We use micro-photoluminescence (PL) and ultrasonic force microscopy to explore the effects of dielectric substrate and capping on optical properties of a few mono-layer MoS2 films. PL lineshapes and peak energies for uncapped films are found to vary widely. This non-uniformity is dramatically suppressed by capping with SiO2 and SiN, improving mechanical coupling of MoS2 with the surrounding dielectrics. Capping also leads to pronounced charging of the films, evidenced from the dominating negative trion peak in PL.
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17 pages, 5 figures