Defect Mo S Misidentified as Mo S2 in Monolayer MoS 2 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 MoS 2 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 MoS 2 -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 MoS 2 . Our results highlight the necessity of considering the defect ionization and associated structural distortion in STEM identification of defects in layered semiconductors.