van der Waals photothermoelectric effect in atomic layer heterojunctions

Two-dimensional (2D) van der Waals (vdW) heterostructures provide exceptional opportunities for new physics and devices due to their unprecedented ability to tune the electronic, optical, magnetic, and spintronic properties by atomic layer stacking and electrostatic gating. Harnessing this versatility requires a fundamental understanding of light-matter interactions and establishing new functionalities for photon-charge and photon-spin conversions. Here, we report the first observation of a highly-tunable vdW photothermoelectric effect in dual-gated MoS₂/graphene junctions with a striking multiple-polarity switching of photocurrent as a function of junction bias and carrier density. In stark contrast to photovoltaic effects arising from excitonic absorption in MoS₂, the vdW photothermoelectric effect originates from photoexcitation of hot electrons in graphene and thermoelectric transport across the vdW junction. Systematic studies of photoconductance reveal vdW photothermoelectric effect as the dominant mechanism for photocurrent generation at room temperature, as opposed to excitonic absorption. These findings, further corroborated by our drift-diffusion bipolar transport model, provide an important step for understanding and control of vdW-interface devices.