Saturation and photon-energy dependence of ultrafast charge transfer in vdW heterostructures

We have directly monitored the interlayer charge transfer after photoexcitation in type-II van der Waals heterostructures of monolayer transition metal dichalcogenides. Charge transfer is found to occur on the fs time scale by recording the electromagnetic transient emitted at THz frequencies using THz time-domain spectroscopy. This approach, insensitive to spectral changes of the materials and competing charge-neutral processes, is ideal for studying the dynamics at high excitation fluences and as a function of excitation photon energy. At low excitation fluence, we observe a linear increase in the interlayer current with fluence. However, as the fluence is increased, we see a clear saturation. We analyze this effect in terms of band re-alignment induced by the electric field from charge transfer itself. Such unavoidable saturation puts a limit on the power handling capability of these heterostructures and needs to be carefully characterized for practical applications. With further calibration, such measurements also offer a new route to determine band offsets. We further report the insensitivity of charge transfer dynamics to excitation photon energy within the instrument response. We discuss its implication on the mechanism of interlayer charge transfer.