Chemical Vapor Deposition Synthesis of MoS₂ Layers from the Direct Sulfidation of MoO₃ Surfaces Using Reactive Molecular Dynamics Simulations

Atomically thin MoS₂ layer, a promising transition metal dichalcogenide (TMDC) material, has great potential for application in next-generation electronic and optoelectronic devices. Chemical vapor deposition (CVD) is the most effective technique for the synthesis of high-quality MoS₂ layers. During CVD synthesis, monolayered MoS₂ is generally synthesized by sulfidation of MoO₃. Although qualitative reaction mechanisms for the sulfidation of MoO₃ have been investigated by previous studies, the detailed reaction processes, including atomic-scale reaction pathways and growth kinetics, have yet to be fully understood. Here, we present quantum-mechanically informed and validated reactive molecular dynamics simulations of the direct sulfidation of MoO₃ surfaces using S₂ gas precursors. Our work clarifies the reaction mechanisms and kinetics of the sulfidation of MoO₃ surfaces as follows: the reduction and sulfidation of MoO₃ surfaces occur primarily at O-termination sites, followed by unsaturated Mo sites; these local reaction processes lead to nonuniform MoO_xS_y surface structures during the CVD process. After annealing the MoO_xS_y samples, the crystallized surface structures contain voids, and three different types of local surface complexes (MoO_x, MoO_xS_y, and MoS₂-like surface regions), depending on the fraction of S ingredients on the MoO_xS_y surface. These results, which have been validated by our reactive quantum molecular dynamics simulations and previous experimental results, provide valuable chemical insights into the CVD synthesis of large-scale and defect-free MoS₂ layers and other layered TMDC materials.