Crucial Role of Crystal Field on Determining the Evolution Process of Janus MoSSe Monolayer: A First-Principles Study

Two-dimensional Janus MXY materials have been successfully synthesized from their parent species by CVD, SEAR, or PLD techniques. However, their detailed evolution process and underlying atomistic mechanism are far from understood conclusively, which are prompts for further research. Here, taking Janus MoSSe as a representation, the evolution process from MoS₂ is systematically investigated by first-principles calculation. The simulation shows that the lowest formation energy of MoS_(2-δ)Se_δ increases with selenylation ratio δ. Unexpectedly, Se atoms prefer to form a pair in next-nearest neighboring state (Se-NN-Se), eventually transferred into a growth rule of (6n + 1) during the evolution process. Particularly, it is demonstrated that the stability of the intermediate is mainly governed by the Mo 4d orbitals in different distorted triangular crystal fields, rendering a different degree of orbital splitting. Both the occupied and unoccupied Mo 4d orbitals of Se-NN-Se are farther from the Fermi level than other cases, which is clearly illustrated by d-band center theory. These findings will be helpful to understand the evolution process and the underlying atomistic mechanism of Janus MXY.