Relativistic first-principles study of the impact of native defects on the electronic and optical properties of monolayer MoS₂

Molybdenum disulfide (MoS₂) monolayers are promising candidates for next-generation optoelectronics due to their direct bandgap and high carrier mobility. However, the performance of practical devices is often limited by the presence of native point defects introduced during synthesis. This work presents a systematic first-principles investigation of sulfur (V_S) and molybdenum (V_Mo) vacancies in monolayer MoS₂ using Density Functional Theory (DFT) with Spin-Orbit Coupling (SOC). A supercell approach was employed to determine the thermodynamic stability, electronic-structure, and optical properties of these defects to explain their role in tuning material performance. Formation energy analysis reveals that the sulfur vacancy is the most stable native defect (+1.49 eV), introducing unoccupied mid-gap states that enable sub-bandgap optical transitions in the infrared region. In contrast, V_Mo is energetically expensive (+4.04 eV) and introduces deep-lying defect states. Calculations show that these states merge with the valence band edge, leading to a metallic-like electronic structure with a broadband optical response, though the high formation energy suggests such features would be rare in equilibrium samples. The dominance of sulfur vacancies and their distinct optical signatures highlight the potential of MoS₂ for defect-assisted infrared photodetection.