Interfacial charge and energy transfer in van der Waals heterojunctions

Van der Waals heterojunctions are fast-emerging quantum structures fabricated by the controlled stacking of two-dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next-generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal-boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Forster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field.