On the Spontaneous Structural Reconstructions of Chalcogen-Depleted MoS$_2$, MoSe$_2$, and Janus MoSSe Monolayers

Abstract

Removing one chalcogen layer from a transition-metal dichalcogenide monolayer leaves an undercoordinated sheet that reconstructs as the remaining chalcogen atoms cross the metal plane. Here, we combine \textit{ab initio} and reactive molecular dynamics (MD) simulations of depleted MoS$_2$, MoSe$_2$, and Janus MoSSe, examining each remaining chalcogen on two parent lattices, to show that the remaining chalcogen type determines the structural reconstruction, with only a minor influence from the parent lattice. In \textit{ab initio} trajectories of the Janus-derived sheets, Se crosses the Mo plane already at 300~K, whereas S stays in place at 300~K and the sheet reconstructs only at 900~K. With a free in-plane cell, the crossings precede an anisotropic lattice contraction, which therefore results from the structural reconstruction. Reactive MD simulations of 50~nm sheets show that, when S remains, the Mo lattice stays crystalline in nanometric domains that disappear with time and temperature, with pores/holes at 900~K. In contrast, when Se remains, the Mo lattice loses crystalline order within the first picosecond, and 36--39\% of the Se atoms cross at every temperature. In a depleted lateral MoS$_2$/MoSe$_2$ heterostructure, both modes coexist in one sheet, each half reconstructing like its single-chalcogen counterpart, suggesting chalcogen patterning as a route to define crystalline and disordered regions in a single monolayer.