Ultrafast interfacial charge transfer and fast interface relaxation in graphene-WS2 van der Waals heterostructures

Graphene-WS2 (Gr-WS2) van der Waals heterostructures have emerged as a versatile platform for ultrafast optoelectronics, offering unprecedented control over interfacial charge dynamics. However, the precise role of intrinsic defect states in modulating these processes remains a critical unresolved issue. In this work, we systematically investigate the carrier dynamics in monolayer WS2 and Gr-WS2 heterostructures using femtosecond transient absorption spectroscopy. By employing pump photon energies both above and below the WS2 bandgap, we provide evidence that ultrafast interfacial charge transfer from graphene to WS2 occurs under sub-bandgap excitation. Under above-gap excitation, the insertion of a graphene layer markedly accelerates the overall carrier population decay compared with pristine WS2. Quantitative analysis of the decay dynamics reveals reductions in both fast and slow effective relaxation components in the Gr-WS2 heterostructure. For pristine WS2, the fast component is mainly associated with surface-defect trapping in the chemical vapor deposition-grown monolayer, whereas its pronounced shortening in the Gr-WS2 heterostructure indicates the emergence of an additional graphene-mediated interfacial relaxation pathway. The slow component is mainly attributed to electron–hole recombination. These findings clarify the interplay between interface-modulated relaxation dynamics and defect-influenced carrier trapping in two-dimensional heterostructures, offering valuable guidance for the development of high-speed optoelectronic devices based on two-dimensional heterostructures.