First-principles study of the oxidation susceptibility of WS₂, WSe₂, and WTe₂ monolayers

The environmental stability of two-dimensional (2D) transition metal dichalcogenide monolayers is of great importance for their applications in electronic, photonic, and energy storage devices. In this study, we focus on understanding the susceptibility of WS₂, WSe₂, and WTe₂ monolayers to oxygen exposure in the form of atomic oxygen and O₂ and O₃ molecules, respectively. Calculations based on the van der Waals-corrected density functional theory predicted that O₂ and O₃ molecules are weakly adsorbed on these monolayers, although atomic oxygen prefers chemisorption accompanied by a significant charge transfer from the surface to oxygen. In the physisorbed molecular configurations consisting of O₂ and O₃, the partially oxidized monolayers retain their geometrical and electronic structures. The calculated transition path as the oxygen approaches the surface shows a high-energy barrier for all cases, thus explaining the photo-induced formation of the oxidized configurations in the experiments. Furthermore, oxidizing the WS₂ monolayer is predicted to modify its electronic structure, reducing the band gap with increasing oxygen coverage on the surface. Overall, the calculated results predict the resilience of WS₂, WSe₂, and WTe₂ monolayers against oxygen exposure, thus ensuring stability for devices fabricated with these monolayers.