Optical, Electrical, and Structural Properties of NiO Thin Films, Derived by Sol–Gel Method

NiO films were successfully deposited by sol-gel spin coating on Si, glass, and ITO-covered glass substrates. The impact of the film thickness (the different number of layers), annealing temperatures (from 300 to 500 °C), and the substrate type on the crystal structure, film morphology, optical, and vibrational properties was investigated. X-ray diffraction (XRD) revealed a polycrystalline structure and the appearance of the cubic NiO phase. Field Emission Scanning Electron Microscopy (FESEM) was applied to explore the surface morphology of NiO films, deposited on glass and ITO substrates. The oxidation states of Ni were determined by X-ray photoelectron spectroscopy (XPS). The presence of Ni²⁺ and Ni³⁺ states was supposed. UV-VIS-NIR spectroscopy revealed that NiO films possessed a high average transparency of up to 74.6% in the visible spectral range when they were deposited on glass substrates, and up to 76.9% for NiO films on ITO substrates. The thermal treatments and the film thickness slightly affected the film transparency in the spectral range of 450-700 nm. The work function (WF) of the samples was determined. This research showed that good properties of sol-gel NiO films can be compared to the properties of those films produced using complicated and expensive techniques.