Joachim Dahl Thomsen: Anisotropic Oxidation of Suspended Graphene: Etch Dynamics and Stability Beyond 1000 °C

Joachim Dahl Thomsen1, Jens Kling2, David M. A. Mackenzie3, Peter Bøggild1, Timothy J. Booth1  1Center for Nanostructured Graphene, Department of Physics, Technical University of Denmark, Lyngby, Denmark  2Center for Electron Nanoscopy, Technical University of Denmark, Lyngby, Denmark  3Department of Electronics and Nanoengineering, Aalto University, Espoo, Finland  We study the oxidation of clean suspended mono- and few-layer graphene in real-time by in situ environmental transmission electron microscopy [1]. At pressures below 0.1 mbar we observe anisotropic oxidation and the formation of hexagonal holes with armchair-oriented edges and edge roughness below 1 nm. At a higher pressure, we observe an increasingly isotropic oxidation, eventually leading to irregular holes at a pressure of 6 mbar. In addition, we find that few-layer suspended flakes are stable against oxidation at temperatures up to at least 1000 °C in the absence of impurities and electron beam-induced defects. These findings show, first, that the oxidation behavior of mono- and few-layer graphene depends critically on the intrinsic roughness, cleanliness and any imposed roughness or additional reactivity from a supporting substrate and, second, that the activation energy for oxidation of pristine suspended few-layer graphene is up to 43% higher than previously reported for graphite. In order to study the oxidative etching of suspended graphene, we have developed a cleaning scheme that results in the near complete removal of hydrocarbon residues over the entire visible sample area (~50 µm2) [1, 2]. These results have implications for applications of graphene where edge roughness can critically affect the performance of devices, and more generally highlights the surprising (meta)stability of the basal plane of suspended bilayer and thicker graphene towards oxidative environments at high temperature.  [1] J. D. Thomsen, J. Kling, D. M. A. Mackenzie, P. Bøggild, T. J. Booth, ACS Nano, 13, 2, 2281-2288 (2019) [2] J. D. Thomsen, T. Gunst, S. S. Gregersen, L. Gammelgaard, B. S. Jessen, D. M. A. Mackenzie, K. Watanabe, T. Taniguchi, P. Bøggild, T. J. Booth, Physical Review B, 96, 014101 (2017). Joachim Dahl Thomsen is a postdoctoral researcher in the Timothy J. Booth group at the Technical University of Denmark (Department of Physics) where we also obtained his PhD in 2018. His interests are in transmission electron microscopy of 2D materials, including nanopatterning, defect engineering, and in-situ electrical characterization of devices, as well as physics of van der Waals heterostructures. He has received a DFF International postdoc grant to work in the Frances Ross group at Massachusetts Institute of Technology (Department of Materials Science and Engineering) starting September 2019.