B-Doped ZnO Nanoparticles: Defect Chemistry, Tensile Strain, and Tunable Optical Response

ZnO and ZnO:5%B nanoparticles produced by sol–gel synthesis exhibit a single-phase wurtzite structure. X-ray diffraction (XRD) investigation reveals crystallite sizes in the range of (Formula presented.) nm and microstrain values on the order of (Formula presented.), despite the Uniform Stress Deformation Model (USDM) indicating the presence of considerable tensile stress. Significant band-tail states are introduced via boron doping, resulting in Urbach energies ranging from (Formula presented.) to (Formula presented.) meV and a narrowed optical band gap of (Formula presented.) eV. With a refractive index range of (Formula presented.), the material exhibits tunable optical characteristics. Violet and blue emissions originating predominantly from zinc interstitials (Znᵢ) and zinc vacancies (V_Zn) dominate the photoluminescence spectra, while oxygen interstitial-related contributions remain relatively weak. A high spin density is confirmed by electron spin resonance measurements, which reveal a strong defect-related signal at (Formula presented.). The formation of Znᵢ/V_Zn defect centers due to charge compensation and ionic size mismatch induced by B³⁺ substitution for Zn²⁺ significantly modifies the band-edge states and optical constants. These defect-engineered properties render the material promising for applications in ultraviolet (UV) photodetectors, transparent conducting oxides, and electron transport layers in organic photovoltaic devices.