Propensity of oxidation of transition metal dichalcogenide monolayers in relation to physical configuration of chalcogen vacancies

Using first-principles calculations, we studied how the physical configuration of chalcogen vacancies impact the initial step of surface oxidation of two-dimensional transition metal dichalcogenides (TMDs) where the ${\mathrm{O}}_{2}$ molecule is adsorbed and dissociated into two O atoms. The effects of surface defects on oxidation are studied by calculating the adsorption energies of the ${\mathrm{O}}_{2}$ molecule and O atoms on the surface as a function of defect size. We showed that the adsorption of ${\mathrm{O}}_{2}$ on the surface with one or more chalcogen vacancies is thermodynamically more favorable compared to the pristine surface. Furthermore, the stability of adsorption of two O atoms increases with the number of chalcogen vacancies. The calculated projected density of states indicates that the dangling bond (in-gap) states created by the chalcogen vacancies are passivated by the adsorption of O atoms. Oxygen adsorption kinetics calculation shows that there exists a significant energy barrier of about 1 eV for the dissociation of ${\mathrm{O}}_{2}$ on a surface with only single chalcogen vacancy, whereas a substantially reduced barrier is observed for a surface with two or more vacancies. Our findings suggest that even though vacancy-free two-dimensional (2D) TMDs might not be practically available, the 2D TMD crystals with mainly single chalcogen vacancy defects should be resistant against oxidation and thus provide critical guidance to researchers regarding optimizing the density and clustering of chalcogen vacancies during the crystal growth.