High-efficiency synthesis of large-area monolayer WS2 crystals on SiO2/Si substrate via NaCl-assisted atmospheric pressure chemical vapor deposition

Synthesis of monolayer WS2 crystals on SiO2/Si substrate has attracted interests due to the advantage of fabrication of field effect transistor without WS2 transfer process. Although substantial efforts have been achieved in recent years, controllable synthesis of uniform and large-area monolayer WS2 crystals on SiO2/Si substrate is still challenging. Herein, we report an elegant method to synthesize monolayer WS2 crystals on SiO2/Si substrate by using NaCl as a growth promoter in one semi-sealed quartz tube. It is found that triangular monolayer WS2 with edge lengths ranged from 10 to 460 mu m can be readily synthesized within 5 min of growth by adjusting the growth temperature and weight ratio of NaCl and WO3. The Raman mapping results indicate that the as-synthesized WS2 crystals exhibit homogeneous distributions of the crystallinity, electron doping and residual strain across the entire triangular domains regardless of their dimensions. However, the smaller WS2 crystals exhibit a higher electron doping and less residual strain compared to the larger one obtained under the same growth conditions. Importantly, the amount of WO3 used in this study is three orders lower than the commonly reported one and the semi-sealed quartz tube can be reused more than 50 times after mildly cleaning.