In Situ Visualization of Precursor Retention and Volatilization in Molten-Salt Chemical Vapor Deposition of WS2

Molten-salt chemical vapor deposition (CVD), using water-soluble metal salts such as Na2MoO4 and Na2WO4 as precursors, has emerged as an important strategy for growing high-quality transition metal dichalcogenides (TMDCs). Although generally regarded as a liquid-mediated process, the role of precursor volatilization remains unclear. Here, an in situ optical visualization CVD platform directly tracks Na2WO4 precursor evolution during monolayer WS2 growth. By tuning the growth temperature, two distinct growth regimes, namely weak-volatilization and strong-volatilization regimes, are identified. Under the weak-volatilization, persistent liquid droplets drive nonuniform lateral expansion, producing irregular domain morphologies and high substrate coverage. In contrast, enhanced vapor-phase precursor transport produces smoother domain edges but lower coverage. Atomic-scale characterization further reveals regime-dependent edge structure evolution and structural uniformity. These findings demonstrate that Na2WO4-mediated WS2 growth is governed by the competition between precursor retention and volatilization rather than retained liquid intermediates alone, establishing a general framework for understanding crystal growth in molten-salt CVD.