Excited-State Carrier Dynamics in Knock-on Damage of Monolayer MoS₂ from First Principles

Electron beam irradiation is an important approach for defect engineering of two-dimensional materials. In the monolayer MoS₂, the sulfur atoms get displaced by the electron-nuclei collisions with low threshold displacement energy (TDE), which are assisted by the electronic excitations caused by the electron-electron collisions. In this study, using the real-time time-dependent density functional theory, the displacements of sulfur atoms are studied with the dynamics of the excited-state carriers comprehensively considered. The carriers rapidly recombine in the case of the band edge excitations with TDE not significantly reduced compared to the ground state, whereas the excitations of deep-level electrons yield low TDE that agrees with the experiment. Interestingly, the recombination of carriers from deep-level excitations can enhance the kinetic energy and boost the displacement. Additionally, the displaced sulfur atoms can carry net charge. These findings suggest that instead of the band edge excitations, the deep excitations should be considered in the analysis of the experiments.