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

• Monolayer WS 2 crystal with edge lengths ranged from 10 to 460 μm can be readily synthesized on SiO 2 /Si substrate. • The WS 2 crystals exhibit homogeneous distributions of the crystallinity, electron doping and residual strain. • The amount of WO 3 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. Synthesis of monolayer WS 2 crystals on SiO 2 /Si substrate has attracted interests due to the advantage of fabrication of field effect transistor without WS 2 transfer process. Although substantial efforts have been achieved in recent years, controllable synthesis of uniform and large-area monolayer WS 2 crystals on SiO 2 /Si substrate is still challenging. Herein, we report an elegant method to synthesize monolayer WS 2 crystals on SiO 2 /Si substrate by using NaCl as a growth promoter in one semi-sealed quartz tube. It is found that triangular monolayer WS 2 with edge lengths ranged from 10 to 460 μm can be readily synthesized within 5 min of growth by adjusting the growth temperature and weight ratio of NaCl and WO 3. The Raman mapping results indicate that the as-synthesized WS 2 crystals exhibit homogeneous distributions of the crystallinity, electron doping and residual strain across the entire triangular domains regardless of their dimensions. However, the smaller WS 2 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 WO 3 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. [ABSTRACT FROM AUTHOR]