Tungsten Oxide Thin Films Research Articles

Correlation between physical properties and electrochromic performances of DC magnetron sputtered a-WOx thin films

Tungsten oxide thin films were grown on different substrates by the conventional process of direct current magnetron sputtering. A series of amorphous WOx (a-WOx) films, with variable stoichiometry and porosity, were achieved with different total pressure (Pt) induced by oxygen flow rate (Q(O2)) variation. The objective of this study is to determine the couple (Q(O2); Pt) necessary to achieve optimal electrochromic performances of WOx and also study the correlation between physical properties and electrochromic performances. The WOx films optical transmittance evolved from opaque to transparent with the increase of Q(O2). A degradation of films refractive index and the increase of their porosity, both mainly due to the increase of Pt, were observed. The Raman spectroscopy indicated the presence of different W valence states in the as deposited a-WOx films. They all contain W4+, but W6+ were found only into transparent or semi-transparent films. A weak peak of W5+ was observed into the semi-transparent film. Electrical properties studied by Metal-Oxide-Semiconductor structures showed a decrease of the oxide capacitance when Q(O2) and Pt increase. The WOx transparent and optically reversible films, deposited with lower values of (Q(O2); Pt), i.e., Q(O2) = 4.5 sccm and 6 sccm, and Pt = 1.13 Pa and 1.37 Pa, respectively, exhibited the highest values of WOx/Si interface traps average density and positive ions density (Nss). The positive Nss reflects the oxygen deficiency amount in the films. These films were resistive to the discoloration process but showed the highest coloration efficiency CE632nm = 44.4 cm2/C and 42.2, respectively. Their coloration times were tc = 19 s and 22 s, and their discoloration times were td = 180 s and 75 s, respectively. They exhibited the highest optical modulation close to Δ T632nm = 76 %.

The effect of gold Nanoparticles on Structural and Electrical properties of WO3 thin films

Chemical spray pyrolysis technique was used at temperature 250˚C with annealing temperature at 400C˚( for 1hour) to deposition tungsten oxide thin film with different doping concentration of Au nanoparticle (0, 10, 20, 30 and 40) wt.% on glass substrate with thickness about 100 nm. The structural and electrical properties were investigated. The structure properties shows that the films at substrate by x-ray diffraction (XRD) temperature (250˚C) was amorphous structure while at annealing temperature have a polycrystalline structure with the preferred orientation of (200) , all the samples have a hexagonal structure for WO3 and Au gold nanoparticles have a cubic structure .The mechanisms of dc-conductivity of un-doped WO3 and doped with Au (10,20,30 and 40) wt.% thin films at the range (303 to 473) K have been discussed , there is decrease in conductivity with the increase in the doping concentration and hall measurements show that all films have a negative hall coefficient and nH increases with the increase of Au dopant ratio and decreasing in carrier mobility (μH) with increasing of Au .

Effect of Gamma Irradiation Induced Modifications in Tungsten Oxide Thin Films and Their Potential Applications

Effect of Gamma Irradiation Induced Modifications in Tungsten Oxide Thin Films and Their Potential Applications

Tungsten oxide thin film for room temperature nitrogen dioxide gas sensing

Tungsten oxide (WO3) thin films for gas sensing have been successfully deposited using reactive direct current (DC) magnetron sputtering at different deposition temperatures (300 °C, 400 °C and 500 °C). The structural, morphological properties, thickness and composition have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS) techniques. To investigate the effect of deposition temperature on the gas sensing properties of deposited thin films on alumina substrates, was conducted using the Kenosistec gas sensing unit. WO3 thin film deposited at 500 °C exhibited a higher response when sensing Nitrogen dioxide (NO2) at room temperature as compared to the thin films prepared at 300 °C and 400 °C, respectively. However, as deposited WO3 thin films exhibited low sensitivity when sensing reducing gases such as hydrogen (H2) and ammonia (NH3), which was an indication of good selectivity properties of WO3 related sensors.

Thickness dependent tungsten trioxide thin films deposited using DC magnetron sputtering for electrochromic applications

DC magnetron sputtering was used to grow tungsten oxide (WO3) thin films on FTO and corning substrates. SEM, XRD, Electrochemical Analyzer, and UV–Vis Spectrometer were used to analyze surface morphology, structural properties, electrochromic characteristics, and optical characteristics. At an 800 nm wavelength, a decrease in thin-film thickness increased optical transmittance from 87 % to 95 %. Furthermore, coloring efficiency was observed to vary with the thickness of thin films for both 500 nm and 375 nm are 10.34 cm2 C-1 to 18.57 cm2 C-1. In comparison to the high-thickness thin film, the lesser-thickness deposited nano-thin film has a higher diffusion coefficient. At 8 × 10-4 mbar partial pressure, the diffusion coefficients for the smaller and the high-thickness thin film are 7.28x10-14 cm2s−1 and 6.0x10-14 cm2s−1, respectively. The diffusion coefficient and coloring efficiency have been found to have a considerable influence on the thickness and surface-to-volume ratio, which could be important in electrochromic applications.

Cycling durability and potentiostatic rejuvenation of electrochromic tungsten oxide thin films: Effect of silica nanoparticles in LiClO4–Propylene carbonate electrolytes

Electrochromic (EC) technology allows control of the transmission of visible light and solar radiation through thin-film devices. When applied to “smart” windows, EC technology can significantly diminish energy use for cooling and air conditioning of buildings and simultaneously provide good indoor comfort for the buildings’ occupants through reduced glare. EC “smart” windows are available on the market, but it is nevertheless important that their degradation under operating conditions be better understood and, ideally, prevented. In the present work, we investigated EC properties, voltammetric cycling durability, and potentiostatic rejuvenation of sputter-deposited WO 3 thin films immersed in LiClO 4 –propylene carbonate electrolytes containing up to 3.0 wt% of ∼7-nm-diameter SiO 2 nanoparticles. Adding about 1 wt% SiO 2 led to a significant improvement in cycling durability in the commonly used potential range of 2.0–4.0 V vs. Li/Li + . Furthermore, X-ray photoemission spectroscopy indicated that O–Si bonds were associated with enhanced durability in the presence of SiO 2 nanoparticles. • Electrochromic properties, durability, and potentiostatic rejuvenation of W oxide thin films are investigated. • The aim was to study the effect of ∼7-nm-diameter silica nanoparticles in LiClO 4 –propylene carbonate electrolytes. • Film degradation was investigated in range between 2.0 and 4.0 V vs. Li/Li + . • Adding about 1 wt% SiO 2 led to a significant improvement in cycling durability. • O–Si bonds were associated with enhanced durability in the presence of SiO 2 nanoparticles.

From waste carbonated beverages to high performance electrochromic devices: a green and low-cost synthetic method for self-doped metal oxides.

Metal oxides with reversible optical modulation capability are in the spotlight for smart windows and other emerging optoelectronic devices. Improving the electrochromic performance at a low cost is the only way to popularize their applications. Herein, we demonstrate a facile and versatile strategy to synthesize high-performance electrochromic metal oxides, in which waste carbonated beverages are used as the raw materials for the first time. It can not only reduce the production cost of electrochromic materials, but also alleviate the environmental pollution caused by such liquid waste. With an ingenious carbonization pre-step, both nanoscale pores and oxygen vacancies are created in an annealed tungsten oxide thin film. Multiscale structure optimization endows the self-doped WO3-x films with excellent electrochromic properties such as large transmittance modulation (81.2%), high coloration efficiency (98.7 cm2 C-1) and good cycling stability. DFT calculations show that oxygen vacancies reduce the Li+ ion insertion energy barrier, which is conducive to the interfacial reaction in coloring and bleaching processes. Moreover, this approach is universal to other oxides such as vanadium pentoxide, molybdenum oxide and nickel oxide. The waste-to-value concept paves the way for cost-effective electrochromic materials and sheds light on the multiscale optimization of superior metal oxides.

Synthesis and characterization of Ti-WO3 films for electrochromic applications

In this work, tungsten oxide thin films with titanium dopant were deposited at various temperatures of substrate and oxygen partial pressures on silicon wafer and indium doped tin oxide glasses using reactive dc-magnetron sputtering. The main objective of the study is to investigate in detail and to discuss the effect of titanium and deposition parameters on morphological and electrochromic studies of the films. With the alteration of deposition parameters, the morphology changed from granular to coarse with coagulation of grains, and the titanium has significant impact in increasing the grain size. The titanium films incorporated, which are analyzed by an energy dispersive spectroscopy. The needle-like grains with columnar topography was observed, and having some voids over the surface analyzed with atomic force microscopy. The films exhibited excellent electrochromic properties. The films coloration efficiency was altered between 31.8 cm2/C to 101.70 cm2/C.

In-situ deposition of tungsten oxide hole-contact by Hot-Wire CVD and its application in dopant-free heterojunction solar cells

The tungsten oxide (WO x ) thin films have been deposited by a novel hot filament oxidation-sublimation process and applied in the dopant-free heterojunction solar cells as the hole selective contacts. The oxygen flow ratio plays a significant role during the deposition process. With increasing the oxygen flow ratio from 1.7% to 6.7%, the morphology of WO x films changes from small cauliflower-like particles to large cluster accumulation, and the ratio of W6+ increases from 76.1% to 91.4% with the ratio of W5+ decreasing from 23.9% to 8.6%. The work function of WO x can be tailored in a range of 5.5–6.1 eV by increasing the oxygen flow ratio. Its optical band gap maintains above 3.2 eV with the conductivity of about 10−5 S·cm−1. We have applied the WO x films in dopant-free silicon heterojunction solar cells as the hole selective contact layer by replacing the p-type amorphous silicon layer. By taking advantage of the highly transparent WO x layer, a high photon-current density of 39.6 mA·cm−2 was achieved with the oxygen flow ratio of 1.7%. It is interesting to find that the optimum cell conversion efficiencies of 14.9% were obtained with the oxygen flow ratio of 1.7% and the thickness of 10–20 nm for the deposition of WO x layer. This work proves the feasibility and good potential of Hot-Wire CVD prepared WO x hole selective contact for efficient dopant-free silicon heterojunction solar cells.

Copper tungsten oxide (CuxWOy) thin films for optical and photoelectrochemical applications deposited by reactive high power impulse magnetron co-sputtering

Copper tungsten oxide films are deposited with the help of reactive high power impulse magnetron sputtering (HiPIMS) in an argon/oxygen gas mixture. Two magnetrons, one equipped with a tungsten target and the other with a copper target, are employed. The HiPIMS discharge is operated with a repetition frequency of f=100 Hz. Pulse widths of 100 and 20 μs separated by 25 μs are chosen for the tungsten and copper target, respectively. Films deposited on two different glass substrates [soda lime glass and fluorine doped tin oxide (FTO) coated glass] are characterized by energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, x-ray diffraction, Raman spectroscopy, and ellipsometry. Photoelectrochemical activity was investigated by linear voltammetry. The composition and crystal structure of as-deposited and annealed films are found to depend on the deposition conditions. Annealed films deposited on FTO glass are composed of WO3 and CuWO4 or Cu2WO4 crystal phases. Films deposited on soda lime glass are subject to sodium diffusion into the films during annealing and the formation of Na2W2O7 and Na2W4O13 phases.

Effect of tungsten oxide thin films deposited on cerium oxide nano rods for electrochromic applications

Cerium oxide Nanorods were synthesized on Fluorine-doped Tin Oxide (FTO) coated glass substrate by hydrothermal process. Optimized Tungsten Oxide (WO3) films were coated on the top of cerium oxide (CeO2) Nanorods by a sputter deposition method. WO3 thin film was deposited at a constant substrate temperature of 300K under partial pressures of oxygen (ppo2) at 8 × 10−4 mbar. X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), UV–Visible spectrometer, and cyclic voltammetry were used to analyse the films for material, optical, surface, and electrochemical properties. The improved CeO2/WO3 films increased the cathodic peak current which is related to their high active surface area and increased H+ ions intercalation in the films. In addition, the CeO2 Nanorods/WO3 hybrid films had a better electrochemical property in terms of the diffusion coefficient of 5.53 × 10−8 cm2/s. These structures produce considerable coloration efficiency when compared to pure WO3 (9.88, 10.19 and, 8.14 cm2/C at 700 nm). This study predicts that how varying concentrations of cerium oxide Nano rods change WO3 electrochemical behaviour which will help to choose the right rare earth elements and concentrations for energy-efficient smart windows.

Optimization of GLAD Angle for E-Beam-Fabricated Tungsten Oxide (WO3) Thin Films Towards Novel Electrochromic Behavior

Optimization of GLAD Angle for E-Beam-Fabricated Tungsten Oxide (WO3) Thin Films Towards Novel Electrochromic Behavior

Effects of deposition parameters on RF-sputtered WO3 thin films

In the absence of additional oxygen, thin films of tungsten (VI) oxide (WO3) were prepared on indium-doped tin oxide conductive glass substrates by radio-frequency (RF) magnetron sputtering. The effects of sputtering power, working air pressure, substrate bias voltage and substrate temperature on the surface morphology, microstructure, optical properties and electrochromic (EC) performance of the films were systematically investigated. The research shows that a sputtering power of 80–100 W can ensure a moderate deposition rate of ∼10−2 nm/s and help obtain non-dense films. Similarly, a working air pressure of 1.0 Pa also leads to the deposition of loose films, which is beneficial for the improvement of the optical transmittance and EC performance of tungsten (VI) oxide thin films. The applied substrate bias has little effect on the optical properties, but it will degrade the coloring and/or bleaching efficiency of tungsten (VI) oxide thin films and greatly reduce their optical modulation. When the substrate temperature rises to 600°C, the film begins to crystallize and exhibits a rod-patterned porous structure, which leads to a small increase in the optical modulation.

Effect of variation in glancing angle deposition on resistive switching property of WO3 thin films for RRAM devices

In this paper, nanostructured tungsten oxide (WO3) thin films are deposited using the RF-magnetron sputtering technique in Glancing Angle (GLAD) arrangement. Variation in the structural, morphological, optical, and resistive switching (RS) characteristics of nanostructured WO3 film is investigated as a function of GLAD angle (60°–80°). Electrical studies on nanostructured WO3 films deposited at room temperature are found to exhibit enhanced bipolar resistive-switching properties in metal–insulator–metal pattern [Au/WO3/ITO]. The RON/ROFF ratio between high and low resistance states was noted to be about 190 besides a minimum set voltage of ∼2.22 V in the case of the WO3 thin film deposited at the 70° glancing angle. A detailed current transport mechanism analysis indicates the existence of ohmic-behavior and trap-assisted space charge limited conduction as the governing mechanisms at the state of low and high applied bias, respectively. Good data-retention characteristics coupled with reproducible and fast RS capabilities obtained with Au/WO3/ITO device structure promise scope of rapid development in future RS-based novel memory device applications.

Investigation of Morphological, Optical, and Dielectric Properties of RF Sputtered WOx Thin Films for Optoelectronic Applications.

Tungsten oxide (WOx) thin films were synthesized through the RF magnetron sputtering method by varying the sputtering power from 30 W to 80 W. Different investigations have been conducted to evaluate the variation in different morphological, optical, and dielectric properties with the sputtering power and prove the possibility of using WOx in optoelectronic applications. An Energy Dispersive X-ray (EDX), stylus profilometer, and atomic force microscope (AFM) have been used to investigate the dependency of morphological properties on sputtering power. Transmittance, absorbance, and reflectance of the films, investigated by Ultraviolet-Visible (UV-Vis) spectroscopy, have allowed for further determination of some necessary parameters, such as absorption coefficient, penetration depth, optical band energy gap, refractive index, extinction coefficient, dielectric parameters, a few types of loss parameters, etc. Variations in these parameters with the incident light spectrum have been closely analyzed. Some important parameters such as transmittance (above 80%), optical band energy gap (~3.7 eV), and refractive index (~2) ensure that as-grown WOx films can be used in some optoelectronic applications, mainly in photovoltaic research. Furthermore, strong dependencies of all evaluated parameters on the sputtering power were found, which are to be of great use for developing the films with the required properties.

Synthesis and characterization of spray deposited nanostructured WO3 thin films for ammonia sensing applications

• This paper presents the effect of substrate temperature on chemically sprayed WO 3 thin films to detect ammonia vapours at room temperature. • Prepared nanostructured thin films at different substrate temperatures of 350–450 °C have been characterised for structural, morphological, optical and gas sensing properties. • The WO 3 film deposited at a substrate temperature of 400 °C has shown better structural, morphological properties and excellent ammonia sensing features at room temperature. • This film has shown good sensitivity with a quick response and recovery times with the least detection limit of 1 ppm. Nanostructured tungsten oxide (WO 3 ) thin films are prepared on glass substrates at different substrate temperatures (350–450 °C) using the chemical spray pyrolysis technique. The effects of substrate temperature on structural, morphological and optical properties of the WO 3 films are systematically studied using X-ray diffraction (XRD), UV–visible photo spectroscopy, Raman spectroscopy and atomic force microscopy (AFM). The XRD analysis of the synthesised WO 3 films have shown crystalline nature and preferred orientation along with (0 1 2) direction. The optical bandgap of the thin films is determined using the absorption spectra obtained with UV–visible spectroscopy. The surface features of the deposited thin films are investigated using AFM with contact mode. The gas-sensing performance of the prepared thin films is investigated using a static method towards various ammonia concentrations at an operating temperature of 27 °C. The WO 3 film deposited at a substrate temperature of 400 °C has shown the best response towards ammonia vapours with a response and recovery times of 61 s and 34 s with the least detection limit of 1 ppm.

Tuning Structural and Optical Properties of WO3 NPs Thin Films by the Fluency of Laser Pulses

In this paper, tungsten oxide thin films were successfully synthesized by the laser pulse deposition (PLD) method using a pulsed laser (ND-YAG) and wavelength (1064 nm) on a glass substrate at different laser fluencies. The effect of increasing laser fluency, on the optical and structural properties of WO3 nanoparticle thin films, was investigated by UV-Visible spectrophotometer, X-Ray diffraction (XRD), atomic force microscope (AFM) and Scanning Electron Microscope (SEM). X-Ray measurements for all samples of WO3 NPs thin films have shown that by increasing the laser fluencies from 5.175 to 6.369 J/cm2, the intensity of the (2 01) diffraction peak increases due to the film continuing to grow with increased crystallization.

Influence of gamma radiation on optical, structural and surface morphological properties of WO3 thin films grown by RF sputtering

In the present study, tungsten oxide (WO3) thin films were grown on glass and silicon substrates using RF sputtering method and thin films were annealed at 550 °C. The prepared WO3 thin films were exposed through gamma irradiation by varying doses at 100, 200, 300, 400 and 500 kGy. The modification in optical, structural, morphological and chemical properties of WO3 thin films before and after gamma irradiation doses were observed from UV-Vis, Photoluminescence (PL) and Raman spectroscopy, Fourier Transform spectroscopy (FTIR), Atomic Force Microscopy (AFM), X-ray diffraction (XRD), Rutherford backscattering (RBS) and X-ray photoelectron spectroscopy (XPS). The grain size after gamma irradiation reduced from 6.56 nm to 3.40 nm. The Raman spectroscopy reveals the monoclinic phase of tungsten oxide thin films after gamma irradiation. XRD pattern shows the variation in crystallite size from 7.2 nm to 33.8 nm. The optical band gap of pristine and gamma irradiated thin films was observed from UV-Vis spectroscopy. The functional groups were confirmed by FTIR spectroscopy. PL emission peaks were observed at 415 nm and 475 nm for pristine and gamma irradiated thin films at excitation wavelength of 380 nm. The XPS study reveals the presence of W and O atoms in pristine and gamma irradiated WO3 thin films.

Influence of substrate temperature on the optical and vibrational properties of RF sputter coated NiO:WO3 thin films for optoelectronic applications

Mixed nickel –tungsten oxide thin films denoted as NiO:WO3 were prepared at 100 W rf power with varying substrate temperatures (Tsub = 100, 200, and 300 °C) using radio frequency (RF) magnetron sputtering technique. The influence of substrate temperature (Tsub) of NiO:WO3 thin films was studied in detail. All the prepared samples were found amorphous in nature with very fine smooth morphology. The average transmittance decreases from 88% to 83% with increasing substrate temperature. The energy band gap (Eg) decreases from 3.43 to 3.21 eV with increasing substrate temperature. AFM study revealed the roughness value of 0.875 nm. The detected binding energy values of Ni, O, and W observed from XPS study were, 853.7 and 871.8 eV with their respective satellite peaks at 859.88 and 878.2 eV corresponding to Ni 2p3/2 and Ni 2p1/2, O 1 s peaks at 529.51 eV due to Ni2+ ions and the higher binding energy peak at 531.13 eV due to the Ni3+ ions, peaks at 35.2 and 37.3 eV, corresponds to W 4f7/2 and W 4f5/2. The relationship between the refractive index and energy band gap proposed by different models was discussed. The refractive index (n) evaluated from these models shows an increase in n values with increasing Tsub owing to an increase in interaction among the molecules of NiO:WO3 and effect of these interactions on light. The derived optical constant k decreases from 0.024 to 0.022 indicating the decreased optical loss. The dielectric constants vary between 5.262 and 5.439 for ε1 and 0.113-0.106 for ε2 with variation in Tsub. The room temperature photoluminescence (RTPL) spectroscopy showed emission bands around 368 nm and 423 nm corresponding to the transition of 3 d8 Ni2+ ions. Micro-Raman analysis shows two broad peaks due to Ni–O bond vibrations at 570 cm−1 and 1100 cm−1, while the peak found at 870 cm−1 is due to a W–O bond vibration. We have aimed to discuss these results and correlate them with possible mechanisms behind the phenomena.

Pseudocapacitive Charge Storage in Electrochromic Transition-Metal Oxide Thin Films

Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turn transparent to sky-blue color in the lithiated state due to the reduction of W6+ to W5+. We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li+ insertion/de-insertion in WO3. The electrochromic thin film’s pseudocapacitive and electrical double layer mechanism was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV s−1. These films exhibit an areal charge density of around 100 mC cm−2 and a capacitance of 80 mF cm−2, superior to most comparable electrochromic materials and supercapacitors. This work combines electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO3.