Influence of Concentration on Structural and Optical Characteristics of Nanocrystalline ZnO Thin Films Synthesized by Sol-Gel Dip Coating Method

Zinc Oxide thin films (ZnO) were deposited successfully on glass substrate by sol-gel dip coating method by varying precursor concentrations. The structural properties of ZnO thin films were investigated by X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) techniques. The optical properties of ZnO thin films were also characterised using UV visible Spectrophotometer, Fourier Transform Infrared Spectrometer (FTIR) and Photoluminescence (PL) studies. The XRD analysis of all ZnO films showed hexagonal structure with c-axis orientation along (002) plane, whereas SEM image of ZnO thin films showed granular surface. The UV visible spectral study showed a higher transmittance in the visible region with a direct band gap value in the range of 3.2 eV to 3.3 eV for prepared Zinc Oxide thin films. FTIR spectra indicated a ZnO stretching mode at 480 cm -1 . The PL spectra of ZnO sample gave an intense visible strongest ultraviolet emission peak centred at 380 nm and weak blue emission peak around 480 nm. In the preparation of ZnO thin films, Zinc acetate dihydrate (Zn(CH3COO)22H2O), ethanol and mono ethanolamine (MEA) were used as starting material, solvent and sol stabilizer respectively. Zinc acetate (0.2M) was first dissolved in ethanol at room temperature, the resulting mixture was stirred at 60℃ for an hour and MEA was added into the solution drop by drop (molar ratio of MEA and zinc acetate was maintained at 1.0). Finally, a clear homogeneous solution was obtained. The ZnO solution was aged for 24 hours at room temperature and then ZnO thin films were prepared by dip-coating method on cleaned and dried glass substrates. Every time the substrate was withdrawn from the ZnO solution and kept in furnace, and allowed to dry and subjected for pre-heat treatment at 300℃ for 5 minutes. The procedure from dip-coating to drying was repeated six times. At last, ZnO thin film was annealed at 550℃ in air for an hour. Two more samples were also prepared by varying the molarities of zinc acetate as 0.3M and 0.4M. The crystallinites of the ZnO thin films were studied by an X-ray diffractometer (RIGAKU, Japan) with CuKα radiation. The surface morphologies of the films were observed using Scanning Electron Microscope (Shimadzu S-3000N). The optical transmittance and band gap of ZnO thin films were found out at room temperature using Spectrophotometer (Shimadzu UV- Vis 1800). The Photoluminescence spectra of ZnO thin films were evaluated by Spectrofluorometer (Perkin Elmer LS55). In addition the thickness of the films was also determined by Stylus Profilometer (Mitutoyo SJ-301).