Enhanced epitaxial growth of two-dimensional monolayer WS₂ film with large single domains

The emerging of graphene has stimulated the exploration of two-dimensional (2D) nanomaterials. As a kind of important semiconducting 2D materials, WS₂ shows great potentials in electronics, optoelectronics, and photonics, etc.; however, the synthesis of high quality, large area, uniform and epitaxial 2D WS₂ monolayer films is still a challenge. Herein, we report the epitaxial growth of WS₂ on sapphire with excellent electrical and optoelectronic properties via an enhanced chemical vapor deposition method. With the assistance of Na₂WO₄ as well as the investigation of related growth mechanism, large single crystal triangular WS₂ monolayer flakes with well-defined orientations are achieved. The corresponding single crystal domain size is larger than 1 mm together with the centimeter-scale continuous film realized. Once fabricated into transistors, the monolayers exhibit excellent device properties, such as high mobility and large on/off current ratio. Also, these devices give respectable photoresponse properties when operated in the photoconductive mode. The peak responsivity can reach 4.6 A W⁻¹. All these results demonstrate the high quality of as-synthesized WS₂ monolayers, indicating the bright future of this synthesis technology for 2D materials towards practical applications.