Electrical gate control of photon-spin and photon-charge conversions in van der Waals heterostructures

Semiconductor heterostructures of dissimilar materials are inherently limited by their interface quality, lattice mismatch, and intrinsic defects. These obstacles are overcome in van der Waals (vdW) heterostructures forming atomically sharp interfaces even between very different 2D materials. Our recent work demonstrates the efficient optically-created spin-polarized charge transfer across monolayer MoS₂/graphene vdW interface, including complete control of spin polarization with photon helicity and photon energy up to room temperature. However, the underlying mechanisms and the qualitative trends in both photon-spin and photon-charge conversions remain elusive. To investigate origins of photon-charge conversion, we build a dual-gated MoS₂/graphene field-effect device which allows multi-variable control of photon energy, bias voltage, and top and bottom gates. We observe an intriguing bias enhancement of photoconductivity, which behaves oppositely across graphene Dirac point. We further investigate the origins of photoconductivity using both above and below MoS₂ band gap photon excitation. Gate dependence and photon intensity dependence indicate strongly towards the dominance of graphene photothermoelectric effect for below gap excitation, while an interplay of MoS₂ photovoltaic effect and graphene photothermoelectric effect mediates above gap excitation. DFT and analytical models connect the role of bias voltage as an independent density of states modulation, which enhances the charge tunneling efficiency across the vdW barrier. In the end, we will briefly discuss the ultrafast photon-spin transfer across monolayer WSe₂/graphene interface, as well as an outlook to the challenges and future of photon-spin and photon-charge conversions based on vdW hybrid systems.