Operando Scanning Electron Microscopy Study of Support Interactions and Mechanisms of Salt-Assisted WS₂ Growth

Salt enhanced chemical vapor deposition of WS₂ and related 2D materials is widespread, and while many mechanisms including vapor-liquid-solid (VLS) mediated growth have been suggested, gaining a more detailed understanding remains challenging. We employ operando scanning electron microscopy to resolve the entire process of salt-assisted CVD of WS₂, focusing on a model system of individual, small (<100 μm), sapphire supported sodium tungstate (Na₂WO₄) salt particles. We reveal support interactions that lead a salt particle to develop a lateral halo interface, driven by surface eutectic melting above 630 °C. This halo dictates the salt wetting as well as Na and W transport, and thus upon gaseous sulfur precursor exposure dominates the spatiotemporal WS₂ nucleation and mono- and multilayer domain expansion kinetics, all of which we can directly track by secondary electron (SE) contrast with a conventional In-Lens SE detector. Unlike for a conventional VLS mechanism, large (>20 μm) monolayer WS₂ formation does not involve the salt droplet directly attached to the growth facets, rather the salt droplet drives WS₂ layer growth in the contiguous halo interface region with a continuous supply of W. We compare this to SiO₂ and NaOH treated sapphire where corrosive surface roughening dictates the salt wetting, and critically discuss our findings in the context of the connected wider literature.