We performed nanoscale confocal photoluminescence (PL), Raman, and absorption spectral imaging measurements to investigate the optical and structural properties of molybdenum disulfide (MoS2) monolayers synthesized by chemical vapor deposition method and subjected to oxygen plasma treatment for 10 to 120 s under high vacuum(1.3 x 10(-3) Pa). Oxygen plasma treatment induced red shifts of similar to 20 nmin the PL emission peaks corresponding to A and B excitons. Similarly, the peak positions corresponding to A and B excitons of the absorption spectra were red-shifted following oxygen plasma treatment. Based on the confocal PL, absorption, and Raman microscopy results, we suggest that the red-shifting of the A and B exciton peaks originated from shallow defect states generated by oxygen plasma treatment. (C) 2015 Elsevier B.V. All rights reserved.