  The outstanding properties of transition metal dichalcogenide (TMD)
monolayers and their van der Waals (vdW) heterostructures, arising from their
structure and the modified electron-hole Coulomb interaction in 2D, make them
promising candidates for potential electro-optical devices. However, the
production of reproducible devices remains challenging, partly due to
variability at the nanometer to atomic scales. Thus, access to chemical,
structural, and optical characterization at these lengthscales is essential.
While electron microscopy and spectroscopy can provide chemical and structural
data, accessing the optical response at the nanoscale through electron
spectroscopies has been hindered until recently. This review focuses on the
application of two electron spectroscopies in scanning (transmission) electron
microscopes, namely cathodoluminescence and electron energy-loss spectroscopy,
to study the nano-optics of TMD atomic layers and their vdW heterostructures.
We discuss how technological advancements that can improve these
spectroscopies, many of which are already underway, will make them ideal for
studying the physics of vdW heterostructures at the nanoscale.
