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.

A new charge transfer pathway in the MoSe2–WSe2 heterostructure under the conditions of B-excitons being resonantly pumped

“…Most studies have focused on the case of resonant pumping of the low energy band-edge A-exciton in TMD HSs. 19,[33][34][35] When the intralayer B-exciton in TMD HSs is pumped with higher energies, the charge transfer process becomes more complicated due to the simultaneous excitation of multiple excitons, but it also provides abundant and useful information for understanding the charge transfer process. Here, using valley-resolved broadband pump-probe spectroscopy to study the ultrafast charge transfer process of MoSe 2 -WSe 2 heterostructure and monolayer WSe 2 under the condition of resonantly pumped B-excitons, we found a new charge transfer pathway, namely charges could be transferred at the higher energy state associated with the B-exciton, not limited to the transfer at the lower energy state associated with the band edge A-exciton.…”

Excitonic Effect Drives Ultrafast Transition in Two-Dimensional Transition Metal Dichalcogenides

“…The transitions between different exciton states have also been proposed in the WSe 2 –WS 2 heterostructure, in which intralayer excitons transform directly into interlayer excitons without an intermediated state . The efficient exciton transition can be achieved by electron–hole exchange interaction through Coulomb interaction of the charges, transition dipoles, Dexter exchange, and wave function overlap effect. , This kind of exchange interaction has also been demonstrated as a driving force for intravalley coherent coupling of A- and B-excitons in monolayer MoS 2 or inverted spin-valley polarization between A- and B-excitons in monolayer WS 2 , . When the pump energy is far above the fundamental bandgap, instead of electron–hole pairs, free carriers are generated, which fill the higher energy level in monolayer TMDCs, and relax from the upper energy level to exciton ground state.…”