Electrochromic and photoluminescence properties of cobalt oxide thin films prepared by spray pyrolysis

Cobalt oxide thin films were prepared by a facile spray pyrolysis technique, using a perfume atomizer with an aqueous solution of hydrated cobalt chloride salt with a molar concentration of 0.025?M as a source of cobalt. The films were deposited onto glass substrates at temperature of 350?°C. The structural, morphological, and electrochromic properties of the obtained films were studied. It was found from X-ray diffraction analysis that the films were polycrystalline in nature with spinel-type cubic structure and preferred orientation along [111] direction. The Scanning Electron Microscopy images revealed a porous structure with the average grain size around 200?nm. The cyclic voltammetry measurements revealed that Cobalt oxide thin film is an anodically coloring electrochromic material with a transmittance variation in the visible range of 31%, and a fast response time (about 2?seconds) and a good cycling stability. These electrochromic performances make cobalt oxide thin film an attractive material for using as an anodic electrochromic material in smart windows devices. The photoluminescence spectra exhibited a strong emission in the visible region confirming the good crystallinity properties of Co₃O₄ thin films.     

Electrochromic phenomena in transition metal oxide thin films prepared by thermal evaporation

We present an experimental study on the electrochromic properties of MoO₃, WO₃ and mixed WO₃-MoO₃ thin films prepared by thermal evaporation. We have constructed symmetric and quasi-symmetric electrochromic cells incorporating the evaporated oxide films as electrochromic layers. Li⁺ doped V₂O₅ films served as ion storage layers. The symmetric cells were found to exhibit significantly improved optical properties compared to the quasi-symmetric ones, with very low luminous transmittance values in the colored state, which makes them suitable for large-area window applications. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Electrochromic and electrochemical properties of amorphous porous nickel hydroxide thin films

Nickel hydroxide films were prepared using the chemical bath deposition (CBD) technique. The amorphous nature of the films was confirmed by X-ray diffraction measurements. X-ray photoelectron spectroscopy (XPS) measurements showed that the films exhibited nickel hydroxide nature. The porosity of the films was studied using optical measurements. The electrochromic properties of the porous nickel hydroxide layers were investigated, using cyclic voltammetry, chronoamperometry, in situ transmittance, UV-vis spectroscopy, and impedance spectroscopy. The change in the optical density (ΔOD) was found to be 0.79 for the as-deposited nickel hydroxide films, whereas it is 0.53 and 0.50 for the films annealed at 150 °C and 200 °C, respectively. The in situ transmittance and chronoamperometry curves revealed that the annealed films had a very fast colouration (t_c < 290 ms) and decolouration (t_b < 130 ms). The measured colouration efficiencies range between 30 and 40 cm²/C. The impedance measurements revealed the faster colouration and good electrochromic properties for the annealed nickel hydroxide films.     

Properties of highly oriented spray-deposited fluorine-doped tin oxide thin films on glass substrates of different thickness

Transparent conducting thin films of fluorine-doped tin oxide (FTO) have been deposited onto the preheated glass substrates of different thickness by spray pyrolysis process using SnCl₄·5H₂O and NH₄F precursors. Substrate thickness is varied from 1 to 6 mm. The films are grown using mixed solvent with propane-2-ol as organic solvent and distilled water at optimized substrate temperature of 475 °C. Films of thickness up to 1525 nm are grown by a fine spray of the source solution using compressed air as a carrier gas. The films have been characterized by the techniques such as X-ray diffraction, optical absorption, van der Pauw technique, and Hall effect. The as-deposited films are preferentially oriented along the (2 0 0) plane and are of polycrystalline SnO₂ with a tetragonal crystal structure having the texture coefficient of 6.19 for the films deposited on 4 mm thick substrate. The lattice parameter values remain unchanged with the substrate thickness. The grain size varies between 38 and 48 nm. The films exhibit moderate optical transmission up to 70% at 550 nm. The figure of merit (φ) varies from 1.36×10⁻⁴ to 1.93×10⁻³ Ω⁻¹. The films are heavily doped, therefore degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance (R_s) of 7.5 Ω is obtained for a typical sample deposited on 4 mm thick substrate. The resistivity (ρ) and carrier concentration (n_D) vary over 8.38×10⁻⁴ to 2.95×10⁻³ Ω cm and 4.03×10²⁰ to 2.69×10²¹ cm⁻³, respectively.     

Optical properties of stannous oxide thin films

Thin films of stannous oxide (SnO) have been prepared by the electron-beam evaporation method. Film properties, such as refractive indexn, extinction indexk, and absorption coefficient, were studied in the wavelength range between 200 nm and 2 000 nm with different methods. The surface roughness of SnO thin films has been found to depend linearly on the film thickness in the range 400 <d < 2 000 nm. The optical transition in SnO thin films shows direct transition corresponding to an optical gap of 2.85 eV.     

Some superconducting and normal state properties of niobium nitride thin films

We have measured the temperature dependence of the resistance, the superconducting transition temperature, and the upper critical field of niobium nitride thin films. Our samples were made by sputtering in a nitrogen-argon ion beam. The pressure during deposition is typically one to two orders of magnitude lower than that of standard sputtering techniques.     

Electrochromic properties of nano-structured nickel oxide thin film prepared by spray pyrolysis method

In this study, we present a simple method to improve the electrochromic properties of a nickel oxide thin film. The method involves a three-step process—(a) conducting indium tin oxide (ITO) nano-particles were first sprayed onto a conducting substrate to form a porous nano-structured ITO layer, (b) nickel oxide film was then deposited onto the nano-structured ITO layer by a spray pyrolysis technique, and (c) the substrate, ITO nano-particles layer and nickel oxide film were annealed at high temperature of 300 °C to improve adhesion of these three layers. The microstructure of the resulting electrochromic cell was investigated using scanning electron microscopy. It is evident that the nickel oxide film covers the surface of the ITO nano-particle layer and forms a nano-structured nickel oxide (NSNO) film. The switching time and contrast were characterized by Autolab PGSTAT12 potentiostat and Jasco V-570 spectrophotometer. The results suggest that the transmittance contrast and switching time of NSNO are slightly superior to those of a conventional nickel oxide (CNO) film. However, the cycling durability of NSNO can be much better than that of CNO.     

Thickness dependent optical properties of titanium oxide thin films

TiO₂ thin films of different thickness were prepared by the Electron Beam Evaporation (EBE) method on crystal silicon. A variable angle spectroscopic ellipsometer (VASE) was used to determine the optical constants and thickness of the investigated films in the spectral range from 300 to 800 nm at incident angles of 60°, 70°, and 75°, respectively. The whole spectra have been fitted by Forouhi–Bloomer (FB) model, whose best-fit parameters reveal that both electron lifetime and band gap of TiO₂ thin film have positive correlation to the film thickness. The refractive indices of TiO₂ thin film increase monotonically with an increase in film thickness in the investigated spectral range. The refractive index spectra of TiO₂ thin films have maxima at around 320 nm and the maxima exhibit a marginally blue-shift from 327.9 to 310.0 nm with an increase in film thickness. The evolution of structural disorder in the TiO₂ thin film growth can be used to explain these phenomena.     

Electrochromic switching of WO3 nanostructures and thin films

We present transmission measurements through tungsten tri-oxide nanostructures and thin films prepared by sol–gel process on micro-contact printed substrates. Identical electrochromic switching times are found for both the nanostructures and the bulk films with equal thicknesses upon intercalation of H⁺ ions. We attribute the large change in the transmission through nanostructures at 632 nm, which can not be solely explained by absorption, to diffraction effects. PACS 73.40.Cg; 73.40.Mr; 78.20.-e; 78.67.n     

Structural properties of transparent Ti-V oxide semiconductor thin films

Transparent oxide semiconducting thin films based on mixed Ti-V oxides were prepared using a modified reactive magnetron sputtering method. Based on structural investigations performed with the help of x-ray diffraction and transmission electron microscopy analysis, two distinct regions in the prepared thin film have been observed: a nanocrystalline TiO₂-V₂O₃-V₂O₅ mixed composition, and a thin layer consisting of amorphous phase and nanocystalline TiO₂ phase deposited directly at the substrate. Optical measurements show excellent transmission in the visible spectral range of 73%, on average. Resistivity of the thin film was found at the order of 10⁵ Ω cm at room temperature. The preparation of mixed Ti-V oxides provides a combination of high transparency and semiconducting properties.     

The influence of the substrate temperature on the photovoltaic properties of spray-deposited CdS:In thin films

Polycrystalline and highly transparent CdS:In thin films were produced by the spray pyrolysis (SP) technique at different substrate temperatures ranging from 350 to 490 °C on glass substrates. The effect of the substrate temperature on the photovoltaic properties of the films was investigated by studying the transmittance measurements, X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) observations and the I-V plots. The transmittance measurements were used to estimate the band gap energy by the linear fit of (αhν)² versus hν. The band gap energy was found to be slightly increasing with the substrate temperature. XRD diffractograms show that a phase transition from the cubic to the hexagonal phase occurs by increasing the substrate temperature, beside more orientation of crystal growth. Also they show that complex cadmium compounds are still present till T_s ≈ 460 °C after which they practically disappear. From the linear I-V plots the resistivity was estimated and found to be strongly decreasing with the substrate temperature.     

Direct observation of infrared photoluminescence of spray-deposited CdS:In thin films

Infrared photoluminescence spectra (in the range 0.9-1.4 eV) of the as-deposited CdS:In thin films prepared by the spray pyrolysis technique were recorded at different laser powers and different film temperatures in the range 24-130 K. The spectra show an infrared band centred at 1.06 eV, which have a structure and asymmetry. The structure might be attributed to active defect states which are produced through the growth of the film and might be partially due to coupling to longitudinal phonon. Gaussian peaks were used to deconvolute the spectrum by using nonlinear square fit. The Gaussian peaks used in the fit are expected to fit the spectra taken at different laser powers and different temperatures. These results are discussed in view of the importance of CdS as a window layer for photovoltaic heterojunction solar cells.     

Preparation and properties of spray-deposited ZnIn2Se4 nanocrystalline thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO)-coated glass substrates using a spray pyrolysis technique. Aqueous solution containing precursors of Zn, In, and Se has been used to obtain good quality deposits at different substrate temperatures. The preparative parameters such as substrate temperature and concentration of precursors solution have been optimized by photoelectrochemical technique and are found to be 325 °C and 0.025 M, respectively. The X-ray diffraction patterns show that the films are nanocrystalline with rhombohedral crystal structure having lattice parameter a=4.05 Å. The scanning electron microscopy (SEM) studies reveal the compact morphology with large number of single crystals on the surface. From optical absorption data the indirect band gap energy of ZnIn₂Se₄ thin film is found to be 1.41 eV.     

Upper critical field of niobium nitride thin films

The temperature dependences of the superconducting transition of niobium nitride (NbN) thin films have been investigated via the first harmonic of the voltage in dc magnetic fields of up to 8 T. The temperature dependence of the second critical field of NbN has been determined. The parameter responsible for the effect of spin paramagnetism in this material and the temperature dependence of the upper critical field that describes well the experimental data have been found in terms of the Werthamer–Helfand–Hohenberg (WHH) theory. The key parameters of the superconductor have been estimated from the transport and optical measurements.     

Electrochromic colour centres in amorphous tungsten trioxide thin films

Amorphous tungsten trioxide films, investigated by the Raman scattering method, are shown to be composed of a spatial network of tightly bound (WO₆)_n·mH₂O clusters with a large number of terminal oxygen W=O and W-O-W bonds between clusters. The injected electrons in an amorphous tungsten trioxide film are localized in the tungsten 5d orbitals in an axially distorted octahedron, as is shown by ESR analysis. The optical absorption of a coloured amorphous tungsten trioxide film, as has previously been proposed, can be satisfactorily described by an intervalence charge-transfer transition between localized W⁵⁺ and W⁶⁺ states.     

Electrochromic properties of nano-composite nickel oxide film

In this study, we develop a nano-composite nickel oxide (NNO) film on the indium tin oxide (ITO)-coated glass substrate for electrochromic applications. The NNO film is composed of the core-shell structure of NiO/conducting ITO nano-particles. High porosity in the NNO film offers large active surface area for redox reaction. Electrochromic electrodes fabricated with the NNO films produce high transmittance variation (66.2% at a wavelength of 550 nm), fast switching speed (coloring: 3.5 s; bleaching: 4 s) and good durability, which are much better than those of ones made with the traditional nickel oxide films. The structure, morphology, and electrochromic properties are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and UV-vis spectroscopy.     

Ion induced silicide formation in niobium thin films

MeV ⁴He backscattering and x-ray diffraction analysis were used to examine the intermixing of niobium thin films on single crystal silicon during ²⁸Si⁺ ion bombardment. The ambient temperature dependence of the intermixing is reported. The dependence cannot be explained by either radiation-enhanced diffusion or cascade mixing alone. The silicides. NbSi₂ and Nb₅Si₃, were both observed. Silicide growth was found to be proportional to the square root of the fluence for the case in which the ion range exceeds the film thickness.