Modification of Mono-layer MoS₂ Through Post-Deposition Treatment and Oxidation for Enhanced Optoelectronic Properties

Atomic layer deposition is a great tool for growing large-scale high-quality mono-layer (ML) MoS₂ and with a dedicated high temperature post-deposition treatment the amount of sulfur vacancies can be controlled. Sulfur-containing atmospheres, such as H₂S, lead to MoS₂ with low number of sulfur vacancies and increased stability against oxidation in air, whereas post-annealing treatments in N₂ increase the number of vacancies and the material is more prone to oxidation. Post-deposition annealing treatments thereby offer a great possibility to tune the properties of the 2D layer.<br/><br/>Here we show that the sulfur vacancy-rich ML MoS₂ films oxidize already at room temperature, which strongly affects the photoluminescence yield, the MoS₂-Al₂O₃ substrate interaction and the structural integrity of the films. We used X-ray photoelectron spectroscopy to monitor the formation of MoO₃ and possibly MoS_2−xO_x after exposure to air and to quantify the number of sulfur defects in the films. Atomic force microscopy (AFM) measurements allow us to pinpoint the exact regions of oxidation and to develop a dedicated low temperature heating procedure to remove the oxidized species, leading to MoO₃-free MoS₂ films. Interestingly, AFM and Kelvin probe force microscopy show that the MoS₂-Al₂O₃ substrate coupling is changed, which also affects the ML MoS₂ work function significantly (variations by approximately 300 meV). In addition, the reduction in MoS₂-substrate coupling leads to a 10-fold increase in the PL intensity, and the ratio between trions and neutral excitons is changed.<br/><br/>Our work highlights the importance of oxidized sulfur vacancies and provides useful methods to measure and manipulate the number of vacancies on MoS₂. Furthermore, the changes in the MoS₂-substrate interaction via sulfur vacancies and oxidation offer a new pathway to tune the optoelectronic properties of the 2D films.