Evolution of Raman scattering and electronic structure of ultrathin molybdenum disulfide by oxygen chemisorption

Molybdenum disulfide (MoS₂) has attracted vast interest in the fields of electronic, optoelectronic, and valleytronic applications due to its unique properties. Because of its low dimensionality, MoS₂ is susceptible to oxygen absorption in ambient environments, which can alter its electrical and optical properties. Here, a new method to introduce oxygen chemisorption in ultrathin MoS₂ by controlled oxygen plasma treatment is presented. Using Raman spectroscopy, a red shift in the frequency of E¹_2g mode with increasing oxygen chemisorption is found, whereas, the frequency of A_1g mode is fixed. Interestingly, the absorption peak in the photoluminescence spectra red shifts, indicating an optical band gap reduction upon oxygen chemisorption. The behaviors of these different shifts are reproduced and elucidated by density functional theory calculations. It is found that the red shift of the E¹ _2g Raman peak is caused by a softening of in-plane Mo-S force constants, while the red shift in the photoluminescence peak is due to a reduction of the electronic band gap in oxygen-chemisorbed MoS₂. The results shine light on the fundamental understanding of chemical interactions between oxygen and MoS₂ and provide an alternative way to achieve band gap engineering in MoS₂.