Highly responsive and broadband photodetectors based on WS2-graphene van der Waals epitaxial heterostructures

van der Waals heterostructures built from two-dimensional (2D) materials have attracted wide attention because of their fascinating electrical and optoelectronic properties. Here, we report a highly responsive and broadband photodetector based on WS2-graphene van der Waals epitaxial heterostructures, which were fabricated by directly growing single crystalline few layer WS2 nanosheets on a polycrystalline graphene film. Upon light illumination, the current apparently reduces because of the transfer of photogenerated electrons from WS2 into the underlying p-type graphene and a photo-gating effect induced by the remaining holes, which is in stark contrast to ordinary semiconducting photoconductors. Thanks to the strong and broadband absorption of WS2, the WS2-graphene heterostructure photodetector exhibits a high responsivity with a maximum of 950 A W-1 at 405 nm and a wide spectrum response ranging from 340 to 680 nm. The high performance can be attributed to the internal built-in electric field at the WS2-graphene interface, which leads to the efficient separation of photogenerated electron-hole pairs. The WS2-graphene heterostructure photodetector may have potential applications in low cost, broadband, and flexible optoelectronics.