Tungsten Oxide Thin Films Research Articles

Effect of gas enhanced metal-semiconductor-metal UV photodetectors based on thermal annealing tungsten oxide thin film prepared by sol–gel method

Metal-semiconductor-metal ultraviolet photodetectors were fabricated on annealed tungsten oxide thin films which were synthesized by the sol–gel reaction. Surface morphology, microstructure, opto-electrical properties and crystal structure tests of the W18O49 thin films were examined by scanning electron microscopy, transmission, photoluminescence, and X-ray diffraction, respectively. The photo-responsivity of the photodetector shows a dependence on atmosphere environments. The photocurrent measured in a vacuum is about ~2 times higher than the illuminated photocurrent in air. The illumination conductance in a vacuum and in air increase by a factor of 100 compared to that in oxygen ambient. Furthermore, the obtained photoresponse curves showed significant changes in dark, under illumination, and during recovery for gas switching experiment. The solid-state process of electron-hole generation/recombination and surface effects of oxygen was discussed to explain the changes in the observed photoresponse.

Preparation of Electrolytic Tungsten Oxide Thin Films as the Anode in Rechargeable Lithium Battery

Tungsten oxide films were prepared by an electrochemical deposition method for use as the anode in rechargeable lithium batteries. Continuous potentiostatic deposition of the film led to numerous cracks of the deposits while pulsed deposition significantly suppressed crack generation and film delamination. In particular, a crack-free dense tungsten oxide film with a thickness of ca. 210 nm was successfully created by pulsed deposition. The thickness of tungsten oxide was linearly proportional to deposition time. Compositional and structural analyses revealed that the as-prepared deposit was amorphous tungsten oxide and the heat treatment transformed it into crystalline triclinic tungsten oxide. Both the as-prepared and heat-treated samples reacted reversibly with lithium as the anode for rechargeable lithium batteries. Typical peaks for the conversion processes of tungsten oxides were observed in cyclic voltammograms, and the reversibility of the heat-treated sample exceeded that of the as-prepared one. Consistently, the cycling stability of the heat-treated sample proved to be much better than that of the as-prepared one in a galvanostatic charge/discharge experiment. These results demonstrate the feasibility of using electrolytic tungsten oxide films as the anode in rechargeable lithium batteries. However, further works are still needed to make a dense film with higher thickness and improved cycling stability for its practical use.

Lithium-Doped Nickel Oxide and Nanostructured Tungsten Oxide Thin Films Made By USP, Toward Improved Electrochromic Performances

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Optical Characteristics of WO<sub>3</sub> Thin Films Prepared Using the Electrodeposition Method

The tungsten oxide (WO3) thin films on the indium tin oxide (ITO) glass in this study were produced by applying the electrodeposition method (ED). This method involves adjusting ED time to control the structure, film thickness, and morphology of WO3thin films. Furthermore, the crystallization of the WO3thin films was controlled by annealing them at 200 °C and 500 °C. The results show that the WO3coated glass with non-annealed and annealed at 200 °C has a high transmittance of visible light and anti-infrared characteristics. However, the WO3coated glass annealed at 500 °C has high infrared transmittance, with the highest infrared transmittance reaching 85% at the 1000 nm wavelength. To improve the heat collecting efficiency of solar water heaters, the high infrared WO3coated glass can be applied to the surface of the heat collecting component.

${\rm WO}_{3}$ Hydrogen Resistive Gas Sensor and Its Wide-Range Current-Mode Electronic Read-Out Circuit

Tungsten oxide (WO3) thin films are prepared via sol-gel route by spinning (WCl6 in ethanol, 0.2 M) on Pt interdigitated Si/Si3N4 substrates and annealed at 300°C for 12, 24, 96, and 200 hours. Films morphology and crystalline phase are characterized through scanning electron microscopy, atomic force microscopy, and glancing angle X-ray diffraction. The increasing of the annealing time shows a positive effect on the degree of crystallization but with no substantial influence on the crystallite size, surface area, and mean roughness of the films. Electrical tests are carried out using a current-mode dedicated read-out circuit to perform gas-sensing measurements of the polycrystalline WO3 films to H2 gas (0-250 ppm) in dry air and operating temperatures ranging from 25 to 250°C. Electrical tests confirmed an n-type response of the films. Although improved sensitivity (S=RA/RSENS) is achieved when decreasing the annealing time, best performances in terms of reproducibility and long-term stability of the response are obtained by annealing the film for 200 hours at 300°C temperature.

An ultra-fast response gasochromic device for hydrogen gas detection

In the present work, gasochromic tungsten oxide (WO3) thin films on glass substrate were fabricated by Pulsed Laser Deposition (PLD) system. Pt was sputtered on the surface of WO3 thin films as a catalyst layer for hydrogen gas detection. The gasochromic properties of Pt/WO3 thin films at room temperature were determined by examining their optical transmittance under various H2–N2 mixture gas exposures. The results show that in low H2 atmosphere (0.5%) the room temperature grown Pt/WO3 thin films still exhibited noticeable transmittance change, within the visible light (550–700nm) range. WO3 thin films have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). XRD and TEM confirmed that the morphology of the best performance WO3 thin film is amorphous, and SEM investigations revealed that the thin films were composed of nanotextured grains.

Electrochromism in surface modified crystalline WO3 thin films grown by reactive DC magnetron sputtering

In the present work, tungsten oxide thin films were deposited at various oxygen chamber pressures (1.0–5.0×10−3mbar) by maintaining the sputtering power density and argon pressure constant at 3.0W/cm2 and 1.2×10−2mbar, respectively. The role of surface morphology and porosity on the electrochromic properties of crystalline tungsten oxide thin films has been investigated. XRD and Raman studies reveal that all the samples post annealed at 450̊C in air for 3.0h settle in monoclinic crystal system of tungsten oxide (W18O49). Though the phase of material is indifferent to oxygen pressure variations (PO2), morphology and film density shows a striking dependence on PO2. A systematic study on plasma (OES), morphology, optical and electrochromic properties of crystalline tungsten oxide reveal that the films deposited at PO2 of 2.0×10−3 mbar exhibit better coloration efficiency (58cm2/C), electron/ion capacity (Qc: −25mC/cm2), and reversibility (92%). This is attributed to the enhanced surface properties like high density of pores and fine particulates (100nm) and to lesser bulk density of the film (ρ/ρo=0.84) which facilitates the process of intercalation/de-intercalation of protons and electrons. These results show good promise toward stable and efficient crystalline tungsten oxide based electrochromic device applications.

Optical Constants of Amorphous, Transparent Titanium-Doped Tungsten Oxide Thin Films

We report on the optical constants and their dispersion profiles determined from spectroscopic ellipsometry (SE) analysis of the 20%-titanium (Ti) doped of tungsten oxide (WO3) thin films grown by sputter-deposition. The Ti-doped WO3 films grown in a wide range of temperatures (25-500 °C) are amorphous and optically transparent. SE data indicates that there is no significant interdiffusion at the film-substrate interface for a W-Ti oxide film growth of ~90 nm. The index refraction (n) at λ = 550 nm vary in the range of 2.17-2.31 with a gradual increase in growth temperature. A correlation between the growth conditions and optical constants is discussed.

Hot-wire chemical vapor deposition of WO3−x thin films of various oxygen contents

We present the synthesis of tungsten oxide (WO3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO3−x, WO3/W and WO3/W/WO3−x are obtained in a single continuous hot-wire chemical vapor deposition process using only ambient air and hydrogen. The air oxidizes resistively heated tungsten filaments and produces the tungsten oxide species, which deposit on a substrate and are subsequently reduced by the hydrogen. The reduction of tungsten oxides to oxides of lower oxygen content (suboxides) depends on the local water vapor pressure and temperature. In this work, the substrate temperature is either below 250 °C or is kept at 750 °C. A number of films are synthesized using a combined air/hydrogen flow at various total process pressures. Rutherford backscattering spectrometry is employed to measure the number of tungsten and oxygen atoms deposited, revealing the average atomic compositions and the oxygen profiles of the films. High-resolution scanning electron microscopy is performed to measure the physical thicknesses and display the internal morphologies of the films. The chemical structure and crystallinity are investigated with Raman spectroscopy and X-ray diffraction, respectively.

Visible light-induced photocatalytic reduction of graphene oxide by tungsten oxide thin films

Tungsten oxide thin films (deposited by thermal evaporation or sol gel method) were used for photocatalytic reduction of graphene oxide (GO) platelets (synthesized through a chemical exfoliation method) on surface of the films under UV or visible light of the environment, in the absence of any aqueous ambient at room temperature. Atomic force microscopy (AFM) technique was employed to characterize surface morphology of the GO sheets and the tungsten oxide films. Moreover, using X-ray photoelectron spectroscopy (XPS), chemical state of the tungsten oxide films and the photocatalytic reduction of the GO platelets were quantitatively investigated. The better performance of the sol–gel tungsten oxide films in photocatalytic reduction of GO platelets as compared to the evaporated tungsten oxide films was assigned to lower W5+/W6+ ratio (i.e., a better stoichiometry) and higher surface water content of the sol–gel film. The GO reduction level achieved after 24h UV-assisted photocatalytic reduction on surface of the sol–gel tungsten oxide film was comparable with the reduction level usually obtainable by hydrazine. The sol–gel tungsten oxide film even showed an efficient photocatalytic reduction of the GO platelets after exposure to the visible light of the environment for 2 days.

Bond and electrochromic properties of WO3 films deposited with horizontal DC, pulsed DC, and RF sputtering

The purpose of this research was to investigate the bond and electrochromic properties of tungsten oxide (WO3) thin films deposited by three different plasmas sources: direct current (DC), pulsed DC and radio frequency (RF). The experimental results show that optical intensity increases with an increasing oxygen percentage and that all films have a 950cm−1 peak which results in a bond of W6+O in the Raman spectra. Hence, the oxygen percentage was changed from 0.3 to 0.7 to study the influence of coloring and bleaching as regards the different sputtering powers. Additionally, WO3 thin films show the best electrochromic properties as regards the oxygen/argon (O2/Ar) gas ratios of 0.7, 0.6 and 0.6 for DC, pulsed DC and RF sputtering, respectively. The transmittances of all films are over 75% as-deposited, and the deposition rates were between 0.8 and 0.1Ås−1. Simultaneously, the transmittance variations (ΔT) were 51%, 57% and 53% for DC, pulsed DC and RF sputtering at a wavelength of 550nm, respectively. The bleached/colored ability of the cyclic voltammograms (CVs) was pulsed DC>DC>RF. Since, the coloration ability of WO3 thin film deposited with plasma source of pulsed DC was better than those of the DC and RF.

H2 sensors based on WO3 thin films activated by platinum nanoparticles synthesized by electroless process

Platinum (Pt) nanoparticles were successfully synthesized on tungsten oxide (WO3) thin films by electroless process without any further post-treatment. The prepared Pt nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy. Gas sensors based on the Pt–WO3 films were found to provide repeatable and significant responses to ppm-level H2. The size of Pt nanoparticles increases with the deposition time and has improved the sensing characteristics of the sensors. The work in this paper paves a facile way to the fabrication of Pt nanoparticles on metal oxide surface at a low temperature (68 °C).

Gas-sensing properties at room temperature for the sensors based on tungsten oxide thin films sputtered on n-type ordered porous silicon

n-type porous silicons are prepared by the electrochemical corrosion method, on which tungsten oxide thin films with different thickness values are sputtered using DC reactive magnetron sputtering. The structures of ordered porous silicons and tungsten oxide thin films are characterized using field emission scanning electron microscope, which show that the pores are pillared and ordered and the thin films cover the porous layer loosely with many pores open to ambient air. The X-ray diffraction characterization indicates that the lattice structure of tungsten oxide thin film is mainly triclinic polycrystalline. The gas-sensing properties at room temperature for both ordered porous silicon and composite structure are studied, which indicate that the latter is much more sensitive to nitrogen dioxide than the former. And there is a critical spurting time of WO3 thin film, which in our case is 10 min. The sensing mechanism of composite structure is discussed and the probable explanation for the improvement of sensitivity to NO2 is the formation of hetero-junctions between the ordered porous silicon layer and the WO3 thin film. In addition, there exists an inversion layer on the surface of the WO3 thin film, which causes the anomalous resistance to change during the gas sensing measurements.

Structural Study of WO<sub>3</sub> and MoO<sub>3</sub> Compound Films in H<sub>2</sub> Gasochromism

Sol-gel tequnique was used to prepare disordered tungsten oxide and molybdenum oxide sols. A series ratio of W:Mo compound sols were obtained via mettalic powder co-peroxided by H2O2 as precursors in ethonal. Compound films were achieved by dip-coating method. Fourier Transform Infrared Spectroscopy, Raman Spectroscopy were taken to characterize the structure of these compound films. Uv-visible Spectroscopy was used to test the gasochromic property. The results showed that the gasochromics property was much different from that of pure tungsten oxide and molybdenum oxide sol-gel thin films. The effect was origined from the structrue alteration, which was not due to the spectrum superposition but the co-reaction of W and Mo.

Thickness Dependent Optical Properties of WO3 Thin Film using Surface Plasmon Resonance

ABSTRACTThe effect of tungsten oxide (WO3) thin film thickness on the surface plasmon resonance (SPR) properties have been investigated. WO3 films of varying the thickness (36 nm, 60 nm, 80 nm, 100 nm, 150 nm and 200nm) have been deposited onto Au coated prism (Au/prism) by radio frequency (RF) magnetron sputtering technique. The SPR responses of bilayer films were fitted with the Fresnel’s equations in order to calculate the dielectric constant of WO3 thin film. The variation of complex dielectric constant and refractive index with the thickness of WO3 thin film was studied.

Physical Properties of Macroporous Tungsten Oxide Thin Films and Their Impact on the Photocurrent Density

Tungsten trioxide (WO3) films were prepared using polystyrene spheres of two different diameters as a template in order to create porous layers. X-ray diffraction data and electron microscopy images show that annealed films exhibit polycrystalline structure with monoclinic phase and pore size of approximately hundred nanometers. The optical band gap energies have been determined by photoacoustic spectroscopy as 3.17 eV, and this value was not affected by sample morphology. Low temperature photoluminescence spectra exhibit broad band in the blue region. Deconvolutions of PL spectra show that there are two transitions which intensity depends on thin film pore size. We discuss the possible origin of this emissions associated with oxygen vacancies and surface states. A comparative study of the WO3films used as photoanodes is presented and correlated with PL results.

The Process Optimization and Structural Analysis of Gasochromic Thin Films Derived by Peroxopolytungstic Acid

The sol-gel method with a combination of dip-coating process was employed to prepare peroxopolytungstic acid gasochromic thin films. The influence of preparation process on the structural and gasochromic durability was then discussed. We found that hydrogen peroxide content shown a significant impact on the structure of tungsten oxide thin films, which directly determined the gasochromic durability

Effect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films

Silver incorporated tungsten oxide thin films are prepared by RF magnetron sputtering technique. The effect of silver incorporation in micro structure evolution, phase enhancement, band gap tuning and other optical properties are investigated using techniques such as x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and UV-Visible spectroscopy. Effect of silver addition in phase formation and band gap tuning of tungsten oxide thin films are investigated. It is found that the texturing and phase formation improves with enhancement in silver content. It is also found that as the silver incorporation enhances the thickness of the films increases at the same time the strain in the film decreases. Even without annealing the desired phase can be achieved by doping with silver. A broad band centered at the wavelength 437 nm is observed in the absorption spectra of tungsten oxide films of higher silver incorporation and this can be attributed to surface plasmon resonance of silver atoms present in the tungsten oxide matrix. The transmittance of the films is decreased with increase in silver content which can be due to increase in film thickness, enhancement of scattering, and absorption of light caused by the increase of grain size, surface roughness and porosity of films and enhanced absorption due to surface plasmon resonance of silver. It is found that silver can act as the seed for the growth of tungsten oxide grains and found that the grain size increases with silver content which in turn decreases the band gap of tungsten oxide from 3.14 eV to 2.70 eV.

Optical Properties of WO3 Thin Films Modeled by Finite-Difference Time-Domain and Fabricated by Glancing Angle Deposition

Optical transmittance spectra between 1.55 eV (800 nm) and 3.10 eV (400 nm) of tungsten oxide (WO3) thin films nanostructured thanks to the Glancing Angle Deposition technique are investigated both experimentally and theoretically, as a function of geometrical parameters. A Finite-Difference Time-Domain code was used to numerically model the films structure and to calculate their optical properties. The corresponding optical index and porosity are considered. It is found that the optical index of columnar structures always follows Cauchy's law as a function of energy and is reduced as the incident angle increases (alpha = 0 to 80 degrees) from n633 = 2.2 to 1.98 for experimental data against 2.1 to 1.75 for those computed with the Finite-Difference Time-Domain code. For zigzag architectures, an increase of the zigzag number from 0.5 to 8, amplifies interference fringes and improves the measured refractive indices. It agrees with modeled optical characteristics since n633 increases from 2.18 to 2.30.

Structural, Optical, and Luminescence Properties of Reactive Magnetron Sputtered Tungsten Oxide Thin Films

Tungsten oxide (WO3) thin films were deposited on to unheated Corning glass and silicon substrates by RF magnetron sputtering of metallic tungsten target at various oxygen partial pressures in the range 4×10-2–1×10-1 Pa. The influence of oxygen partial pressure on the structure and surface morphology and the optical and photoluminescence properties of the films were investigated. X-ray diffraction studies revealed that the deposited films were amorphous in nature. Fourier transform infrared transmission spectra confirmed that the presence of stretching vibration of W-O-W and deformation of W-O bonds related to the WO3. The optical transmittance of the films at wavelengths >500 nm increased from 62% to 85% with the increase of oxygen partial pressure. The optical band gap of the films increased from 3.00 to 3.14 eV and the refractive index of the films decreased from 2.26 to 2.08 with the increase of oxygen partial pressure from 4×10−2 to 1×10−1 Pa, respectively. The photoluminescence studies indicated that the intense blue emission which was assigned to band-to-band transition was observed at oxygen partial pressure of 6×10−2 Pa.