Oliver L A Monti

Interests

Research

In LabMonti(TM), we seek new ways of making electronic devices more energy efficient by many orders of magnitude, much smaller and therefore faster, and to harvest renewable energy from the sun. We do this by combining molecular materials with amazing so-called 2D materials which are just a few atoms tall but extremely large laterally. TO accomplish this, we are at the forefront of research in the fundamental chemistry and physics of working out the behavior of electrons in such materials. We are able to probe directly excited states of films of large pi-conjugated organic molecules and their dynamics from less than 1 fs to thousands of s and from thin films to single molecules. We complement these efforts by making use of both synchrotron and “bench-top” electron spectroscopies to investigate electronic structure in the core- and valence-levels, augmented by a precise structural understanding obtained from low-temperature scanning tunneling microscopy (LT-STM). These experiments shed light on the complex many-body interactions of excitations at interfaces, and seek to provide test-cases and conceptual frameworks for much-needed theoretical advances in the physics of organic semiconductors and a fundamental understanding of the new frontier of hybrid interfaces of functional materials. Undergraduate researchers gain knowledge and skills in machine learning, computational chemistry, materials growth and a broad array of advanced experimental tools.

Teaching

Mathematical methods in physics, physical chemistryPhysical chemistryCondensed matter physicsquantum mechanicsthermodyanmics

Courses

Scholarly Contributions

Journals/Publications

• Schaibley, J. R., LeRoy, B. J., Yu, H., Monti Masel, O. L., Watanabe, K., Taniguchi, T., Mandrus, D. G., Koehler, M., Raglow, S., Alfrey, A., Idi, I., Badada, B., Muccianti, C., Shanks, D. N., & Mahdikhanysarvejahany, F. (2020). Temperature dependent moiré trapping of interlayer excitons in MoSe2-WSe2 heterostructures. npj 2D Materials and Applications, 5, 67. doi:10.1038/s41699-021-00248-7
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  MoSe2-WSe2 heterostructures host strongly bound interlayer excitons (IXs) which exhibit bright photoluminescence (PL) when the twist-angle is near 0° or 60°. Over the past several years, there have been numerous reports on the optical response of these heterostructures but no unifying model to understand the dynamics of IXs and their temperature dependence. In this letter, we perform a comprehensive study of the temperature, excitation power, and time-dependent PL of IXs. We observe a significant decrease in PL intensity above a critical temperature that we attribute to a transition from localized to delocalized IXs. We conclude that this temperature dependence is a result of IX-IX interactions, which is suppressed by the moiré potential trapping IXs at low temperature. Astoundingly, we find a simple inverse relationship between the IX PL energy and the critical temperature, which exhibits opposite power dependent behaviors for near 0° and 60° samples.
• Schaibley, J. R., Monti Masel, O. L., LeRoy, B. J., Binder, R., Mandris, D., Koehler, M., Alfrey, A., Badada, B., Eads, C., Garlant, A., Zachritz, S., & Muccianti, C. (2020). Coupled 2D Semiconductor−Molecular Excitons with Enhanced Raman Scattering. J. Phys. Chem. C, 27637 - 27644.
• Monti Masel, O. L. (2017). Publications. List.
• Ilyas, N., Harivyasi, S., Zahl, P., Cortez, R., Hofmann, O., Sutter, P., Zojer, E., & Monti, O. L. (2016). Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111). Journal of Physical Chemistry C, 120, 7113-7121.
• Wu, B., Ivie, J., Johnson, T., & Monti Masel, O. L. (2017). Uncovering Hierarchical Data Structure in Single Molecule Transport. Journal of Chemical Physics, 146, 092321.
• Gruenewald, M., Schirra, L., Winget, P., Kozlik, M., Ndione, P., Sigdel, A., Berry, J. J., Forker, R., Bredas, J., Fritz, T., & Monti Masel, O. L. (2015). Integer Charge Transfer and Hybridization at an Organic Semiconductor / Conductive Oxide Interface. Journal of Physical Chemistry C.
• Jakobs, S., Narayan, A., Stadtmueller, B., Droghetti, A., Rungger, I., Hor, Y. S., Klyatskaya, S., Jungkenn, D., Stoeckl, J., Laux, M., Monti, O. L., Aeschlimann, M., Cava, R., Ruben, M., Mathias, S., Sanvito, S., & Cinchetti, M. (2015). Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules. Nano Letters. doi:10.1021/acs.nanolett.5b02213
• Kelly, L., Racke, D., Kim, H., Ndione, P., Sigdel, A., Berry, J., Graham, S., Monti Masel, O. L., & Nordlund, D. (2015). Hybridization-Induced Carrier Localization at the C60 / ZnO Interface. Advanced Materials.
• Maughan, B., Zahl, P., Sutter, P., & Monti Masel, O. L. (2015). Selective Cooperative Self-Assembly between an Organic Semiconductor and Native Adatoms on Cu(110). Journal pf Physical Chemistry C.
• Racke, D., Kelly, L. L., & Monti, O. L. (2015). The Importance of Gap States for Energy Level Alignment at Hybrid Interfaces. Journal of Electron Spectroscopy and Related Phenomena.
• Racke, D., Kelly, L., Kim, H., Schulz, P., Sigdel, A., Berry, J. J., Graham, S., Nordlund, D., & Monti, O. L. (2015). Disrupted Attosecond Charge Carrier Delocalization at a Hybrid Organic/Inorganic Semiconductor Interface. Journal of Physical Chemistry Letters.
• Huang, Y., Sutter, E., Sadowski, J. T., Cotlet, M., Monti, O. L., Racke, D. A., Neupane, M. R., Wickramaratne, D., Lake, R. K., Parkinson, B. A., & Sutter, P. (2014). Tin Disulfide-An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics. ACS NANO, 8(10), 10743-10755.
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  Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.
• Ilyas, N., & Monti, O. L. (2014). Interplay of local and global interfacial electronic structure of a strongly coupled dipolar organic semiconductor. PHYSICAL REVIEW B, 90(12).
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  We investigate the consequences of strong electronic coupling at the organic semiconductor/metal interface in both the ground and excited state manifold for the case of chloro-boron subphthalocyanine on Cu(111). Using a combination of low-temperature scanning tunneling microscopy and ultraviolet photoelectron spectroscopy and angle-resolved two-photon photoemission, we are able to connect local electronic interactions at the interface with thin film structure despite complex growth in the submonolayer regime. We show that strong coupling leads to charge transfer from the surface to the molecule, and we are able to correlate this observation with the specific molecular adsorption geometry. Strong coupling further results in molecular excited state anion resonances and is responsible for autoionization of highly excited image potential states, relating to the heterogeneous electronic environment in these thin films. This study provides a step towards disentangling interfacial electronic interactions at complex organic/metal interfaces.
• Racke, D. A., & Monti, O. L. (2014). Persistent non-equilibrium interface dipoles at quasi-2D organic/inorganic semiconductor interfaces: The effect of gap states. SURFACE SCIENCE, 630, 136-143.
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  We investigate the role of gap states in the quasi-2D van der Waals crystal SnS2 and their influence on the electronic structure formed at the hybrid interface between SnS2 and several different organic semiconductors. We demonstrate that this density of states creates trapped carriers, generating an interfacial electric field that transiently alters the energy level alignment at the hybrid interface. The trapped carriers are extremely long-lived due to the weak interlayer coupling that is characteristic of quasi-2D materials. We suggest that these effects, observed here by photoemission spectroscopy, likely play a role for many different van der Waals materials with moderate screening lengths, with direct impact on optoelectronic and transport properties in the (quasi-)2D limit. (C) 2014 Elsevier B.V. All rights reserved.
• Racke, D. A., Neupane, M. R., & Monti, O. L. (2016). Valence and conduction band structure of the quasi-two-dimensional semiconductor SnS2. PHYSICAL REVIEW B, 93(8).
• Schulz, P., Kelly, L. L., Winget, P., Li, H., Kim, H., Ndione, P. F., Sigdel, A. K., Berry, J. J., Graham, S., Bredas, J., Kahn, A., & Monti, O. L. (2014). Tailoring Electron-Transfer Barriers for Zinc Oxide/C-60 Fullerene Interfaces. ADVANCED FUNCTIONAL MATERIALS, 24(46), 7381-7389.
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  The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next-generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C-60. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier-free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy-conversion and sensor technologies.
• Winget, P., Schirra, L. K., Cornil, D., Li, H., Coropceanu, V., Ndione, P. F., Sigdel, A. K., Ginley, D. S., Berry, J. J., Shim, J., Kim, H., Kippelen, B., Bredas, J., & Monti, O. L. (2014). Defect-Driven Interfacial Electronic Structures at an Organic/Metal-Oxide Semiconductor Heterojunction. ADVANCED MATERIALS, 26(27), 4711-+.
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  The electronic structure of the hybrid interface between ZnO and the prototypical organic semiconductor PTCDI is investigated via a combination of ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) and density functional theory (DFT) calculations. The interfacial electronic interactions lead to a large interface dipole due to substantial charge transfer from ZnO to 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which can be properly described only when accounting for surface defects that confer ZnO its n-type properties.
• Ilyas, N., Kelly, L. L., & L., O. (2013). Electronic structure and dynamics of quasi-2D states of vanadyl naphthalocyanine on Au(111). Molecular Physics, 111(14-15), 2175-2188.
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  Abstract: We investigate the evolution of the interfacial electronic structure and dynamics of thin films of the organic semiconductor vanadyl naphthalocyanine on Au(111). Using angle-resolved two-photon photoemission, a comprehensive coverage- and excitation-energy-dependent characterisation of the electronic structure and the resulting dynamics of short-lived image potential resonances (IPRs) on Au(111) are presented. The study of these quasi-two-dimensional (quasi-2D) bands is enabled by molecular adsorption and reveals a significant lengthening of their lifetimes. The resonances remain, however, significantly coupled to the continuum of bulk bands of Au(111) even in the presence of the organic adsorbate, giving rise to Fano-like quantum interference and 'intensity switching' effects. Coupling to the continuum is also responsible for providing excitation pathways to the image potential manifold above and below optical resonance with the Shockley surface state. The organic semiconductor interface and quasi-2D bands investigated here provide a model for understanding the role of quantum effects in ultrafast dynamics of confined systems and at interfaces such as those that are relevant e.g. for interfacial charge-transfer processes in organic electronics. © 2013 Taylor & Francis.
• Steil, S., Großmann, N., Laux, M., Ruffing, A., Steil, D., Wiesenmayer, M., Mathias, S., L., O., Cinchetti, M., & Aeschlimann, M. (2013). Spin-dependent trapping of electrons at spinterfaces. Nature Physics, 9(4), 242-247.
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  Abstract: Hybrid ferromagnetic metal/organic interfaces - also known as spinterfaces - can exhibit highly efficient spin-filtering properties and therefore present a promising class of materials for the future development of new spintronic devices. Advancing the field depends critically on elucidating the fundamental microscopic processes that eventually determine the spin-filtering properties in such hybrid structures. Here, we study the femtosecond spin dynamics at the prototypical interface between cobalt and the metalorganic complex tris(8-hydroxyquinolinato)aluminium. To disentangle the microscopic origin of spin filtering, we optically generate a transient spin polarization in a well-defined hybrid interface state that we follow with a spin-resolved real-time pump-probe two-photon photoemission experiment. We find that the electrons are trapped at the interface in a spin-dependent manner for a surprisingly long time of the order of 0.5-1 ps. We conclude that ferromagnetic metal/organic interfaces act as spin filters because electrons are trapped in hybrid interface states by spin-dependent confining potentials. Copyright © 2013 Macmillan Publishers Limited. All rights reserved.
• Hong, L. i., Schirra, L. K., Shim, J., Cheun, H., Kippelen, B., L., O., & Bredas, J. (2012). Zinc oxide as a model transparent conducting oxide: A theoretical and experimental study of the impact of hydroxylation, vacancies, interstitials, and extrinsic doping on the electronic properties of the Polar ZnO (0002) surface. Chemistry of Materials, 24(15), 3044-3055.
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  Abstract: The technology-relevant zinc-terminated zinc oxide (0002) polar surface has been studied at the density-functional theory level using both Perdew-Burke-Ernzerhof (PBE) and hybrid Heyd-Scuseria-Ernzerhof (HSE06) functionals. We have considered a number of surface conditions to better understand the impact of surface hydroxylation and intrinsic and extrinsic surface defects, including zinc vacancies, oxygen vacancies, zinc interstitials, and aluminum dopants on the surface electronic properties. Our calculations point to large variations in surface work function and energy band gap as a function of the surface model; these variations can be attributed to changes in surface charge carrier density and to additional surface states induced by the defects. The calculated shifts in O(1s) core-level binding energy of the surface oxygens in different bonding configurations are in good agreement with experimental X-ray photoelectron spectroscopy data and point to the presence of two distinct OH-species on the ZnO surface. Our results also show that the electron-compensation centers induced by zinc vacancies can be stabilized by intrinsic and/or extrinsic n-type doping near the surface; such n-type doping can lead to better performance of organic opto-electronic devices in which zinc oxide is used as an electron-selective interlayer. © 2012 American Chemical Society.
• Kelly, L., Racke, D., Schulz, P., Li, H., Winget, P., Kim, H., Ndione, P., Sigdel, A., Bredas, J., Berry, J., Graham, S., & Monti Masel, O. L. (2016). Spectroscopy and Control of Near-Surface Defects in Conductive Thin Film ZnO. Journal of Physics: Condensed Matter.
• L., O. (2012). Understanding interfacial electronic structure and charge transfer: An electrostatic perspective. Journal of Physical Chemistry Letters, 3(17), 2342-2351.
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  Abstract: The challenge of understanding electronic structure and dynamics at organic semiconductor interfaces arises from the richness and importance of weak interactions in thin films of extended π-conjugated molecules. In this Perspective, I discuss a conceptually simple electrostatic approach toward a molecular-level description of the electronic structure and dynamics at a subset of such interfaces. Self-assembled monolayers of oriented dipolar molecules physisorbed on metal surfaces generate sizable collective electric fields, and electrostatics determines the key factors for energy level alignment and molecular electronic structure. A rigorous quantum mechanical treatment of such interfaces supports this conclusion and sheds light on the subtle interplay of the different interfacial interactions. The electrostatic model of the interface has the potential to offer also insights into the role of strong collective electric fields on interfacial charge-transfer dynamics. © 2012 American Chemical Society.
• Baker, T. A., L., O., & Nesbitt, D. J. (2011). Kinetic studies of the photogeneration of silver nanoparticles. Journal of Physical Chemistry C, 115(20), 9861-9870.
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  Abstract: Stable, intensely Raman active silver nanoparticles are photogenerated by visible light from silver ions in a thin polymer film within a diffraction-limited focal area. The emission is resolved both spectrally and temporally to demonstrate that the source of the signal is surface-enhanced Raman scattering (SERS) from multiple silver nanoparticles generated in the diffraction-limited spot. The time evolution of the SERS signal is sigmoidal in shape and well described by Avrami phase transformation kinetics. The rate constant for the Avrami transformation depends linearly on illumination intensity, consistent with single photon photoreduction of the silver percholorate startingmaterial as the limiting step to form silver nanoparticles. The asymptotic kinetic growth SERS signal exhibits a linear dependence on illumination intensity. Avrami analysis of the kinetics indicates that transformation is constrained to two dimensions, consistent with the ̃10 nm thin film nature of the sample. The technique presented provides a novel route to large-scale periodic molecular sensor arrays with long-term stability, diffraction-limited resolution (
• Steele, M. P., Kelly, L. L., Ilyas, N., & L., O. (2011). Resonance and localization effects at a dipolar organic semiconductor interface. Journal of Chemical Physics, 135(12).
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  PMID: 21974547;Abstract: The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0 - 1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage. Near a coverage of one monolayer, a hybrid image potentialanion state is also formed, showing progressive localization with coverage. Intensities for all these features develop rather differently with molecular coverage, pointing towards the different types of charge-transfer interactions at play at this interface. © 2011 American Institute of Physics.
• Terentjevs, A., Steele, M. P., Blumenfeld, M. L., Ilyas, N., Kelly, L. L., Fabiano, E., Monti, O. L., & Sala, F. D. (2011). Interfacial electronic structure of the dipolar vanadyl naphthalocyanine on Au(111): "Push-Back" vs dipolar effects. Journal of Physical Chemistry C, 115(43), 21128-21138.
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  Abstract: We investigate the interfacial electronic structure of the dipolar organic semiconductor vanadyl naphthalocyanine on Au(111) in a combined computational and experimental approach to understand the role of the permanent molecular dipole moment on energy-level alignment at this interface. First-principles Density Functional Theory (DFT) calculations on such large systems are challenging, due to the large computational cost and the need to accurately consider dispersion interactions. Our DFT results with dispersion correction show a molecular deformation upon adsorption but no strong chemical bond formation. Ultraviolet photoelectron spectroscopy measurements show a considerable workfunction change of -0.73(2) eV upon growth of the first monolayer, which is well reproduced by the DFT calculations. This shift originates from a large electron density "push-back" effect at the gold surface, whereas the large out-of-plane vanadyl dipole moment plays only a minor role. © 2011 American Chemical Society.
• Blumenfeld, M. L., Steele, M. P., & L., O. (2010). Near- and far-field effects on molecular energy level alignment at an organic/electrode interface. Journal of Physical Chemistry Letters, 1(1), 145-148.
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  Abstract: We investigate the evolution of the interfacial electronic structure at the interface of (sub)monolayer vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite (HOPG). Both the vacuum level and molecular energy levels show a significant but fundamentally different dependence on coverage, resulting in an overall change of the ionization potential with coverage. We use a simple model to show how this effect arises from the differential sensitivity of these levels to the near- and far-field properties of the dipole-layer-generated interfacial electric potential. These results help to unravel the electronic structure at organic/electrode interfaces, with direct implications for organic electronic devices. © 2009 American Chemical Society.
• Blumenfeld, M. L., Steele, M. P., Ilyas, N., & Monti, O. L. (2010). Interfacial electronic structure of vanadyl naphthalocyanine on highly ordered pyrolytic graphite. Surface Science, 604(19-20), 1649-1657.
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  Abstract: We present a core and valence region spectroscopic analysis of the interfacial electronic structure of thin films of vanadyl naphthalocyanine (VONc) deposited onto highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy indicates the predominantly ionic character of the vanadyl metal center coordinated by the heterocycle and affords the bandgap in the thin VONc films. Valence band photoelectron spectroscopy points to the existence of three different adsorption geometries of VONc on the HOPG surface. The distribution of the different geometries can be systematically influenced in a simple post-deposition processing step, with an immediate effect on the interfacial electronic environment. We find spectroscopic evidence in the valence levels that VONc grows on HOPG most likely in a 2D-gas fashion rather than by nucleation and growth of islands. These data allow us to predict accurately the interface dipole in the case of a broad class of dipolar organic semiconductors, based simply on molecular dipole moment, polarizability and molecular diameter. This ability provides an important step towards rational optimization of energy level alignment in organic electronics. © 2010 Elsevier B.V. All rights reserved.
• L., O., & Steele, M. P. (2010). Influence of electrostatic fields on molecular electronic structure: Insights for interfacial charge transfer. Physical Chemistry Chemical Physics, 12(39), 12390-12400.
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  PMID: 20714606;Abstract: Molecular and interfacial electronic structure at organic semiconductor interfaces shows a rich and subtle dependence on short- and long-range electrostatic interactions. Interface dipoles can be controlled making use of the anisotropic charge distribution at the interface, often with direct consequences also for the molecular electronic structure. In this Perspective, we will discuss the emerging understanding of how local and collective electrostatic effects control energy level alignment and molecular electronic structure at organic semiconductor interfaces and highlight some of the ramifications for interfacial charge-transfer dynamics. Attention is paid to the validity of the underlying assumptions inherent to the classical electrostatic approach. © the Owner Societies 2010.
• Steele, M. P., Blumenfeld, M. L., & L., O. (2010). Image states at the interface with a dipolar organic semiconductor. Journal of Chemical Physics, 133(12).
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  PMID: 20886959;Abstract: Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage. Above 1 ML, a new image state develops, corresponding to the screened interaction at the organic semiconductor/substrate interface. We show that the evolution of the observed image states can be understood on the basis of resonance-enhanced anion formation in the presence of strong electric fields. These data represent a step toward understanding the influence of electrostatic fields on electronic structure at organic semiconductor interfaces. © 2010 American Institute of Physics.
• Steele, M. P., Blumenfeld, M. L., & Monti, O. L. (2010). Experimental determination of the excited-state polarizability and dipole moment in a thin organic semiconductor film. Journal of Physical Chemistry Letters, 1(13), 2011-2016.
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  Abstract: We report the evolution of the electronic structure of excited (sub)monolayer films of vanadyl naphthalocyanine (VONc) at the interface with highly oriented pyrolytic graphite (HOPG). Using two-photon photoemission spectroscopy, an unoccupied state corresponding to the lowest unoccupied molecular orbital of VONc is observed. The energy of this state shows a significant dependence on coverage, interpreted in the context of the electrostatic environment at the interface. On the basis of a simple electrostatic model, we were able to determine the excited-state polarizability and dipole moment of VONc at the interface with HOPG. The results suggest that local electric fields may have a major influence on interfacial energy level alignment in the excited-state manifold, with direct consequences for interfacial charge-transfer dynamics. © 2010 American Chemical Society.
• Blumenfeld, M. L., Steele, M. P., & L., O. (2009). Interfacial structure and dynamics at the electrode/organic interface. ACS National Meeting Book of Abstracts.
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  Abstract: We have investigated interfacial structure and ultrafast charge transfer dynamics at the interface between highly structured oligothiophene films and Au(111) and Cu (111). The frontier orbitals at these interfaces play an important role in charge transfer processes in organic photovoltaic devices. We observe the evolution of electronic structure in both HOMO and LUMO as a function of interface thickness and structure, and correlate this information with device performance. In addition, we observe ultrafast dynamics of electronic states formed at these interfaces and discuss the implications for charge injection dynamics in organic photovoltaics.
• Blumenfeld, M. L., Tackett, B. S., Schirra, L. K., Tyler, J. M., & L., O. (2009). Confocal single molecule fluorescence spectroscopy in ultrahigh vacuum. Review of Scientific Instruments, 80(10).
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  PMID: 19895048;Abstract: We have constructed an ultrahigh vacuum confocal fluorescence microscope with the purpose of performing single molecule spectroscopy under highly defined conditions. The microscope is designed for high stability while affording the capability of sample preparation, sample transfer, and optical detection in ultrahigh vacuum. It achieves near-diffraction-limited performance and allows the observation of single molecule fluorescence dynamics over extended periods of time. The design of the microscope is discussed in detail and the performance is demonstrated with single molecule fluorescence images and trajectories of N, N′ -dibutylperylene-3,4,9,10-dicarboxyimide deposited onto several different surfaces. This instrument further enhances the array of available surface science techniques, permitting spectroscopic investigations of molecule-surface interactions at the single molecule level and on insulating surfaces. © 2009 American Institute of Physics.
• L., O., & Prezhdo, O. V. (2009). Proceedings of SPIE - The International Society for Optical Engineering: Introduction. Proceedings of SPIE - The International Society for Optical Engineering, 7396, vii.
• L., O., Schirra, L. K., Blumenfeld, M. L., Tackett, B. S., & Tyler, J. M. (2009). Interfacial charge transfer at the single molecule level: Insights for dye-sensitized solar cells. ACS National Meeting Book of Abstracts.
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  Abstract: We present the first measurements of forward- (FET) and back-electron transfer (BET) dynamics at the single molecule level performed under highly controlled, well-characterized ultrahigh vacuum conditions. FET from an excited perylene bisimide sensitizer to the conduction band of a single crystalline acceptor (GaN or Al2O3) and BET to the dye ground state were monitored by single molecule fluorescence intermittency. In the case of GaN, the surface was covered with a heteroepitaxial insulating layer of Sc2O3 which serves as a tunable electron injection barrier between sensitizer and semiconductor. The Sc2O3 spacer layer allowed isolation of the sensitizer/semiconductor distance dependence on FET and BET. The observed charge transfer dynamics was correlated with surface structure characterized by AFM, UPS and XPS. Our results point to the origin of non-exponential charge transfer kinetics in dye-sensitized solar cells that persist even on single crystalline surfaces.
• Schirra, L. K., Tackett, B. S., Blumenfeld, M. L., & Monti, O. L. (2009). Single molecule power-law behavior on a crystalline surface. Journal of Chemical Physics, 131(12).
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  PMID: 19791907;Abstract: Single perylene bisimide molecules deposited onto Al2 O 3 (0001) and investigated under controlled ultrahigh vacuum conditions display fluorescence intermittency behavior characteristic of an interfacial charge transfer process. Remarkably, even though the molecules are deposited on a crystalline surface with reduced disorder, power-law-distributed bright and dark periods are observed. These data can be understood based on activated formation of localized small polaron states in Al2 O 3 (0001). We present a kinetic scheme capable of explaining the occurrence of power-law distributions for both "on" and "off" periods for single molecules on the sapphire substrate. These findings represent a first step toward understanding interfacial charge transfer processes under controlled conditions on crystalline surfaces and at the single molecule level. © 2009 American Institute of Physics.
• Blumenfeld, M. L., Steele, M. P., Tackett, B. S., & L., O. (2008). Local interfacial electronic structure of thin pentacene films on Si(111). ACS National Meeting Book of Abstracts.
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  Abstract: Organic photovoltaic (OPV) devices offer a promising route to low-cost solar energy harvesting. These systems have demonstrated heterogeneous ultrafast charge transfer dynamics in part due to complex micro and nanoscale disorder at semiconductor domain interfaces. We have investigated the effects of local morphology on interfacial electronic structure using the model system of pentacene grown on Si(111). Spatially resolved two photon photoemission spectroscopy was used to map local variations in interfacial dipole formation and intraband state densities. These results are correlated with film topography as measured by AFM and bulk surface character as measured by conventional UPS. Findings are discussed in the framework of their implications on charge transfer dynamics and successful OPV development.
• Blumenfeld, M. L., Tackett, B. S., & L., O. (2008). Toward microscale electronic structure in polymer - Nanoparticle composites for solar energy conversion. ACS National Meeting Book of Abstracts.
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  Abstract: Solar energy conversion must arguably play a central role in sustainably providing large-scale power to a growing global population currently undergoing a massive surge in industrialization. At present, this goal remains elusive owing to the absence of means for economical solar power generation. Organic and hybrid organic / inorganic photovoltaic cells (OPV) have the potential to fill this gap, but are currently unable to meet the efficiency required for a significant impact on the energy market. The delicate balance between high interfacial area required for efficient exciton dissociation and high mobility of both types of carriers poses fundamental challenges to device architecture that require novel spectroscopic insights on microscopic length-scales. Scanning photoionization microscopy (SPIM) is capable of providing frontier orbital position at high spatial resolution, thus generating a detailed map of interfacial electronic structure at sub-micron length-scales. We will discuss recent advances to implement this form of microscopy in the context of nano- and meso-structured hybrid materials for solar energy conversion.
• Schirra, L. K., Blumenfeld, M. L., Tackett, B. S., Tyler, J. M., & L., O. (2008). Toward single molecule interfacial charge transfer dynamics in a dye-sensitized solar cell model. Proceedings of SPIE - The International Society for Optical Engineering, 7034.
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  Abstract: Using confocal fluorescence microscopy under ultrahigh vacuum conditions, we investigate the heterogeneous interactions between a perylene bisimide fluorophore and single crystalline Al2O3 (0001) at the single molecule level. We find that the dye molecules undergo reversible transitions to long-lived dark states, with bright and dark periods lasting from several hundred milliseconds to many tens of seconds. These periods are power-law distributed and point towards charge tunneling processes from the molecule to the substrate. The fluorescence intensity levels show a bimodal distribution, indicating different classes of adsorption sites on the sapphire surface. This study is aimed at obtaining a better understanding of interfacial structure and dynamics in order to address ultimately both the growth of organic semiconductor films on inorganic surfaces and the heterogeneous nature of charge transfer in excitonic solar cells.
• Tackett, B. S., Schirra, L. K., Blumenfeld, M. L., Tyler, J. M., & L., O. (2008). Single molecule interfacial charge transfer dynamics in a dye-sensitized solar cell model. ACS National Meeting Book of Abstracts.
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  Abstract: We present the first measurements of forward- (FET) and back-electron transfer (BET) dynamics in a model dye-sensitized solar cell performed under highly controlled, well-characterized ultrahigh vacuum conditions. FET from a perylene bisimide (PTCDI) sensitizer excited state to the conduction band (CB) of single crystalline GaN and BET to the dye ground state were monitored by single molecule fluorescence intermittency. The GaN surface was covered with a heteroepitaxial insulating layer of Sc2O3 which serves as a tunable electron injection barrier between sensitizer and semiconductor. The Sc2O3 spacer layer allowed isolation of the sensitizer/semiconductor distance dependence on FET and BET while slowing the charge transfer (CT) reactions to single-molecule fluorescence measurement time scales. The GaN/Sc2O3 substrate and GaN/Sc2O3/PTCDI system were extensively characterized by AFM, XPS, UPS and LEED to correlate the observed distribution of CT kinetics with surface morphology and electronic structure.
• Blumenfeld, M. L., Tyler, J. M., & L., O. (2007). Interfacial structure and dynamics in organic photovoltaics: From solar cells to single molecules. ACS National Meeting Book of Abstracts.
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  Abstract: Organic solar cells based on bulk heterojunction architecture have displayed a complex interplay of morphology, interfacial physics and charge transport characterisitics. We are building a multi-layered research program through collaboration and innovation aimed at understanding how these complexities affect organic photovoltaic function. At the core of this research program is the development of novel experimental methodologies that address the fundamental heterogeneity of organic solar cells. Scanning photoionization microscopy (SPIM) is demonstrated as a tool for mapping the local energy structure of the bulk heterojunction with the chemical contrast of resonant multiphoton ionization and the spatial resolution of diffraction-limited excitation. Femtosecond time-resolved SPIM is introduced as a technique for measuring exciton dissociation dynamics in films based on novel directed architectures employing magnetically- directed assembly by functionalized ferromagnetic cobalt nanoparticles. Single molecule microscopy of fluorescent probes deposited on well-defined wide bandgap semiconductors is demonstrated as a tool for investigating the fundamental energy pathways that govern photovoltaic efficiencies. We will present first results obtained from microscopic studies based on these experimental platforms.
• Monti, O. L., Schirra, L. K., Blumenfeld, M. L., Tyler, J. M., & Tackett, B. S. (2007). Interfacial structure and dynamics in molecular solar cells. Proceedings of SPIE - The International Society for Optical Engineering, 6643.
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  Abstract: A novel approach to studying interfacial processes in dye-sensitized solar cells is presented. In order to reduce the complexities of heterogeneity at the heterojunction in such cells, charge transfer is investigated from single fluorescent molecules (alkyl-perylene bisimide) to a highly defined single-crystalline wide-bandgap semiconductor (GaN) using confocal fluorescence microscopy under ultrahigh vacuum conditions. We report detailed studies on the energy level alignment between the perylene bisimide and GaN, characterize the nature of the surfaces involved and demonstrate confocal fluorescence microscopy in an ultrahigh vacuum set-up. The results reported here indicate that the excited state in the chromophore lies at 0 = 100 meV with respect to the bulk conduction band minimum of GaN.
• Monti, O. L., Baker, T. A., & Nesbitt, D. J. (2006). Imaging nanostructures with scanning photoionization microscopy. Journal of Chemical Physics, 125(15).
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  Abstract: We report detailed studies of local electronic properties in nanostructured thin metallic films using scanning photoionization microscopy. This novel form of microscopy combines the advantages of diffraction-limited optical excitation with the ability to detect both photons and low kinetic energy photoelectrons, permitting sensitive characterization of heterogeneous surfaces under vacuum conditions. Using this technique, correlated measurements of multiphoton photoemission cross section and optical penetration depth are reported for Au films supported on Pt. These results present a first step toward combining confocal fluorescence or Raman microscopy with time-resolved photoelectron imaging spectroscopy in complex metal film environments, which should be ideally suited to investigating local plasmonic effects in nanostructures. © 2006 American Institute of Physics.
• Cardoza, D., Langhojer, F., Trallero-Herrero, C., L., O., & Weinacht, T. (2004). Changing pulse-shape basis for molecular learning control. Physical Review A - Atomic, Molecular, and Optical Physics, 70(5 B), 053406-1-053406-4.
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  Abstract: The results of a molecular fragmentation learning control experiment were analyzed. An amplified Ti:sapphire laser system capable of producing 30-fs pulses with an energy of ∼1 mJ at a repetition rate of 1 kHz was used for the experiment. The experiments involved fragmentation of trifluoroacetone. A time-of flight mass spectrometer (TOFM) was used to evaluate the pulse shapes. The time domain basis was found to be natural for cases where periodic dynamics play a key role in selective fragmentation.
• Monti, O. L., Fourkas, J. T., & Nesbitt, D. J. (2004). Diffraction-limited photogeneration and characterization of silver nanoparticles. Journal of Physical Chemistry B, 108(5), 1604-1612.
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  Abstract: Visible-light-induced photogeneration of silver nanoparticles in a diffraction-limited focal region is demonstrated. The photochemical growth depends quadratically on illumination intensity indicative of a multiphoton generation process, with the identity of the silver nanoparticles confirmed by UV/vis absorption spectroscopy. Mie simulations of the absorption spectrum reveal a size distribution dominated by Ag particles with radii in the range of a few nanometers. Spectrally resolved laser excitation and emission studies demonstrate that the likely luminescence source is surface-enhanced Raman scattering from silver nanoparticles, with spectral jumps occurring on a time scale comparable to that of fluctuations in the total luminescence intensity. Possible routes for the photogeneration process as well as identity of the Raman-active species are discussed. Such diffraction-limited photoproduction methods for luminescent silver nanoparticles offer novel routes toward optical data storage and nanometer-scale molecular sensing.
• Samuels, D. A., Weinacht, T. C., Monti, O. L., Gershgoren, E., Leone, S. R., Kapteyn, H. C., Murnane, M. M., & Bartels, R. A. (2003). Making and measuring vibrational wave packets in small molecules through non-resonant impulsive stimulated Raman scattering. Springer Series in Chemical Physics, 71, 91-93.
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  Abstract: We demonstrate the creation and measurement of vibrational wavepackets in small polyatomic molecules using nonresonant impulsive excitation. Modeling reveals features due to vibrational dephasing and rephrasing, rotational dephasing, and dissociation.
• Monti, O. L., Cruse, H. A., Softley, T. P., & MacKenzie, S. R. (2001). High resolution photoionisation spectroscopy of vibrationally excited Ar · NO. Chemical Physics Letters, 333(1-2), 146-152.
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  Abstract: Mass-analysed threshold ionisation (MATI) spectra of the Ar·NO complex have been obtained for the first time. These spectra have been used to determine unambiguously the nature of three bands detected by resonance-enhanced multiphoton ionisation (REMPI) spectroscopy via the à state of Ar·NO. The features are shown to originate from vibrationally excited states of Ar·NO in its electronic ground state. The assignment is in agreement with recent theoretical calculations. © 2001 Elsevier Science B.V.
• Monti, O. L., Cruse, H. A., Softley, T. P., & Mackenzie, S. R. (2001). Spatial discrimination of Rydberg tagged molecular photofragments in an inhomogeneous electric field. Journal of Chemical Physics, 115(17), 7924-7934.
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  Abstract: A new approach for obtaining spatial and energetic information is described. It is shown how the field configuration and other experimental parameters can be optimized so as to obtain the best spatial resolution. For demonstration purposes, an electrostatic octople, comprising eight cylindrical electrodes of alternating polarity equally spaced around a circle is considered.
• Beil, A., Hollenstein, H., L., O., Quack, M., & Stohner, J. (2000). Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF: Experiment and theory. Journal of Chemical Physics, 113(7), 2701-2718.
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  Abstract: The complete vibrational spectra of CDBrClF, analyzed from the far infrared range (FIR) to the near infrared (NIR), are reported. The results in the fundamental region are compared to ab initio second order Moller-Plesset perturbation theory (MP2) and density functional Becke3-Lee-Yang-Parr (B3LYP) calculations. The experimental overtone spectra are analyzed in terms of effective Hamiltonians involving three and four vibrational degrees of freedom and in terms of vibrational variational calculations on the corresponding ab initio potential energy and dipole moment surfaces.
• Monti, O. L., Dickinson, H., Mackenzie, S. R., & Softley, T. P. (2000). Rapidly fluctuating anisotropy parameter in the near-threshold photodissociation of NO₂. Journal of Chemical Physics, 112(8), 3699-3709.
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  Abstract: A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J = 17/2) and O3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate. © 2000 American Institute of Physics.

Presentations

• Monti Masel, O. L. (2017, Winter). Presentations. List.