Inside the Microreactor: In Situ Real-Time Observation of Vapor-Liquid-Solid Growth of Monolayer TMDCs

Real-time observation of molten droplet-driven crystal growth provides an unprecedented in situ window into the formation of atomically thin transition metal dichalcogenides (TMDCs). Materials such as MoS₂ and WS₂ exhibit remarkable optoelectronic properties arising from their monolayer structures, enabling advanced applications that exploit valley degrees of freedom. Among various synthetic approaches, vapor-liquid-solid (VLS) growth from a low-melting molten source containing alkali, transition metal, halide, and oxygen atoms has proven highly effective for producing large single-crystal monolayer TMDCs, while also yielding distinct growth regimes including molten particle-driven nanoribbon formation. A chemical vapor deposition method is recently developed that integrates VLS growth with the spatial confinement provided by a substrate-stacked microreactor; however, the precise role of confinement and droplet dynamics remains unclear. Here, in situ the VLS growth of TMDCs inside such microreactors is directly captured using an infrared heating furnace. The microreactor, formed by sealing a transparent sapphire substrate with a Na₂WO₄-coated SiO₂/Si wafer, enables continuous observation of growth mode transitions governed by the balance of sulfur and Na₂WO₄ supply. The findings demonstrate that fine control over precursor supply rates is essential for engineering the size, morphology, and crystallinity of monolayer TMDCs in the VLS regime.