Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2.

Two-dimensional MoS2 is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS2 monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS2 monolayer growth at the atomic level. We find that solid MoO3 precursor sublimates forming ringlike molecules, such as Mo3O9, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS6 molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS2-layer edge, with the release of a S4 molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS2 CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.