Inward growth of monolayer MoS2 single crystals from molten Na2MoO4 droplets

Two dimensional molybdenum disulfide (MoS2) holds great application prospects for next generation electronics. In order for controllable production of large-scale and high-quality MoS2 monolayer, a full picture of the growth process in all aspects is essential which remains to be developed. Recent study [Nat. Mater., 17, 535-542 (2018)] has shown that chemical vapor deposition (CVD) growth of MoS2 can involve a crawling movement of Na-Mo-C droplet in a vapor-liquid-solid (VLS) growth mechanism mediated by surface diffusion. Here, we report the inward growth of monolayer MoS2 can occur by re-evaporation of the Na-Mo-C droplets when surface diffusion is inhibited. During this regasification process, the nuclei formed at droplet surfaces are pushed away by underneath vapor until the vapor pressure drops to zero at the last atomic layer, which ensures the easy formation of large quantities of monolayers. The as-prepared MoS2 monolayers exhibit same crystal quality as those obtained by traditional CVD methods. The advancement of understanding this VLS growth process on different substrates enables us to better control the preparation of atomic thin MoS2 films on various nanoelectronic devices.