Fig. 2. Scheme and characterization of Gr/WS₂ heterostructure.

(A) Scheme showing photoexcited hot electron transfer from graphene to WS₂ underneath before electron-hole coalescence. (B) Optical image of a representative Gr/WS₂ heterostructure. Scale bar, 5 μm. (C) Atomic force microscopy height profile of a Gr/WS₂ heterostructure showing a ~0.9-nm height difference between WS₂ and substrate and ~0.4 nm between graphene and WS₂. (D) Scheme of quasi-particle band alignment between graphene and WS₂. The barrier height between the graphene Dirac point and WS₂ CBM is estimated to be ~0.9 eV without considering excitonic effect or interfacial effect. VBM, valance band maximum. (E) Reflectance contrast (δR) spectra of graphene, WS₂ monolayers, and Gr/WS₂ heterostructure. The summed spectrum of isolated graphene and WS₂ monolayers is also shown for comparison, showing little contribution from interlayer CT transition.