Preparation, Microstructure and Optical Absorption Behaviour of NiO Thin Films

Abstract

Nickel oxide thin films have been prepared by spin coating on glass, silicon(111) and quartz substrate using a solution of nickel acetate tetrahydrate in 2-methoxyethanol and mono-ethanolamine and subsequent annealing at 300–600 °C for 2 h in air. These films have been characterized with regard to phase, microstructure and optical absorption using X-ray diffraction, atomic force microscopy and UV-visible spectrophotometry, respectively. It is shown that NiO thin films have polycrystalline nature and possess an f.c.c. (NaCl-type) structure with lattice parameter varying with annealing temperature (T) and solution molarity; typical value being a = 4.186 Å with T = 500 °C and molarity 0.5 M. The microstructure clearly reveals the formation of ellipsoids with average projected major and minor diameters as ∼58 and ∼38 nm, respectively and nano-rods of average diameter ∼12nm with aspect ratio of ∼5.2. On the other hand, thin films formed by dip coating with the same solution contain spherical particles of average diameter ∼28 nm. NiO thin films exhibit (i) high optical transmittance (80–95%) in the wave length range of 400–800 nm, (ii) sharp absorption in the interval 300–400 nm like that of semiconductor/insulator, (iii) decrease of energy band gap, Eg (value lies in the range 3.66–3.83 eV; bulk value being 3.55 eV) with increase of annealing temperature and molarity both. The higher values of band gap have been attributed to the reduced average size of the crystallites.