Optically active defect states and valley depolarization in monolayer MoS₂ induced by high-energy electron-beam...

Structural defects in 2D transition metal dichalcogenides are critical in modulating their optical and electrical behavior. Nevertheless, precise defect control within the monolayer regime poses a significant challenge. Herein, a high-energy (1 MeV) electron-beam irradiation strategy is harnessed to induce defects in monolayer MoS₂. Systematic variation of electron-beam irradiation time controls the electron fluence delivered to the sample and tunes the defect density, as reflected by the evolution of defect-mediated photoluminescence characteristics. The optically active defect emission appearing at ≈200-300 meV below the A exciton at 85 K exhibits a gradual increase in intensity with prolonged exposure and saturates at higher laser excitation power. Circular polarization-resolved photoluminescence spectroscopy reveals strong suppression of valley polarization of the A exciton after irradiation. Complementary X-ray photoelectron spectroscopy identifies enhanced Mo-O bonding signatures in MoS₂ following irradiation. Kelvin probe force microscopy indicates the transition to p-type doping behaviour. Detailed temperature and power-dependent photoluminescence measurements further elucidate the optical behaviour of these defect states. Density functional theory calculations using these configurations establish that the transition between the conduction band and acceptor states within the bandgap accounts for the defect emission. This work presents the fluence-dependent evolution of defect-mediated excitonic and valley responses in monolayer MoS₂ under MeV electron irradiation, thereby providing insights into the practical design constraints for optoelectronic devices operating in high-radiation environments.