Visualizing Moiré Excitons in WS₂/WSe₂ Heterostructures Using Low-Loss EELS

In heterostructures of two-dimensional (2D) van der Waals semiconductors, the moiré superlattice reconstruction modulates the electronic and excitonic properties. The signature of moiré excitons in rotationally-aligned WS₂/WSe₂ is in the form of emergent intralayer excitonic resonances in the optical absorption [1,2]. Theoretically, the excitons corresponding to these new resonances are predicted to have a distinct character compared to the A exciton. The moiré excitons can exhibit spatially modulated Wannier character and an intralayer charge-transfer character. While the new resonances have been probed using optical techniques, variations within the moiré unit cell cannot be determined this way due to the optical diffraction limit . Low-loss electron energy loss spectroscopy (EELS) is a unique experimental technique that can measure the imaginary part of the dielectric constant and provide information similar to optical absorption, but with nanoscale spatial resolution. Here we use rotationally aligned WS₂/WSe₂ heterostructures as a model system to probe excitonic spectra variations within a moiré unit cell at 77K using low-loss EELS. We determine the variation of excitons on a length scale of about ~1-2 nm and correlate it with the different atomic stacking configurations within the moiré supercell using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).<br/>[1] C. Jin et al. Nature 567, 76-80 (2019)<br/>[2] Y. Tang et al. Nature 579, 353–358 (2020)