Diffusion and Surface Effects on Sodium-Promoted MoS Growth Observed in Operando.

Understanding precursor diffusion and substrate interaction is key to advancing chemical vapor deposition (CVD) of transition metal dichalcogenides (TMCs), yet direct observation has remained a challenge due to limited real-time observation. Here, MoS growth is directly monitored to investigate the kinetics and influence of the precursor/sodium droplet eutectic (SODE). Serving as a catalyst, SODE migrates from the basal plane to the edges and substrate interface, promoting growth and enabling grain translation and rotation. Kinetic analysis shows MoS grows more readily on its own surface than on SiO, indicating a thermodynamic-kinetic interplay supported by density functional theory calculations. Notably, larger SODE droplets enhance such grain dynamics, while submicron-scale SODE exhibits extended diffusion, enabling uniform, large-area growth. These findings highlight the critical role of molten metal diffusion in growth continuity and provide new insights for optimizing scalable, cost-effective TMC fabrication.