Salt-Assisted MoS2 Growth: Molecular Mechanisms from the First Principles

Two-dimensional transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS₂), have aroused substantial research interest in recent years, motivating the quest for new synthetic strategies. Recently, halide salts have been reported to promote the chemical vapor deposition (CVD) growth of a wide range of TMD. Nevertheless, the underlying promoting mechanisms and reactions are largely unknown. Here, we employ first-principles calculations and ab initio molecular dynamics (AIMD) simulations in order to investigate the detailed molecular mechanisms during the salt-assisted CVD growth of MoS₂ monolayers. The sulfurization of molybdenum oxyhalides MoO₂X₂ (X = F, Cl, Br, and I)─the form of Mo-feedstock dominating in salt-assisted synthesis─has been explored and displays much lower activation barriers than that of molybdenum oxide present during conventional "saltless" growth of MoS₂. Furthermore, the rate-limiting barriers appear to depend linearly on the electronegativity of the halogen element, with oxyiodide having the lowest barrier. Our study reveals the promoting mechanisms of halides and allows growth parameter optimization to achieve even faster growth of MoS₂ monolayers in the CVD synthesis.