Defect MoS Misidentified as MoS2 in Monolayer MoS2 by Scanning Transmission Electron Microscopy: A First-Principles Prediction

The defect types in layered semiconductors can be identified by matching the scanning transmission electron microscopy (STEM) images with the structures from first-principles simulations. In a PVD-grown MoS2 monolayer, the MoS2 antisite (one Mo replaces two S) is recognized as being dominant, because its calculated structure matches the distortive structure in STEM images. Therefore, MoS2 has received much attention in MoS2-related defect engineering. We reveal that MoS (one Mo replaces one S) may be mistaken for MoS2, because ionized MoS also has similar structural distortion and can easily be ionized under electron irradiation. Unfortunately, the radiation-induced ionization and associated structural distortion of MoS were overlooked in previous studies. Because the formation energy of MoS is much lower than that of MoS2, it is more likely to exist as the dominant defect in MoS2. Our results highlight the necessity of considering the defect ionization and associated structural distortion in STEM identification of defects in layered semiconductors.