Richard Stehle

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• Hou, Y., Li, X., Qing, F., & Stehle, R. (2022). Co‐Localized Characterization of Aged and Transferred CVD Graphene with Scanning Electron Microscopy, Atomic Force Microscopy, and Raman Spectroscopy. Advanced materials and technologies. doi:10.1002/admt.202200596
• Li, X., Shen, C. H., & Stehle, R. (2022). Raman Spectroscopy Investigation on the Stability of C-Isotope Labeled Twisted and AB-Stacked Bilayer Graphene. Materials Science Forum. doi:10.4028/p-596y29
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  Bilayer graphene has been widely studied in recent years due to its intriguing physical properties and potential engineering applications. Here, we report on the stability measurements of isotope-labeled bilayer graphene with different stacking sequences. The results showed evidence of different defect intensity after the Ar plasma treatment. We found that the AB stacked bilayer graphene shows better stability when compared to twisted bilayer and monolayer graphene. However, for the protection of the under layer graphene, the twisted bilayer graphene showed better results. Our work demonstrates that the stability of bilayer graphene strongly depends on the layer stacking sequence.
• Chen, Y., Li, X., Niu, X., Qing, F., Stehle, R., Wang, B., & Zheng, B. (2021). Unconventional Reaction Phase Diagram for the Penetration Etching/Growth of Graphene Adlayers. Chemistry of Materials. doi:10.1021/acs.chemmater.1c03402
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  Cu has shown an advantage in growing monolayer graphene due to the very low C solubility and surface-mediated self-limiting growth, which hinders the growth of multilayer graphene. This work reports an unconventional penetration etching/growth of graphene adlayers tuned by oxygen beyond the self-limiting growth, supported by the C isotope labeling results. The effect of oxygen is nonmonotonic, i.e., with the increase of oxygen, graphene adlayers are etched without damaging the top layer, then shift to growth, and finally, all layers are etched. In addition, the reaction did not seem to reach equilibrium in the time range of the experiment but continued as if oxygen was increasing with respect to time. An oxygen-assisted exchange penetration model is proposed to interpret the growth mechanism. Oxygen etches the top shield layer, which is simultaneously healed by consuming the C species around adlayers and results in adlayer decomposition due to the break of equilibrium. Additionally, oxygen assists penetration of C from the gaseous agents into the shield layer for adlayer growth. The domination of one over the other depends on the concentration of oxygen, resulting in overall etching or growth. Finally, the synthesis of large-area monolayer and bilayer graphene films with good uniformity is demonstrated.
• Chen, Y., Li, X., Niu, Y., Qing, F., Stehle, R., & Zhang, Y. (2020). Towards large-scale graphene transfer. Nanoscale. doi:10.1039/d0nr01198c
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  The transfer process is crucial for obtaining high-quality graphene for its large-scale industrial application.
• Hou, Y., Li, X., Qing, F., & Stehle, R. (2019). Chemical vapor deposition synthesis of graphene films. APL Materials. doi:10.1063/1.5078551
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  Chemical vapor deposition synthesis of graphene films has developed over the past decade and has been used in both academia and industry. This perspective discusses the major areas of focus for this topic, including the recent achievements and the challenges to be overcome.
• Mao, S. S., Niu, X., Qing, F., Shen, C., Stehle, R., Xu, M., Yan, X., & Zhang, W. (2019). Criteria for the growth of large-area adlayer-free monolayer graphene films by chemical vapor deposition. Journal of Materiomics. doi:10.1016/j.jmat.2019.01.009
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  Abstract Homogeneity is important to material applications for good performance of individual devices, for making AB-stacked bilayer graphene in a layer-by-layer stacking order, and from the point of view of industrial production. Among many properties to be controlled, for the case of graphene, the thickness (or layer number) uniformity is the prerequisite. Chemical vapor deposition (CVD) of C precursors on Cu substrates is the most popular method to produce large-area graphene films. To date, precise control on the number of graphene layers as well as the uniformity over a large area is still very challenging. In this work, with a further understanding of the factors affecting adlayer growth, the synthesis of large-area adlayer-free monolayer graphene (MLG) films was achieved up to tens of squared centimeters in area by just using untreated Cu foil and a normal CVD process. We found that keeping equal C precursor concentration on the two sides of the Cu substrate is a criterion in addition to other factors such as the ratio of H:C and the substrate surface morphology for the growth of adlayer-free MLG. This finding is not only of great significance for the industrial production of large-area adlayer-free MLG films but also instructive for the synthesis of homogeneous few-layer graphene.
• Chang, H., Gao, L., Jia, Y., Qing, F., Stehle, R., Xu, M., Xu, J., Yue, W., Zhan, L., & Zhang, W. (2018). Preparation of Ultra-Smooth Cu Surface for High-Quality Graphene Synthesis. Nanoscale Research Letters. doi:10.1186/s11671-018-2740-x
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  As grown graphene by chemical vapor deposition typically degrades greatly due to the presence of grain boundaries, which limit graphene's excellent properties and integration into advanced applications. It has been demonstrated that there is a strong correlation between substrate morphology and graphene domain density. Here, we investigate how thermal annealing and electro-polishing affects the morphology of Cu foils. Ultra-smooth Cu surfaces can be achieved and maintained at elevated temperatures by electro-polishing after a pre-annealing treatment. This technique has shown to be more effective than just electro-polishing the Cu substrate without pre-annealing. This may be due to the remaining dislocations and point defects within the Cu bulk material moving to the surface when the Cu is heated. Likewise, a pre-annealing step may release them. Graphene grown on annealed electro-polished Cu substrates show a better quality in terms of lower domain density and higher layer uniformity than those grown on Cu substrates with only annealing or only electro-polishing treatment.