K.K. Nagaraja

Effect of annealing on the structural and nonlinear optical properties of ZnO thin films under cw regime

We report on the studies of the effects of annealing on the structural and third-order nonlinear optical properties of ZnO thin films deposited on quartz substrates by the RF magnetron sputtering technique. The films were annealed in the temperature range 400–1000 °C. The third-order nonlinear optical studies were carried out using the z-scan technique under continuous wave (cw) He–Ne laser irradiation at 633 nm wavelength. The effects of annealing on the structural properties were examined using x-ray diffraction and atomic force microscopy (AFM). The (0 0 2) preferred orientation increased with increase in annealing temperature up to 800 °C. The crystalline phases of SiO2 were observed at higher annealing temperatures. The appearance of an extraneous phase was confirmed by AFM images and optical transmittance spectra. The samples exhibited nonlinear absorption and nonlinear refraction under the experimental conditions. The negative sign of the nonlinear refractive index n2 indicated that the films exhibit self-defocusing property due to thermal nonlinearity. The nonlinear refractive index n2, the nonlinear absorption coefficient βeff and the third-order optical susceptibility χ(3) were found to be of the highest orders. The estimated value of third-order optical susceptibility χ(3) was of the order of 10−3 esu. Multiple diffraction rings were observed when the samples were exposed to the laser beam. The appearance of rings was due to the refractive index change and thermal lensing. With increase in laser intensity, the variations of the self-diffraction ring patterns were studied experimentally. The films also exhibited strong optical limiting properties under cw laser excitation, and reverse saturable absorption was the dominant process leading to the observed nonlinear behaviour.

Fabrication and performance evaluation of hybrid supercapacitor electrodes based on carbon nanotubes and sputtered TiO2

We report a simple and eco-friendly method for the fabrication of a titanium dioxide/functionalized multiwalled carbon nanotube (TiO2/FMWCNT) composite electrode for use in supercapacitors. The nanocomposite electrodes were formed by depositing titanium dioxide onto FMWCNTs using reactive magnetron sputtering, thus providing a green roue for the formation of the binder-free composite electrode. It is shown that the electrochemical performance of the fabricated electrodes can be altered by tuning the thickness of the titanium dioxide overlayer. The integrated nanocomposite electrode showed an improved specific capacitance of 90 Fg(-1) in two-electrode configuration.

Structural, linear, and nonlinear optical properties of radio frequency-sputtered nitrogen-doped ZnO thin films studied using z-scan technique

The third-order nonlinear optical properties of undoped and nitrogen-doped ZnO thin films were evaluated using the z-scan technique. The films were sputter-deposited on glass substrates using radio frequency power. The He–Ne continuous wave laser operating at 633 nm was used as an irradiation source. A change in the growth mode in the nitrogen-doped films was observed. The grain size and roughness were found to be dependent on the nitrogen concentration, as shown by atomic force microscopy analysis. The optical band gap was determined and found to increase with nitrogen concentration in the films. Both nonlinear absorption and refraction nonlinearities were exhibited by the deposited films. The nonlinear refractive index n2, the nonlinear absorption coefficient β eff and the third-order nonlinear optical susceptibility χ (3) were determined and found to be largest. Multiple diffraction ring patterns were observed when the samples were made to interact with the laser beam and were attributed to refractive index change and thermal lensing. Further, optical power-limiting experiments were performed to determine the optical-limiting threshold and clamping values for undoped and nitrogen-doped ZnO films.

Polymer assisted preparation and characterization of ZnO and Sn doped ZnO nanostructures

Zinc oxide (ZnO) and tin doped ZnO are wide band gap semiconducting materials with excellent optoelectronic properties. In the present study ZnO and Sn: ZnO films are prepared using polymer assisted sol gel process. The thermal behaviour of the dried gel sample studied using DTA and TG analysis. TG-DTA result shown that most of the organic of PVA and CH3COO group of zinc acetate and other volatiles are removed below 500°C. The effect of Sn on the crystallinity, microstructral properties of the deposited films was investigated. XRD patterns of undoped and Sn doped ZnO films indicate enhanced intensities for the peak corresponding to (002) plane, resulting preferential orientation along the c-axis. The SEM images confirm that the grown films are composed of nanorods.

Polymer Assisted Synthesis and Characterization of Fine ZnWO4

Zinc tungstate is a scintillator material with excellent photocatalytic and photoluminescent properties. In the present study, fine sized ZnWO4 powders with monoclinic wolframate structure were prepared by chemical precipitation route assisted with polymer evaporation. DTA/TG and XRD were employed to investigate the thermal behaviour and crystallite size evaluation of ZnWO4 .The particle size and morphology of the powder heat treated at different temperature was observed and characterized by SEM. The FTIR spectrum has been used to study the stretching and bending frequencies of the ZnWO4. The result shows that the process provides a technically simple route for the precipitation of fine sized narrow particle distribution and free dense agglomerate ZnWO4 particles.

Aluminum Doped ZnO Thin Films by RF Sputtering of Coaxial ZnO and Al Targets

Transparent conducting aluminum doped zinc oxide (AZO) films were deposited on glass substrates by radio (RF) frequency magnetron sputtering employing zinc oxide and aluminum targets. The targets are fixed coaxially in one cathode, by using a center hollow aluminum disc. Gas pressure was kept constant and the sputter power was varied. The nature of AZO film was found to be polycrystalline with hexagonal structure and a preferred orientation along c‐axis. The Al content in the films is determined using EDXA analysis and it is found to vary with the applied power. Surface morphology of the films was found to be uniform and has fine grained structure. Electrical resistivity of the deposited films was found to be as low as 26×10−4 Ω‐cm for the film deposited at 250 W. The average transparencies up to 85% in the visible region were obtained for all the films. Optical band gap of the films show a slight blue shift as indicated by the (αhν)2 v/s hν plots. In the present investigation we have controlled Al conten...

Fabrication and characterization of ZnO/AI/ZnO multilayers by simultaneous DC and RF magnetron sputtering

The present investigation reports the fabrication and characterization of multilayered transparent electrodes by simultaneous DC and RF magnetron sputtering on glass substrates. The multilayer structure consists of three layers (ZnO/Al/ZnO). The influence of Al layer thickness on the electrical and optical properties was investigated. Optimum thickness of Al was determined for high transmittance and good electrical conductivity. High quality films having resistance as low as 25 Ω/sq with optical transmittance upto 65% were obtained at room temperature.

Growth and temperature dependent electrical properties of ZnO nanostructures

Zinc oxide (ZnO) nanostructured films are grown using Polyvinyal alcohol (PVA)-citric acid modified sol gel process and the effect of (0, 1, and 2Wt%) PVA content on growth, crystallanity, microstructure and electrical properties are investigated. The XRD patterns confirm that the films are polycrystalline nature. 1 Wt% assisted grown samples are found to be good crystallanity with (002) preferred orientation. The SEM micrographs confirm that the 1 Wt% PVA assisted grown films are composed of vertically oriented ∼1.2 μm length nanorods. The temperature dependent electrical resistivity of the films has been measured using the four-probe method. The electrical resistivity found to be 7.08 Ω-cm at room temperature for vertically oriented nanorods composed films, which are grown using 1Wt% PVA.