Due to the extremely large surface to bulk ratio,the properties of single layer (SL) MoS2 are largely determined by its interaction with environment.One critical interaction process that has been intensively investigated is the oxidation process of MoS2.Despite numerous previous explorations,the detailed mechanism regarding how MoS2 reacts with oxygen is still not well understood.In this work,we systematically investigate the impact of intrinsic and pre-created defects on the oxidation process of SL MoS2.For pristine SL MoS2,the oxidation is found to initiate near point defects and grain boundaries,leading to the formation of triangle pits in the basal plane and cracks near the grain boundaries.The pre-created defects introduced by ion irradiation are found to serve as the oxidation center,resulting in a more uniform oxidation process.The oxidation is found to introduce p-type doping in the SL MoS2,leading to the blue shift of Raman and photoluminescence (PL) spectra.The shift is found to be more for the region near the grain boundary and for the samples with more pre-created defects.Our results suggest that the presence of defects can strongly promote the oxidation reaction of SL MoS2 in ambient condition,which significantly affects the stability and functionality of materials.