Sub-Nanometer Hyperspectral Imaging of Exciton Confinement within a Moiré Unit Cell in a WSe₂/WS₂ Heterostructure

Atomically precise heterostructures of two-dimensional crystals like graphene and transition metal dichalcogenides can be prepared without the constraints of epitaxy by stacking monolayers that are held together by van der Waals forces. The optical and electronic properties of such heterostructures are sensitive to the moiré potential created by the lattice mismatch or relative angular alignment between the constituent monolayers. Here, we use transmission electron microscopy and spectroscopy at cryogenic temperatures to simultaneously image the structure and the excitonic response of the lattice mismatched stack of WSe₂/WS₂. We observe structural reconstruction such that the area of the highest energy stacking site is minimized. Together with optical spectroscopy and ab initio calculations, we discern that the intralayer exciton center of mass wavefunction is localized around this highest energy stacking site to a radius of around 2 nm.