Phase transition of a MoS₂ monolayer through top layer desulfurization by He⁺ ion irradiation

Two-dimensional (2D) metal monochalcogenides have recently attracted significant interest following the extensive and intensive research into transition metal dichalcogenides (TMDs). However, the formation of transition metal monochalcogenide remains relatively unstudied. Here, we investigate the structural and electronic changes of the MoS2 monolayer by removing the top sulfur layer using low-energy He+ ion sputtering. As a result, the substoichiometric MoS x surface induces semiconducting to the metallic phase transition. Under ambient conditions, the oxidized MoS x surface restores a semiconducting state with narrowed bandgap, p-type conduction, or possibly a semimetallic state. Our findings provide an effective way to form and improve the functionality of Janus TMD monolayers.