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 E-2g(1) 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 E-2g(1) 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.