Structural, morphological, and nonlinear optical characteristics of NiO thin films prepared by electron -beam evaporation

Nickel oxide (NiO) nanostructures were synthesized by thermally oxidizing thin nickel (Ni) films deposited on glass substrates via electron beam evaporation under a controlled vacuum environment. Thermal oxidation was performed at 400 °C, 500 °C, and 600 °C in ambient air. X-ray diffraction (XRD) confirmed mixed phases of Ni and NiO at 400 °C and 500 °C, whereas a single-phase NiO with improved crystallinity was obtained at 600 °C. XRD data were analyzed using Rietveld refinement to confirm phase composition and structural evolution. Field emission scanning electron microscopy (FESEM) revealed that higher oxidation temperatures promote grain growth and induce a porous morphology. Energy-dispersive X-ray spectroscopy (EDX) confirmed the chemical purity of the oxidized films, showing only nickel and oxygen. Optical absorption analysis demonstrated a redshift in the bandgap with increasing temperature, consistent with defect-mediated electronic transitions. X-ray photoelectron spectroscopy (XPS) of the Ni 2p core level exhibited characteristic Ni 2p₃/₂ and Ni 2p₁/₂ peaks along with satellite features, evidencing the coexistence of Ni²⁺ and Ni³⁺ oxidation states. Nonlinear optical behavior was probed using the Z-scan technique at 532 nm, yielding third order nonlinear susceptibility (χ3) values in the range of 3.64 × 10^–4–1.67 × 10^–4 esu. The films displayed strong nonlinear absorption and thermal lensing effects, with an optical limiting threshold as low as 0.26 kJ/cm². These findings highlight the potential of NiO thin films as efficient candidates for nonlinear optical limiting applications, where structural tuning via oxidation temperature plays a critical role in enhancing performance.