Diffusion and Surface Effects on Sodium-Promoted MoS₂ 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, MoS₂ 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 MoS₂ grows more readily on its own surface than on SiO₂, 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.