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 32.37–39.63 nm and microstrain values on the order of (1.98–8.03)×10−4, 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 110 to 193 meV and a narrowed optical band gap of 3.216 eV. With a refractive index range of 2.05–2.71, the material exhibits tunable optical characteristics. Violet and blue emissions originating predominantly from zinc interstitials (Znᵢ) and zinc vacancies (VZn) 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 g≈2.294. The formation of Znᵢ/VZn 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.