Moiré-Tunable Localization of Simultaneous Type I and Type II Band Alignment in a Heterobilayer

Moiré heterobilayers exhibiting spatially varying band alignment and electron and hole localization that can be precisely controlled through the twist angle have emerged as exciting platforms for studying complex quantum phenomena. While most heterobilayers of transition metal dichalcogenides (TMDs) have a type II band alignment, the introduction of type I band alignment could enable stronger light-matter coupling and enhanced radiative emission. Here, our first principles calculations based on the GW plus Bethe Salpeter equation (GW-BSE) approach show that contrary to previous understanding, the MoSe2/WS2 heterobilayer has type I band alignment at large twist angles and simultaneous regions of type I and type II band alignment due to the structural reconstruction at different high symmetry regions at small twist angles. Moreover, despite the near degeneracy of the conduction bands of the two layers, no excitonic hybridization occurs, suggesting that previously observed absorption peaks in this material arise from lattice reconstruction. In tr-ARPES experiments, long-lived electron population in both layers is observed in samples with small twist angles, but for large twist angles long-lived signal is only observed in the MoSe2 layer, consistent with the calculations. Our findings clarify the complex energy landscape in MoSe2/WS2 heterostructures, where the coexistence of type I and type II band alignment opens the door to moiré-tunable optoelectronic devices with intrinsic lateral heterojunctions.