Momentum-space indirect interlayer excitons in transition-metal dichalcogenide van der Waals heterostructures

Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are dominated by tightly bound electron-hole pairs, called excitons. Creating van der Waals heterostructures by deterministically stacking individual monolayers can tune various properties via the choice of materials(1) and the relative orientation of the layers(2,3). In these structures, a new type of exciton emerges where the electron and hole are spatially separated into different layers. These interlayer excitons(4-6) allow exploration of many-body quantum phenomena(7,8) and are ideally suited for valleytronic applications(9). A basic model of a fully spatially separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is usually applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first-principles calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS2/WSe2 heterostructures where the hole resides at the Gamma point and the electron is located in a K valley. We control the emission energy of this new type of momentum-space indirect, yet strongly bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in van der Waals heterostructures and devices.