Spin-Conserved Hot Charge Transfer Exciton Formation and Cooling at the Two-Dimensional Semiconductor Interface

Understanding the spin and charge transfer (CT) process at the 2D semiconductor interface is of both fundamental and practical importance for photocatalysis and optoelectronics. However, experimentally resolving the interfacial CT dynamics with both species, temporal, and energy information remains challenging. Herein, we exploit the spin degree of freedom to directly visualize the formation and cooling of hot interlayer CT excitons at the 2D interface using spin-resolved ultrafast transient absorption (TA) spectroscopy. We reveal a universal two-stage exciton interfacial process: an initial ultrafast (∼0.1 ps) spin-conserved electron injection forming a weakly bound delocalized hot CT exciton with a hot electron in the accepting layer, followed by a slow hot electron intralayer cooling over hundreds of femtoseconds to yield a lowest-energy tightly bound CT exciton with a band-edge electron and hole. The markedly slower cooling relative to the transfer step indicates that the electron and hole at the 2D type II interface can maintain a transient loosely bound and delocalized phase, which can promote the long-range charge separation and light-to-charge conversion. Indeed, spin-resolved TA measurements on ternary heterostructures directly confirm the spin-conserved long-range electron transfer across multiple interfaces. This study establishes a unified picture of spin-dependent interfacial charge transfer and cooling at a 2D semiconductor interface and provides guiding principles in next-generation light-harvesting and photon-to-charge conversion devices.