Atomic-scale description of 2D Janus MoSO and MoSeO formation: oxidation patterns and band-gap engineering

Transition metal dichalcogenides (TMDs) have attracted attention due to their broad-ranging physical properties. Their semiconducting characteristics make them attractive for nanotechnology applications. In particular, molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂) possess direct band gaps of 1.62 and 1.45 eV, respectively. Both monolayers are prone to oxidation in oxygen-rich environments. In this sense, we have studied the oxidation process in these 2D systems using first-principles calculations based on density functional theory. The stability of several oxidized structures under different growth conditions was analyzed via a formation-energy study, where the Janus oxidized phases are stable in oxygen-rich environments. The oxidation process is not random. Instead, it has a well-defined pattern, forming diagonal structures before reaching a complete monolayer. We have observed a systematic band-gap reduction as oxygen content increases, reaching 1.12 eV for MoSO and 0.83 eV for MoSeO, and a direct-to-indirect band-gap transition occurs at the early stages of oxidation. Our study is a step further towards designing new monolayers with engineered electronic properties and increasing reactivity towards molecules with a positive polarity on the O side of the monolayers.