  Two-dimensional (2D) compounds provide unique building blocks for novel
layered devices and hybrid photonic structures. However, large
surface-to-volume ratio in thin films enhances the significance of surface
interactions and charging effects requiring new understanding. Here we use
micro-photoluminescence (PL) and ultrasonic force microscopy to explore the
influence of the dielectric environment on optical properties of a few
monolayer MoS2 films. PL spectra for MoS2 films deposited on SiO2 substrates
are found to vary widely. This film-to-film variation is suppressed by
additional capping of MoS2 with SiO2 and SiN, improving mechanical coupling of
MoS2 with surrounding dielectrics. We show that the observed PL
non-uniformities are related to strong variation in the local electron charging
of MoS2 films. In completely encapsulated films, negative charging is enhanced
leading to uniform optical properties. Observed great sensitivity of optical
characteristics of 2D films to surface interactions has important implications
for optoelectronics applications of layered materials.
