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 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 MoS (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 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.

Automated summary

The defect types in layered semiconductors can be determined by comparing scanning transmission electron microscopy (STEM) images with first-principles simulations. The Mo-S2 antisite is found to be the dominant defect in PVD-grown MoS2 due to its matching structure in STEM images. However, MoS can also be mistaken for Mo-S2, as both have similar structural distortion and can be easily ionized under electron irradiation. The overlooked radiation-induced ionization and structural distortion of Mo-S highlight the necessity of considering these factors in STEM identification of defects in layered semiconductors.