This restriction may be lifted by integrating graphene into the TMDC photodetector. In this configuration, the low-energy photon-induced hot carriers from graphene can be injected into the TMDC and the spectral window of the TMDC photodetector is broadened. The generated charge 20 carriers in graphene can relax via carrier-carrier scattering without the assistance of the lattice. For pure graphene, this advantage becomes apparent in terms of an intrinsic response time of 1.5 ps at room temperature. However, pure graphene exhibits high dark currents and, thus, low detectivities. Combining graphene with WS2 to a van-der-Waals heterostructure preserves the ultrafast hot carrier cooling and results in a response time of 1.2 ps under 1560 nm excitation (Figure ). This photodetector combines the advantage of the low dark current in TMDCs and the fast hot carrier cooling as well as infrared sensitivity of graphene. If the extrinsic limits of these devices could be lifted at least to some extent, these ultrafast intrinsic response times suggest that GHz device operation should be easily feasible with TMDCs, even in the infrared regime.