Fabrication and STM characterization of spiral MoS2-graphene/SiC heterostructure

Two-dimensional (2D) heterostructures comprising of differently stacking atomic layers are attractive owing to its flexible composition as well as the emerging new physicochemical properties. However, so far many 2D vertical heterojunctions are constructed through transfer methods, inevitably introducing interfacial impurities and thus hindering detailed atomic-level studies. In this work, we have developed a clean two-step fabrication strategy by combining ultrahigh vacuum (UHV) molecular beam epitaxy (MBE) growth with ambient chemical vapor deposition (CVD). We firstly grew single crystalline graphene film on a SiC substrate under UHV condition, and then synthesized MoS2 films on the graphene-SiC surface through CVD under inert atmosphere, thus successfully realized the construction of a well-defined MoS2-graphene/SiC heterojunction with clean surface. Particularly, we observed the MoS2 can not only grow into monolayer flakes but also form spiral structures, the latter showing layer-by-layer stacks with reduced bandgap down to ∼1.0 eV.