Formation mechanism of mirror twin grain boundaries in molecular beam epitaxy grown monolayer WSe₂–MoSe₂ lateral heterojunctions

Mirror twin grain boundary (MTB) defects, being a special type of high-symmetry one-dimensional (1D) defects in two-dimensional atomically thin transition metal dichalcogenides (TMDCs), have received considerable interest due to their unique structures and intriguing 1D properties. However, formation and distribution of MTBs in hybrid TMDC materials such as heterojunction remain scarcely studied. Herein, we investigate the spatial distribution, lattice registry and formation mechanism of MTBs in molecular beam epitaxy grown monolayer WSe₂–MoSe₂ lateral heterojunctions using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM). MTBs manifest a much higher density in MoSe₂ than in WSe₂ domains with a few of them spanning coherently across the domain interface. Compositionally, a Mo-dominant rather than W-dominant configuration was observed in those MTBs located in WSe₂ domains and its origin can be attributed to the preferable Mo substitution to W along the MTBs occurring at the later MoSe₂ growth period. This proposed mechanism is supported by ab-initio density functional theory calculations and substitution dynamics captured by in-situ ADF-STEM. The present study deepens our understanding of MTBs in heterostructured TMDCs, which may also serve as an excellent platform for the exploration of intriguing 1D physics.