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.
Document Type: Research Article
Publication date: 2008-08-01
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