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The excitonic states of transition metal dichacolgenide (TMD) monolayers areheavily influenced by their external dielectric environment based on thesubstrate used. In this work, various wide bandgap dielectric materials, namelyhexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride(Si$_3$N$_4$), under different configurations as support or encapsulationmaterial for WS$_2$ monolayers are investigated to disentangle the factorscontributing to inhomogeneous broadening of exciton absorption lines in TMDsusing electron energy loss spectroscopy (EELS) in a scanning transmissionelectron microscope (STEM). In addition, monolayer roughness in eachconfiguration was determined from tilt series of electron diffraction patternsby assessing the broadening of diffraction spots by comparison withsimulations. From our experiments, the main factors that play a role inlinewidth broadening can be classified in increasing order of importance by:monolayer roughness, surface cleanliness, and substrate-induced chargetrapping. Furthermore, because high-energy electrons are used as a probe,electron beam-induced damage on bare TMD monolayer is also revealed to beresponsible for irreversible linewidth increases. \textit{h}-BN not onlyprovides clean surfaces of TMD monolayer, and minimal charge disorder, but canalso protect the TMD from irradiation damage. This work provides a betterunderstanding of the mechanisms by which \textit{h}-BN remains, to date, themost compatible material for 2D material encapsulation, facilitating therealization of intrinsic material properties to their full potential.

Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening