Stacking-independent ultrafast charge transfer in TMD heterostructures. (a) Band alignment of MoS₂/WS₂ bilayers. After pumping MoS₂ A-exciton with ultrafast laser, the electron remains in MoS₂ while the hole will transfer to a lower energy at WS₂, resulting in an efficient electron–hole separation. (b) Photoluminescence spectra of MoS₂ monolayer and MoS₂/WS₂ bilayers. The two obvious peaks in MoS₂/WS₂ bilayers correspond to direct A-exciton transitions from MoS₂ (peak I) and WS₂ (peak II), respectively. (c) Transient absorption spectra of MoS₂/WS₂ bilayers by selectively probing with a higher energy light at WS₂ A-exciton resonance. (d) TEM image of an AA stacking MoS₂/WS₂ bilayer. In addition to the energy-favorable AA₁ stacking, there exists high-energy state of AA₃ stacking due to interlayer stretching and shifting. (e) Schematic of charge transfer process at the interface of AA stacking MoS₂/WS₂ bilayers, where multi-channels coexist. The apparent transfer time is mainly determined by the fastest channel. Reprinted with permission from Ref. [87]. Copyright 2017, American Chemical Society.