Moir\'e-Tunable Localization of Simultaneous Type I and Type II Band Alignment in a MoSe2/WS2 Heterobilayer

TL;DR

This study reveals that MoSe2/WS2 heterobilayers can exhibit both type I and type II band alignments depending on the twist angle, enabling tunable optoelectronic properties and complex quantum phenomena.

Contribution

It demonstrates, through combined theoretical and experimental methods, the moiré-tunable coexistence of type I and type II band alignments in MoSe2/WS2 heterobilayers, challenging previous assumptions.

Findings

Type I band alignment occurs at large twist angles.

Regions of both type I and type II alignments coexist at small twist angles.

No excitonic hybridization despite near degeneracy of conduction bands.

Abstract

Moir\'e 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, we show through a combination of first-principles GW plus Bethe Salpeter equation (GW-BSE) calculations and time- and angle-resolved photoemission spectroscopy (tr-ARPES) measurements that contrary to previous understanding, the MoSe2/WS2 heterobilayer has a type I band alignment at large twist angles and simultaneous regions of type I and type II band alignment due to the structural reconstruction in different high symmetry regions at small twist angles. In tr-ARPES, consistent with our calculations, a long-lived electron population is only observed in MoSe2 for samples with large twist angles, while in samples with small twist angles, signals from two distinct long-lived excitons are observed. 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. 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 moir\'e-tunable optoelectronic devices with intrinsic lateral heterojunctions.