Fluence-dependent evolution of crystallinity, phonon behaviour, and optical properties in 90 MeV oxygen ion irradiated NiO films

Swift heavy ion (SHI) irradiation is an effective approach for controlled defect engineering in oxide semiconductors. In this work, NiO thin films prepared by the sol–gel spin coating technique were irradiated with 90 MeV O⁷⁺ ions at fluences ranging from 2 × 10¹¹ to 2 × 10¹³ ions cm⁻². X-ray diffraction analysis confirms the formation of cubic NiO and reveals improved crystallinity, increased crystallite size, and reduced microstrain with increasing irradiation fluence, indicating defect annealing and lattice relaxation. Raman and FTIR studies demonstrate phase stability and enhanced lattice ordering, accompanied by reduced defect-induced vibrational disorder at higher fluence. Optical measurements show a systematic decrease in transmittance and an increase in absorbance due to irradiation-induced defect states. The optical band gap exhibits initial narrowing at low fluence followed by partial recovery at higher fluence, suggesting defect reorganization and dynamic annealing. An increase in Urbach energy further confirms the formation of localized band-tail states. These findings highlight the potential of oxygen ion irradiation for tuning the structural and optical properties of NiO thin films for optoelectronic applications.