Exciton-driven two-step charge transfer in MoS 2 –WSe 2 heterostructures revealed by valley-resolved broadband pump–probe spectroscopy

The strong excitonic effects in transition metal dichalcogenide (TMD) heterostructures (HSs) play a critical role in interlayer charge transfer. Using valley-resolved broadband pump-probe spectroscopy, we investigate the charge dynamics in MoS₂-WSe₂ HSs. Electrons transferred from WSe₂ to MoS₂ exhibit delayed valley polarization build-up, indicating a two-step process: initial population of a high-energy spin-degenerate state in MoS₂via transition from WSe₂ intralayer excitons to hot interlayer excitons, followed by sub-picosecond relaxation into lower-energy states. Holes in WSe₂ show analogous relaxation dynamics, corresponding to the relaxation of interlayer excitons. Temperature-dependent measurements confirm that these processes are nearly insensitive to thermal variations, which align well with the Dexter-type electron exchange mechanism. Our findings underscore the role of excitons in enhancing interlayer coupling and enabling robust charge separation, demonstrating the capability of valley-resolved spectroscopy for probing ultrafast processes in TMD HSs.