Joachim Dahl Thomsen: Suppression of Intrinsic Roughness in Encapsulated Graphene

J. D. Thomsen [1], T. Gunst [1], S. S. Gregersen [1], L. Gammelgaard [1], B. S. Jessen [1], D. M. A. Mackenzie [1], K. Watanabe [2], T. Taniguchi [2], P. Bøggild [1], and T. J. Booth [1] [1] Center for Nanostructured Graphene, Department of Micro- and Nanotechnology, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan Roughness in graphene is known to contribute to scattering effects which lower carrier mobility [1]. Encapsulating graphene in hexagonal boron nitride (hBN) leads to a significant reduction in roughness and has become the de facto standard method for producing high-quality graphene devices. We have fabricated graphene samples encapsulated by hBN that are suspended over apertures in a substrate and used electron diffraction in a transmission electron microscope to measure the roughness of encapsulated graphene inside such structures [2]. We furthermore compare the roughness of these samples to suspended bare graphene and suspended graphene on hBN. The suspended and encapsulated graphene displays a root mean square (rms) roughness down to 12 pm, considerably less than that previously reported for both suspended graphene [3] and graphene on any substrate. Our first-principles calculations of the phonon bands in graphene/hBN heterostructures show that the flexural acoustic phonon mode is localized predominantly in the hBN layers upon hBN encapsulation. These results could lead to new strategies for device fabrication in applications requiring ultimately high performance, and show that layer roughness in artificially fabricated van der Waals heterostructures approaches that in naturally occurring bulk crystals. [1] E.V. Castro et al, Phys. Rev. Lett. 105, 266601 (2010) [2] J. D. Thomsen et al, Phys Rev. B, 96, 014101 (2017) [3] D. A. Kirilenko et al, PRB 84, 235417 (2007) Joachim Dahl Thomsen is currently a Ph.D. student at the Technical University of Denmark (DTU) in the Nanocarbon group. He is working primarily with in-situ transmission electron microscopy (TEM) experiments involving nanopatterning, defect engineering and in-situ electrical characterisation of graphene and other 2D materials, using microfabricated sample platforms compatible with TEM sample holders. Joachim received his M.Sc. degree in Physics and Nanotechnology from DTU in 2014 where he worked with cleanroom micro-fabrication of flexible arrays of photodetectors in his final project.