Impact of precursor solution on structure, optical and nonlinear optical properties of spray pyrolyzed nickel oxide thin films

A comprehensive study on structural, surface morphology, optical properties of spray-deposited nickel oxide (NiO) thin films derived from different precursor salts is presented. All the samples were prepared at a solution molarity of 0.05 M and a glass substrate temperature of 450 \:^∘\: C. XRD analysis confirmed the cubic NiO phase for all samples with a crystallite size of 20 – 30 nm, which was further verified by Raman spectroscopy. FESEM analysis revealed a granular surface morphology. AFM analysis showed that the NiO films prepared from the nickel nitrate precursor exhibited relatively higher surface roughness. Linear optical studies revealed high optical transparency in the visible region, and the estimated bandgap was found to be in the range of 3.5–3.6 eV. Photoluminescence spectra exhibited emission peaks including near-band-edge emission along with defect-related emissions, suggesting the presence of defect-related states within the NiO films. The nonlinear optical (NLO) properties were investigated using the Z-scan technique under two excitation regimes: continuous-wave (CW) laser excitation at 532 nm and femtosecond (fs) laser excitation at 800 nm. Open-aperture Z-scan results revealed reverse saturable absorption behaviour under both excitation conditions. Closed-aperture Z-scan measurements showed self-defocusing nonlinearity for CW laser excitation and self-focusing behaviour under fs laser excitation. The third-order NLO susceptibility [χ (3) ] obtained using the CW laser is ~ 10⁻¹¹ m²/V², and that using the fs laser is ~ 10⁻¹⁸ m²/V². The observed RSA behaviour was further employed to evaluate the optical limiting threshold of the deposited NiO thin films.