Low-Energy Electron Microscopy contrast of stacking boundaries: comparing twisted few-layer graphene and strained epitaxial graphene on silicon carbide

TL;DR

This paper investigates how Low-Energy Electron Microscopy (LEEM) contrast reveals stacking domain boundaries in graphene, showing that contrast mechanisms depend on boundary depth and local stacking, with implications for imaging layered materials.

Contribution

It provides a detailed analysis of LEEM contrast mechanisms for stacking boundaries in graphene, including a general rule-of-thumb for interpreting contrast in Van der Waals heterostructures.

Findings

Amplitude contrast effectively images top-layer boundaries.

Phase contrast enhances visibility of small domains.

Contrast mechanisms depend on boundary depth and stacking configuration.

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 to determine local variations in twisted bilayer graphene. Here, we analyse the mechanisms causing stacking domain boundary contrast in Bright Field Low-Energy Electron Microscopy (BF-LEEM) for both 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 phase contrast, where electrons from different parts of the unit cell interfere causes a very strong contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials.