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

Salt enhanced chemical vapour deposition of 2D transition metal dichalcogenides (TMDs) is widespread, and while many mechanisms including vapour-liquid-solid (VLS) mediated growth have been suggested, gaining a more detailed understanding remains challenging. We employ operando scanning electron microscopy (SEM) [1] to resolve the entire process of salt-assisted CVD of two widely studied TMDs, namely WS₂ and MoS₂, focusing on a model system of individual, small (&lt;100 µm), sapphire supported sodium tungstate (Na₂WO₄) salt particles.[2] We reveal support interactions that lead a salt particle to develop a lateral halo interface, that we propose is driven by sodium aluminate formation and surface eutectic melting above 630 °C. This hot salt corrosion dictates the salt wetting as well as Na and W transport, and thus upon gaseous sulphur precursor exposure dominates the spatio-temporal WS₂ nucleation and mono- and multi-layer domain expansion kinetics, all of which we can directly track by secondary electron (SE) contrast with a conventional In-Lens detector. Unlike to a conventional VLS mechanism, large (&gt;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 vicinal halo interface region with a continuous supply of W. Sapphire supported, salt-assisted MoS₂ growth shows similar behaviour. We systematically also explore support interactions on SiO2, MgO and graphite. Our insights here make a strong case that support interactions should be considered in much more detail for a deeper understanding of salt-assisted TMD CVD and when discussing the opportunities and trade-offs of processing opportunities that salts can bring.<br/>[1] Lomonosov et al. <i>Nano Lett</i> <b>2024</b>, <i>24</i>, 7084<br/>[2] Yang et al., submitted (2024)