  The integration of two-dimensional transition metal dichalcogenide crystals
(TMDCs) into a dielectric environment is critical for optoelectronic and
photonic device applications. Here, we investigate the effects of direct
deposition of different dielectric materials (Al$_2$O$_3$, SiO$_2$, SiN$_x$)
onto atomically thin (monolayer) TMDC WS$_2$ on its optical response. Atomic
layer deposition (ALD), electron beam evaporation (EBE), plasma enhanced
chemical vapour deposition (PECVD), and magnetron sputtering methods of
material deposition are investigated. The photoluminescence (PL) measurements
reveal quenching of the excitonic emission after all deposition processes. The
reduction in neutral exciton PL is linked to the increased level of charge
doping and associated rise of the trion emission, and/or the localized (bound)
exciton emission. Furthermore, Raman spectroscopy allows us to clearly
correlate the observed changes of excitonic emission with the increased levels
of lattice disorder and defects. Overall, the EBE process results in the lowest
level of doping and defect densities and preserves the spectral weight of the
exciton emission in the PL, as well as the exciton oscillator strength.
Encapsulation with ALD appears to cause chemical changes, which makes it
distinct from all other techniques. Sputtering is revealed as the most
aggressive deposition method for WS$_2$, fully quenching its optical response.
Our results demonstrate and quantify the effects of direct deposition of
dielectric materials onto monolayer WS$_2$, which can provide a valuable
guidance for the efforts to integrate monolayer TMDCs into functional
optoelectronic devices.
