Structural relaxation driven exciton spectral and spatial signatures in moiré superlattices of WS₂/WSe₂

Artificial moiré superlattices of transition metal dichalcogenides formed by stacking individual monolayers have emerged as a promising platform to tune the optoelectronic properties with twist angle as an additional degree of freedom. The interplay between intralayer strain and interlayer stacking energy with varying twist angles governs the structural relaxation and the corresponding lattice symmetries. The previous studies have shown the spatial localization of moiré excitons in a twisted bilayer of WS₂/WSe₂ using monochromated scanning transmission electron microscope- electron energy loss spectroscopy (STEM-EELS) but with spectral resolution of 100 meV merging all fine spectral features. In this work, we image the fine structure of these excitons with a tenfold higher spectral resolution. Through simultaneous structural imaging, we correlate the spectral and spatial variation of individual moiré excitons of WSe₂ and WS_2, demonstrating the significance of lattice relaxation effects. We find that the lowest energy moiré exciton of WSe₂ exhibits localization at AA sites only when significant lattice reconstruction occurs; otherwise, the different moiré exciton peaks delocalize across different stacking sites.