Origin of Trapped Intralayer Wannier and Charge-Transfer Excitons in Moiré Materials.

Abstract

Moiré materials provide a versatile platform for engineering excitons, enabling next-generation optoelectronic applications. Continuum models are widely used to study moiré excitons due to their efficiency, but they often disagree with ab initio many-body approaches, as seen for intralayer excitons in WS₂/WSe₂ heterobilayers. Here, we resolve these discrepancies using an atomistic, quantum-mechanical framework based on the Bethe-Salpeter equation with Wannier functions as the electronic structure basis, showing that dielectric screening from hBN encapsulation is essential to reproduce experimentally observed exciton features. Our analysis reveals that exciton behavior emerges from a subtle competition between Wannier and charge-transfer characters, driven by stacking-dependent intralayer bandgap variations and environment-tuned electron-hole interactions. We show that the lowest-energy bright intralayer excitons are Wannier-like in WS₂/WSe₂ heterobilayers but charge-transfer-like in twisted WSe₂ homobilayers, despite comparable moiré sizes. These results establish atomistic modeling as a powerful tool for understanding and controlling excitonic phenomena in moiré materials.