Low-energy electron microscopy contrast of stacking boundaries: Comparing twisted few-layer graphene and strained epitaxial graphene on silicon carbide

Abstract

Stacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological edge states, imaging them has been instrumental in determining local variations in twisted bilayer graphene. Here, we analyze the mechanisms causing stacking domain boundary contrast in bright field low-energy electron microscopy (BF-LEEM) both for graphene on SiC, where domain boundaries are caused by strain, and for twisted few-layer graphene. We show that when domain boundaries are between the top two graphene layers, BF-LEEM contrast is observed due to amplitude contrast and corresponds well to calculations of the contrast based purely on the local stacking in the domain boundary. Conversely, for deeper-lying domain boundaries, amplitude contrast only provides a weak distinction between the inequivalent stackings in the domains themselves. However, for small domains, electrons from different parts of the unit cell interfere, causing a very strong phase contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials.