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T. A. de Jong: Imaging strain and twist in van der Waals materials

posted 6 Aug 2019, 08:34 by info admin
T.A. de Jong, E.E. Krasovskii, C. Ott, H. Yoo, P. Kim, R.M. Tromp, S.J. van der Molen, J. Jobst Leiden Institute of Physics 
Leiden University, The Netherlands 

Both strain and twist between individual layers are yet another handle to tune the properties of in Van der Waals structures and have led to new device concepts and exciting correlated electron effects. To
optimize and fully utilize this handle, measurements and statistics on the magnitude and spread of strain
and twist angle within a sample are vital. Here, we demonstrate two applications of Low Energy Electron Microscopy (LEEM) to measure those properties as well as their statistics, by combining diffraction and real space information.
First, we study the inherent strain between the two layers in bilayer graphene on SiC, which is induced by the growth process. We show that the strain thermally relaxes to form domains of AB and AC variants of Bernal stacking [1]. By automated gathering of statistics on the size and shape of those domains, we show that the strain is locally highly anisotropic and strongly influenced by the step edges of the SiC substrate. Second, we show that we can distinguish the precise stacking order and relative twist angle in exfoliated flakes of transition metal dichalcogenides [2]. The methods and findings presented here are very general and apply to all other strained or twisted Van der Waals structures.
[1] T.A. de Jong et al., Phys. Rev. Mater. 2, 104005 (2018).
[2] T.A. de Jong et al., physica status solidi (b) 255.12 (2018): 1800191

Tobias A. de Jong is currently a PhD candidate in the Van der Molen group at Leiden University. He applies Low Energy Electron Microscopy to study structural and electronic properties of Van der Waals materials, including stacking domains and Charge Density Waves. He obtained a BSc degree in both Physics and Mathematics from Leiden University in 2014 and a cum laude MSc degree in Physics from the same university. Email: jongt@physics.leidenuniv.nl