Atomic-scale control of electronic states in polar 2D oxyhalides via Br-site defects

Defect engineering is a powerful approach for tuning the physical properties of two-dimensional materials, enabling control over their electronic, optical, and quantum behavior for applications in nanoelectronics, optoelectronics, and energy devices. While defects in well-known covalent systems such as graphene and transition metal dichalcogenides have been extensively studied, considerably less attention has been devoted to low-symmetry ionic–covalent materials, including polar 2D metal oxyhalides. For these systems, the relationship between atomic-scale defects and electronic structure remains insufficiently understood, despite the potential for unusual responses arising from mixed bonding and structural asymmetry.In this work, we use density functional theory to systematically investigate Br-site point defects in the AlOBr monolayer, a representative polar oxyhalide. Three classes of defects are examined: vacancies, substitutional doping, and atomic displacements. Our results show that removing a Br atom induces a semiconductor-to-metal transition driven by localized electronic states near the Fermi level. Substitutional doping further demonstrates a strong dependence on atomic size. In the 2×2×1 supercell, both smaller atoms (C, Si, Ge) and larger atoms (Ba, Sr, Te) produce metallic behavior even at 50%–100% concentrations. However, when the concentration is reduced using a 3×3×1 supercell, Ba uniquely preserves semiconducting behavior, exhibiting a direct band gap of 2.31 eV, whereas all other substituents retain metallic characteristics. Electron Localization Function analysis shows that smaller dopants promote electron delocalization, while larger atoms introduce tensile strain that alters bonding character and charge localization.Finally, displacement defects created by slight shifts in Br positions maintain the semiconducting state but decrease the band gap from 4.92 to 3.47 eV, demonstrating a more gradual and controllable modulation. These findings highlight that in 2D AlOBr, the type of defect is more critical than defect presence alone, offering new insights for tailoring electronic properties in polar 2D oxyhalides and guiding defect-engineered material design.