Tungsten Oxide Layer Research Articles

Towards a micro-system for monitoring ethylene in warehouses

By means of an adapted screen-printing technique, sensitive layers of tin and tungsten oxide (pure and doped with 1% in weight of Au, Pt and Pd) were deposited on silicon micromachined substrates with low thermal inertia. Each chip contains four membranes with different active layers. The sensor response to ethylene, ethanol, ammonia and their binary mixtures was studied. The sensor reaction to humidity was also investigated. A 4-element microsensor array is designed (by selecting the active materials and noble metal loading) to selectively detect ethylene, ethanol, ammonia and their binary mixtures. It is shown that these species can be easily discriminated, even at low concentrations (1 ppm), which makes the microarray designed suitable to monitor climacteric fruit during storage in warehouses or storage chambers.

Layered Tungsten Oxide-Based Organic−Inorganic Hybrid Materials: An Infrared and Raman Study

Tungsten oxide-organic layered hybrid materials have been studied by infrared and Raman spectroscopy and demonstrate a difference in bonding nature as the length of the interlayer organic "spacer" molecule is increased. Ethylenediamine-tungsten oxide clearly displays a lack of terminal -NH3(+) ammonium groups which appear in hybrids with longer organic molecules, thus indicating that the longer chains are bound by electrostatic interactions as well as or in place of the hydrogen bonding that must be present in the shorter chain ethylenediamine hybrids. The presence of organic molecules between the tungsten oxide layers, compared with the layered tungstic acid H2WO4, shows a decrease in the apical W=O bond strength, as might be expected from the aforementioned electrostatic interaction.

Self-organized porous WO 3 formed in NaF electrolytes

In the present work we show that by anodization of W in NaF electrolytes porous tungsten oxide layers can be formed under a range of experimental conditions. The morphology of the layers depends strongly on the NaF concentration as well as the formation potential. Under optimized electrochemical conditions self-organized porous tungsten oxide layers can be formed. The single pore openings are very regular and have an average pore diameter of 100 nm.

A Model of Cu-CMP

AbstractCMP has been described qualitatively in terms of alternating cycles of chemical formation and mechanical removal of a thin layer on the wafer surface. A quantitative model of CMP has been developed2-7 which is based on mechanisms for surface kinetics, treating mechanical removal as one step in the mechanism. This model has been used successfully to explain removal rates for tungsten and thermal oxide CMP. In particular, for tungsten CMP the removal rate increases steeply with increasing oxidizer concentration at low concentrations, and then approaches an asymptotic maximum removal rate at high concentrations. The model explains this by starting with the assumption that mechanical abrasion removes only tungsten oxide but not tungsten metal. It then focuses on the fraction of wafer surface covered by a tungsten oxide layer. At low oxidizer concentrations, the oxide formation rate is small compared the removal rate, so only a small fraction of the surface is oxidized and the removal rate is small. At high oxidizer concentrations, the oxide formation rate is large compared to the removal rate, so most of the surface is oxidized and the removal rate is large. Increasing the oxidizer concentration in the high oxidizer concentration region does not significantly increase the surface fraction of tungsten oxide, and the removal rate approaches a constant value.

Microstructural characterization of thick YBa2Cu3O7−δ films on improved rolling-assisted biaxially textured substrates

The microstructural changes associated with the reduced dependence of critical current density (Jc) versus thickness of thick, epitaxial YBa2Cu3O7–δ (YBCO) films on rolling-assisted biaxially textured substrates (RABiTS) were investigated. Pulsed laser deposited YBCO films varying in thickness from 1.0 to 6.4 ?m on RABiTS with an architecture of Ni–3 at.% W/Y2O3/yttrium-stabilized-zirconia/CeO2/YBCO were prepared for cross-sectional transmission electron microscopy studies. Dramatic improvements in physical properties and microstructural quality were observed resulting from the use of Ni–3 at.% W substrates, which provided a sharper texture over earlier Ni substrates, and replacement of CeO2 with Y2O3 as the seed layer within the buffers. The YBCO films showed exceptional orientation up to 6.4 μm thickness, with no misoriented grains or dead layers observed and only limited reaction between the YBCO and CeO2 cap layer. The high quality of the films was also attributed in part to the formation of a tungsten oxide layer forming at the top of the Ni–3% W substrate, limiting the growth of deleterious NiO into the conductor.

Syntheses, structures, and second-harmonic generating properties in new quaternary tellurites: A2TeW 3O 12 ( A=K, Rb, or Cs)

The syntheses, structures, and characterization of a new family of quaternary alkali tungsten tellurites, A 2TeW 3O 12 ( A=K, Rb, or Cs), are reported. Crystals of the materials were synthesized by supercritical hydrothermal methods using 1 M AOH ( A=K, Rb, or Cs), TeO 2, and WO 3 as reagents. Bulk, polycrystalline phases were synthesized by standard solid-state methods combining stoichiometric amounts of A 2CO 3, TeO 2, and WO 3. Although the three materials are not iso-structural, each exhibits a hexagonal tungsten oxide layer comprised of corner-sharing W 6+O 6 octahedra. Te 4+O 3 groups connect the WO 6 layers in K 2TeW 3O 12, whereas the same groups cap the WO 6 layers in Rb 2TeW 3O 12 and Cs 2TeW 3O 12. This capping results in non-centrosymmetric structures for Rb 2TeW 3O 12 and Cs 2TeW 3O 12. Powder second-harmonic generation measurements on Rb 2TeW 3O 12 and Cs 2TeW 3O 12 revealed strong SHG efficiencies of 200 and 400×SiO 2, respectively. These values indicate an average non-linear optical susceptibility, 〈 d eff〉 exp of 16 and 23 pm/V for Rb 2TeW 3O 12 and Cs 2TeW 3O 12, respectively. Crystallographic information: K 2TeW 3O 12, monoclinic, space group P2 1/ n (No. 14), a=7.3224(13) Å, b=11.669(2) Å, c=12.708(2) Å, β=90.421(3)°, Z=4; Rb 2TeW 3O 12, trigonal, space group P31 c (No. 159), a= b=7.2980(2) Å, c=12.0640(2) Å, Z=2.

Fabrication and electrochromic properties of double layer WO 3(V)/V 2O 5(Ti) thin films prepared by pulsed laser ablation technique

Single and double layers of tungsten oxide and vanadium oxide thin films have been deposited on In 2O 3:SnO 2 (ITO) glass using the pulsed laser deposition (PLD) technique at a substrate heating temperature of 200°C and in rich oxygen pressure. The structural properties and chemical composition of as-deposited WO 3(V) and V 2O 5(Ti) thin films were analyzed using X-ray diffraction (XRD), Raman spectrum (RS), and X-ray photo-electron spectroscopy (XPS). The as-deposited V 2O 5 doped WO 3 thin film on ITO glass showed an amorphous structure. Highly oriented growth nano-crystalline TiO 2 doped V 2O 5 thin films with polycrystalline orthorhombic structure were successfully synthesized on ITO glass using the scanning laser ablation technique at a deposition temperature as low as 200°C. Cyclic voltammograms, at a sweep rate of 50 mV s −1, show that long-term degradation was noticed for the as-deposited WO 3(V) thin film, and no long-term degradation was noticed for the as-deposited double layer WO 3(V)/V 2O 5(Ti) thin film. Durability was verified to 8000 cycles in the voltage range between −1.0 and 1.0 V. It is demonstrated that these double layer WO 3(V)/V 2O 5(Ti) thin films on ITO are good candidates for cathode thin films for rechargeable batteries and electrochromic devices. As the additional top layer of V 2O 5(Ti) possesses nano-crystalline and c-axis oriented structure, which is suitable for Li-ions transport, the cycle stability and reversibility of WO 3(V) films have been improved. Double layer WO 3(V)/V 2O 5(Ti) thin films provide a neutral brownish blue electrochromic color. Therefore, this double layer film maybe suitable for most building applications.

A simple fabrication method for tunnel junction emitters

The tunnel junction emitter is a novel vacuum electron source, based on ballistic transmission of electrons through ultra-thin foils. This source can operate at low voltages and is expected to have a high brightness and low energy spread. In this paper, we will present a simple method to fabricate tunnel junction emitters in the form of a solid state device. These devices consist of a sharp tungsten tip with a tungsten oxide layer casted in an epoxy resin. The apex of the tip is in contact with a 20 nm thick gold foil, which is deposited on the resin surface. Such a device is similar to a conventional metal-oxide–metal cathode. The difference is that for this device, the lateral size of the tunnel contact is very small. Results are presented on the tunnel current and the emission current as a function of the bias voltage between tip and foil. The threshold voltage above which electrons are emitted is in agreement with the work function of the gold foil.

Failure modes of sol–gel deposited electrochromic devices

Long-term charge injection and extraction cycling performance of electrochromic devices, and the degradation of the devices has been studied. The electrochromic devices have been constructed using sol–gel deposited tungsten oxide layer, sol–gel deposited vanadium oxide layer and a polymer electrolyte. Both electrodes were cycled in a solution of 1 M-LiClO 4 and propylene carbonate prior to device fabrication. The devices were sealed using a rubber O-ring so that disassembly and further electrochemical and optical testing of each component after use in a device could identify the reason for device degradation. During the tests, the charge state of the device and device transmittance were recorded by a computer controlled experimental system. The results show three main modes of degradation, which we have denoted as electrolyte failure, counterelectrode failure and working electrode failure. Each failure mode can be identified on a device history graph which shows charge and the ratio of transmittance in the bleached to the transmittance in the coloured state (contrast ratio) as functions of cycle number.

Development of electrochromic cells by the sol–gel process

These investigations have the development of electrochromic cells via the sol–gel process as their objective. Syntheses routes for tungsten oxide, WO3, coating sols and cerium oxide–titanium oxide sols, CeO2–TiO2, were developed, which enabled indium–tin oxide (ITO) coated glass substrates to be coated homogeneously and without visible cracking. Optoelectrochemical measurements were employed to determine the variation of the electrochromic properties (change in optical density, ΔOD, and switching times) of WO3 layers, investigated as a function of coating parameters, such as chamber humidity and the temperature of heat treatment. A novel nanocomposite electrolyte system, based on organically modified silanes, was developed, which has an ionic conductivity of 10−4–10−5Scm−1 at 25°C. Values of diffusion coefficients for lithium ions in tungsten oxide layers could be determined by electrochemical impedance spectroscopy (EIS) and lie in the range 10−11–10−12cm2/s. The values obtained are comparable with literature values for WO3 layers deposited by sputtering. The cells which have been constructed to date show a typical reduction in transmission from 80% to 20% with a half-life of less than 30s, where half-life is here defined as the time necessary for half of the change in transmission to be obtained, i.e. from 80% to 50%.

A New and Simple Method for Manufacturing Electrochromic Tungsten Oxide Films

A new and simple method for the preparation of electrochromic tungsten oxide film is presented. This novel approach can be realized by introducing a nanoporous textured polyacrylic acid (PAA) layer between tungsten oxide layer and indium tin oxide (ITO) one by dip-coating process. Homogeneous film with a thickness of about 0.8 μm is made by single dipping a PAA-coated ITO glass in the WO3-NH4OH solution and shows an X-ray amorphous structure. During the cathodic and anodic processes by chronopotentiometry, the present films exhibit highly reversible electrochemical insertion of lithium ions; the corresponding inserted charge of 12.5 mC/cm2gives rise to a marked blue color yielding the change in optical density (AOD) of 0.65 at 550 nm and a coloration efficiency(η)of 51.8 cm2/C.

Hydrothermal syntheses and crystal structures of new layered tungsten(VI) methylphosphonates, M2(WO3)3PO3CH3(M = NH4, Rb, Cs)

The hydrothermal syntheses of Cs2(WO3)3PO3CH3, Rb2(WO3)3PO3CH3 and (NH4)2(WO3)3PO3CH3, three new non-centrosymmetric, layered tungsten(VI)-containing phases, are described. The crystal structures of Cs2(WO3)3PO3CH3 and (NH4)2(WO3)3PO3CH3 were refined by using Rietveld analysis of powder diffraction data. These phases are built up from hexagonal tungsten-oxide-like layers of vertex-sharing WO6 groups, capped by P—CH3 entities (as methylphosphonate groups) on one side of the tungsten oxide layer. Caesium, rubidium or ammonium cations provide interlayer charge compensation for the anionic layers. IR, Raman and thermogravimetry (TG) data for these phases are also presented. Crystal data: Cs2(WO3)3PO3CH3, Mr= 1055.36, trigonal, space group R3 (no. 146), a= 7.25791(9)Å, c= 20.2762(4)Å, V= 925.00(4)Å3, Z= 3, Rp= 2.37%, Rwp= 3.12%(2943 neutron powder data). (NH4)2(WO3)3PO3CH3, Mr=825.63, trigonal, R3 (no. 146), a= 7.22851(7)Å, c= 19.3471(3)Å, V= 875.48(3)Å3, Z= 3, Rp= 8.14%, Rwp= 10.12%(2665 X-ray powder data).

Reactive ion etching and plasma etching of tungsten

Tungsten etching has been studied in audio frequent plasmas with NF 3-O 2 and SF 6-O 2 mixtures. The influence of etch mode, gas flows, pressure and power has been investigated. It is possible to etch tungsten chemically but large concentrations of free fluorine are needed. These are supplied more easily by NF 3 than by SF 6. During chemical etching with NF 3 fluorine diffuses into the tungsten. When in contact with air, the fluorine is replaced by oxygen, forming a tungsten oxide layer, which withstands further chemical etching.

Structure of the intercalate Na1.5WO2Cl2 by powder neutron diffraction

The crystal structure of the intercalation compound Na1.5WO2Cl2 has been refined using powder neutron diffraction. Refinement was carried out in the tetragonal space group, I4/mmm, with a= 3.8809(4)A, c= 15.102(2)A, V= 227.46(7)A3, Z= 2. The final R factors are Rwp= 4.84%, Rex= 2.43%, χ2= 4.00 for 20 basic variables with 2699 observations corresponding to 54 reflections. The structure, which is closely related to that of the unintercalated host WO2Cl2, consists of layers of tungsten oxide sandwiched between chloride ions to form trilayer blocks of general formula [ClWO2Cl]n. These blocks stack in the c direction to generate the three-dimensional structure, which is stabilised by van der Waals interactions. Na+ ions are located outside the van der Waals gap within the trilayer blocks and are coordinated to four chlorine and four oxygen atoms in a square antiprismatic coordination.

The Disordered Structure of WO2Cl2: A Powder Diffraction Study

The crystal structure of the intercalation host WO2Cl2, Fw = 286.85, has been refined using powder neutron diffraction. Refinement was carried out in the orthorhombic space group, Immm, with a = 3.8414(1), b = 3.8851(1), c = 13.8720(4) Å, V = 207.03 Å3, Z = 2. The final R factors are Rwp = 5.22%, Rex = 4.74%, X2 = 1.21 for 26 basic variables with 7000 observations corresponding to 292 reflections. The structure consists of layers of tungsten oxide sandwiched between chloride ions to form trilayer blocks of general formula [Cl-WO2-Cl]n. These blocks stack in the c -direction to generate the three-dimensional structure. Only weak van der Waals forces hold the blocks together and as a result the compound is an excellent host for intercalation. The structure shows a distorted W environment accompanied by significant disorder in the layer planes.

A convenient method for removing surface oxides from tungsten STM tips

A convenient treatment that can be applied to tungsten scanning tunneling microscope tips prepared by electrochemical etching in aqueous base is described. The treatment removes the majority of the tungsten oxide layer present after etching and produces a very stable tunnel junction and a high success rate in preparing tips for atomic-resolution imaging.

Characterization of tungsten tips for STM by SEM/AES/XPS

For the first time, both X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) techniques were applied in analysis of surface contamination of electrochemically etched Scanning Tunneling Microscope (STM) tungsten tips. Carbon monoxide, graphite, tungsten carbide and tungsten oxide were found as main surface contaminations of STM tungsten tips. The thickness of tungsten oxide layers was estimated to be about 1–3 nm. Quantitative analysis of surface and bulk concentration of carbon, oxygen and tungsten has been performed.

Charge-density-wave instabilities expected in monophosphate tungsten bronzes.

On the basis of tight-binding band calculations, we examined the electronic structures of the tungsten oxide layers found in the monophosphate tungsten bronze (MPTB) phases. The Fermi surfaces of these MPTB phases consist of five well-nested one- and two-dimensional pieces. We calculated the nesting vectors of these Fermi surfaces and discussed the expected charge-density-wave instabilities.

Sol–gel synthesis of WO3thin films

Versatile molecular precursors for the sol–gel synthesis of tungsten oxide thin films can be obtained via the reaction of tungsten oxychloride WOCl4 with alcohols. Oligomeric species [WOCl4 –x(OR)x]n are formed. Their molecular structure is analysed by infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy (1H, 13C, 183W), X-ray absorption spectroscopy (EXAFS) and small-angle X-ray scattering (SAXS). Hydrolysis of these precursors leads to the formation of tungsten oxide colloidal solutions. They can be easily deposited by dipcoating. Thin films ca. 3000 A thick are obtained. Their morphology depends on the nature of the alcohol. Homogeneous films are obtained with bulky alkyl groups such as PriOH. These amorphous tungsten oxide layers exhibit electrochromic properties and could be used for display devices or smart windows. They can also be easily transformed, at room temperature, into crystalline hydrates WO3·nH2O (n= 1 or 2) when left in a humid atmosphere.

Similarity of the electronic properties of the monophosphate tungsten bronzes

The electronic structures of the perovskite-type tungsten oxide (W-O) layers of the monophosphate tungsten bronzes (MPTB) were examined by performing tight-binding band calculations. The authors study shows that all known MPTB phases have one- and two-dimensional metallic bands regardless of the difference in the thickness of their W-O layers and in their octahedral distortions. Concerning this dimensionality of the electronic properties, the bond valence sum analysis is found to give erroneous predictions. They examine the origin of this failure and also that of the remarkable similarity in the MPTB electronic structures. 20 refs., 5 figs., 1 tab.