Japanese Journal of Applied Physics.
• Armstrong, N. R. (1995). Ordered ultrathin films of perylenetetracarboxylic dianhydride (PTCDA) and dimethylperylenebis(dicarboximide) (Me-PTCDI) on Cu(100): Characterization of structure and surface stoichiometry by LEED, TDMS, and XPS. Journal of Physical Chemistry.
• Armstrong, N. R. (1995). Periodic Multilayers of Perylene-3,4:9,10-tetracarboxylic Dianhydride and Chloroindium Phthalocyanine: Limitations to Long-Term Stability. Chemistry of Materials.
• Armstrong, N. R. (1995). Photoelectron and optical spectroscopic investigations of the electronic structure of oligo(p‐phenylenevinylene)s in the solid state. Advanced Materials.
• Armstrong, N. R. (1995). Spectra of third-order optical nonlinear susceptibilities of epitaxial chloro-indium-phthalocyanines. Applied Physics Letters.
• Hammond, C., Back, A., Lawrence, M., Nebesny, K., Lee, P., Schlaf, R., & Armstrong, N. R. (1995). Growth of layered semiconductors by molecular-beam epitaxy: Formation and characterization of GaSe, MoSe ₂, and phthalocyanine ultrathin films on sulfur-passivated GaP(111). Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 13(3), 1768-1775.
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  Abstract: Studies of the sulfur passivation of GaP(111) surfaces with solution treatments, and the subsequent molecular-beam epitaxy growth and characterization of ultrathin films of layered semiconductors on those surfaces are presented in this article. Simple solution passivation procedures allow formation of a S-GaP(111) semiconductor surface which will support the growth of highly ordered layers of GaSe (E band gap=2.05 eV). These thin GaSe layers then permit the further growth of ordered layers of a semiconductor like MoSe 2 (E band gap=1.0 eV) or an organic dye such as a phthalocyanine. Reflection high energy electron diffraction and angle-resolved x-ray photoelectron spectroscopy have been used to determine the structure and composition of the surface layers formed at each stage of the deposition process. Absorption spectra are used to characterize the band edge and A 1,2 exciton energies of the MoSe 2/GaSe S-GaP(111) thin films, and the position and shape of the phthalocyanine Q band. © 1995 American Vacuum Society.
• Schlaf, R., Louder, D., Lang, O., Pettenkofer, C., Jaegermann, W., Nebesny, K. W., Lee, P. A., Parkinson, B. A., & Armstrong, N. R. (1995). Molecular beam epitaxy growth of thin films of SnS ₂ and SnSe ₂ on cleaved mica and the basal planes of single-crystal layered semiconductors: Reflection high-energy electron diffraction, low-energy electron diffraction, photoemission, and scanning tunneling microscopy/atomic force microscopy characterization. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 13(3), 1761-1767.
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  Abstract: SnS 2 and SnSe 2 thin films were deposited by molecular beam epitaxy (MBE) methods on a variety of layered semiconductor substrates (freshly cleaved SnS 2, SnSe 2, WSe 2, MoS 2, MoTe 2, GaSe, InSe) and cleaved mica, for investigation of the interfaces formed as a result of MBE growth. These ultrathin films were characterized in situ by x-ray photoelectron spectroscopy and surface reflection high energy and low energy electron diffraction techniques. The growth modes were verified ex situ by scanning tunneling microscopy and/or atomic force microscopy [scanning probe microscopies (SPM)]. Despite the chemical and structural similarities between SnS 2 and SnSe 2, SPM measurements show that the two materials as ultrathin films have different growth morphologies in the first few monolayers. Photoelectron spectroscopy (PES) measurements lead to the conclusion that both materials grow on the basal planes of most layered substrates in an epitaxial, layer-by-layer mode. Small deviations from ideal Frank-van der Merwe growth for these thin films could be determined from PES intensity ratios versus coverage. Deposition of both materials on freshly cleaved mica, where the lattice mismatch between the substrate and the overlayer surface unit cell dimension exceeds 40, yields exclusively 500-1000 Å diameter epitaxial islands with threefold symmetry. © 1995 American Vacuum Society.
• Schmidt, A., Anderson, M. L., & Armstrong, N. R. (1995). Electronic states of vapor deposited electron and hole transport agents and luminescent materials for light-emitting diodes. Journal of Applied Physics, 78(9), 5619-5625.
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  Abstract: For most organic materials used in light-emitting devices (LEDs) the energies of the electronic states are not well known. This paper is aimed at filling the gap of experimental data for a few already widely used or promising luminescent materials, hole transport agents, and electron transport agents. UV photoelectron spectroscopy (UPS) was used for the estimation of the energy of the occupied electronic states. The combination of UPS, UV-vis absorbance and photoluminescence spectroscopy allows a determination of the binding energy of the states responsible for the absorbance and luminescence (mainly excitons in organic materials) and the approximation of the energies for the unoccupied electronic states. The knowledge of these states allows a discussion of possible optimization of materials for LEDs in order to decrease the driving voltage of the devices and ensure balanced injection conditions for electrons and holes. Another possible application of these data can be seen in the discussion of the formation of the heterojunctions where recombination takes place, in order to avoid quenching of the electroluminescence near the conducting injection electrodes of ultrathin film LEDs.
• Schmidt, A., Anderson, M. L., Dunphy, D., Wehrmeister, T., Müllen, K., & Armstrong, N. R. (1995). Photoelectron and optical spectroscopic investigations of the electronic structure of oligo(p-phenylenevinylene)s in the solid state. Advanced Materials, 7(8), 722-726.
• Schmidt, A., Chau, L. -., Valencia, V. S., & Armstrong, N. R. (1995). Periodic multilayers of perylene-3,4:9,10-tetracarboxylic dianhydride and chloroindium phthalocyanine: Limitations to long-term stability. Chemistry of Materials, 7(4), 657-662.
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  Abstract: The linear optical properties of periodic ultrathin multilayers of perylene-3,4:9,10-tetracarboxylic dianhydride (PTCDA) and chloroindium phthalocyanine (ClInPc) change after exposure to ambient environments. This discoloration extends over a period of months. The B- and Q-bands of ClInPc decrease significantly in intensity, while the PTCDA absorbance spectra are unchanged. ClInPc and PTCDA multilayers alone are stable in air over the same period of time. UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS) are in agreement with a destruction of the Pc chromophore. This reaction may be initiated by reaction of the anhydride groups of PTCDA with ambient moisture, forming carboxylic acids, as seen by XPS. Control experiments show that PTCDA hydrolyzes in the presence of water and that ClInPc degrades if exposed to acids. These results impact the device implications for long-term use of PTCDA/phthalocyanine assemblies.
• Schmidt, A., Schlaf, R., Louder, D., Chau, L. -., Chen, S. -., Fritz, T., Lawrence, M. F., Parkinson, B. A., & Armstrong, N. R. (1995). Epitaxial growth of the ionic polymer fluoroaluminum phthalocyanine on the basal plane of single crystal tin disulfide. Chemistry of Materials, 7(11), 2127-2135.
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  Abstract: The epitaxial growth of fluoroaluminum-phthalocyanine (FAlPc) thin films on the hep (0001) tin disulfide (SnS2) surface is reported. Growth conditions can be achieved that lead to FAlPc films of polymer-like aggregates (FAlPc)n with the long axis of the aggregate apparently standing perpendicular to the surface. When the substrate is held at a temperature of 260°C during deposition, reflection high-energy electron diffraction of the resultant thin films showed that FAlPc grows epitaxially on SnS2 in a square lattice structure, starting with the first monolayer, with a unit cell dimension of 13.2 ± 0.8 Å. One axis of the resulting FAlPc square lattice overlayer is oriented along the main axis of the basal plane SnS2. Scanning tunneling microscopy (STM) showed elongated crystallites growing along the main crystallographic axes of the substrate, and these same STM data indicate that phthalocyanine film growth may be initiated at step sites on the SnS2 surface. The development of H-aggregate (FAlPc)n in these thin films is confirmed by the position of the Q-band absorbance maximum observed for these thin films (642 nm), which is blue-shifted from the monomer absorbance maximum at ca. 675 nm. The Q-band spectrum of the epitaxial film is much narrower (fwhm 28 nm) than comparable spectra from less-ordered thin films of (FAlPc)n aggregates deposited on sapphire (fwhm 55 nm). The polarization dependence of these Q-band spectra was consistent with individual FAlPc molecules in these thin films, on average, lying parallel to the surface plane. Dye sensitization/photoelectrochemistry showed photocurrent action spectra that paralleled the absorbance spectra and made possible the characterization of even monolayer-level coverages. Quantum efficiencies for charge injection from the FAlPc aggregates into the SnS2 conduction band (≤5%) was lower by at least a factor of 10, relative to previously explored phthalocyanine sensitizers which do not form linear cofacial aggregates. © 1995 American Chemical Society.
• Schmidt, A., Schuerlein, T. J., Collins, G. E., & Armstrong, N. R. (1995). Ordered ultrathin films of perylenetetracarboxylic dianhydride (PTCDA) and dimethylperylenebis(dicarboximide) (Me-PTCDI) on Cu(100): Characterization of structure and surface stoichiometry by LEED, TDMS, and XPS. Journal of Physical Chemistry, 99(30), 11770-11779.
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  Abstract: Ordered monolayer and multilayer thin films of perylene-3,4:9,10-tetracarboxylic dianhydride (PTCDA) have been grown on Cu(100) and MoS2(0001) single crystals. Low-energy electron diffraction (LEED) investigations have shown that PTCDA grows on Cu(100) in a commensurate rectangular lattice (4√2×5√2)R = 45°, whose dimensions (b1 = 14.5 Å, b2 = 18.1 Å) are significant different from those seen on the (0001) cleavage faces of single-crystal MoS2, where a coincident rectangular lattice with b1 = 13.1 Å and b2 = 21.2 Å is formed. This latter structure is much closer to the packing in the (102) plane of the bulk crystal of PTCDA. X-ray photoelectron spectra (XPS) and thermal desorption mass spectroscopy (TDMS) show that the first monolayer of PTCDA is strongly chemisorbed on the Cu surface. During the reaction with the copper surface each PTCDA molecule apparently loses the two bridging oxygen atoms in the anhydride groups, leading to a molecular system with close to the same dimensions and symmetry as the parent molecule. XPS data show that the adsorption on the surface of the weakly interacting metal dichalcogenide MoS2 is not accompanied by such a surface reaction. Support for this hypothesis was obtained by deposition of N,N′-dimethylperylene-3,4:9,10-bis(dicarboximide) (Me-PTCDI) on the Cu(100) surface. Me-PTCDI forms a rectangular (6 × 8) structure on Cu(100), with b1 = 15.3 Å and b2 = 20.4 Å. XPS data obtained for the deposition of this molecule on clean copper show that Me-PTCDI apparently loses both nitrogen imide groups, for the first monolayer adsorbed. TDMS data confirm this reactivity of Me-PTCDI. Me-PTCDI thin films appear to grow as islands, starting with the second monolayer, in contrast to PTCDA, which appears to adopt a layer-by-layer growth mode for the first six monolayers. Differences in the bulk crystal structures of these two molecules are believed to lead to these differences in growth mode; the layered bulk structure of PTCDA lends itself to a layered growth mode in multilayer ultrathin films. © 1995 American Chemical Society.
• Schuerlein, T. J., Schmidt, A., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1995). Large molecule epitaxy on single crystal metals, insulators and single crystal and MBE-grown layered semiconductors. Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes & Review Papers, 34(7 B), 3837-3845.
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  Abstract: We review the packing structures for a series of aromatic hydrocarbons, deposited by vacuum deposition methods as ordered monolayers →→multilayers, on a variety of metal, semiconductor and insulator surfaces. New results are presented for the adsorption of monolayers of perylenes, phthalocyanines, coronene, and pentacenes on the Cu(100) surface, along with the implications of these studies for the formation of ordered multilayers of these molecular systems. Aromatic molecules without heteroatoms appear to pack in a flat-lying motif, and exhibit approximately hexagonal close packing, even on a substrate with four-fold symmetry such as Cu(100). In general, aromatic systems whose bulk structures lend themselves to layer-by-layer growth during vacuum deposition appear to be the best candidates for ordered multilayer growth.
• Tajalli, H., Jiang, J. P., Murray, J. T., Armstrong, N. R., Schmidt, A., Chandross, M., Mazumdar, S., & Peyghambarian, N. (1995). Spectra of third-order optical nonlinear susceptibilities of epitaxial chloro-indium-phthalocyanines. Applied Physics Letters, 67(12), 1639-1641.
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  Abstract: A report is presented of the measurement of the spectrum of third harmonic generation from epitaxially grown chloro-indium-phthalocyanine thin films over the fundamental wavelength range of 970-1650 nm. Various resonances were observed and are compared with theoretical calculations that take into account Coulomb interaction effects. The experiment-theory comparison allows for the assignment of the observed resonances to two- and three-photon resonances of χ(3).
• Anderson, M. L., Williams, V. S., Schuerlein, T. J., Collins, G. E., England, C. D., Chau, L. -., Lee, P. A., Nebesny, K. W., & Armstrong, N. R. (1994). RHEED and optical characterization of ordered multilayers of phthalocyanine{plus 45 degree rule}C₆₀ and phthalocyanine/perylene-tetracarboxylicdianhydride (PTCDA). Surface Science, 307-309(PART A), 551-558.
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  Abstract: Ordered monolayers → multilayers of trivalent metal phthalocyanines, C60, and the perylenetetracarboxylic dianhydride (PTCDA) have been formed by molecular beam epitaxy processes on both single crystal MoS2 and SnS2, and on MBE-deposited SnS2 thin films. The bulk packing structures for the trivalent metal chloride phthalocyanines lend themselves to layered growth during deposition, as do the structures for C60 and PTCDA. RHEED data collected during the formation of [(InPc-Cl)1-4 ML/(C60)1-4 ML]1-20/SnS2/mica multilayers suggests that the structures observed during the formation of single component thin films can be sustained during multilayer formation, up to several lattice periods. Absorbance spectra of Pc/C60 and Pc/PTCDA multilayers show Pc Q-bands with maxima and full widths consistent with the formation of ordered Pc layers, provided that the Pc layer thickness is kept at 1-2 monolayers throughout the formation of the multilayer assembly. © 1994.
• Armstrong, N. R. (1994). Correlation of frontier orbital positions and conduction type of molecular semiconductors as derived from UPS in combination with electrical and photoelectrochemical experiments. Journal of physical chemistry.
• Armstrong, N. R. (1994). Dye Sensitization with Octasubstituted Liquid Crystalline Phthalocyanines. Langmuir.
• Armstrong, N. R. (1994). Electrochemical Characterization of the Oxidation of LB Films of Polyphthalocyaninatosiloxane. Macromolecules.
• Armstrong, N. R. (1994). Electronic structure of oligorylenes in thin solid films. Journal of Physical Chemistry®.
• Armstrong, N. R. (1994). Epitaxial Thin Films of Large Organic Molecules: Characterization of Phthalocyanine and Coronene Overlayers on the Layered Semiconductors MoS2 and SnS2. Langmuir.
• Armstrong, N. R. (1994). Fabrication and Characterization of Low-Loss, Sol-Gel Planar Waveguides. Analytical Chemistry.
• Armstrong, N. R. (1994). Formation and characterization of epitaxial phthalocyanine and perylene monolayers and bilayers on Cu(100): Low-energy electron diffraction and thermal desorption mass spectrometry studies. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1994). Organic/inorganic molecular beam epitaxy (O/L-Mbe): Formation and characterization of ordered phthalocyanine thin films - photoelectrochemical processes. Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals.
• Armstrong, N. R. (1994). RHEED and optical characterization of ordered multilayers of phthalocyanine{plus 45 degree rule}C60 and phthalocyanine/perylene-tetracarboxylicdianhydride (PTCDA). Surface Science.
• Chau, L. -., Osburn, E. J., Armstrong, N. R., O'Brien, D., & Parkinson, B. A. (1994). Dye sensitization with octasubstituted liquid crystalline phthalocyanines. Langmuir, 10(2), 351-353.
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  Abstract: New liquid crystalline octaamide (C12) and octaester (C12) copper phthalocyanines (LC-Pc), which form self-assembled structures in solutions and in cast films, act as effective sensitizers for the photoelectrochemical response of the layered semiconductors, n-SnS2. The photocurrent yield spectra of these cast films, at a wide range of coverages, show a single sharp Q-band at a position corresponding to close packed monolayer assemblies of flat-lying Pc's. Quantum yields per absorbed photon are high (QYAP ≈ 44%) for the close packed monolayers. In contrast, films from a short-chain substituted and an unsubstituted CuPc on SnS2 show Q-band photocurrent yield spectra due to both the monomer and H-aggregate forms. These phenomena suggest that the efficiency of exciton transport through a self-assembled LC-Pc film strongly depends on the separation distance between the chromophores. The interfacial structure of the mesogenic molecules at the semiconductor or metal surface appears to be critical in determining the overall exciton transport efficiency in columnar mesophases.
• England, C. D., Collins, G. E., Schuerlein, T. J., & Armstrong, N. R. (1994). Epitaxial thin films of large organic molecules. Characterization of phthalocyanine and coronene overlayers on the layered semiconductors MoS₂ and SnS₂. Langmuir, 10(8), 2748-2756.
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  Abstract: Ordered overlayers of large organic molecules have been studied on the basal planes of freshly cleaved metal dichalcogenides such as MoS2 or Sn/S2. Low-energy electron diffraction (LEED) has been used to characterize the growth of copper (CuPc) and chlorindium (InPcCL) phthalocyanine and coronene on these substrates, at coverage up a ca. two monolayers. InPcCl and CuPc films form square lattice nets on both SnS2 and MoS2, with lattice vectors aligned along (Φ = 0°, MoS2) or near (Φ = ±4°, SnS2) the principal lattice vectors of the metal dichalcogenide (0001) surface, resulting in 3-6 equivalent Pc domains. Ordered coronene films on MoS2 consists to two hexagonal lattice domains rotated by ±13.9° from each principal axis of the (0001) surface of MoS2. Modeling studies were conducted for adsorbed coronene on MoS2 and for InPcCl on both MoS2 and SnS2, where purely van der Waals forces were presumed to be responsible for the orientation of the epitaxial layers of these organic materials. By summing van der Waals interactions over surface lattices of up to nine molecules, it is possible to show that small difference in the binding site, repulsive and attractive interactions between molecules, and sulfur-sulfur spacing in the MoS2 or SnS2 basal plane may be responsible for the orientations of the close-packed monolayers.
• Ferencz, A., Armstrong, N. R., & Wegner, G. (1994). Electrochemical characterization of the oxidation of LB films of polyhthalocyaninatosiloxane. Macromolecules, 27(6), 1517-1528.
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  Abstract: Highly ordered PcPS (phthalocyaninatopolysiloxane) thin films have been produced on a variety of substrates using Langmuir-Blodgett thin-film deposition techniques, with coverages ranging from 1 to 100 molecular layers. Siloxane-polymerized cofacial phthalocynines, with alkoxyhydrocarbon side chains on the individual Pc units, yield molecules capable of forming highly ordered, densely packed LB films. These ultrathin films shows facile electron and ion transport during electrochemical and chemical oxidation and notable stability of the PC cation radicals in the polymer chains.
• Mazumdar, S., Williams, V. S., Armstrong, N. R., & Peyghambarian, N. (1994). Biexcitons in phthalocyanine thin films. Proceedings of the International Quantum Electronics Conference (IQEC'94), 33-.
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  Abstract: We investigate both these issues within the context of molecular exciton theory, and perform femtosecond pump-probe experiments to compare with theoretical predictions. Furthermore, we prove the nature of the lowest excited state in metallophthalocyanines, in order to compare to earlier experiments that suggested subgap two-photon states in these systems. It is therefore predicted within the theoretical model that if the ground state absorption in the dimer is to the red of the monomer absorption, excited state absorption should occur on the high energy side of the linear absorption energy.
• Schlettwein, D., & Armstrong, N. R. (1994). Correlation of frontier orbital positions and conduction type of molecular semiconductors as derived from UPS in combination with electrical and photoelectrochemical experiments. Journal of physical chemistry, 98(45), 11771-11779.
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  Abstract: He I photoelectron spectra collected from selected phthalocyanine thin films on Au substrates have been used to determine the position of the highest occupied orbitals and threshold ionization potentials in the solid state. Relative as well as absolute positions are given. Chemical modifications of these dyes (substitution of nitrogen heteroatoms to create the pyrido- or pyrazinotetraazaporphyrin complexes, addition of electron-withdrawing substituents to the ring periphery) and changes in the crystal structure of the solid affect the size of the frontier orbital (band) gap and especially the absolute HOMO and LUMO energies. The influence of different substituents with varying electron-donating or electron-accepting properties is investigated in detail. Photoemission data are compared with redox potentials and molecular orbital (MO) calculations from the literature. The frontier orbital positions strongly correlate with the conduction type of the thin film, the nature of electrical property changes seen during interactions with gases such as O2 and NH3, and the photoelectrochemical properties in different aqueous electrolytes.
• Schmidt, A., Armstrong, N. R., Goeltner, C., & Muellen, K. (1994). Electronic structure of oligorylenes in thin solid films. Journal of Physical Chemistry®, 98(45), 11780-11785.
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  Abstract: A series of oligorylenes with 2, 3, and 4 naphthalene units, symmetrically substituted by tert-butyl groups, was investigated in thin solid films by ultraviolet photoelectron spectroscopy, UV/vis absorbance, and fluorescence spectroscopy. The electronic structure of these oligorylenes was determined from these measurements and compared to theoretical and experimental data available for this class of materials. The ionization energy decreases with increasing length of the aromatic system from 5.46 to 4.59 eV, while the apparent electron affinity increases, from 2.77 to 3.01 eV. We predicted from this series the values for the polymer poly(peri-naphthalene) (PPN) and obtained an ionization energy of 3.69 eV. The fit for the predicted band gap of PPN was ca. 0.45 eV; therefore, the electron affinity should reach 3.24 eV. The polarization energy is 1.5 eV for all oligorylenes. There is a linear relation between the solid-state ionization potential and the electrochemical first oxidation potential. The slope is larger than 1, however, which may be explainable as arising from a solution solvation effect. © 1994 American Chemical Society.
• Yanagi, H., Kanbayashi, Y., Schlettwein, D., Wöhrle, D., & Armstrong, N. R. (1994). Photoelectrochemical investigations on naphthalocyanine derivatives in thin films. Journal of Physical Chemistry, 98(17), 4760-4766.
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  Abstract: Photoelectrochemical properties of Zn-naphthalocyanine (ZnNc) and its modified compound, Zn-2,3-tetraquinoxalinotetraazaporphyrin (ZnTQP) were investigated in thin films. The ZnNc electrode vacuum-deposited on an indium tin oxide (ITO) substrate exhibited almost an ohmic I-V curve in the dark and small cathodic photocurrents under illumination, which was typical for the p-type semiconduction of the ZnNc layer, in a photoelectrochemical cell. The drop-casted film electrode of ZnTQP, on the other hand, showed rectified I-V characteristics in the dark and high anodic photocurrents under illumination, which was attributed to the n-type semiconducting character of the ZnTQP layer. These different photoelectrochemical behaviors were characterized by photocurrent action spectra and ultraviolet photoelectron spectroscopy. The substitution with quinoxalino groups into the Nc macrocyclic system gave rise to lowering of the HOMO energy level for the TQP molecule. This electronic energy shift generated donor electron states in the band gap after contact with the substrate or electrolyte and enabled the photooxidation at the valence band edges corresponding to the Soret-and Q-band excitations. The pH dependence of the anodic photocurrents for the ZnTQP electrode indicated that the photooxidation occurred through a hole injection from the HOMO and underlying sub-HOMO states, separately, for the Soret- and Q-band excitations. For the ZnNc electrode, the Q-band illumination produced cathodic photocurrents, whereas the Soret band produced anodic photocurrents. It suggested that under the Soret-band excitation holes generated in the sub-HOMO state directly contributed to the photooxidation. © 1994 American Chemical Society.
• Yang, L., Saavedra, S. S., Armstrong, N. R., & Hayes, J. (1994). Fabrication and characterization of low-loss, sol-gel planar waveguides. Analytical Chemistry, 66(8), 1254-1263.
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  PMID: 8210042;Abstract: Applications of planar integrated optical wareguide (IOW) technology to problems in surface spectroscopy and optical chemical sensing have been partly limited by the difficulty of producing high-quality glass IOWs. The fabrication of IOWs by the sol-gel method from methyltriethoxysilane and titanium tetrabutoxide precursors is described here. The physical, chemical, and optical properties of the films during and after high-temperature annealing were studied using a variety of analytical techniques. The results show that the catalyst used to accelerate the sol-gel reaction strongly influenced the optical quality of the IOW. HCI catalysis produced waveguides with propagation losses of ∼1 dB/cm, whereas in the case of SiCl4 catalysis, propagation losses were
• Armstrong, N. R. (1993). Epitaxial phthalocyanine thin films and phthalocyanine/C60 multilayers. Synthetic Metals.
• Armstrong, N. R. (1993). Photoconductivity/dark conductivity studies of chlorogallium phthalocyanine thin films on interdigitated microcircuit arrays. Journal of Physical Chemistry.
• Armstrong, N. R. (1993). Phthalocyanine aggregates on metal dichalcogenide surfaces: Dye sensitization on SnS2 semiconductor electrodes by ordered and disordered InPc-Cl thin films. Journal of Physical Chemistry.
• Armstrong, N. R. (1993). Thin Film Sensors: The Role of Defects And Impurity Sites In Controlling Sensor Response and Selectivity. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Chau, L. -., Arbour, C., Collins, G. E., Nebesny, K. W., Lee, P. A., England, C. D., Armstrong, N. R., & Parkinson, B. A. (1993). Phthalocyanine aggregates on metal dichalcogenide surfaces: Dye sensitization on SnS₂ semiconductor electrodes by ordered and disordered InPc-Cl thin films. Journal of Physical Chemistry, 97(11), 2690-2698.
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  Abstract: Photoelectrochemical dye sensitization processes have been explored for various coverages of ultrathin films of chloroindium phthalocyanine (InPc-Cl), on single crystal metal dichalcogenide (SnS2) surfaces. Films were prepared by conventional vacuum deposition and by a process which led mainly to flat-lying, epitaxial Pc layers, exhibiting a "layer-by-layer" growth mode. These experiments were carried out in parallel with optical characterization of InPc-Cl layers grown on nonconductive transparent SnS2 thin films, where both the Pc and SnS2 thin films were grown by a molecular beam epitaxy (MBE) process on freshly cleaved mica. Surface electron diffraction data were collected during the organic/inorganic-molecular beam epitaxy (O/I-MBE) experiments, on both bulk and MBE-grown SnS2 and on bulk MoS2. These data suggest that (a) InPc-Cl can deposit in a flat-lying ordered monolayer, consisting of one or two sets of 3-fold rotations of coincident 3 × 2 square lattice domains aligned along the principal axes of the metal dichalcogenide, the ordering of which can be extended to multilayers of InPc-Cl, or (b) the InPc-Cl can deposit in a strict three-dimensional growth mode, which produces less ordered multilayers. Low coverages of both types of InPc-Cl ultrathin films sensitize the SnS2 photoelectrochemical response with quantum yields for charge injection per absorbed photon (QYAP) ≥ 10%. Low coverage (below monolayer) InPc-Cl deposits produce photocurrent yield spectra nearly as narrow as the solution absorption spectra (fwhm 35 nm). Both the highly ordered and less ordered InPc-Cl multilayers continue to produce sizeable QYAP values out to coverages of tens of monolayers. The "epitaxial" deposits show both absorbance and photocurrent yield spectra which are much narrower (fwhm ≤ 60 nm) than those for polycrystalline deposits, consistent with the more homogeneous Pc environment in these ultrathin films. © 1993 American Chemical Society.
• Chau, L., England, C. D., Chen, S., & Armstrong, N. R. (1993). Visible absorption and photocurrent spectra of epitaxially deposited phthalocyanine thin films: Interpretation of exciton coupling effects. Journal of Physical Chemistry, 97(11), 2699-2706.
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  Abstract: The Q-band absorbance spectra or photocurrent spectral maxima from epitaxially grown AlPc-Cl, GaPc-Cl, and InPc-Cl thin films, grown on the metal dichalcogenide SnS2, were compared with the spectral peak positions predicted from an extension of the molecular exciton model, adapted for thin film aggregates of these same Pc'S. These comparisons are made possible by the narrow line shapes in the Q-band spectra of these molecules when ordered Pc films are produced. Such narrow line shapes result from the highly homogeneous environments for these dyes. Correlations between the spectral shift in the Q-band and the number of monolayers in the ultrathin film were made and suggest that reasonable approximations can be made to the perturbation in exciton energies in closely spaced 4-fold symmetric chromophores such as the phthalocyanines. The calculations also show that the Q-band position and full width are sensitive to the packing geometries of adjacent chromophores, which may be useful in characterizing a variety of highly ordered thin film dyes and organic superlattices. © 1993 American Chemical Society.
• Collins, G. E., Williams, V. S., Chau, L. -., Nebesny, K. W., England, C., Lee, P. A., Lowe, T., Fernando, Q., & Armstrong, N. R. (1993). Epitaxial phthalocyanine thin films and phthalocyanine/C₆₀ multilayers. Synthetic Metals, 54(1-3), 351-362.
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  Abstract: Ordered ultrathin films of various dye molecules can be created by vacuum deposition onto a variety of substrates where the interaction with the overlayer is weak and intermolecular forces dominate the ordering process. Examples are shown for the surface electron diffraction data obtained from ordered monolayers of phthalocyanines on metal dichalcogenide surfaces and the corresponding linear optical changes which result from the formation of thin multilayers of those dyes. Organic/organic' multilayer assemblies are also presented, consisting of alternating layers of the fullerene C60 and InPc-Cl, which show promise for the formation of true organic superlattices, where excitonic interactions are confined in two dimensions. © 1993.
• Pankow, J. W., Arbour, C., Dodelet, J. P., Collins, G. E., & Armstrong, N. R. (1993). Photoconductivity/dark conductivity studies of chlorogallium phthalocyanine thin films on interdigitated microcircuit arrays. Journal of Physical Chemistry, 97(32), 8485-8494.
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  Abstract: Studies of dark conductivity and photoconductivity are reported for ultrahigh-vacuum-prepared, highly purified thin films of the trivalent metal phthalocyanine (GaPc-Cl) on gold interdigitated array microcircuit (MC) electrodes. These GaPc-Cl/MC assemblies were subsequently exposed to O2, NO2, and NH3 from ca. 10-8 Torr to atmospheric pressure and/or ultrathin films of TCNQ. It was found that the dark current-voltage properties, the rise time to steady-state photocurrents upon first illumination, and the ratio of photoconductivity/ dark conductivity (σph/σdk) are strongly dependent upon the initial purification of the Pc, the surface cleanliness of the microcircuit, and the extent of exposure to gases like O2. Ohmic current-voltage behavior is seen for less pure GaPc-Cl films or in those cases where trace contiminants are left on the MC surface. Space charge limited current-voltage behavior was seen at low applied fields in all other cases. Light intensity dependencies of the photocurrent response in the Ohmic region indicate that trap levels are distributed uniformly as a function of energy in the "purified" Pc thin films but that this distribution is made strongly inhomogeneous after extensive exposure to electron acceptors such as O2 and NO2. The photocurrent generation process appears to be assisted by the formation of charged species during exposure to O2 and is strongly enhanced by illumination during those O2 exposures. These results suggest that doping of GaPc-Cl thin films with near atmospheric pressures of O2 occurs by means of a photoassisted charge-transfer process, Pc* + O2 ⇄ Pc•+ + O2•-. A similar, but smaller, enhancement in photoconductivity is observed following formation of ultrathin films of TCNQ on the GaPc-Cl/MC surface. Exposure to NH3 can eliminate some of the traps present in the less pure Pc films, decreasing both dark conductivity and photoconductivity but raising the σph/σdk ratio. These results are useful in rationalizing some of the chemical sensor and photoelectrochemical studies of GaPc-Cl and related Pc thin films, where the presence of traps and impurities can control both photoconductivity and the photopotentials observed in barriertype cells. © 1993 American Chemical Society.
• Sandalphon, ., Williams, V. S., Meissner, K., Armstrong, N. R., & Peyghambarian, N. (1993). Polarization-dependent femtosecond dynamics of MBE-grown phthalocyanine organic thin films. Springer Series in Chemical Physics, 379-380.
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  Abstract: Ultrafast bleaching of the lowest energy π-π state and photoinduced absorption from a sub-gap state is reported in chloro-indium-phthalocyanine grown by molecular beam epitaxy.
• Armstrong, N. R. (1992). Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy. Thin Solid Films.
• Armstrong, N. R. (1992). Femtosecond excited-state dynamics in fluoro- and chloroaluminum phthalocyanine thin films. Journal of Physical Chemistry.
• Armstrong, N. R. (1992). Orientation and structure of rnonolayer -> multilayer phthalocyanine thin films on layered semiconductor (MoS2 and SnS2 surfaces. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1992). Ultrafast polarization-dependent spectral hole-burning in a nearly amorphous fluoro-aluminum phthalocyanine thin film. Chemical Physics Letters.
• Armstrong, N. R., Chau, L. K., Collins, G. E., Chen, S. Y., Nebesny, K. W., Williams, V. A., Lee, P. A., Arbour, C., Danziger, J. L., Osburn, E., O'Brien, D., & Parkinson, B. A. (1992). Organic/inorganic molecular beam epitaxy: photoelectrochemical and optical properties of epitaxially deposited single-component and multiple-component organic superlattices (Invited Paper). Proceedings of SPIE - The International Society for Optical Engineering, 1729, 2-12.
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  Abstract: We discuss here our most recent results with the characterization of epitaxial deposits of various phthalolcyanine dyes formed by vacuum deposition (O/I-MBE) or solution deposition on the surface of metal dichalcogenide semiconductors, such as SnS2. Surface electron diffraction techniques used during the vacuum deposition process help to verify the type and extent of long range ordering of these dyes. SnS2 semiconductor substrates allow for the photoelectrochemical characterization of the dye layers, starting with the deposition of submonolayer amounts of material. High quantum yields per absorbed photon are seen for ultrathin films of InPc-Cl, VOPc, and CuPc on SnS2, and the photocurrent spectra suggest similar ordering at the monolayer level, even though multilayer structures are quite different. Ordered Pc thin films are also obtained for a new class of liquid crystalline phthalocyanines (LC-Pc), where the hydrocarbon side chains are attached to the Pc ring by amide linkages. Deposition of ultrathin films of these materials produces photocurrent spectra which are quite similar to those obtained for low coverages of the vacuum deposited Pc. Photocurrent spectra on SnS2 show that the first monolayer of material may have a completely different surface structure than the bulk of the multilayer LC-Pc thin film. The nature of dye/dye' interfaces and their effect on exciton dissociation events has also been explored using vacuum deposited materials. Superlattices of Pcs were formed by vacuum deposition, where the active dye was sandwiched between various spacer molecules with thicknesses down to a few molecular layers. Transient photocurrent yield spectra from such assemblies suggests that exciton dissociation events in such materials can be confined to within a few molecular layers of the dye/dye' interface.
• Armstrong, N. R., Nebesny, K. W., Collins, G. E., Chau, L. -., Lee, P. A., England, C., Diehl, D., Douskey, M., & Parkinson, B. A. (1992). Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy. Thin Solid Films, 216(1), 90-95.
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  Abstract: We describe the formation of ordered ultrathin films of chloroindium phthalocyanine (InPcCl) on thin films of the layered semiconductor SnS2, both of which are deposited by a molecular beam epitaxy process to form InPcCl/ SnS2/mica assemblies. The resultant thin films show markedly narrowed linear absorbance spectra versus those seen previously for Pc thin films. Low energy electron diffraction and reflection high energy electron diffraction data collected for the first few monolayers of InPcCl deposited on the SnS2 substrate suggest that three (equally populated) ordered Pc domains are formed, with the Pc rings parallel to the SnS2 surface, aligned along the principle axes of this hcp SnS2 substrate. The red-shift and peak shape of the absorbance spectra of these thin films (about 758 nm, full width at half maximum of about 60 nm) are consistent with the type of splitting of the exciton energies expected of Pc aggregates in which the adjacent Pc rings are cofacial, but staggered in both x and y directions by about half a molecular diameter. This is the structure determined from related trivalent metal Pc's by single crystal X-ray diffraction. Loss of the flat-lying configuration of Pc in these ultrathin films is immediately apparent from the surface electron diffraction data. © 1992.
• Williams, V. S., Mazumdar, S., Armstrong, N. R., Ho, Z. Z., & Peyghambarian, N. (1992). Femtosecond excited-state dynamics in fluoro- and chloroaluminum phthalocyanine thin films. Journal of Physical Chemistry, 96(11), 4500-4505.
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  Abstract: Photoinduced absorption on the high-energy side of the Q-band has been observed and time-resolved in polycrystalline/amorphous thin films of both fluoroaluminum and chloroaluminum phthalocyanine. The films were pumped with ≈100-fs optical pulses at various wavelengths inside the inhomogeneously broadened Q-band, including 620 and 760 nm. The induced absorption signal is accompanied by a bleaching signal on the low-energy edge of the Q-band. The bleaching signal resembles a blue-shifted monomer absorbance spectrum. The induced absorption signal develops as the absorption bleaching signal decays, suggesting exciton decay into a subgap state. The different bimolecular decay dynamics observed for the absorption bleaching (τeff ≈ 700 fs) and induced absorption (τeff ≈ 2 ps) signals support this conclusion. Analyses of the results and possible origins of the indicated subgap state are discussed. © 1992 American Chemical Society.
• Williams, V. S., Sokoloff, J. P., Ho, Z. Z., Arbour, C., Armstrong, N. R., & Peyghambarian, N. (1992). Ultrafast polarization-dependent spectral hole-burning in a nearly amorphous fluoro-aluminum phthalocyanine thin film. Chemical Physics Letters, 193(5), 317-322.
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  Abstract: We report the observation of a polarization-dependent spectral hole burned in the Q-band absorption peak of a nearly amorphous fluoro-aluminum phthalocyanine thin film by 130 fs optical pulses at room temperature. The hole is spectrally coincident with the pump pulse and observed only when the pump and probe pulses are temporally overlapped and polarized parallel. Coherent coupling effects appear to be responsible for the spectral hole. The observation of the polarization-dependent spectral hole implies that the two orthogonal polarization states of the system are decoupled, unlike inorganic semiconductors. The initially polarized population created by the pump pulse rapidly relaxes to the bottom of the excited-state manifold and depolarizes. © 1992.
• Armstrong, N. R. (1991). Electrochemical and Photoelectrochemical Processes on Thin Films of Perylenetetracarboxylic Dianhydride. Chemistry of Materials.
• Armstrong, N. R. (1991). Highly photoactive chemically modified thin films of chloroaluminum (and bromoaluminum) phthalocyanines probed by NEXAFS and UPS: Determination of the electronic structure and the molecular orientation. Journal of Physical Chemistry.
• Armstrong, N. R. (1991). Organic/Inorganic Molecular Beam Epitaxy: Formation of an Ordered Phthalocyanine/SnS2 Heterojunction. Chemistry of Materials.
• Armstrong, N. R. (1991). Oxide formation on Fe and Ti thin films and on Fe thin films modified with ultrathin layers of Ti. Surface and Interface Analysis.
• Armstrong, N. R. (1991). Oxide formation on rare earth/transition metal and bimetallic transition metal thin films: modeling the effect of fourth element modifiers on O2 and H2O surface chemistries. Journal of Magnetism and Magnetic Materials.
• Armstrong, N. R. (1991). Tetracyanoquinodimethane Thin Films on Cu, Au, Pt, and SnS2: Characterization by X-ray Photoelectron Spectroscopy. Langmuir.
• Armstrong, N. R., Nebesny, K. W., Collins, G. E., Lee, P. A., Chau, L. K., Arbour, C., & Parkinson, B. (1991). O/I-MBE: formation of highly ordered phthalocyanine/semiconductor junctions by molecular-beam epitaxy: photoelectrochemical characterization. Proceedings of SPIE - The International Society for Optical Engineering, 1559, 18-26.
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  Abstract: Highly ordered phthalocyanine (Pc)/inorganic semiconductor heterojunctions have been created by a process of molecular beam epitaxy in ultrahigh vacuum environments (O/I- MBE). The layered metal dichalcogenides, MoS2, and SnS2, have been used as substrates, either as melt grown crystals or as thin films, created by the MBE process, immediately prior to the deposition of the organic layer. Surface electron diffraction (RHEED) is used to ascertain the type and extent of ordering of the Pc layer. Absorption or reflection spectroscopies are used in the visible wavelength region to characterize the shape and position of the Q-band absorbance of the ordered dye layers, which confirm the ordering seen by RHEED. When deposited on doped SnS2 crystals, and immersed in aqueous electrolytes, the photocurrent yield of these dye layers can also be measured, which provides an alternative means of obtaining the Q-band spectrum for ultrathin dye layers.
• Danziger, J., Dodelet, J. -., & Armstrong, N. R. (1991). Electrochemical and photoelectrochemical processes on thin films of perylenetetracarboxylic dianhydride. Chemistry of Materials, 3(5), 812-820.
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  Abstract: Thin films of perylene derivative perylenetetracarboxylic dianhydride (PTCDA) have been examined as electrodes and photoelectrodes on both metal and metal oxide substrates. These thin films gave only anodic photocurrents (reverse bias condition) in contact with various aqueous electrolytes and facile dark currents negative of ca. -0.1 V vs. Ag/AgCl (forward bias condition). The origin of this photoeffect appears to be due to differential rates of charge injection at the PTCDA/solution interface and/or an asymmetric distribution of traps at that same interface. Electron microscopy of the PTCDA films indicated that they were deposited as elongated crystallites, with spaces between individual crystallites that strongly affected their dark and photoelectrochemical behavior, especially on Au substrates. Electrochemical polymerization with α-naphthol was carried out to passivate sites that were electrochemically active in the dark, a treatment that greatly enhanced the overall photoelectrochemical activity of the PTCDA thin films. Cathodic polarization of the PTCDA thin films caused surface bound electrochemically active redox couples to be produced at the film's surface, and this polarization step further enhanced the photoelectrochemical activity. These experiments suggest that the PTCDA photoelectrochemical activity is strongly dependent upon near-surface composition and morphology and that, because of the highly porous nature of these thin films, the photoactive interfaces extend throughout most of the bulk of the PTCDA film. © 1991 American Chemical Society.
• Danziger, J., Dodelet, J. -., Lee, P., Nebesny, K. W., & Armstrong, N. R. (1991). Heterojunctions formed from phthalocyanine and perylene thin films: Photoelectrochemical characterization. Chemistry of Materials, 3(5), 821-829.
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  Abstract: Bilayer and multilayer structures created from thin films of vanadylphthalocyanine (VOPc) and perylenetetracarboxylic dianhydride (PTCDA) or copper phthalocyanine (CuPc) and PTCDA have been created, and their absorbance spectra and electrochemical/photoelectrochemical properties explored. Vacuum-deposited bilayers of these thin films show rectification in their current-voltage behavior over a narrow potential window. The open-circuit photopotential appears to be determined by the junction potential formed at the Pc/PTCDA interface. The magnitude of this junction is estimated from ultraviolet photoelectron spectroscopy (UPS) of the pure materials and previous electrochemical studies of solutions of comparable molecules. Photocurrent yield spectra, from two different illumination directions, indicated that a narrow region near the VOPc/PTCDA or CuPc/PTCDA interface was responsible for most of the photocurrent production. This was confirmed through the characterization of multilayer assemblies, where it was found that the transient photocurrent (using a modulated light source) was directly proportional to the number of Pc/PTCDA interfaces present. © 1991 American Chemical Society.
• Guay, D., Tourillon, G., Gastonguay, L., Dodelet, J. P., Nebesny, K. W., Armstrong, N. R., & Garrett, R. (1991). Highly photoactive chemically modified thin films of chloroaluminum (and bromoaluminum) phthalocyanines probed by NEXAFS and UPS: Determination of the electronic structure and the molecular orientation. Journal of Physical Chemistry, 95(1), 251-257.
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  Abstract: Near edge X-ray absorption fine structure (NEXAFS) at the C and N K edges and UV photoelectron spectroscopy (UPS) at variable excitation energy (21.2 and 120 eV) have been used to characterize the electronic structure and the molecular orientation of chloroaluminum phthalocyanine (ClAlPc) thin films deposited on glass or SnO2 substrates and subjected to chemical transformations. Transformation H occurs when the films are soaked in an aqueous acidic solution, while transformation I is observed if I3-/I- is added to the solution. It is demonstrated that the protonation of the aza nitrogen atoms of ClAlPc happens during both transformations. It modifies the valence band of the compound but has no effect on the unoccupied levels of the material. Besides the protonation, the uptake of I3- in the phthalocyanine film during transformation I leads to the formation of a charge-transfer complex between the phthalocyanine and the iodine species with a modification of both the occupied and unoccupied levels of the material. The band gap is reduced by 0.3 eV and 1.5 eV for transformations H and I, respectively. No orientation-dependent effect is observed when the chloroaluminum phthalocyanine is deposited on SnO2. On the contrary, strong polarization effects are observed in the NEXAFS spectra of phthalocyanine deposited on a float glass substrate. In the as-deposited film, the molecules are lying almost flat on the substrate surface. They are switching to a perpendicular to the substrate orientation after transformation H, while they are becoming randomly oriented after transformation 1. Despite the amorphous structure found after transformation I, the photoelectrochemical activity of the film is three times as large as the photoactivity of the untreated ClAlPc. This increase of photoactivity is related to the formation of a charge-transfer complex and is explained in terms of a better interaction between the film and the redox (I3-/I-) in solution. Bromoaluminum phthalocyanine (BrAlPc) films were also probed by NEXAFS. All modifications observed for ClAlPc were also obtained for BrAlPc. © 1991 American Chemical Society.
• Lee, P. A., & Armstrong, N. R. (1991). Oxide formation on rare earth/transition metal and bimetallic transition metal thin films: modeling the effect of fourth element modifiers on O₂ and H₂O surface chemistries. Journal of Magnetism and Magnetic Materials, 93(C), 159-163.
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  Abstract: The chemical reactions undergone by both O2 and H2O on (a) rare earth (Tb), (b) rare-earth transition metal alloy (FeTbCo), (c) transition metal (Fe and Ti), and (d) transition metals, modified with thin overlayers of another metal (Fe/Ti), have been studied by surface photoelectron spectroscopies, starting with the clean metals in UHV environment. Various degrees of reactivity are indicated, especially for adsorption and dissociation of H2O, depending upon whether the pure metals are studied, or mixtures of metals, which change the type and extent of dissociation of H2O (oxide vs. hydroxide formation). © 1991.
• Lee, P. A., Stork, K. F., Maschhoff, B. L., Nebesny, K. W., & Armstrong, N. R. (1991). Oxide formation of Fe and Ti thin films and on Fe thin films modified with ultrathin layers of Ti. Surface and Interface Analysis, 17(1), 48-56.
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  Abstract: Reaction of O2 and H2O to form oxides on Fe, Ti and Ti-modified Fe thin films have been studied by XPS, following O2 and H2O exposures in the range 0-600 L, to form oxides with thickness of < 40 A. XPS lineshape analysis is used, utilizing a special combination of reflection electron energy-loss spectroscopy (REELS) and nonlinear least-squares fitting routines to model the intrinsic and extrinsic energy losses that accompany photoemission for the Ti and Fe 2p lines. During the formation of the thinnest oxides, this approach yields a unique picture of the composition of the oxide, while for the thicker layers, there is little significant difference between this fitting approach and (1) fitting approaches using an integral background approach or (2) direct deconvolution methods using the REELS data. Both Fe and Ti are quite reactive to O2, yielding a surface oxide that is apparently an FeO-dominated FeO/Fe3O4 bilayer on Fe surfaces, and a surface oxide that is predominantly TiO2 on the Ti surfaces. Pure Fe and Ti surfaces are unreactive to H2O. Predosing of these surfaces with low levels of O2 (5 L) does not increase the reactivity appreciably toward H2O. Ti dispersed on the Fe surface as an adatom layer, with an equivalent thickness of 3 A, greatly suppresses the reactivity of Fe Fe toward O2, while the Ti is oxidized primarily to TiO2. In contrast to the pure Ti layers, these Ti adatom layers are quite reactive toward H2O, yielding oxides (no evidence for hydroxide) in a uniform distribution of oxidation states (Ti+2, Ti+3 and Ti+4).
• Maschhoff, B. L., & Armstrong, N. R. (1991). Thin oxide layers on clean iron surfaces. Formation under vacuum and characterization by photoelectron spectroscopy and electrochemical reactions of probe molecules at the oxide/electrolyte interface. Langmuir, 7(4), 693-703.
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  Abstract: Exposure of atomically clean polycrystalline iron surfaces to low pressures of O2 produces oxide films which UV photoelectron spectroscopy and X-ray photoelectron spectroscopy reveal are predominantly bilayers of FeO and Fe3O4. FeO is the principal product indicated in the first 10 L of oxygen exposure. Fe3O4 formation is initiated following this exposure level. The Fe3O4 layer appears to constitute up to 25% of the total oxide composition, which is predominantly FeO. Following the growth of several oxide thin films on iron under vacuum, at different O2 exposure times and pressures, transfer under ultrahigh vacuum to an electrochemical cell was carried out, whereupon electron transfer rates (ETR) of benzoquinone (BQ) reduction and ferrocene (Fc) oxidation were explored in acetonitrile. Minimal chemical reaction of the iron or oxidized iron surfaces with acetonitrile was demonstrated. The voltammetrically determined ETR of BQ and Fc both showed a different dependence on oxide thickness than predicted for a tunneling-controlled process.
• Nebesny, K. W., Collins, G. E., Lee, P. A., Chau, L. -., Danziger, J., Osburn, E., & Armstrong, N. R. (1991). Organic/inorganic molecular beam epitaxy: Formation of an ordered phthalocyanine/SnS₂ heterojunction. Chemistry of Materials, 3(5), 829-837.
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  Abstract: We report the growth of thin films of the metal dichalcogenide, SnS2, on cleaved mica surfaces, followed by the deposition of monolayers and multilayers of chloroindium phthalocyanine, InPc-Cl. Reflection high-energy electron diffraction (RHEED) was used to show that the SnS2 thin films grew with a principal axis oriented along a principal axis of the mica substrate, even at submonolayer coverages of SnS2. X-ray photoelectron spectroscopy (XPS), reflection electron energy loss spectroscopies (REELS), and optical spectroscopies were used to confirm that the stoichiometry and electronic properties of these SnS2 thin films were those of 2H-SnS2. As with the deposition of InPc-Cl on melt-grown, bulk SnS2, the first monolayer of InPc-Cl on SnS2/mica gave RHEED patterns consistent with the formation of a closest packed monolayer, with the Pc's lying flat on the surface. On the MBE-grown SnS2, InPc-Cl multilayer films were grown that gave the same RHEED patterns as seen for the monolayer deposits. These diffraction data are consistent with the buildup of each Pc monolayer in registry with the initial monolayer. Transmission optical spectra of these Pc thin films show a strong, narrow absorption spectrum, in sharp contrast to those seen for polycrystalline InPc-Cl deposits, suggesting that the Pc chromophores sit in essentially equivalent environments. © 1991 American Chemical Society.
• Patterson, T., Pankow, J., & Armstrong, N. R. (1991). Tetracyanoquinodimethane thin films on Cu, Au, Pt, and sns₂. Characterization by X-ray photoelectron spectroscopy. Langmuir, 7(12), 3160-3166.
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  Abstract: Tetracyanoquinodimethane (TCNQ) thin films, prepared by vacuum deposition on Cu, Au, and Pt and the layered semiconductor SnS2, have been characterized using X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy studies showed that TCNQ reduction occurred, to distances within the TCNQ film which corresponded to coverages beyond monolayer, on Cu and Au thin films prepared by evaporation onto freshly cleaved mica, and on the layered compound SnS2. Chemical shifts of the C(1s) and N(1s) lines were consistent with the formation of at least two forms of metal/TCNQ charge transfer salt, and the coverage of this product on these smooth surfaces suggests that there is facile charge exchange over several monolayers of TCNQ. At a critical coverage of TCNQ (ca. 20 A on the Cu/ mica surface), the bulk form begins to predominate, setting an upper coverage limit on this charge exchange process. Similar reactivities were seen for TCNQ on SnS2 surfaces, with no indication of intercalation. On clean Pt and Au surfaces prepared by sputter cleaning of the metal foils, reduction of TCNQ occurred by a similar mechanism, but conversion to the bulk form of TCNQ occurred at much lower apparent coverages, which we propose is due to the roughened nature of those surfaces. A reproportionation mechanism is proposed which rationalizes the reaction of incoming TCNQ with the multiple anion forms present in the first monolayer.
• Armstrong, N. R. (1990). Chemiresistor Gas Sensors Based on Photoconductivity Changes in Phthalocyanine Thin Films: Enhancement of Response toward Ammonia by Photoelectrochemical Deposition with Metal Modifiers. Analytical Chemistry.
• Armstrong, N. R. (1990). Surface Chemical and Electrochemical Processes of Well-Characterized Lithium Thin Films: Electrochemical Reactions in SO2/CH3CN. Journal of the Electrochemical Society.
• Armstrong, N. R. (1990). Surface chemistries of lithium: Detailed characterization of the reactions of H2S, SO2, and SO2Cl2 using XPS and EELS. Surface Science.
• Brina, R., Collins, G. E., Lee, P. A., & Armstrong, N. R. (1990). Chemiresistor gas sensors based on photoconductivity changes in phthalocyanine thin films: Enhancement of response toward ammonia by photoelectrochemical deposition with metal modifiers. Analytical Chemistry, 62(21), 2357-2365.
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  Abstract: Determination of NH3 in flowing N2 or air is demonstrated by monitoring changes in the photoconductivity of chloroindium phthalocyanine (InPc-Cl) and chlorogallium phthalocyanine (GaPc-Cl) thin films vacuum deposited on platinum silicide interdigitated array microcircuits. Photoconductivity changes were measured, as opposed to the more commonly measured dark conductivity changes, yielding reasonable sensitivity to the adsorption of submonolayer amounts of gas analytes such as NH3. Photoelectrochemical modification of Pc-coated microcircuits was carried out in order to obtain submonolayer coverages of a variety of different metals (Ag, Au, Cu, Pt, and Hg) on the Pc film. This modification apparently provides new chemisorption sites for NH3, thus improving the performance of these microcircuit assemblies as NH3 sensors in N2 and in air. The Hg-modified microcircuits demonstrated a superior performance compared to the microcircuits modified with other metals, by showing a linear photocurrent response as a function of NH3 concentration in N2 at room temperature over the range from ca. 1.5 to 5.4 ppm, and a logarithmic relationship above that level (up to ca. 84 ppm). Photoconductivity measurements in NH3-air mixtures were carried out on microcircuit assemblies "as-prepared" and after photoelectrochemical modification. The presence of the metal modifier on the InPc-Cl film markedly improves the assembly performance in air, providing a detection limit for NH3 in air of ca. 1.0 part per thousand (ppt). The effect on the photoconductivity of O2 and other impurities present in GaPc-Cl and InPc-Cl is discussed, and a mechanism is proposed to explain the variations observed in the photocurrent response to NH3 in these assemblies before and after modification.
• Zavadil, K. R., & Armstrong, N. R. (1990). Surface chemical and electrochemical processes of well-characterized lithium thin films. Electrochemical reactions in SO₂/CH₃CN. Journal of the Electrochemical Society, 137(8), 2371-2378.
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  Abstract: The electrochemical reactivity of lithium and similar alkali metals has been widely explored because of the interest in these materials for high energy density batteries both primary and secondary cells. The studies presented in this paper focus on the electrochemical characterization of lithium thin films prepared in an atomically clean state in an ultra-high vacuum chamber and then transferred to an electrochemical cell consisting of SO2-saturated acetonitrile. The role of prior oxidation with small and large doses of gaseous O2 and H2O in the UHV preparation chamber is also explored, along with the role that prolonged solution immersion times of these same surfaces prior to discharge has on the ultimate electrochemical reactivity.
• Zavadil, K. R., & Armstrong, N. R. (1990). Surface chemistries of lithium: Detailed characterization of the reactions of H₂S, SO₂, and SO₂Cl₂ using XPS and EELS. Surface Science, 230(1-3), 61-73.
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  Abstract: The surface chemical reactions of clean lithium with H2S, SO2 and SO2Cl2 have been studied by a combination of XPS and EELS. The reactions with H2S lead entirely to the formation of sulfide product layers, which propagate only to the extent of a few monolayers. The reactions with SO2 are more complicated, the first monolayer consists of oxide and sulfide from the full dissociation of the SO2 molecule. Subsequent reaction occurs to form sulfide, sulfite and SO2 (ads), probably as the radical anion form. At exposures of SO2 up to atmospheric pressure, the product layer composition, within the XPS sampling depth, becomes exclusively sulfite. The reaction with SO2Cl2 occurs along similar pathways. The first monolayer is exclusively the oxide, sulfide and chloride, while subsequent reactions occur to form sulfite and SO2 (ads), in the presence of additional chloride. These studies are important first steps toward the understanding of electrochemical processes of these interfaces. © 1990.
• Zavadil, K. R., & Armstrong, N. R. (1990). Surface chemistries of lithium: Detailed characterization of the reactions with O₂ and H₂O using XPS, EELS, AND microgravimetry. Surface Science, 230(1-3), 47-60.
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  Abstract: The surface chemical reactions of O2 and H2O on clean lithium have been studied by a combination of XPS, EELS and microgravimetry. Reactions with O2 produce a monolayer of oxide which does not passivate the surface and which allows for the growth of several monolayers of additional oxide, probably as a result of the mixing of zero-valent metal into the oxide layer. The reaction of H2O with the clean lithium surface results in the complete dissociation of the molecule and loss of hydrogen to form one monolayer of the oxide. This is followed by the formation of multilayers of hydroxide/oxide mixtures which are shown to be unstable over periods of minutes, converting back to the oxide form predominantly. © 1990.
• Armstrong, N. R. (1989). Comparison of Supramolecular Aggregate Structure and Spectroscopic and Photoelectrochemical Properties of Tetravalent and Trivalent Metal Phthalocyanine Thin Films. Chemistry of Materials.
• Armstrong, N. R. (1989). Interactions of NH3 and O2 with the Surfaces of Chlorogallium Phthalocyanine Thin Films: Microcircuit Photoconductivity and Quartz-Crystal Microgravimetry Studies. Langmuir.
• Armstrong, N. R. (1989). Quantitation of Auger and X-ray Photoelectron Spectroscopies. Analytical Chemistry.
• Armstrong, N. R. (1989). Reactions at the interface between multi-component glasses and metallic lithium films. Journal of Materials Research.
• Armstrong, N. R. (1989). The interaction of lithium with sodium and potassium silicate glasses. Surface and Interface Analysis.
• Armstrong, N. R. (1989). Voltammetry at the thin-film mercury electrode (TFME): Anodic and cathodic stripping voltammetries and simple potentiostat construction. Journal of Chemical Education.
• Maschhoff, B. L., & Armstrong, N. R. (1989). Interaction of lithium with sodium and potassium silicate glasses. Surface and Interface Analysis, 14(1-2), 76-82.
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  Abstract: The interaction of lithium with sodium and potassium glass surfaces in UHV has been characterized with XPS. Earlier studies of the Li/SiO2 system have shown the formation of a partially passivating layer upon initial Li deposition. Following breakdown of this layer, substantial reduction of the glass matrix occurs. In this study, several compositions of sodium silicate and potassium silicate glasses were found to be more resistant than SiO2 to lithium attach owing to the presence of non-bridging oxygens in the matrix. The proposed corrosion mechanism involves charge transfer from Li to the modifying cations at the glass surface followed by desorption of neutral Na or K and interdiffusion of Li+ into the glass. For thick (metallic) Li films on sodium silicates, the formation of a dilute Li-Na intermetallic and sodium metal are identified from distinct plasmon structure in the Na(1s) line.
• Nebesny, K. W., Maschhoff, B. L., & Armstrong, N. R. (1989). Quantitation of Auger and X-ray photoelectron spectroscopies. Analytical Chemistry, 61(7), 469 A-481 A.
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  Abstract: Complex sample matrix and instrumental parameters make quantitation of surface electron spectroscopies a real challenge. Kenneth W. Nebesny, Brian L. Maschhoff, and Neal R. Armstrong of the University of Arizona describe approaches that make it possible to obtain reliable compositional information. © 1989 American Chemical Society.
• Sims, T. D., Pemberton, J. E., Lee, P., & Armstrong, N. R. (1989). Comparison of supramolecular aggregate structure and spectroscopic and photoelectrochemical properties of tetravalent and trivalent metal phthalocyanine thin films. Chemistry of Materials, 1(1), 26-34.
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  Abstract: The intermolecular interactions between trivalent metal and tetravalent metal phthalocyanines in vacuum-deposited thin films lead to at least two different crystallite types with different optical and electronic properties. UV-visible, infrared, and Raman spectroscopies, X-ray diffraction, scanning electron microscopy (SEM), and photoelectrochemical metal deposition followed by SEM have been used in combination on the same thin films to characterize these different phases and to differentiate their photoelectrochemical activity. Two distinct crystallite forms are evident from these data, which we hypothesize to be (a) a cofacially stacked linear assembly, aligned along the metal-halide or metal-oxo bond or (b) an assembly where adjacent molecules are staggered to form a slipped-stack arrangement and a unique crystallite in the thin film. This latter stacking arrangement leads to a red-shifted Q band in the visible absorbance spectrum, and these crystallites appear to be photoelectrochemically more active by a factor of at least 10. © 1989 American Chemical Society.
• Waite, S., Pankow, J., Collins, G., Lee, P., & Armstrong, N. R. (1989). Interactions of NH₃ and O₂ with the surfaces of chlorogallium phthalocyanine thin films: Microcircuit photoconductivity and quartz-crystal microgravimetry studies. Langmuir, 5(3), 797-805.
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  Abstract: The chemisorption of O2 and NH3 on the surface of GaPc-Cl vacuum-deposited thin films has been studied by a combination of photoconductivity changes of interdigitated array microcircuits (MC) and mass uptake on quartz-crystal microbalances (QCM). Measurement of photoconductivity versus dark conductivity changes gives a higher sensitivity to changes caused by the adsorption of monolayer (or less) levels of these gases. The simultaneous monitoring of mass changes and photoconductivity changes shows clearly that there exists more than one type of chemisorption site for both O2 and NH3 and that these molecules compete for at least one of these sites. The response of the GaPc-Cl-coated microcircuit to NH3 in room temperature air is poor, because of this competition with O2 . Photoelectrochemical modification (Ag0 deposition at submonolayer coverages) of the Pc-coated microcircuit is possible, however, which then provides for a reasonable room temperature response to NH3, presumably by producing a unique chemisorption site for this molecule.
• Zavadil, K. R., Armstrong, N. R., & Peden, C. H. (1989). Reactions at the interface between multi-component glasses and metallic lithium films. Journal of Materials Research, 4(4), 978-989.
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  Abstract: The reactions of vacuum deposited thin films of lithium with various complex glasses have been explored using x-ray photoelectron spectroscopy (XPS). In contrast to lithium reactions with simple glasses such as silica or boron oxides, the reactions are predominantly those of the network modifiers such as sodium, potassium, and magnesium. XPS and x-ray induced Auger lineshapes indicate the migration of the network modifier to the near surface region followed by its reduction. In the case of magnesium, there is evidence for stable alloy formation with unreacted lithium following these migration and reduction steps.
• Armstrong, N. R. (1988). Chemical modification of GaPc-Cl thin film photoelectrodes. Journal of Electroanalytical Chemistry.
• Armstrong, N. R. (1988). Probing of basal planes of MoS2 by scanning tunneling microscopy. Applied Physics Letters.
• Armstrong, N. R. (1988). Quantitation of surface electron spectroscopies: problems in correct description of intrinsic lineshape and extrinsic energy loss processes. Spectrochimica Acta Part B: Atomic Spectroscopy.
• Armstrong, N. R. (1988). Quantitative surface electron spectroscopies: Application to the study of corrosion of metals and glasses. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1988). Summary Abstract: Nonlinear least-squares fitting employing fast Fourier transform background generation for the x-ray photoelectron spectroscopy analysis of iron oxidation. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Maschhoff, B. L., Nebesny, K. W., Zavadil, K. R., Fordemwalt, J. W., & Armstrong, N. R. (1988). Quantitation of surface electron spectroscopies: problems in correct description of intrinsic lineshape and extrinsic energy loss processes. Spectrochimica Acta Part B: Atomic Spectroscopy, 43(4-5), 535-550.
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  Abstract: Methods for improved description of lineshapes and energy loss backgrounds in the surface electron spectroscopies. Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS), are discussed. For simple homogeneous non-metallic materials in the near surface region, the energy loss profiles are a small contribution to the total spectral intensity and can be adequately described by simple background functions. For many pure metals or metal alloys, with or without thin adsorbate overlayers, it is necessary to use more realistic models for the energy loss profiles. These can be generated from functions which describe the electron emission event on a microscopic scale, and/or from electrons beams which are scattered from the surface at the kinetic energy of the photoemission or Auger process of interest. Deconvolution schemes (particularly using Fast Fourier Transform approaches) and non-linear least squares fitting routines are compared for preparing the XPS or AES lineshapes for further analysis. © 1988.
• Nanthakumar, A., & Armstrong, N. R. (1988). Chemical modification of GaPc-Cl thin film photoelectrodes. Journal of Electroanalytical Chemistry, 248(2), 349-362.
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  Abstract: Modification of chlorogallium phthalocyanine (GaPC-Cl) thin film photoelectrodes with thin films of polyvinylferrocene (PVF), thin films of PVF with incorporated ferricyanide, and thin films of a coprecipitated ferricenium-ferricyanide salt is demonstrated. The analysis of the coprecipitated ferriceniuin-ferricyanide salt demonstrates the formation of a Prussian Blue (PB)-like electroactive film. PVF films on bare gold were first characterized to demonstrate incorporation of ferricyanide during formation of ferricenium ions. This was followed by demonstration of the direct interaction of ferrocenium and ferricyanide, without the polymer, on bare gold, and on the Pc surface. GaPc-Cl films overlayed with PB thin films formed from ferricenium/ferricyanide can produce either negative or positive photopotentials, depending upon the dopant condition of the Pc layer, relying only upon the photoelectrochemical redox activity of the PB layer. On the PVF-modified GaPc-Cl electrodes, the ferrocene redox activity could be depleted in the dark through oxidation at potentials positive of the apparent flat band potential of the p-type molecular semiconductor. Following this, reduction of the ferricenium centers was suppressed by the rectifying nature of the Pc layer, until solution contact was made with a reductant such as ascorbate. Redox activity of the ferrocene (oxidation of ferrocene in PVF) could be restored in relation to the solution concentration of the ascorbate. © 1988.
• Sarid, D., Henson, T. D., Armstrong, N. R., & Bell, L. S. (1988). Probing of basal planes of MoS₂ by scanning tunneling microscopy. Applied Physics Letters, 52(26), 2252-2254.
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  Abstract: Atomically resolved images of MoS2 have been obtained using scanning tunneling microscopy with both positively and negatively biased samples yielding the hexagonal symmetry of the surface of the crystal. Also measured were curves of tunneling current as a function of bias voltage, from which the density of states of the valence and conduction bands can be inferred.
• Armstrong, N. R. (1987). Photoelectrochemical and spectroscopic characterization of thin films of titanyl phthalocyanine: Comparisons with vanadyl phthalocyanine. Journal of Physical Chemistry.
• Armstrong, N. R. (1987). Spectroscopic and photoelectrochemical studies of trivalent phthalocyanine thin films. The role of gaseous dopants (O2 and H2) in determining photoelectrochemical response. Journal of Physical Chemistry.
• Klofta, T. J., Danziger, J., Lee, P., Pankow, J., Nebesny, K. W., & Armstrong, N. R. (1987). Photoelectrochemical and spectroscopic characterization of thin films of titanyl phthalocyanine: Comparisons with vanadyl phthalocyanine. Journal of Physical Chemistry, 91(22), 5646-5651.
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  Abstract: The photoelectrochemical properties of vacuum-deposited thin films of titanyl phthalocyanine (TiOPc) are described. Continuous films between 60 and 400 nm in thickness show behavior like that of a highly doped p-type semiconductor, with open-circuit photopotentials (Voc) that vary linearly in magnitude with the formal potential of the contacting redox couple (up to 0.7 V). Comparison of photocurrent action spectra and transmission absorbance spectra showed that two distinct phases of TiOPc were deposited from vacuum, having different absorbance maxima at ca. 720 and 820 nm. The TiOPc phase with the more red-shifted absorbance appeared to possess higher photoactivity. X-ray photoelectron spectroscopy (XPS) of both TiOPc and vanadyl phthalocyanine films (VOPc) showed clearly the presence of extra oxygen forms for films vacuum deposited in moderate (10-6 Torr) vacuums, as opposed to the stoichiometric quantities of oxygen found in films deposited under ultrahigh vacuum. © 1987 American Chemical Society.
• Klofta, T. J., Sims, T. D., Pankow, J. W., Danziger, J., Nebesny, K. W., & Armstrong, N. R. (1987). Spectroscopic and photoelectrochemical studies of trivalent phthalocyanine thin films. The role of gaseous dopants (O₂ and H₂) in determining photoelectrochemical response. Journal of Physical Chemistry, 91(22), 5651-5659.
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  Abstract: Thin films of vacuum-deposited chlorogallium phthalocyanine (GaPc-Cl) and other trivalent-metal Pc's show widely variable photoelectrochemical properties depending upon vacuum deposition conditions and postdeposition doping with either O2 or H2. Combination of scanning electron microscopy (SEM), infrared spectroscopy (ATR-FTIR), visible absorbance spectroscopies, and X-ray diffraction indicates that typical GaPc-Cl films consist of a mixture of at least two different phases, with different proportions of each depending upon growth conditions. High-temperature O2 doping of GaPc-Cl can turn a film that behaves like a lightly doped semiconductor into a p-type material; H2 doping can reverse the effect. ESR experiments indicate the presence of high concentrations of radical species (ca. 1017 cm-3) whose populations can be increased or decreased with O2 or H2 doping, respectively. Photocurrent yield spectra of lightly doped or p-type GaPc-Cl films confirm that photocurrents are limited by hole transport in the first instance and electron transport in the second instant. © 1987 American Chemical Society.
• Maschhoff, B. L., Zavadil, K. R., & Armstrong, N. R. (1987). REACTIONS OF EVAPORATED LITHIUM FILMS ON OXIDE AND GLASS SURFACES.. Proceedings - The Electrochemical Society, 87-1, 212-223.
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  Abstract: The corrosion reactions of UHV evaporated lithium films with SiO//2, B//2O//3, and multi-component glass surfaces studied with XPS are used to model the corrosion of glass headers by electrodeposition of lithium in Li/SO//2 batteries. SiO//2 exhibits little resistance to matrix breadown within the XPS sampling region at modest temperatures ( less than 80 degree C), whereas B//2O//3 appears to form a passivating layer which limits the extent of reaction. Reactions of lithium deposited on multi-component glasses are characterized by increased resistance to network former reduction, yet show unusual activity for alkali and alkaline earth modifiers in the surface region.
• Armstrong, N. R. (1986). Characterization of Pt/TiO2 surfaces by means of photoelectron spectroscopy of adsorbed xenon. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1986). Deconvolution of auger electron spectra for lineshape analysis and quantitation using a fast fourier transform algorithm. Journal of Electron Spectroscopy and Related Phenomena.
• Armstrong, N. R. (1986). Effect of Crystallite Size and Hydrogen and Oxygen Uptake in the Photoelectrochemistries of Thin Films of Chlorogallium Phthalocyanine. Journal of the Electrochemical Society.
• Armstrong, N. R. (1986). Electrochemical Oxide Formation on Titanium and Titanium Hydride (Deuteride) Thin-Film Surfaces. Langmuir.
• Armstrong, N. R. (1986). Oxides Formed on Polycrystalline Titanium Thin-Film Surfaces: Rates of Formation and Composition of Oxides Formed at Low and High O2 Partial Pressures. Langmuir.
• Armstrong, N. R. (1986). Problems in quantitation in x-ray photoelectron spectroscopy (xps): The use of data reduction techniques to obtain peak areas. Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1986). Reactions of evaporated lithium films with silica surfaces. Applied Surface Science.
• Armstrong, N. R., Zavadil, K. R., & Maschhoff, B. L. (1986). SURFACE CHEMICAL PROCESSES OF CLEAN LITHIUM: SMALL MOLECULE AND SOLID STATE CORROSION REACTIONS RELEVANT TO THE Li/SO//2 BATTERY.. Proceedings - The Electrochemical Society, 86-7, 387-401.
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  Abstract: Self-discharge/corrosion reactions of clean lithium in environments which model Li/SO//2 batteries have been studied. Removal of the interface in question to ultrahigh vacuum, subsequent dosing with gases and liquids, and examination by XPS has been carried out to characterize the near surface regions, which are short-lived and difficult to study in real-world systems. Semi-passive layers which form on the lithium surface in contact with SO//2, SO//2-saturated CH//3CN, etc. are different than previously diagnosed by post-mortem examination of electrochemical cells. Corrosion of SiO//2 by Li degree proceeds by reduction of the matrix and formation of products not predicted from purely thermodynamic considerations.
• Burrell, M. C., & Armstrong, N. R. (1986). Electrochemical oxide formation on titanium and titanium hydride (deuteride) thin-film surfaces. Langmuir, 2(1), 37-42.
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  Abstract: The electrochemical formation of oxides on the surfaces of titanium and titanium hydride (deuteride) thin films has been studied. The thin films were prepared in an ultrahigh vacuum environment and transferred in this environment into a cell where they are exposed to atmospheric pressure high purity argon and then to the aqueous electrolyte. Control experiments were conducted with Auger electron spectroscopy (AES) and quartz-crystal microgravimetry (QCM) to quantitate the oxide that would form on these surfaces prior to immersion in the electrolyte. The onset potential for further oxide formation was displaced from the formal potential for the Ti3+/Ti0 redox couple by ca. +0.5 V. The current/voltage curves showed that the rate of oxide formation was linear with applied potential (39-45 Å/V), consistent with a constant field growth mechanism which has been postulated for the gas-phase oxidation of this type of active metal. The onset potential for oxidation in the current/voltage curves is as close to the formal potential for active dissolution of the metal as is possible to obtain with the present apparatus, without resorting to continuous mechanical abrasion of the metal surface. © 1986 American Chemical Society.
• Burrell, M. C., & Armstrong, N. R. (1986). Oxides formed on polycrystalline titanium thin-film surfaces: Rates of formation and composition of oxides formed at low and high O₂ partial pressures. Langmuir, 2(1), 30-36.
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  Abstract: The room temperature surface oxidation of evaporated titanium thin films was investigated by a quartz crystal microbalance and Auger electron spectroscopy. The initial oxidation reaction was characterized by a linear oxygen adsorption rate and the formation of a TiO2/Ti2O3 layer and ends when ca. three monolayers of oxygen have been adsorbed. Continued oxidation occurs at a logarithmic rate, and predominantly TiO2 is formed. The rate of oxygen adsorption and the limiting oxide thickness were dependent on the constant field growth mechanism proposed by Fehlner and Mott (1970). The results of these studies are used as controls in the investigation of the electrochemical oxidation of titanium-the subject of the following paper in this issue. © 1986 American Chemical Society.
• Maschhoff, B. L., Zavadil, K. R., & Armstrong, N. R. (1986). Reactions of evaporated lithium films with silica surfaces. Applied Surface Science, 27(3), 285-298.
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  Abstract: Reactions of thin films of clean lithium with freshly cleaved silica surfaces have been characterized by X-ray photoelectron spectroscopy (XPS). Reaction rates (near room temperature) are low enough that spectroscopic analysis is possible during the entire course of the reaction. The major product of the reaction is composed of oxidized lithium, a metasilicate (SiO2-3) species, and a unique form of reduced Si, possibly a lithium-rich alloy phase. For confined regions, where the lithium concentration can be made extremely high (as in these thin film depositions) the reaction with the active metal gives a product distribution not predicted by previous thermodynamic arguments. © 1986.
• Maschhoff, B. L., Zavadil, K. R., Nebesny, K. W., Fordemwalt, J. W., & Armstrong, N. R. (1986). PROBLEMS IN QUANTITATION IN X-RAY PHOTOELECTRON SPECTROSCOPY (XPS): THE USE OF DATA REDUCTION TECHNIQUES TO OBTAIN PEAK AREAS.. Proceedings of SPIE - The International Society for Optical Engineering, 690, 112-121.
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  Abstract: Problems in lineshape analysis for quantitation of XPS data on metals are reviewed. Comparison is made of deconvolution versus curve fitting approaches. Deconvolution methods successfully applied to AES data, using electron-energy-loss spectra, are not similarly successful with XPS data. An optimum approach is data fitting using previously formulated lineshapes that account for the large intrinsic energy losses which cause significant distortion of metal core level spectra.
• Nebesny, K. W., & Armstrong, N. R. (1986). Deconvolution of auger electron spectra for lineshape analysis and quantitation using a fast fourier transform algorithm. Journal of Electron Spectroscopy and Related Phenomena, 37(3), 355-373.
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  Abstract: A Fast Fourier Transform algorithm for the deconvolution of electron energy loss processes and instrumental broadening functions from Auger lineshapes is presented. This method uses a scaled electron backscatter spectrum at the energy of the Auger transition to approximate the spectral features in the data due to energy-loss processes and instrumental broadening effects (the sample-instrument response function, SIRF). Auger spectra, following deconvolution of the SIRF, show little variation to changes in the scaling of the low energy loss features of this backscatter spectrum, and little sensitivity to the choice of the truncation point of the SIRF in the frequency domain during apodization. Auger transitions for a series of sulfur oxyanions standards are comparable to transitions expected from independent molecular orbital calculations. The areas of the deconvolved oxygen (KLL) and sulfur (LMM) spectra were used to calculate oxygen-to-sulfur atomic ratios which were then corrected using standard theories for electron escape depth, electron impact ionization cross section and backscattering coefficient. These relative atomic ratios agree with the stoichiometry obtained from the molecular formulas of the standard compounds. © 1986.
• Armstrong, N. R. (1985). An Ultrahigh Vacuum Reaction Chamber/Transfer Mechanism Attached to an Auger Electron Spectrometer for the Study of the Reactions of clean Lithium Surfaces. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1985). Deuterium uptake in titanium thin films: t ssc00621 the effect of oxide, and the metal (Ti and Fe) overlayers. Surface Science.
• Armstrong, N. R. (1985). Evidence for charge trapping at the gold/chlorogallium phthalocyanine interface using photocurrent spectroscopy with one or two illumination sources. Journal of Physical Chemistry.
• Armstrong, N. R. (1985). Photoelectrochemical studies of vanadyl phthalocyanine (VOPc) thin film electrodes. Journal of Electroanalytical Chemistry.
• Armstrong, N. R. (1985). Reactions of Clean Lithium Surfaces with SO2: Molecular Auger Line-Shape Analysis and Reaction Kinetics^t. Langmuir.
• Armstrong, N. R. (1985). Reactions of clean lithium surfaces with SO2, O2 and H2O; auger lineshape analysis and X-ray photoelectron spectroscopic analysis of the initial product layers. Surface Science.
• Armstrong, N. R. (1985). The Au/GaPc-Cl/Ferri, Ferrocyanide/GaPc-Cl/Pt Photoelectrochemical Cell. Journal of the American Chemical Society.
• Armstrong, N. R. (1985). Tri-and Tetravalent Phthalocyanine Thin Film Photoelectrodes: Comparison with Other Metal and Demetallated Phtholocyonine Systems. Journal of the Electrochemical Society.
• Burrell, M. C., & Armstrong, N. R. (1985). Deuterium uptake in titanium thin films: t ssc00621 the effect of oxide, and the metal (Ti and Fe) overlayers. Surface Science, 160(1), 235-252.
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  Abstract: Titanium thin films prepared in UHV were reacted with deuterium (PD2 〈1 × 10-5 Torr) to various loadings, as determined by microgravimetry using a quartz crystal microbalance. The kinetics of deuterium absorption favor a mechanism in which an α-phase surface deuteride forms on the film during the early stages of the reaction, resulting in a constant rate of deuterium uptake during most of the reaction. Surface characterization by AES and ELS, however, demonstrated spectral changes which were dependent on the bulk film stoichiometry. Electron-beam decomposition of the surface deuteride during AES analysis is postulated to explain this results. Oxidation of the titanium film surface caused a decrease in the deuterium absorption rate, completely inhibiting the reaction when oxide of thickness 20 Å or greater were formed. Fresh titanium layers on top of the oxide renewed the ability of the Ti film to take up D2 at the previous rate. Iron adlayers were found to accelerate the D2 absorption rate if Ti films, or to likewise reactive oxidized Ti surfaces. © 1985.
• Buttner, W. J., Rieke, P. C., & Armstrong, N. R. (1985). Evidence for charge trapping at the gold/chlorogallium phthalocyanine interface using photocurrent spectroscopy with one or two illumination sources. Journal of Physical Chemistry, 89(7), 1116-1121.
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  Abstract: Photocurrent vs. wavelength spectra were reported for thin films of chlorogallium phthalocyanine (GaPc-Cl) on optically transparent gold substrates in contact with aqueous hydroquinone solutions. Charge carriers were produced which depending upon the absorptivity of the film and direction of illumination, were localized near either the Au interface or the solution interface. Under positive bias (hole harvesting at the solution interface), with illumination of the Au/Pc interface first, photons near the absorbance maximum in the visible range (600-750 nm) produced charge which was trapped at the Au/Pc interface. This trapping arose because of the formation of a potential well brought about by a mismatch in the Fermi levels of bare gold and the Pc film. The Fermi level of the Pc layer before contact with either phase is ca. 0.5 eV above he valence band edges allowing the formation of space charge layers at both interfaces. The potential well at the Au/Pc interface extends ca 250 nm into the 1-μm-thickness films for bias potentials 200-300-mV positive of the flat-band potential (e.g the equilibrium potential for H2O/BQ couple). Carriers generated by the addition of a second CW light source attenuated the effects of the potential well at the Au/Pc interface and thus enhanced the photocurrent yields from the primary, modulated light source. © 1985 American Chemical Society.
• Buttner, W. J., Rieke, P. C., & Armstrong, N. R. (1985). The Au/GaPc-Cl/ferri,ferrocyanide/GaPc-Cl/Pt photoelectrochemical cell. Journal of the American Chemical Society, 107(12), 3738-3739.
• Klofta, T. J., & Armstrong, N. R. (1985). PHOTOELECTROCHEMICAL STUDY OF p-TYPE VANADYL PHTHALOCYANINE (VOPc) THIN-FILM ELECTRODES.. Electrochemical Society Extended Abstracts, 85-1, 252-.
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  Abstract: Extensive research in this laboratory has focused upon a variety of tri- and tetravalent metal phthalocyanine compounds to be used as electrode materials in possible photovoltaic device applications. The thin-film electrodes with thicknesses on the order of one micron are fabricated from vanadyl phthalocyanine (VOPc) by sublimation upon gold coated, metallized plastic optically transparent electrodes. After numerous attempts in achieving the optimum growth conditions with chlorogallium phthalocyanine, it was found that the most photoactive films were grown at rates which required 24 to 72 hours to prepare a film with a thickness on the order of one micron. The most effective VOPc films were grown at slightly accelerated rates which required approximately 3 hours with thicknesses of the same dimensions. The films were then studied using cyclic voltammetry and a variety of redox couples with and without illumination in order to map out the energetics of the system.
• Klofta, T. J., Rieke, P. C., Linkous, C. A., Buttner, W. J., Nanthakumar, A., Mewborn, T. D., & Armstrong, N. R. (1985). TRI- AND TETRAVALENT PHTHALOCYANINE THIN FILM PHOTOELECTRODES: COMPARISON WITH OTHER METAL AND DEMETALLATED PHTHALOCYANINE SYSTEMS.. Journal of the Electrochemical Society, 132(9), 2134-2144.
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  Abstract: A series of trivalent and tetravalent metal phthalocyanines (Pc) has been examined in thin film form on metal substrates by electron microscopy, UV-visible spectrophotometry, and electrochemical and photoelectrochemical techniques. These Pcs are compared with other divalent metal and demetallated phthalocyanines. The tri- and tetravalent phthalocyanines show enhanced photoresponses vs. the divalent metal and demetallated systems. This enhanced activity correlates with their broadened and red-shifted visible spectra and their tendency to grow as block-like or platelet-like microcrystals in the thin film. The divalent and demetallated Pcs grow as smaller, needle-like deposits with poor photoresponses. This work is of interest to solar cell research.
• Klofta, T., Linkous, C., & Armstrong, N. R. (1985). Photoelectrochemical studies of vanadyl phthalocyanine (VOPc) thin film electrodes. Journal of Electroanalytical Chemistry, 185(1), 73-92.
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  Abstract: Photoactive vanadyl phthalocyanine (VOPc) thin films have been prepared by vacuum deposition on gold metallized plastic optically transparent electrode (MPOTE) substrates. The morphology of the film as determined by scanning electron microscopy (SEM) revealed the existence of two types of crystallites at the surface. Photoelectrochemical studies of these thin films were undertaken using different redox couples in aqueous solution. For redox couples with standard electrochemical potentials negative of +0.33 V vs. Ag/AgCl, positive photopotentials were observed (i.e., the films behave as photocathodes). Those redox couples with E°'s situated positive of +0.33 V exhibited reversible electrochemical behavior independent of illumination at the VOPc electrode. These results suggest a clear p-type semiconducting nature for the VOPc system. Mott-Schottky capacitance data revealed an acceptor density of 6×1018 cm-3 which is speculated to be attributed to the inclusion of dioxygen within the VOPc lattice as supported by X-ray photoelectron spectroscopy (XPS). A power conversion efficiency of 0.1% was observed for the photoassisted evolution of hydrogen which did not require the addition of micro-islands of platinum to the VOPc surface. © 1985.
• Nebesny, K. W., & Armstrong, N. R. (1985). REACTIONS OF CLEAN LITHIUM SURFACES WITH SO//2: MOLECULAR AUGER LINE-SHAPE ANALYSIS AND REACTION KINETICS.. Langmuir, 1(4), 469-477.
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  Abstract: The products of the reaction of SO//2 with clean metal surfaces have been examined using Auger electron spectroscopy. The Auger spectra were deconvolved of sample and instrument broadening effects, which allowed for detailed fingerprint spectra of several sulfur oxyanions to be distinguished. The rate of reaction of SO//2 with the clean Li metal (at low surface coverages) was measured for temperatures between 298 and 357 K. An activation energy of 2-5 kcal/mol was determined for the initial reaction to form the Li//2S/Li//2O monolayer. A model for the growth and stability of the layer is presented.
• Nebesny, K. W., Zavadil, K., Burrow, B., & Armstrong, N. R. (1985). Reactions of clean lithium surfaces with SO₂, O₂ and H₂O; auger lineshape analysis and X-ray photoelectron spectroscopic analysis of the initial product layers. Surface Science, 162(1-3), 292-297.
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  Abstract: The reactions of clean lithium with gas phase oxidants such as SO2, O2 and H2O have been explored as a means of understanding the reactions that lithium undergoes in electrochemical environments. Auger electron spectra for sulfur and oxygen in the SO2 reaction indicated the presence of a lithium sulfide/lithium oxide layer as the product. XPS studies confirm this reaction sequence for SO2, and that complete oxidation occurs to the first monolayer by dissociative chemisorption of SO2 and O2 and H2O. Reaction with SO2 does not completely cease beyond the initial layer, and higher oxidation states of sulfur are formed on top of either the Li2S/Li2O layer (SO2) or on the Li2O layer (O2). Auger lineshapes of the reaction products of SO2, at partial pressures exceeding 10-3 Torr, up to atmospheric pressure, show a mixture of sulfur-oxyanion salts in the product layer. © 1985.
• Rieke, P. C., & Armstrong, N. R. (1985). PULSED LASER PHOTOCOULOSTATIC STUDIES OF THE GOLD/PHTHALOCYANINE FE(CN)//6 OVER BAR **3**/** minus **4 PHOTOCONDUCTOR ELECTRODE SYSTEM.. Electrochemical Society Extended Abstracts, 85-1, 819-.
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  Abstract: The photoconductor system under study was of a metalized plastic, optically transparent electrode. A nonporous polycrystalline layer of chloro-gallium phthalocyanine was sublimed onto this substrate. This electrode was placed in contact with an equimolar solution (1 mM) of Fe(CN)//6 OVER BAR **3**,** minus **4, maintained at a pH of 4. In the pulsed laser photocoulostatic experiments, the electrode was first biased at some initial potential. The emission wavelength of the dye laser closely matched the absorption maxima of the phthalocyanine dye. A band model summarizing the results is shown for an equilibrium bias potential.
• Rieke, P. C., & Armstrong, N. R. (1985). Pulsed laser coulostatic studies of phthalocyanine photoconductor electrodes. Journal of Physical Chemistry, 89(7), 1121-1126.
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  Abstract: The Au/chlorogallium phthalocyanine/Fe(CN)63-,4- photoconductor system was investigated by pulsed dye laser photocoulostatics. By variation of the direction and intensity of illumination, the Schottky barriers formed between the photoconductor dye and the contacting phases could be studied separately. The potential gradients at each interface were found to oppose one another such that a potential well formed in the dye layer, consistent with previous photocurrent spectroscopy studies The depth of this well was quantitated and the nonlinear potential gradients across the film defined for any bias potential This potential well decreases photoelectrochemical efficiencies in the Pc film by the trapping of mobile charges This effect is most pronounced when there is a large difference between the work function of the conductive substrate and the intrinsic Fermi level of the Pc layer. © 1985 American Chemical Society.
• Armstrong, N. R. (1984). Chlorogallium phthalocyanine thin-film photoelectrochemistry. Effect of surface coverage and physical distribution of microcrystallites on the photoelectrochemical response. Journal of Physical Chemistry.
• Armstrong, N. R. (1984). Light-Assisted, Aqueous Redox Reactions at Chlorogallium Phthalocyanine Thin-Film Photoconductors: Dependence of the Photopotential on the Formal Potential of the Redox Couple and Evidence for Photoassisted Hydrogen Evolution. Journal of the American Chemical Society.
• Armstrong, N. R. (1984). Photoelectrochemical Response of GaPc-Cl Thin Film Electrodes Using Two Photon Sources and Two Illumination Directions. Journal of the Electrochemical Society.
• Armstrong, N. R. (1984). Photoelectrochemical studies of thin films of monomer, dimer and trimer stacked-ring silicon phthalocyanines. Journal of Electroanalytical Chemistry.
• Armstrong, N. R. (1984). Quantitative Auger electron spectroscopy and Rutherford backscattering of potassium-implanted silicon, silica and sodium trisilicate. Applications of Surface Science.
• Armstrong, N. R. (1984). Voltammetric, chronoamperometric and chronoabsorptometric studies of the nucleation of n-heptyl viologen films on SnO2, silane-modified SnO2 and ion-beam-treated ito-metallized polymer films. Journal of Electroanalytical Chemistry.
• Burrow, B. J., Armstrong, N. R., Quinn, R. K., Bunker, B. C., Arnold, G. W., & Salmi, D. R. (1984). Quantitative Auger electron spectroscopy and Rutherford backscattering of potassium-implanted silicon, silica and sodium trisilicate. Applied Surface Science, 20(1-2), 167-180.
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  Abstract: Samples of silicon, silica, and sodium trisilicate, which were ion implanted with potassium, were analyzed by Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectroscopy (RBS). Depth profiles obtained by both techniques showed that approximately 50% of the implanted potassium migrated from the subsurface implant region to the surface for both the silica and sodium trisilicate samples. Less migration was detected for the silicon samples. Analyses of the results suggest that some of the ion migration observed in Auger depth profiles is induced by the analysis technique. With this exception, it is demonstrated that a combination of AES and RBS techniques can provide quantitative compositional analyses for altered glass surfaces. © 1984.
• Chan, K. W., & Armstrong, N. R. (1984). FARADAIC IMPEDANCE STUDIES ON THIN-FILM MONOMERIC SILICON PHTHALOCYANINE AND CHLORO-GALLIUM PHTHALOCYANINE ELECTRODES.. Electrochemical Society Extended Abstracts, 84-1, 565-.
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  Abstract: The photo-oxidation of hydroquinone to benzoquinone at thin-film monomeric silicon phthalocyanine and chloro-gallium phthalocyanine electrodes has been studied using the faradaic impedance method. The Randles equivalent circuit was used as a starting point to explain the faradaic impedance behavior of these photoelectrochemical systems. Under illumination, the charge transfer resistance decreased and the double layer capacitance increased. This behavior was consistent with results that have previously been observed by cyclic voltammetry and with the model that was presented in the discussion of photocoulometric measurements on these systems.
• Chan, K. W., & Armstrong, N. R. (1984). PHOTOCOULOMETRIC STUDIES OF THIN-FILM MONOMERIC SILICON PHTHALOCYANINE ELECTRODES.. Electrochemical Society Extended Abstracts, 84-1, 564-.
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  Abstract: The photo-oxidation of hydroquinone to benzoquinone at thin-film monomeric silicon phthalocyanine electrodes has been studied using the photocoulometric method. This technique consists of irradiating an electrochemical cell with a pulsed laser and measuring both the current-time behavior (the photocurrent) and the total amount of charge transfer (the photocharge). A block diagram of the apparatus for these measurements is illustrated. The dependence of the photocharge on several experimental variables was explored. The results of these measurements are summarized below.
• Cieslinski, R., & Armstrong, N. R. (1984). Voltammetric, chronoamperometric and chronoabsorptometric studies of the nucleation of n-heptyl viologen films on SnO₂, silane-modified SnO₂ and ion-beam-treated ito-metallized polymer films. Journal of Electroanalytical Chemistry, 161(1), 59-73.
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  Abstract: Nucleation of the first monolayers of the n-heptyl viologen-bronude or -biphthalate films on SnO2 electrodes was found to be accelerated by the addition of one molecular layer of dimethyldiethoxy (dmde)-silane to the SnO2 surface. Partitioning of the reactantviologen into the non-polar surface layer and lowering of the surface tension against nucleation site growth in the silane layer, may both be responsiblefor the enhanced nucleation rates. Similar acceleration of the nucleation of n-HV+ was found on ion-beam-etched indium-tin oxide-metallized polymer electrodes (MPOTE). A second nucleation site with a much lower; and distinguishable rate of nucleation, was produced on this type of electrode surface. Chronoamperometry and chronoabsorptometry were used to confirm that the nucleation of n-heptyl viologen cation-radical salts follows an instantaneous rather than progressive nucleation mechanism, on any of the surfaces studied. © 1984.
• Mezza, T., Armstrong, N. R., & Kenney, M. (1984). Photoelectrochemical studies of thin films of monomer, dimer and trimer stacked-ring silicon phthalocyanines. Journal of Electroanalytical Chemistry, 176(1-2), 259-273.
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  Abstract: Thin films of silicon phthalocyanines (SiPc) on SnO2 and Au substrates have been studied as photoelectrodes. These SiPc molecules were the same monomer, dimer and trimer stacked-ring Pc's (m-SiPc, d-SiPc and t-SiPc) which had been previously characterized by voltammetry in CH2Cl2 solutions. As thin solid films, the m-SiPc proved to yield high photocurrent quantum efficiencies for oxidation of hydroquinone and ferrocyanide, up to 10% at surface coverages below 40 equivalent monolayers. Transmission spectra and photoaction spectra, using frontside and backside illumination, showed that the m-SiPc films appeared to consist of at least two distinguishable phases on the SnO2 surface where as d-SiPc and t-SiPc films appeared to consist of only one phase at all coverages. d-SiPc and t-SiPc films on SnO2 substrates photoenhanced the reduction of benzoquinone over the oxidation of hydroquinone, while on m-SiPc on SnO2, the hydroquinone oxidation was preferentially photoenchanced. The two phases of m-SiPc on SnO2 appear to be first deposited as a porous, ordered photoactive phase of less than 100 equivalent monolayer thickness, and second as a randomly oriented phase, with lower photoactivity. This last phase filters light incident first on the m-SiPc/electrolyte interface, producing a photocurrent strongly dependent upon illumination direction. © 1984.
• Rieke, P. C., & Armstrong, N. R. (1984). LIGHT-ASSISTED, AQUEOUS REDOX REACTIONS AT CHLOROGALLIUM PHTHALOCYANINE THIN-FILM PHOTOCONDUCTORS: DEPENDENCE OF THE PHOTOPOTENTIAL ON THE FORMAL POTENTIAL OF THE REDOX COUPLE AND EVIDENCE FOR PHOTOASSISTED HYDROGEN EVOLUTION.. Journal of the American Chemical Society, 106(1), 47-50.
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  Abstract: Recent studies have shown that thin films of ordered chlorogallium phthalocyanine (GaPc-Cl) on metal substrates behave as efficient photoconductors in aqueous media, with photopotentials that are dictated by the relative chemical potentials of an electron in the substrate and in the solution redox couple. The magnitudes of the photopotentials are comparable to those seen for photoelectrodes consisting of thin films of a-SiH. When the surface of the phthalocyanine is modified by the addition of submonolayer quantities of platinum, photoassisted hydrogen evolution is observed at an underpotential of 450 mV. The power conversion efficiency of this process is low (less than 0. 1%), compared to that seen on other semiconductor materials, but it encourages further development because of the polycrystalline, organic, and potentially more economic nature of the photoelectrodes employed.
• Rieke, P. C., Linkous, C. L., & Armstrong, N. R. (1984). Chlorogallium phthalocyanine thin-film photoelectrochemistry. Effect of surface coverage and physical distribution of microcrystallites on the photoelectrochemical response. Journal of Physical Chemistry, 88(7), 1351-1357.
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  Abstract: Chlorogallium phthalocyanine (GaPc-Cl) thin films have been prepared by vacuum deposition on gold-metallized plastic substrates. Depending on deposition rate, two types of films resulted: type A GaPc-Cl films, consisting of small crystallites, 100-300 nm in diameter, which completely covered the substrate; and type B films, consisting of larger crystallites, 500-700 nm, isolated on the exposed Au substrate. Photoelectrochemical studies of the redox reactions of hydroquinone and ferrocyanide solutions showed that the type A films behave as photoconductors with a film thickness too small to support an appreciable photopotential, while the type B films can be made to exhibit a photopotential of -220 mV vs. a Pt cathode in the hydroquinone solutions. The difference in behavior of these two chemically equivalent photoelectrodes was attributable to differences in the thickness of the films in relation to the thickness of the space charge regions formed in the film. However, the exposed gold substrate on the type B films was found to diminish photopotentials by acting as a back-reaction site for the solution redox reaction. Photopotentials could be increased on the type B films by electrochemically polymerizing phenol to form an insulating layer over the gold substrate. Photoaction spectra identified holes as the majority charge carriers with an estimated mean free path of 150 nm. © 1984 American Chemical Society.
• Armstrong, N. R. (1983). A sequential method for removing the inelastic loss contribution from Auger electron spectroscopic data. Applications of Surface Science.
• Armstrong, N. R. (1983). Photoactivated Electrolysis on Nonporous Chlorogallium Phthalocyanine Thin Film Electrodes. Journal of the Electrochemical Society.
• Armstrong, N. R. (1983). Stoichiometry and thickness of the initial oxide formed on clean titanium surfaces determined by quantitative Auger electron spectroscopy, electron energy loss spectroscopy, and microgravimetry. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films.
• Armstrong, N. R. (1983). Synthesis, Characterization, and Electrochemistry of the Bis-Bridged Complexes [formula-ommited] and [formula-ommited]. Molecular Structures of [formula-ommited] Where n = 1 and 3. Organometallics.
• Burrell, M. C., & Armstrong, N. R. (1983). A sequential method for removing the inelastic loss contribution from Auger electron spectroscopic data. Applications of Surface Science, 17(1), 53-69.
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  Abstract: A method for removing the contribution of inelastically scattered Auger electrons from an Auger electron spectrum is presented. Each spectral data point is defined as a delta function which is convolved with an inelastic scattering function. The total observed loss contribution at any point in the spectrum is treated as a composition sum of the inelastic loss functions from all spectral data points at higher kinetic energies. The functional form of the individual contributors to the total loss contribution is obtained by utilizing the inelastic portion of electron backscatter spectra taken using primary electron beam energies close to the energy of the spectral feature under consideration. Data treated by this method are shown to be useful for quantification of surface composition and for line shape analysis. The method presented herein offers an alternative to the Van Cittert deconvolution of the entire electron backscatter function from AES data. © 1983.
• Burrell, M. C., & Armstrong, N. R. (1983). STOICHIOMETRY AND THICKNESS OF THE INITIAL OXIDE FORMED ON CLEAN TITANIUM SURFACES DETERMINED BY QUANTITATIVE AUGER ELECTRON SPECTROSCOPY, ELECTRON ENERGY LOSS SPECTROSCOPY, AND MICROGRAVIMETRY.. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 1(4), 1831-1836.
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  Abstract: The initial oxidation of a clean polycrystalline titanium film was studied by Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS), while simultaneously monitoring the total oxygen uptake using the quartz crystal microbalance (QCM). Quantitation of the O KLL and Ti LMM AES intensities reveals that the surface stoichiometry at the initial stages of oxidation ( less than 100 L O//2 at 1 multiplied by 10** minus **5 Torr) corresponds to that expected for Ti//2O//3, and the ELS results also suggest the presence of the lower oxidation states of titanium ( plus 2, plus 3) on the surface. The microgravimetric results indicate that the oxidation is not confined to the region sampled by the electron spectroscopic methods, and a surface layer of Ti//2O//3 whose thickness is approximately 40 A is consistent with the observed combination of results.
• Linkous, C. A., & Armstrong, N. R. (1983). PHOTOELECTROCHEMICAL COMPARISON OF CHLORO-GALLIUM PHTHALOCYLANINE TO OTHER PHTHALOCYANINE DERIVATIVES.. Electrochemical Society Extended Abstracts, 83-1, 792-793.
• Linkous, C., Klofta, T., & Armstrong, N. R. (1983). PHOTOACTIVATED ELECTROLYSIS ON NONPOROUS CHLOROGALLIUM PHTHALOCYANINE THIN FILM ELECTRODES.. Journal of the Electrochemical Society, 130(5), 1050-1055.
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  Abstract: Various metal and semiconductor substrates demonstrated greatly enhanced activity toward the electrolysis of the benzoquinone (Q) hydroquinone (H//2Q) redox couple when coated with multimolecular layers of chlorogallium phthalocyanine (GAPc-Cl) and irradiated with visible wavelength light. These electrodes were nearly reversible toward H//2Q in the light, but were quite inert in the dark, even suppressing corrosion of the metal when silver substrates were used. Exchange current densities on GaPc-Cl/Au electrodes irradiated with approximately 100 mW/cm**2 polychromatic light were ca. 10**3 times greater than on plain Au. The improved voltammetric response of these electrodes over other Pc film electrodes are attributable in part to the sublimation conditions employed, distinguished primarily by a deposition rate of tens of angstroms per minute or less. Varying degrees of film porosity were obtained as evidenced by cyclic voltammetry and x-ray photoelectron spectroscopy.
• Rieke, P. C., Linkons, C. L., & Armstrong, N. R. (1983). PHOTOELECTROLYSIS AT ORDERED POLYCRYSTALLINE GaPc-Cl THIN FILM SURFACES.. Electrochemical Society Extended Abstracts, 83-1, 797-.
• Wright, M. E., Mezza, T. M., Nelson, G. O., Armstrong, N. R., Day, V. W., & Thompson, M. R. (1983). Synthesis, characterization, and electrochemistry of the bis-bridged complexes Me₂Si[η⁵-C₅H₄Fe(CO)] ₂[Ph₂P(CH₂)_n PPh₂] (n = 1-3) and [Me₂Si[(η⁵-C₅H₄) ₂Fe₂(CO)₃]]₂[Ph .... Organometallics, 2(12), 1711-1718.
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  Abstract: Full title: Synthesis, characterization, and electrochemistry of the bis-bridged complexes Me2Si[η5-C5H4Fe(CO)] 2[Ph2P(CH2)n PPh2] (n = 1-3) and [Me2Si[(η5-C5H4) 2Fe2(CO)3]]2[Ph2P(CH 2)nPPh2] (n = 2, 3). Molecular structures of Me2Si[η5-C5H4Fe(CO)] 2[Ph2P(CH2)nPPh2] where n = 1 and 3. Photolysis of Me2Si[η5-C5H4Fe(CO) 2]2 (1) with a series of bis(phosphines) (L = Ph2P(CH2)nPPh2, where n = 1, 2, and 3) leads to the formation of substituted dinuclear and tetranuclear compounds of the form Me2Si[η5-C5H4Fe(CO)] 2[L] and [Me2Si[(η5-C5H4) 2Fe2(CO)3]]2[L], respectively. The ratio of these two types of compounds is dependent upon the ligand size (number of methylene units), stoichiometry, and the overall concentration employed. With the small ligands and dilute conditions, formation of the dinuclear product is favored. Under more concentrated reaction conditions and when an excess of 1 is used, the tetranuclear type of product becomes dominant. The electrochemistry of the dinuclear compounds indicate that both the silyl and the bis(phosphine) bridges are capable of stabilizing a two-electron oxidation product. This is demonstrated in the case of L = dppm where we observe, by fast-scan cyclic voltammetry, two, one-electron reversible oxidation steps. The structure of Me2Si[η5-C5H4Fe(CO)] 2[dppm] (3a) is determined by X-ray diffraction. It crystallizes in the space group Pnma (No. 62) with a = 13.754 (4) Å, b = 17.707 (4) Å, c = 13.771 (3) Å and α, β, and γ = 90.00° with Z = 4. The structure was refined to R1 = 0.041 and R2 = 0.045 for 2226 independent reflections having I > 3σ(I). The structure of Me2Si[η5-C5H4Fe(CO)] 2[dppp] (3c) is also determined by X-ray diffraction. It crystallizes in the space group P1 (No. 2) with a = 11.638 (11) Å, b = 11.833 (11) Å, c = 16.289 (14) Å, α = 111.22 (6)°, β = 72.67 (7)°, and γ = 107.02 (7)° with Z = 2. The structure is refined to R1 = 0.067 and R2 = 0.076 for 4115 independent reflections having I > 3σ(I). The overall geometry of 3a compared to 3c implies that some distortion occurs from the steric requirements of the larger dppp ligand. This includes a significant lengthening of the Fe-Fe bond in complex 3c by 0.024 (3) Å relative to 3a. © 1983 American Chemical Society.
• Armstrong, N. R. (1982). Data Acquisition and Processing Modes for Quantitative Auger Electron Spectroscopy. Analytical Chemistry.
• Armstrong, N. R. (1982). Photoelectrolysis using linear arrays of chemically modified semitransparent electrodes. Journal of Electroanalytical Chemistry.
• Armstrong, N. R. (1982). Reactions of Lithium Surfaces with Gas-Phase and Solution Phase SO2. Journal of the Electrochemical Society.
• Armstrong, N. R. (1982). Voltammetric characterization of gold, metallized-plastic electrodes, following exposure to ion beams or RF plasmas. Journal of Electroanalytical Chemistry.
• Armstrong, N. R., & Shepard Jr., V. R. (1982). Photoelectrolysis using linear arrays of chemically modified semitransparent electrodes. Journal of Electroanalytical Chemistry, 131(C), 113-120.
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  Abstract: Chemical modification of semitransparent electrodes with various dye molecules allows photoassisted electrolysis to occur with visible wavelength excitation and without an optically dense solution of the dye adjacent to the electrode. Linear, multi-electrode stacks of the semitransparent electrodes allow for more efficient capture of light normally lost by transmission. A ratio of photocurrent to photon flux ( nc np) of ca. 1-2% is possible for certain dye modified electrode stacks, when used in the photoassisted oxidation of hydroquinone. Stacks of electrodes with more than one bound dye allow for the use of more of the visible wavelength spectrum than with a single dye. © 1982.
• Armstrong, N. R., & White, J. R. (1982). Voltammetric characterization of gold, metallized-plastic electrodes, following exposure to ion beams or RF plasmas. Journal of Electroanalytical Chemistry, 131(C), 121-136.
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  Abstract: Voltammetric analysis for the formation and reduction of gold oxide was used to probe surface compositional changes caused by argon-ion beams or oxygen RF plasmas. Gold, metallized plastic film electrodes were explored because of their reproducible surface composition and electrochemical characteristics, and because of our interest in exploiting the carbonaceous composition of their surface. Low doses of ions (
• Burrell, M. C., Kaller, R. S., & Armstrong, N. R. (1982). DATA ACQUISITION AND PROCESSING MODES FOR QUANTITATIVE AUGER ELECTRON SPECTROSCOPY.. Analytical Chemistry, 54(14), 2511-2517.
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  Abstract: Auger electron spectroscopy has been applied to the quantitative surface analysis of a series of metals (Ag, Cd, In, Sn), indium-tin alloys, indium and tin oxides, and indium-tin oxide (ITO) films. Spectra were obtained by two methods: (1) the conventional modulation technique, which results in the derivative spectrum, and (2) a direct current measurement, which gives an undifferentiated spectrum. For data collected in the latter mode, an instrumental approach and secondary electron background correlation are discussed. Quantitative results obtained by using this approach are shown to be more accurate than the traditional measurement method.
• Mezza, T. M., Armstrong, N. R., W., G., Iafalice, J. P., & Kenney, M. E. (1982). Electrochemical studies on stacked-ring silicon phthalocyanines. Journal of Electroanalytical Chemistry, 137(2), 227-237.
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  Abstract: Voltammetric and spectroscopic data on monomeric, dimeric, and trimeric t-butyldimethylsilyl silicon phthalocyanines (SiPc) are compared. These phthalocyanines are bonded through axial OSiO linkages with an interplanar spacing of about 0.34 nm. The through-space orbital interactions which result from the cofacial orientation of the SiPc rings causes a large stabilization of the one-electron oxidation product of the dimer and trimer SiPc compared to the monomer SiPc and a slight destabilization of their one-electron reduction products. The electrochemical reactions have significance in correlation with the conductivity, photoconductivity, and solid-state orientation of phthalocyanines which aggregate or polymerize. © 1982.
• Armstrong, N. R. (1981). CHEMICALLY MODIFIED ELECTRODES FOR PHOTOELECTROCHEMICAL AND ELECTROCHROMIC REACTIONS.. American Chemical Society, Division of Petroleum Chemistry, Preprints, 26(3), 757-.
• Armstrong, N. R. (1981). Characterization of the materials comprising the reactive interfaces in the li(si)/fes2 primary battery. Journal of the Electrochemical Society.
• Armstrong, N. R. (1981). Differential-capacitance studies of silane-modified SnO2 electrodes at low modulation frequencies. Journal of Physical Chemistry.
• Armstrong, N. R. (1981). Improved photoelectrochemical efficiencies at phthalocyanine-modified SnO2 electrodes. Journal of Electroanalytical Chemistry.
• Armstrong, N. R., & Shepard Jr., V. R. (1981). Differential-capacitance studies of silane-modified SnO₂ electrodes at low modulation frequencies. Journal of Physical Chemistry, 85(20), 2965-2970.
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  Abstract: Differential-capacitance studies, at low modulation frequencies, were conducted on a variety of silane-modified SnO2 electrodes. Silanes with amine functionality, when added to the SnO2 surface, perturb the ionic population of the gel-like surface layer. This phenomenon and the nonideal capacitance behavior observed for SnO2 electrodes at low modulation frequencies can be explained by the presence of a surface-state capacitance term in parallel with the normal space-charge capacitance. © 1981 American Chemical Society.
• Burrow, B. J., Nebesny, K. W., Armstrong, N. R., Quinn, R. K., & Zurawski, D. E. (1981). CHARACTERIZATION OF THE MATERIALS COMPRISING THE REACTIVE INTERFACES IN THE Li(Si)/FeS//2 PRIMARY BATTERY.. Journal of the Electrochemical Society, 128(9), 1919-1926.
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  Abstract: Photomicroscopic, electron-microscopic, and surface analytical studies have been conducted on Li(Si)LiCl multiplied by (times) KCl/FeS//2 electrochemical cells before and after discharge, and on individual anode and cathode components. Physical and chemical compositional changes were observed at both the anode and cathode interface that have significance in the discharge mechanism. Oxygen-rich species were found at the surface of the FeS//2 cathode particles and migration of sulfur away from the cathode during discharge was also observed. Thermal shock of FeS//2 single crystals confirmed the tendency for sulfur to move away from the bulk with the application of heat to the surface. Exposure of the Li(Si) alloy to atmosphere and/or thermal shock resulted in the formation of a lithium-rich surface in the carbonate or oxide form.
• Mezza, T. M., Linkous, C. L., Shepard, V. R., Armstrong, N. R., Nohr, R., & Kenney, M. (1981). Improved photoelectrochemical efficiencies at methalocyanine-modified SnO₂ electrodes. Journal of Electroanalytical Chemistry, 124(1-2), 311-320.
• Quinn, R. K., Zurawski, D. E., & Armstrong, N. R. (1981). ELECTROCHEMICAL AND SURFACE ANALYSIS OF THE REACTIVE INTERFACES IN THE Li(Si)-FeS//2 PRIMARY BATTERY.. Power Sources: Research and Development in Non-Mechanical Electrical Power Sources, 305-321.
• Armstrong, N. R. (1980). Voltammetric studies of silane-modified SnO2 surfaces in selected aqueous and non-aqueous media. Journal of Electroanalytical Chemistry.
• Armstrong, N. R., & Shepard Jr., V. R. (1980). Voltammetric studies of silane-modified SnO₂ surfaces in selected aqueous and non-aqueous media. Journal of Electroanalytical Chemistry, 115(2), 253-265.
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  Abstract: Voltammetric studies in selected aqueous and non-aqueous media were conducted for a series of silane-modified SnO2 electrodes which had been further modified with a variety of metal phthalocyanines and metal cations. It is shown that tetrasulfonated, transitionmetal phthalocyanines can be both adsorbed and covalently attached to silane-modified surfaces where the silane has a coordinating ligand functionality such as a primary or secondary amine. It is further shown that the presence of the silane on the SnO2surface causes the appearance of a reversible voltammetric process following exposure of the surface to solution cation species. This reversible voltammetric wave is hypothesized to be due to the reduction of a surface bound Sn(IV) species to Sn(II) in a strongly hydroxylated environment. © 1980.
• Blasse, G., Fromherz, P., Armstrong, N. R., Meyerstein, D., Goodenough, J. B., Mackor, A., Grätzel, M., Albery, W. J., Archer, M. D., Chamberlain, G. A., Ginley, D. S., Bockris, J., Groot, M. D., Peter, L. M., Faulkner, L. R., Richoux, M. C., Pichat, P., Gerischer, H., Foulds, A. W., , Bartlett, P. N., et al. (1980). General discussion. Faraday Discussions of the Chemical Society, 70, 403-427.
• Cieslinski, R. C., & Armstrong, N. R. (1980). VOLTAMMETRIC AND ELECTROCHROMIC BEHAVIOR OF N-HEPTYLVIOLOGEN ON CHEMICALLY MODIFIED METAL OXIDE ELECTRODES.. Journal of the Electrochemical Society, 127(12), 2605-2610.
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  Abstract: Voltammetric and spectroelectrochemical results are presented for the one-electron reduction of n-heptylviologen dibromide on clean and silane-modified, tin oxide, and on indium tin oxide metallized-plastic optically transparent electrodes (MPOTE). The amount of n-heptyviologen dibromide reduced and the rate of coloration due to the formation of the cation-radical film can strongly be influenced by the state of the electrode surface. The initial 200-500 msec of viologen deposition are not mass transport controlled. Calculations for the amount of viologen deposited vary, depending on whether a spectrophotometric or coulometric estimated is used. Spectrophotometric data indicate that the amount of viologen deposited in the first 200 msec of chronoamperometric reduction from a concentrated viologen solution is ca. 10-14 monolayers and is increased at least 60% by the addition of a monolayer of the silane modifier to the electrode surface.
• Quinn, R. K., & Armstrong, N. R. (1979). SURFACE ANALYSIS OF SIGNIFICANT INTERFACES IN THE Li(Si)/FeS//2 BATTERY SYSTEM.. Journal of Geophysical Research, 79-2, 225-229.
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  Abstract: Studies of the composition of the interfaces in the anode and cathode regions of lithium alloy/metal sulfide battery systems should lead to a clearer understanding of overall battery performance. Using techniques developed for studies of other active metals and their oxides, a study was made of these critical interfaces in order to correlate the results to battery performance data. The electrochemical system of interest for the thermally-activated batteries consists of a lithium alloy anode, a potassium chloride-lithium chloride electrolyte with binder material, and an iron disulfide cathode material. The results of studies by electron and photomicroscopy, electron microprobe (EM) analysis, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) for the anode/electrolyte-binder (EB) interface and the cathode/electrolyte-binder interface are discussed. The results are outlined according to the two active electrochemical components for various states of discharge.
• Shepard Jr., V. R., & Armstrong, N. R. (1979). Electrochemical and photoelectrochemical studies of copper and cobalt phthalocyanine-tin oxide electrodes. Journal of Physical Chemistry, 83(10), 1268-1276.
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  Abstract: Tetrasulfonated copper and cobalt phthalocyanines have been bound to SnO2 electrode surfaces modified by either (γ-aminopropyl)triethoxysilane or 3-(N-(2-aminoethyl)aminopropyl)trimethoxysilanes. The electrochemical behavior of the surface bound dye was compared with that of the solution and absorbed forms at SnO2 in Me2SO and aqueous media. A stable reversible redox couple was seen on the cobalt phthalocyanine electrodes/SnO2 in aqueous media. Concentrations of surface bound phthalocyanines were easily detected by monitoring the Cu(2p1/2,3/2) and N(1s) X-ray photoelectron transitions. SnO2 electrodes with bound phthalocyanine and SnO2 electrodes in contact with solutions of phthalocyanines showed similar photoelectrochemical response to the oxidation of oxalate or ascorbate. © 1979 American Chemical Society.
• Hawn, D. D., & Armstrong, N. R. (1978). Electrochemical adsorption and covalent attachment of erythrosin to modified tin dioxide electrodes and measurement of the photocurrent sensitization to visible wavelength light. Journal of Physical Chemistry, 82(11), 1288-1295.
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  Abstract: Erythrosin (2,4,5,7-tetraiodofluorescein) has been attached to SnO2 electrode surfaces either through electrochemical adsorption or covalent attachment. SnO2 electrodes modified with mercaptopropyltrimethoxysilane, 3-(2-aminoethyl)propyltriethoxysilane, or γ-aminopropyltrimethoxysilane showed a considerable enhancement of the currents passed during oxidation of erythrosin and a deposition of several monolayers of active chromophore. Erythrosin was covalently attached to the silane-modified SnO2 surfaces by means of an amide bond or by the formation of a thiol bond. Small surface concentrations of the adsorbed and covalently attached dye were easily detected by monitoring the I(3d3/2,5/2) x-ray photoelectron transitions. Sensitization of the SnO2 current/voltage response to visible wavelength light was observed for both the adsorbed and covalently attached dye electrodes. Enhanced stability and efficiency of the photocurrent response was observed for the covalently attached vs. adsorbed dye molecules. © 1978 American Chemical Society.
• Quinn, R. K., & Armstrong, N. R. (1978). ELECTROCHEMICAL AND SURFACE ANALYTICAL CHARACTERIZATION OF TITANIUM AND TITANIUM HYDRIDE THIN FILM ELECTRODE OXIDATION.. J Electrochem Soc, 125(11), 1790-1796.
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  Abstract: The oxidation of the active, high energy density materials, titanium and titanium hydride, has been studied by electrochemical and electron spectroscopic techniques. The apparent thickness of the oxide was controlled by the magnitude of the applied potential across thin (200-6000 A) films of these materials. Our electrochemical measurements indicated that after irreversible oxide formation, the thin films behaved as typical semiconducting oxide electrodes. A technique of electrochemical aging is described which permits comparison with oxide thicknesses produced by thermal aging. Post mortem analysis by XPS and AES was used to characterize the extent of oxidation of the film surface. The results of the experiments are compared for titanium and titanium hydride and are examined with respect to a model for oxide formation and growth.
• Sayers, C. N., & Armstrong, N. R. (1978). X-ray photoelectron spectroscopy of TiO₂ and other titanate electrodes and various standard Titanium oxide materials: Surface compositional changes of the TiO₂ electrode during photoelectrolysis. Surface Science, 77(2), 301-320.
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  Abstract: Surface compositional changes were observed for TiO2 single crystal electrodes used for photoelectrolysis of water. Surface stoichiometries of several types of TiO2, SrTiO3 and BaTiO3 electrodes were characterized by XPS and compared with a variety of titanium, titanium oxide and titanium hydride standard materials. Reduction of the electrode surface in a hydrogen atmosphere results in an oxygen deficient surface composition. Photoelectrolysis at current densities of 10-15 mA cm2 for periods up to 8 h appears to return the electrode surface to a nearly stoichiometric oxygen-to-metal ratio. Reduction of the titanium oxide surfaces was also observed by exposure to an argon ion beam. Analysis of the electrode surface by a combination of XPS and ion-sputter profiling was still possible by simultaneous analysis of standard materials. © 1978.
• Armstrong, N. R., & Quinn, R. K. (1977). Auger and X-ray photoelectron spectroscopic and electrochemical characterization of titanium thin film electrodes. Surface Science, 67(2), 451-468.
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  Abstract: Auger (AES) and X-ray photoelectron spectroscopic (XPS) characterizations of electrochemically oxidized titanium are described. Surface oxides on thin (200-250 Å) vacuum deposited titanium films were formed under conditions of linear potential scan in 1 N KClO4, 1 N HClO4 and 1 N H2SO4. Current/voltage, capacitance/voltage and surface conductance/voltage relationships confirmed the irreversible formation of the surface oxide at thickness of 20-30 Å/V, for low applied potentials. Post moretem analysis of the thin films by AES and XPS indicated a mixture of metal and metal oxides (TiO2, Ti2O3, TiO) on each surface, with the higher oxide states predominating on the electrochemically oxidized films. Observation of the LIIIM2,3M4,5, N(E) signal shape in the Auger spectra of the potentially oxidized oxidized films showed a suboxide TiO-like surface rather than an TiO2 surface state. Deconvolution of the Ti(2p 1 2, 3 2) XPS spectra confirmed the coexistence of multiple oxidation states of Ti during electrochemical or atmospheric oxidation of the films. Ion sputtering of each surface was used to characterize the subsurface metal/metal oxide composition and to correlate the oxygen to metal atomic ratio with electrochemical pretreatment. © 1977.
• Armstrong, N. R., Qulnn, R. K., & Vanderborgh, N. E. (1977). Spectroelectrochemical investigations of the reduction of benzaldehyde and p-cyano-and p-phenylbenzaldehyde in sulfolane. Journal of Physical Chemistry, 81(7), 657-659.
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  Abstract: Spectroelectrochemical investigations are presented for the reduction of benzaldehyde and p-cyano- and p-phenylbenzaldehyde in the solvent sulfolane. Two reduction intermediates were observed for the p-cyanobenzaldehyde reduction. One intermediate was the radical anion which was consumed in a dimerization reaction, k2 = 85-88 M-1 s-1. The other intermediate was the electrochemically inactive complex between the radical anion and the unreduced parent. The verification of this complexation clarifies previous voltammetric studies of this type of reduction mechanism.
• Knotek, M. L., Quinn, R. K., Armstrong, N. R., & Vanderborgh, N. E. (1977). AUGER ELECTRON SPECTROSCOPIC DEPTH PROFILING TECHNIQUES APPLIED TO ULTRATHIN ELECTROCHEMICALLY DEPOSITED METAL LAYERS.. J Vac Sci Technol, 14(2), 705-710.
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  Abstract: The Auger electron spectroscopic depth profiling technique is applied to the study of the initial equivalent monolayer of electrodeposited Ag on thin-film Pt electrodes. The ultimate depth resolution of this technique is determined by examining ultrathin layers of Ag which have been ion deposited from a zeolite source in ultrahigh vacuum. The exponential decay apparent in the examination of such layers is explained from elementary considerations of the sputtering process. These are seen to be the resolution-limiting factor for these layers. The depth resolution is shown to be of the order of 5 Angstrom. Examination of monolayer and submonolayer films of Ag electrodeposited from an AgClO//4/sulfolane solution shows that there is an adsorbed overlayer of S (of solvent origin), followed by the electrodeposited Ag film, which is rich in Cl(of solute origin), followed by the Pt substrate, which also contains high levels of Cl.
• Lin, A. W., Armstrong, N. R., & Kuwana, T. (1977). X-RAY PHOTOELECTRON/AUGER ELECTRON SPECTROSCOPIC STUDIES OF TIN AND INDIUM METAL FOILS AND OXIDES.. Analytical Chemistry, 49(8), 1228-1235.
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  Abstract: X-ray photoelectron spectroscopy (XPS or ESCA) and Auger electron spectroscopy (AES) have been applied to the surface analysis of ″standards″ of metal foils and oxides of two elements, tin and indium. For the metal foils, the surface was initially in an oxidized state which could be removed by argon ion sputtering to reveal the pure metal. On the surfaces of the tin and indium foils, the oxygen to metal atomic ratios were close to the expected values for SnO//2 and In//2O//3. These ratios changed with depth profiling until the pure metal was exposed. The energetics and the atomic ratios were also obtained for several powder samples of SnO//2, SnO, and In//2O//3 standards. The problems and approaches to the identification of elemental composition and to quantification of oxygen to metal atomic ratios are discussed.
• Armstrong, N. R., Lin, A. W., Fujihira, M., & Kuwana, T. (1976). ELECTROCHEMICAL AND SURFACE CHARACTERISTICS OF TIN OXIDE AND INDIUM OXIDE ELECTRODES.. Analytical Chemistry, 48(4), 741-750.
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  Abstract: The electrochemical characteristics of heavily doped tin oxide and indium oxide thin film electrodes have been correlated with results of surface analyses by x-ray photoelectron (ESCA) and Auger spectroscopy. From measurement of current, capacitance, and surface conductance as a function of the applied electrode potential, regions of potential where surface reactions were possibly occurring could be delineated. ESCA/Auger analyses of these electrodes, which were poised in various potential regions, confirmed the changes in the stoichiometry of the metal oxides at the surface. Similar analyses were performed on tin oxide surfaces which had been modified by derivatization of the surface. Such modifications resulted in the lowering of the effective carrier density of the surface.
• Armstrong, N. R., Vanderborgh, N. E., & Quinn, R. K. (1976). The electrochemistry of nitrobenzene and p-nitrobenzaldehyde studied by transmission spectroelectrochemical methods in sulfolane. Journal of Physical Chemistry, 80(25), 2740-2745.
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  Abstract: Transmission spectroelectrochemistry experiments have been used to study the electrochemical reductions of nitrobenzene and p-nitrobenzaldehyde in sulfolane. The reduction intermediates of both these aromatic compounds are sufficiently long-lived in sulfolane to be amenable to study by these techniques. Nitrobenzene undergoes a single one-electron, chemically reversible reduction at Ep,c = -1.875 V vs. AgRE. This was verified by analysis of the visible absorption spectrum for the reduction species; two maxima were observed at 348 nm (ε 5.7 × 103 M-1 cm-1) and 465 nm (ε 4.1 × 103 M-1 cm-1). p-Nitrobenzaldehyde undergoes two one-electron reductions (Ep,1 = -1.528 V, Ep,2 = -2.114 V vs. AgRE), which are accompanied by following chemical reactions. A mechanism is shown involving formation of a parent-radical anion complex and an unusually low reproportionation reaction rate (kf = 5.4 ± 0.7 × 102 M-1 s-1 at 50°C). Verification of the proposed mechanism is assisted by digital simulation techniques and a newly developed differential treatment of the spectroelectrochemical data.
• Quinn, R. K., Knotek, M. L., Armstrong, N. R., & Vanderborgh, N. E. (1976). STUDIES OF THE Ag-Pt INTERFACE FORMED BY Ag DEPOSITION FROM SULFOLANE SOLUTIONS ON THIN FILM ELECTRODES.. Journal of the Electrochemical Society, 123(12), 1794-1799.
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  Abstract: Vapor deposition, thin film fabrication techniques have been applied to the preparation of Pt electrodes of various topographies. Large variations were observed in the electrical resistivities of these electrodes as a function of porosities. The effect of these film preparation techniques on Ag electrodeposition from solutions of AgClO//4 in sulfolane was examined. Roughness factors, obtained from differential capacitance measurements, varied from 1. 0 to 32. 8 over the range from bulk Pt to the most porous films. Analysis of cyclic voltammetric data and double potential step chronocoulometric results indicate silver deposition occurs without underpotential deposition in sulfolane in contrast to behavior found in aqueous medium.
• Armstrong, N. R., Vanderborgh, N. E., & Quinn, R. K. (1975). ROTATING RING-DISK ELECTRODE STUDIES OF THE ELECTROCHEMISTRY OF AROMATIC CARBONYL COMPOUNDS IN THE SOLVENT SULFOLANE.. Journal of the Electrochemical Society, 122(5), 615-619.
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  Abstract: Benzaldehyde undergoes an EC reduction mechanism with an anion dimerization as the following reaction. Digital simulation techniques give values of the second order rate coefficients of k//2 equals 2. 4 multiplied by 10**3 liters/mole-sec. Para-cyanobenzaldehyde undergoes a mixed reduction mechanism. At low disk currents in the reduction intermediate is proposed to be a parent-anion complex. At higher fluxes, dimerization becomes the significant following process; k//2 equals 45 liters/mole-sec. A single electroactive intermediate is observed for the first one-electron reduction of p-nitrobenzaldehyde. The single, chemically, reversible redox mechanism for this compound in sulfolane agrees with spectroelectrochemical results.
• Armstrong, N. R., Quinn, R. K., & Vanderborgh, N. E. (1974). Voltammetry in sulfolane: The electrochemical behavior of benzaldehyde and substituted benzaldehydes. Analytical Chemistry, 46(12), 1758-1764.
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  Abstract: The electrochemical reduction of a series of para-substituted benzaldehydes in the aprotic solvent sulfolane has been explored by linear sweep and cyclic voltammetry. A single one-electron process is seen for the carbonyl reduction. The potential for the benzaldehyde reduction is shifted cathodically from previously reported potentials in other solvents and the chemical reaction following reduction is slow enough so that the reoxidation of the benzaldehyde intermediate is detectable at a sweep rate of 83.3 mV/sec. Benzaldehydes with an inductive substituent show an enhanced stability of the reduction intermediate. Rate coefficients for the disappearance of the intermediate are presented (e.g., k2 for the dimerization following reduction of benzaldehyde = 2.4 × 103 l./m-sec). The contribution of the dielectric constant, and low Bronsted acidity of sulfolane to this reduction behavior is discussed.
• Quinn, R. K., Armstrong, N. R., & Vanderborgh, N. E. (1974). ELECTROCHEMISTRY AT THIN SOLID FILMS.. J Vac Sci Technol, 12(1), 160-168.
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  Abstract: Application of thin deposited films to spectroelectrochemical studies are reviewed. Simultaneous spectroscopic and voltametric results are shown for Pt films deposited under various sputtering conditions. Results are presented on the effect of the electrode (Pt, Pd, and Au) surface structure and conditions on charge transfer rates of a model compound in organic electrolytes. The application of these studies to catalytic and electrocatalytic processes is discussed. the interaction of electrodeposited Ag with vapor deposited Pt films has been studied.
• Vanderborgh, N. E., Armstrong, N. R., & Spall, W. D. (1970). A cryoscopic study of the association of phenolic compounds in benzene. Journal of Physical Chemistry, 74(8), 1734-1741.
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  Abstract: The cryoscopic behavior of phenol, positional isomers of chlorophenol and cresol, and 2,5-, 2,6-, 3,4-, and 3,5-dimethyl phenol were studied in benzene, and equilibrium constants describing this behavior in terms of association were determined for the concentration range 0-0.8 m. The results indicate that substitution of a ring hydrogen of phenol by either chlorine or methyl decreases the amount of association of the parent phenol, chloro isomers having less association than methyl isomers. The degree of association is qualitatively related to the effects of the substituents on the π electron cloud of the phenyl ring.

Proceedings Publications

• Armstrong, N. R. (2013). Critical interfaces in emerging thin film photovoltaic technologies: From basic science to policy and economic issues. In Optics InfoBase Conference Papers.
• Armstrong, N. R. (2012). Doping effect on chloroindium phthalocyanine (ClInPc)/C60 solar cells. In Materials Research Society Symposium Proceedings.
• Armstrong, N. R. (2012). Understanding energy level alignment in PCDTBT:PC70BM solar cells. In Conference Record of the IEEE Photovoltaic Specialists Conference.
• Armstrong, N. R. (2011). Interface science of emerging organic solar cell platforms: Electrochemical and spectroelectrochemical approaches to the characterization of interfacial charge transfer processes. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2011). The interface science of interlayer materials and contacts in organic solar cells. In Conference Record of the IEEE Photovoltaic Specialists Conference.
• Armstrong, N. R. (2010). Advances in potential-modulated attenuated total reflectance (PM-ATM) spectroscopy for characterization of transparent metal oxide/semiconductor interfaces relevant to solar energy conversion. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2010). Electrodeposited poly(thiophene) thin film contacts for organic photovoltaics and organic light emitting diodes. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2010). Polymers and composites for efficient production storage and utilization of energy. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2010). Small molecule organic solar cells with enhanced near-IR photoactivity: The role of texturing and molecular architecture of the active layers on solar cell performance. In Conference Record of the IEEE Photovoltaic Specialists Conference.
• Armstrong, N. R. (2009). Colloidal polymerization of polymer coated ferromagnetic nanoparticles: Chemistry on the mesoscale for energy storage and conversion. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2009). Waveguide-based chemical and spectroelectrochemical sensor platforms. In ECS Transactions.
• Armstrong, N. R. (2008). A planar integrated photometer/refractometer using an organic light emitting diode light source and an organic photovoltaic detector. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (2008). Critical interfaces in new solar cell materials: Organic heterojunctions and heterojunctions involving semiconductor nanoparticles. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (2008). Electrochemically formed semiconductor nanoparticle/polymer composite thin films. In ACS National Meeting Book of Abstracts.
• Armstrong, N. R. (2008). Photovoltaic devices created from electrodeposited "nano- textured" poly(thiophene) films. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (2006). Discotic mesophase materials: Their use in organic field-effect transistors and organic photovoltaics. In Proceedings - Electrochemical Society.
• Armstrong, N. R. (2004). Modeling of organic photovoltaic cells with large fill factor and high efficiency. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (2004). Patterned deposition of tobacco mosaic virus on mica surfaces. In Materials Research Society Symposium Proceedings.
• Armstrong, N. R. (2000). Ordered thin films of perylenetetracarboxylicdianhydride-bisimide and bis-(N-alkyl)-quinacridone dyes. In Materials Research Society Symposium - Proceedings.
• Armstrong, N. R. (2000). Self-assembly processes for organic led transport layers and electrode passivation. In Materials Research Society Symposium - Proceedings.
• Armstrong, N. R. (1999). Approaches to organic light-emitters via layer-by-layer self-assembly. In American Chemical Society, Polymer Preprints, Division of Polymer Chemistry.
• Armstrong, N. R. (1999). Determination of the electronic structure of organic Schottky contacts by photoemission spectroscopy. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1999). Hybrid bilayer organic light-emitting devices based on high Tg hole transport polymers. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1999). Organic electroluminescent devices: aluminum alkali-halide composite cathode for enhanced device performance. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1999). Self-assembly processes for organic LED transport layers and electrode passivation. In Materials Research Society Symposium - Proceedings.
• Armstrong, N. R. (1998). Effects of insulating layers on the performance of organic electroluminescent devices. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1998). Electrochemical processes of the polyphthalocyaninatosiloxanes and related cofacially aggregated phthalocyanine assemblies. In American Chemical Society, Polymer Preprints, Division of Polymer Chemistry.
• Armstrong, N. R. (1998). Molecular self-assembly approaches to multilayer organic light-emitting diode structures. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1998). Molecular self-assembly routes to electroluminescent multilayer structures. In American Chemical Society, Polymer Preprints, Division of Polymer Chemistry.
• Armstrong, N. R. (1997). Biexcitons in mixed-stack charge-transfer solids, conjugated polymers, and molecular aggregates. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1997). Chemical and biochemical sensors based on sol-gel derived, laminate planar waveguide structures. In Polymeric Materials Science and Engineering, Proceedings of the ACS Division of Polymeric Materials Science and Engineering.
• Armstrong, N. R. (1997). Molecular self-assembly routes to optically functional thin films: Electroluminescent multilayer structures. In Materials Research Society Symposium - Proceedings.
• Armstrong, N. R. (1994). Biexcitons in phthalocyanine thin films. In Proceedings of the International Quantum Electronics Conference (IQEC'94).
• Armstrong, N. R. (1992). Organic/inorganic molecular beam epitaxy: photoelectrochemical and optical properties of epitaxially deposited single-component and multiple-component organic superlattices (Invited Paper). In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1992). Oxidation processes in magneto-optic and related materials. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1991). O/I-MBE: formation of highly ordered phthalocyanine/semiconductor junctions by molecular-beam epitaxy: photoelectrochemical characterization. In Proceedings of SPIE - The International Society for Optical Engineering.
• Armstrong, N. R. (1987). REACTIONS OF EVAPORATED LITHIUM FILMS ON OXIDE AND GLASS SURFACES.. In Proceedings - The Electrochemical Society.
• Armstrong, N. R. (1986). SURFACE CHEMICAL PROCESSES OF CLEAN LITHIUM: SMALL MOLECULE AND SOLID STATE CORROSION REACTIONS RELEVANT TO THE Li/SO//2 BATTERY.. In Proceedings - The Electrochemical Society.
• Armstrong, N. R. (1985). PHOTOELECTROCHEMICAL STUDY OF p-TYPE VANADYL PHTHALOCYANINE (VOPc) THIN-FILM ELECTRODES.. In Electrochemical Society Extended Abstracts.
• Armstrong, N. R. (1984). FARADAIC IMPEDANCE STUDIES ON THIN-FILM MONOMERIC SILICON PHTHALOCYANINE AND CHLORO-GALLIUM PHTHALOCYANINE ELECTRODES.. In Electrochemical Society Extended Abstracts.
• Armstrong, N. R. (1983). PHOTOELECTROCHEMICAL COMPARISON OF CHLORO-GALLIUM PHTHALOCYLANINE TO OTHER PHTHALOCYANINE DERIVATIVES.. In Electrochemical Society Extended Abstracts.
• Armstrong, N. R. (1981). CHEMICALLY MODIFIED ELECTRODES FOR PHOTOELECTROCHEMICAL AND ELECTROCHROMIC REACTIONS.. In American Chemical Society, Division of Petroleum Chemistry, Preprints.
• Armstrong, N. R. (1981). ELECTROCHEMICAL AND SURFACE ANALYSIS OF THE REACTIVE INTERFACES IN THE Li(Si)-FeS//2 PRIMARY BATTERY.. In Power Sources: Research and Development in Non-Mechanical Electrical Power Sources.

Others

• Armstrong, N. R. (1999). Aluminum composite cathodes: A new method for the fabrication of efficient and bright organic light-emitting devices. Optics and Photonics News. http://www.scopus.com/inward/record.url?eid=2-s2.0-0032615475&partnerID=MN8TOARS