Abstract
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 SixNy, 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.
Highlights
Two-dimensional (2D) compounds provide unique building blocks for novel layered devices and hybrid photonic structures
We focus on interaction of MoS2 films with SiO2 and SixNy commonly used in photonic devices, and report low temperature PL measurements on over a 100 thin films, enabling detailed insight in interactions of MoS2 with its dielectric surrounding
We find that high mechanical coupling between a MoS2 film and the surrounding layers is only possible for capped films on thermally grown SiO2, whereas more complex morphology and poorer contact with the surrounding layers is observed for uncapped films, the effect further exacerbated for films on higher roughness plasma-enhanced chemical vapor deposition (PECVD) substrates
Summary
Two-dimensional (2D) compounds provide unique building blocks for novel layered devices and hybrid photonic structures. We find that high mechanical coupling between a MoS2 film and the surrounding layers is only possible for capped films on thermally grown SiO2, whereas more complex morphology and poorer contact with the surrounding layers is observed for uncapped films, the effect further exacerbated for films on higher roughness PECVD substrates. Following this observation, we show a direct correlation between the enhanced mechanical coupling of MoS2 with the surrounding dielectric layers and increased negative charging of the films, directly affecting spectral characteristics of PL due to the presence of the pronounced PL peak of a negatively charged www.nature.com/scientificreports trion. By comparison with recent work where MoS2 films were controllably charged by applying voltage[16], we estimate that observed charging lead to electron densities of the order of 1012 cm[22]
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