Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moire superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at similar to 1.35 eV in the WS2/WSe2 heterobilayer to an indirect Gamma-K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures. The twist-angle-modulated excitons in vertically stacked WS2/WSe2 heterobilayers are proven to be an indirect G-K intralayer transition, with a nearly in-plane transition dipole moment.

Automated summary

Stacking atomically thin films allows for the construction of van der Waals heterostructures with exotic functionalities. Here, we investigate the optical transitions in WS2/WSe2 heterobilayers and attribute a specific transition to an indirect Gamma-K transition. Our findings provide new insights into the properties of twist-angle-dependent excitons.