The properties of transition-metal dichalcogenides (TMDCs) are highly sensitive to doping and surface-state defects, making it crucial to fabricate high-performance nanoelectronic devices from defect-free materials and gate dielectrics that have a low interface-state density. In this work, the optical and structural properties of mechanically exfoliated mono-, bi-, and trilayer thick TMDCs, with Al₂O₃, Si₃N₄, or SiO₂ as a potential gate dielectric layer, are investigated. The photoluminescence (PL) and micro- Raman results indicate that all the dielectrics investigated increase the doping of the TMDC monolayers, quench the emission of neutral excitons, and enhance the trion emission. Plasma-enhanced chemical vapor deposition was found to generate more defects in the monolayer TMDCs than atomic layer deposition. We establish the relationship between the dielectric deposition process and the optical properties of TMDCs, which could be of interest for future nanoelectronics based on 2D materials.