(a) Simplified band diagram illustrating the internal photoemission process taking place at a metal–semiconductor interface. Non-thermalized photoexcited carriers in metal with sufficient energy to overcome the Schottky barrier ΦB can be injected into the semiconductor before they lose their initial energy (within 100 fs for conventional metals10). The portion of the energy band filled by electrons and the bandgap of the semiconductor are shaded in blue and pale orange, respectively. Low (high) energy photon and the electronic transition following their absorption are represented by red (green) sinusoidal and vertical arrows. The out-of-equilibrium electron distributions n(E) resulting from these processes are illustrated on the left-hand side with the corresponding colours. Photoexcited electrons are depicted by blue dots and their possible transfer path is represented by blue dashed arrows. (b) Simplified band diagram of the PTI effect at a G/WSe2 interface. The ultrafast thermalization of photoexcited carriers in graphene gives rise to a hot-electron distribution n(E) with a lifetime longer than 1 ps. As the number of electrons in the hot tail (yellow shaded area) of n(E) increases, more electrons are emitted over the Schottky barrier ΦB, which generates a larger thermionic current (represented by the horizontal arrow). The colour gradient from blue to yellow illustrates the heat contained in the electron distribution. The offset between the graphene neutrality point and WSe2 conduction edge is denoted by Φ0 and was experimentally determined to be 0.54 eV (ref. 28). (c) Schematic representation of the heterostructure on a 285-nm-thick SiO2/Si substrate, to which a gate voltage (VG) is applied to modify the Fermi-level μ of the bottom graphene. An interlayer bias voltage (VB) between the top (GT) and bottom (GB) graphene flakes can be applied, and current or photocurrent flowing through GB is measured. (d) Optical image of a heterostructure composed of a 28-nm-thick WSe2 flake. The top and bottom hBN flakes are 10 and 70 nm thick, respectively. For clarity, graphene flakes are shaded in grey and outlined by a black dashed line, whereas WSe2 is coloured in orange and outlined by an orange line. Scale bar, 5 μm.