Vacancy-induced Formation and Growth of Inversion Domains in Transition-metal Dichalcogenide Monolayer

Sixty degree grain boundaries in semiconducting transition-metal dichalcogenide (TMDC) monolayers have been shown to act as conductive channels that have profound influence on both the transport properties and exciton behavior of the monolayers. Here, we show that annealing TMDC monolayers at high temperature induces the formation of large-scale inversion domains surrounded by such 60° grain boundaries. To study the formation mechanism of such inversion domains, we use the electron beam in a scanning transmission electron microscope to activate the dynamic process within pristine TMDC monolayers. The electron beam acts to generate chalcogen vacancies in TMDC monolayers and provide energy for them to undergo structural evolution. We directly visualize the nucleation and growth of such inversion domains and their 60° grain boundaries atom-by-atom within a MoSe2 monolayer and explore their formation mechanism. Combined with density functional theory, we conclude that the nucleation of the inversion domains and migration of their 60° grain boundaries are driven by the collective evolution of Se vacancies and subsequent displacement of Mo atoms, where such a dynamical process reduces the vacancy-induced lattice shrinkage and stabilizes the system. These results can help to understand the performance of such materials under severe conditions (e.g., high temperature).