Interfaces determine the nucleation and growth of large NbS2 single crystals

Abstract

The role of interfaces in facilitating the growth of conventional 1D nanostructures is well established. Apparently, the presence of interfaces is also decisive in the growth of 2D transition metal dichalcogenides, but their role remains ambiguous. Here, we show that the molten Na–Nb–O liquid precursor would create microscale vapor–liquid and liquid–solid interfaces, which determine the nucleation and growth of large single-crystal monolayer 2D metallic NbS₂ by vapor–liquid–solid (VLS) method. We discover that reaction and growth are separated at the vapor–liquid interface and the liquid–solid interface, respectively, due to the insolubility between S and the liquid precursor. The vapor–liquid interface reaction reduces the concentration of NbS_x monomers in droplets, thus effectively controlling the nucleation density. The liquid–solid interface lowers the diffusion barrier of NbS₂ monomers, which increases the growth rate. We establish a 2D diffusion model and calculate and conclude that the growth rate in VLS mode is approximately 8 times that in vapor–solid (VS) mode, which is consistent with our experimental results. This finding reveals that two interfaces are the determining factors that control the nucleation and growth of large NbS₂ single crystals, which will likely contribute to the development of the wafer-scale growth of 2D materials for further device applications.