  Atomically thin transition metal dichalcogenides (TMDs) have distinct
opto-electronic properties including enhanced luminescence and high on-off
current ratios, which can be further modulated by making more complex TMD
heterostructures. However, resolution limits of conventional optical methods do
not allow for direct optical-structural correlation measurements in these
materials, particularly of buried interfaces in TMD heterostructures. Here we
use, for the first time, electron beam induced cathodoluminescence in a
scanning transmission electron microscope (CL-STEM) to measure optical
properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between
layers of hBN. We observe dark areas resulting from localized (~ 100 nm)
imperfect interfaces and monolayer folding, which shows that the intimate
contact between layers in this application-relevant heterostructure is required
for proper inter layer coupling. We also realize a suitable imaging method that
minimizes electron-beam induced changes and provides measurement of intrinsic
properties. To overcome the limitation of small electron interaction volume in
TMD monolayer (and hence low photon yield), we find that encapsulation of TMD
monolayers with hBN and subsequent annealing is important. CL-STEM offers to be
a powerful method to directly measure structure-optical correspondence in
lateral or vertical heterostructures and alloys.
