Hyperspectral imaging of excitons within a moir\'e unit-cell with a sub-nanometer electron probe

TL;DR

This study uses advanced electron microscopy and spectroscopy to directly visualize how atomic-scale structures in moiré heterostructures influence excitonic states, revealing nanoscale confinement and potential for engineered excitonic lattices.

Contribution

It provides the first direct experimental correlation between sub-nanometer structural reconstructions and excitonic localization in moiré materials.

Findings

Excitons are confined within ~2 nm around stacking sites.

Atomic reconstructions create strongly confining moiré potentials.

Nanoscale strain engineering can tailor excitonic lattices.

Abstract

Electronic and optical excitations in two-dimensional moir\'e systems are uniquely sensitive to local atomic registries, leading to materials- and twist-angle specific correlated electronic ground states with varied degree of localization. However, there has been no direct experimental correlation between the sub-nanometer structure and emergent excitonic transitions, comprising tightly-bound pairs of photoexcited electrons and holes. Here, we use cryogenic transmission electron microscopy and spectroscopy to simultaneously image the structural reconstruction and associated localization of the lowest-energy intralayer exciton in a rotationally aligned heterostructure of WS2 and WSe2 monolayers. In conjunction with optical spectroscopy and ab initio calculations, we determine that the exciton center-of-mass wavefunction is strongly modulated in space, confined to a radius of ~ 2 nm around the highest-energy stacking site in the moir\'e unit-cell, forming a triangular lattice. Our results provide direct evidence that atomic reconstructions lead to the strongly confining moir\'e potentials and that engineering strain at the nanoscale will enable new types of excitonic lattices.