Oxygen as a Dual-Function Regulator in MoS2 CVD Synthesis: Enhancing Precursor Evaporation While Modulating Reaction Kinetics.

Molybdenum disulfide (MoS2) is a promising 2D transition metal dichalcogenide (TMD) for optoelectronics and quantum technologies, but scalable synthesis and defect engineering remain challenging. Oxygen-assisted chemical vapor deposition (O-CVD), which introduces in situ oxygen during growth, shows excellent potential in resolving both issues at once; however, oxygen's underlying mechanistic role remains unclear. Here, we combine oxygen dosing experiments, density functional theory (DFT), computational fluid dynamics (CFD), and ab initio molecular dynamics (AIMD) to uncover the dual role of oxygen in O-CVD. First, AIMD reveals that oxygen increases MoO3 sublimation and enhances Mo3O9 supply. Concomitantly, DFT reveals that sulfur oxides (bulkier than pure S2) limit the formation of reactive MoS6 intermediates. Subsequently, by experimentally varying the oxygen flow parameters and correlating them with CFD, we decouple oxygen's roles in source-poisoning prevention (MoO3 evaporation) and growth regulation. We find that a low sulfur-to-oxygen (S:O2) ratio at the MoO3 boat and substrate during nucleation, and a high S:O2 ratio at the substrate during growth, is the key to obtaining large-area high-quality monolayer MoS2, confirmed by our optical measurements. Based on our understanding, we present a kinetic phase diagram that establishes controlled oxygen dosing as a tuning parameter for scalable, defect-controlled monolayer MoS2 synthesis.