Defect Mo_S Misidentified as Mo_S2 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 Mo_S2 antisite (one Mo replaces two S) is recognized as being dominant, because its calculated structure matches the distortive structure in STEM images. Therefore, Mo_S2 has received much attention in MoS₂-related defect engineering. We reveal that Mo_S (one Mo replaces one S) may be mistaken for Mo_S2, because ionized Mo_S also has similar structural distortion and can easily be ionized under electron irradiation. Unfortunately, the radiation-induced ionization and associated structural distortion of Mo_S were overlooked in previous studies. Because the formation energy of Mo_S is much lower than that of Mo_S2, 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.