First principles investigation of structural stability electronic properties and defect formation in two-dimensional BiOBr monolayer

Two-dimensional bismuth oxybromide (BiOBr) has emerged as a promising semiconductor for optoelectronic and photocatalytic applications due to its unique layered structure and tunable electronic properties. Using first-principles density functional theory (DFT) calculations with LDA, GGA-PBE, and hybrid HSE06 functionals, we systematically investigate the structural and electronic properties of pristine and defective monolayer BiOBr. We examine the effects of supercell size ( \(1\times 1\times 1\) , \(2\times 2\times 1\) , and \(3\times 3\times 1\) ) and four types of Br-related point defects: vacancies ( \(V_\text {Br}\) ), displacements, and substitutions with nitrogen ( \(N_\text {Br}\) ) and sulfur ( \(S_\text {Br}\) ). Phonon dispersion calculations and ab initio molecular dynamics simulations confirm the dynamical and thermal stability of all investigated configurations, with no imaginary frequencies or structural degradation observed across the pristine and defective supercells, validating the robustness of the defect-engineered structures at finite temperatures. Our results reveal that the pristine BiOBr exhibits an indirect band gap of 1.43 eV (LDA), 1.60 eV (GGA-PBE), and 4.50 eV (HSE06) in the primitive cell, demonstrating the well-known underestimation of band gaps by LDA and GGA functionals. The HSE06 value of 4.50 eV provides a more accurate description of the electronic structure. This indirect band gap character is maintained in all supercell sizes examined. Defect analysis (using HSE06) shows that Br vacancies reduce the band gap from 1.16 eV to 1.31 eV as the size of the supercell increases, while inducing direct gap character. Sulfur substitution preserves the direct gap with minimal variation (from 1.26 eV to 1.28 eV), while nitrogen substitution induces metallic behavior by introducing mid-gap states. Displacement defects cause moderate band gap reduction (from 1.27 eV to 1.09 eV) without altering the direct gap nature. Bader charge analysis quantifies the charge redistribution mechanisms underlying these defect-induced modifications, revealing that N substitution induces the most dramatic charge transfer (0.50 e per N atom), followed by S substitution (0.15 e ), Br vacancies (0.07 e per neighboring Bi atom), and Br displacement (0.05 e ). Effective mass calculations show that defect engineering reduces carrier effective masses by up to 35–40% for electrons, with S substitution offering the best balance of band gap tunability and transport preservation, while exciton binding energies of 0.08–0.13 eV ensure exciton stability at room temperature. These findings demonstrate that defect engineering can effectively tune the electronic structure of 2D BiOBr, with nitrogen doping emerging as a particularly promising strategy for creating metallic contacts. The results provide fundamental insights into defect physics in bismuth oxyhalides and establish design principles for optimizing BiOBr-based devices for photocatalysis, optoelectronics, and quantum technologies.