  The excitonic states of transition metal dichacolgenide (TMD) monolayers are
heavily influenced by their external dielectric environment based on the
substrate used. In this work, various wide bandgap dielectric materials, namely
hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride
(Si$_3$N$_4$), under different configurations as support or encapsulation
material for WS$_2$ monolayers are investigated to disentangle the factors
contributing to inhomogeneous broadening of exciton absorption lines in TMDs
using electron energy loss spectroscopy (EELS) in a scanning transmission
electron microscope (STEM). In addition, monolayer roughness in each
configuration was determined from tilt series of electron diffraction patterns
by assessing the broadening of diffraction spots by comparison with
simulations. From our experiments, the main factors that play a role in
linewidth broadening can be classified in increasing order of importance by:
monolayer roughness, surface cleanliness, and substrate-induced charge
trapping. Furthermore, because high-energy electrons are used as a probe,
electron beam-induced damage on bare TMD monolayer is also revealed to be
responsible for irreversible linewidth increases. \textit{h}-BN not only
provides clean surfaces of TMD monolayer, and minimal charge disorder, but can
also protect the TMD from irradiation damage. This work provides a better
understanding of the mechanisms by which \textit{h}-BN remains, to date, the
most compatible material for 2D material encapsulation, facilitating the
realization of intrinsic material properties to their full potential.
