Dexter‐Type Exciton Transfer in van der Waals Heterostructures

Abstract Van der Waals (vdW) heterostructures (HSs) built on 2D materials provide an ideal platform for research of energy migration at the nanoscale. However, the underlying charge transfer mechanism in type II vdW HSs is still not well understood. Here, ultrafast exciton dynamics are investigated in trilayer‐WS 2 ‐MoS 2 ‐WSe 2 and trilayer‐MoS 2 ‐WSe 2 ‐WS 2 HSs by broadband pump‐probe spectroscopy. A two‐step process of exciton transfer in trilayer‐WS 2 ‐MoS 2 ‐WSe 2 is directly observed when the band edge exciton of WSe 2 is excited. The electrons in WSe 2 are initially transferred to the high lying electronic state of MoS 2 ‐WS 2 on a time scale of tens of femtoseconds, and then electrons eventually relax into the conduction band minimum of MoS 2 within 1 ps. Furthermore, the transfer of interlayer excitons is observed for the first time in trilayer‐MoS 2 ‐WSe 2 ‐WS 2 . Both transfer processes can be better understood by the Dexter charge exchange model. Due to the nature of Dexter type transfer that the exchange rate exponentially depends on the donor−acceptor distance, the interlayer exciton transfer rate is nearly a hundred times slower than that of exciton transition in bilayer HSs. The results deepen the understanding of charge transfer in 2D vdW HSs and also indicate that the exciton effect and orbital hybridization make HS a strong coupling system.