Charge transfer and dynamics in van der Waals trilayer hybrid heterostructures

Ultrafast interlayer charge transfer has been generally observed in a wide range of van der Waals heterostructures formed by two monolayer semiconductors. Previous studies have revealed that the transferred carriers form interlayer excitons with rather large binding energies. While such interlayer excitons are interesting systems on their own, generating free charge carriers is desired for various optoelectronic applications. For this goal, charge transfer and dynamics in heterostructures formed by F8ZnPc, few-layer transition metal dichalcogenide, and graphene are investigated by femtosecond pump-probe measurements. In such F8ZnPc/WS2/graphene samples, holes excited in F8ZnPc are found to transfer to graphene to minimize their energy. Electrons, however, reside in F8ZnPc due to the energy barrier presented by the WS2 layer. By increasing the thickness of WS2 from monolayer to 4 layers, the Coulomb interaction between the layer-separated electrons and holes is reduced, resulting in unipolar-like diffusion of holes in graphene, as evident by spatially resolved transient absorption microscopy. The finding illustrates the possibility of using band-alignment engineering with thickness control to generate free charge carriers in van der Waals heterostructures.