Impact of quantum point defects on the electronic and optical properties of NbOX[formula omitted] (X: Cl, Br, I) monolayers

Abstract

Monolayer niobium oxychloride (NbOCl2), synthesized as reported in Guo et al. (2023), exhibits potential as a quantum light source. Our density functional theory (DFT) study assesses niobium oxyhalide monolayers (NbOX2, X = Cl, Br, I), focusing on quantum point defects’ impact on electronic behavior. Vacancy-induced magnetism is observed in these monolayers; oxygen and niobium vacancies trigger magnetism, contrasting with non-magnetic halogen vacancies. The energy difference between magnetic and non-magnetic states is contingent on vacancy type and halogen mass, with oxygen vacancies showing an energy increase and niobium vacancies a decrease as halogen mass rises. Doping effects are notable; halogen and oxygen vacancies result in n-type doping, while niobium vacancies lead to p-type. In the NbOI2 monolayer, the Nb+1 vacancy is found to be more stable than the neutral vacancy. Conversely, for other charge states and vacancies, the neutral state remains the most stable. Magnetization is significantly enhanced by oxygen and halogen bivacancies, with spin energy difference and magnetization scaling with the halogen’s atomic number. The optical spectrum analysis shows that defects can extend absorption into the infrared, with anisotropic optical properties dependent on light polarization direction. These findings highlight the tunability of niobium oxyhalide monolayers for quantum technology applications, emphasizing the role of defects in modulating electronic and optical characteristics.