Ultrafast hot carrier transfer in WS2/graphene large area heterostructures

Charge transfer processes in two-dimensional van der Waals heterostructures enable upconversion of low energy photons and efficient charge carriers extraction. Here we use broadband ultrafast optical spectroscopy to track charge transfer dynamics in large-area 2D heterostructures made of epitaxial single-layer tungsten disulfide (WS2) grown by chemical vapour deposition on graphene. Selective carrier photoexcitation in graphene, with tunable near-infrared photon energies as low as 0.8 eV (i.e. lower than half of the optical bandgap of WS2), results in an almost instantaneous bleaching of the WS2 excitonic peaks in the visible range, due to the interlayer charge transfer process. We find that the charge transfer signal is strongly non-linear with the pump fluence and it becomes progressively more linear at increasing pump photon energies, while the interlayer photoinjection rate is constant in energy, reflecting the spectrally flat absorbance of graphene. We ascribe the interlayer charge transfer to a fast transfer of hot carriers, photogenerated in graphene, to the semiconducting layer. The measured sub-20-fs hot-carrier transfer sets the ultimate timescale for this process. Besides their fundamental interest, our results are technologically relevant because, given the capability of large-area deterministic growth of the heterostructure, they open up promising paths for novel 2D photodetectors, also potentially scalable to industrial platforms.