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

Abstract

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.