(Sub)nanoscale Visualization of Reconstruction-Driven Moiré Exciton Localization and Delocalization

Abstract

Spectral fingerprints in optical absorption and emission have typically been used as a signature of exciton localization in twisted moiré bilayers. However, the mechanism by which excitons become confined to specific stacking sites and their associated optical signature is experimentally unresolved. Here, we directly visualize in real space how tuning the change in extent of structural reconstruction leads to localization and delocalization of moiré excitons in the WSe2/WS2 moiré superlattice. Using cryogenic monochromated electron energy loss spectroscopy, together with first-principles GW-Bethe Salpeter equation calculations and optical spectroscopy, we uncover the physical mechanism that drives the correlation between twist-angle-dependent structural transformations, the real-space localization of moiré excitons, and their optical signatures. Surprisingly, and in contrast to the prevailing understanding, we show that the emergence of new moiré exciton resonances in the optical spectra alone is insufficient to establish exciton localization. Instead, the extent of structural reconstruction and external strain drives exciton localization, leading to new design principles for engineering moiré excitons and strain-aware quantum optoelectronic devices.