Reversed charge transfer in a type I MoS2/PtSe2 heterostructure probed by ultrafast two-dimensional electronic spectroscopy

Abstract

Charge transfer processes are crucial for determining the optoelectronic properties of heterostructures constructed from atomically thin transition metal dichalcogenides. Despite significant studies of the resulting inter-layer excitons in type II heterostructures, much less is known about the charge transfer mechanism and dynamics in type I heterostructures. Here, we reveal a two-step reversed charge transfer process in a type I MoS₂/PtSe₂ heterostructure using ultrafast two-dimensional electronic spectroscopy with a broadband pulse that simultaneously excites MoS₂ and PtSe₂. Unique cross-peaks located at low excitation energy and high detection energy indicate primary hot carrier transfer within 1 ps and secondary Auger-assisted carrier transfer occurring between 6 and 100 ps from the small bandgap PtSe₂ to the large bandgap MoS₂. This charge transfer creates an out-of-plane interfacial electric field across the heterostructure interface, which dynamically blue shifts the exciton energy of MoS₂via the combined effect of the Stark effect and the reduction of exciton binding energy. Charge transfer from the large bandgap MoS₂ to the small bandgap PtSe₂ is found to be not significant, possibly due to the resonant optical excitation that creates strongly bound excitons in the MoS₂ layer, making the forward charge transfer inefficient. The results demonstrate the opportunity for functional optoelectronic and photocatalytic materials with type I band alignment under sub-bandgap excitation conditions by making use of the characteristics of hot carriers generated by low energy photons.