Optical investigation of the natural electron doping in thin MoS2 films deposited on dielectric substrates

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 Si_xN_y, 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.