The effect of WO3 source stability on the properties of films deposited by electron beam deposition

This study investigates the impact of high temperature and vacuum on the properties of WO₃ powder during electron beam deposition and evaluates the consequential effects on the as-deposited films. Therein, the grain size and the crystallinity of the source were observed to increase and become non-stoichiometric tungsten oxide (WO_3−x) due to the high temperature and vacuum during the first deposition. As a result of these changes in the source, the optical band gap, E_g, of the deposited film decreased from 3.11 eV to 3.07 eV, and the absorbance was observed to increase. The coloration efficiencies of the deposited films decreased from 23 to 16 cm² C⁻¹. WO₃-incorporated carbon nanotubes (WO₃/CNT) were observed in the source after electron beam deposition if there were some initial carbon impurities in the source prior to deposition.     

Preparation and characterization of spray deposited n-type WO3 thin films for electrochromic devices

The n-type tungsten oxide (WO₃) polycrystalline thin films have been prepared at an optimized substrate temperature of 250 °C by spray pyrolysis technique. Precursor solution of ammonium tungstate ((NH₄)₂WO₄) was sprayed onto the well cleaned, pre-heated fluorine doped tin oxide coated (FTO) and glass substrates with a spray rate of 15 ml/min. The structural, surface morphological and optical properties of the as-deposited WO₃ thin films were studied. Mott-Schottky (M-S) studies of WO₃/FTO electrodes were conducted in Na₂SO₄ solution to identify their nature and extract semiconductor parameters. The electrochromic properties of the as-deposited and lithiated WO₃/FTO thin films were analyzed by employing them as working electrodes in three electrode electrochemical cell using an electrolyte containing LiClO₄ in propylene carbonate (PC) solution.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Synthesis, formation mechanism and sensing properties of WO3 hydrate nanowire netted-spheres

Tungsten oxide hydrate nanowire netted-spheres were successfully synthesized in the glycol solution using a facile solvothermal approach. The nanowires with uniform diameter of 4-6 nm are actually a kind of tungsten oxide hydrate/surfactant hybrid materials. The influence of surfactant, solvent, time and temperature on tailoring morphology was investigated in detail. The possible formation process of WO₃ hydrate nanowire netted-sphere was proposed. Sensing properties of such WO₃ hydrate sensor show that the desirable sensing characteristics towards 100 ppm ammonia gas at 320 °C were obtained, such as rapid response (18.3 s), high sensitivity, good reproducibility and stability.     

WO3 thin film prepared by PECVD technique and its gas sensing properties to NO2

Tungsten oxide films have been successfully fabricated from tungsten oxychloride (WOCl₄) precursor by using plasma enhanced vapor deposition (PECVD) technique. The films were deposited onto silicon substrates and ceramic tubes maintained at 100°C under a constant operating pressure of He-O₂ gas mixtures. The compositions and the structures of the thin films have been investigated by means of anaysis methods, such as XRD, XPS, UV and IR. The as-deposited WO₃ thin films were amorphous state and became crystalline after annealing above 400°C. The surface analysis of the films indicates that stoichiometry O/W is 2.77 : 1. The gas sensing measurements of the WO₃ thin film sensors indicate that these sensors have a high sensitivity, excellent selectivity and quick response behavior to NO₂.     

Investigation of electrochromic properties of nanocrystalline tungsten oxide thin film

Thin films of tungsten oxide were grown by organometallic chemical vapor deposition (OMCVD) using tetra(allyl)tungsten, W(η³-C₃H₅)₄. X-Ray diffraction (XRD) analyses showed amorphous films at substrate temperatures (T_s) <350°C and polycrystalline films at T_s>350°C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed grain sizes in the range 20–40 nm. In situ electrochemical reduction of WO_3.2/ITO (2.0 M HCl) produced a faint blue color in less than 1 s. The maximum coloration efficiency (CE) was found to be 22 cm²/mC at 630 nm. The density of the films decreases from 4.53 to 4.29 g/cm³ after annealing. An optical bandgap (E_g) of ∼3.2 eV was estimated for both as-deposited and annealed films.     

Effect of annealing on molybdenum doped indium oxide thin films RF sputtered at room temperature

Thin films of molybdenum doped indium oxide (IMO) were deposited on glass at room temperature using an in-built three-source RF magnetron sputtering. The films were studied as a function of oxygen volume percentage (O₂ vol. %; ranging from 0.0 to 17.5%) in the sputtering chamber. The as-deposited amorphous films were crystallized on post-annealing. The as-deposited films are low conducting and Hall coefficients were undetectable; whereas post-annealed films possess fairly high conductivity. The lowest transmittance (11.96% at 600 nm) observed from the films deposited without oxygen increased to a maximum of 88.01% (3.5 O₂ vol. %); whereas this transmittance was decreased with the increasing O₂ vol. % to as low as 81.04% (15.6 O₂ vol. %); a maximum of 89.80% was obtained from the films annealed at 500 °C in open air (3.5 O₂ vol. %). The optical band gap of 3.80 eV obtained from the films deposited without oxygen increased with increasing O₂ vol. % to as high as 3.91 eV (17.5 O₂ vol. %). A maximum of 3.92 eV was obtained from the films annealed at 300 °C in N₂:H₂ gas atmosphere (17.5 O₂ vol. %).     

Comparative study of gasochromic and electrochromic effect in thermally evaporated tungsten oxide thin films

Substoichiometric tungsten oxide films (WO_3 − y, 0.49 ≥ y ≥ 0.15) were prepared by non-reactive thermal evaporation of WO₃ powder in vacuum. The thin film composition, structure and optical properties were investigated with the purpose to establish their dependence on the deposition conditions and to prove a possible correlation between electrochromic and gasochromic colouration. An analogy in the dependencies of the maximum achievable optical density on the thin film oxygen content for gasochromically and electrochromically coloured films was observed.In-situ performed XPS measurements suggested that the main mechanism of gasochromic colouration is charge transfer between W⁶⁺ and W⁵⁺ states, i.e., similar to the electrochromic effect.     

Substoichiometric hot-wire WOx films deposited in reducing environment

Substoichiometric tungsten oxide films were deposited on Si substrates by heating metallic filaments at temperatures of 920, 1020 and 1070 K at a total pressure of 133 Pa and H₂ partial pressure of 13.3 Pa at or near room temperature. Due to their substoichiometry and deposition method samples were named hot-wire WO_x (hwWO_x) films (x < 3). No optical gap was identified by optical (spectroscopic ellipsometry) measurements made on hwWO_x films and resistivity was found to increase with temperature. This metallic character of hwWO_x films was attributed to substoichiometry, which causes the creation of electronic states within the band gap and shifts the Fermi level in the conduction band. The growth of hwWO_x films occurred from substoichiometric WO_x vapors produced in two steps: a) by the H₂ reduction of the superficial (native) oxide of the heated tungsten filament to WO_x and b) evaporation of the last. Vapors were composed of WO_x particles with dimensions of 5-6 nm or clusters of such particles and condensing on the cold substrate form hwWO_x films. The substoichiometry of hwWO_x samples was related to the presence of many unsaturated bonds in them, which re-constructed easily upon thermal annealing above 420 K or even after exposure to the electron beam of the transmission electron microscope leading to the formation of crystallites with dimensions of 5 nm within samples.     

The effect of heating time on growth of NaxWO3 nanowhiskers

A simple method for synthesis of Na_xWO₃ nanowhiskers on tungsten thin films with 40 nm thickness sputtered on soda lime substrate as a source of sodium atoms has been reported for the first time. After heat treatment of the W thin films at 650 °C in N₂ ambient for different times (15, 80 and 180 min), crystalline Na_xWO₃ nanowhiskers with [0 0 1] direction were obtained. scanning electron microscopy (SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and optical transmission/reflection measurements were employed to determine various properties of the grown nanowhiskers. Experimental results revealed that size and density of nanowhiskers were dependent on the annealing time and found that the 80-min heat treatment was a proper time for the growth of sodium-doped tungsten oxide nanowhiskers, in our experimental conditions. According to SEM observations, the synthesized nanowhiskers have 70-300 nm in width and 1-10 μm in length. It was also observed that by increasing the heating time to 180 min resulted in diffusion of the nanowhiskers into the substrate.     

In2O3 films prepared by thermal oxidation of amorphous InSe thin films

In₂O₃ thin films were prepared by the thermal oxidation of amorphous InSe films in air atmosphere. The structure, morphology and composition of the thermal annealed products were characterized by X-ray diffraction (XRD), scanning electron microscopy and energy-dispersive spectroscopy, respectively. The XRD patterns indicate that the as-deposited InSe films were amorphous and they fully transformed into polycrystalline In₂O₃ films with a cubic crystal structure in the preferential (222) orientation at a temperature around 600 °C. The optical energy gap of 3.66 eV was determined at room temperature by transmittance and reflectance measurements using UV-vis-NIR spectroscopy. A preliminary characterization shows that these films have a promising response towards NO₂ gas at a working temperature around 180 °C.     

Magnetron sputtered tungsten oxide films activated by dip-coated platinum for ppm-level hydrogen detection

Platinum-activated tungsten oxide (Pt-WO₃) films are prepared for hydrogen (H₂) sensing applications. In this study, WO₃ films were fabricated by reactive magnetron sputtering and Pt clusters were deposited on them by dip-coating. The microstructure, chemical composition and phase structure of Pt-WO₃ films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction, respectively. It was observed that a reconditioning period is required after a rest period of the sensor for obtaining a stable signal. A thermal treatment at 450 °C for 24 h is proposed to solve this problem.     

Structural properties of TiO2–WO3 thin films prepared by r.f. sputtering

TiO₂–WO₃ thin films were prepared by radio frequency (r.f.) reactive sputtering from metallic target. Structural and morphological properties of the thin films have been studied through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The influence of the annealing on the phase composition TiO₂–WO₃ system was studied. The binding energies of titanium and tungsten are characteristic for Ti⁴⁺ and W⁶⁺. The influence of tungsten on anatase–rutile phase transition in TiO₂ was observed. The structural modeling has been performed to account the preferred orientation in tungsten doped titanium oxide.     

Iodization of antimony thin films: XRD, SEM and optical studies of nanostructured SbI3

Antimony films of thicknesses of 25, 40 and 80 nm deposited using thermal evaporation technique were annealed at 200 °C for 6 h. Programmed iodization was carried out at room temperature for periods ranging from 5 min to 9 h on both as-deposited and annealed films. X-ray diffraction studies on iodized films reveal that antimony tri-iodide nanoparticles grow only on annealed antimony films. Surface morphology as revealed through SEM consists of antimony and antimony tri-iodide nanoparticles are 25 nm and 1 μm, respectively. Optical absorption of Sb and SbI₃ nanoparticles carried out at room temperature. As-deposited antimony film of thickness 25 nm exhibits a sharp rise in the absorption near ultraviolet region while post-deposition annealed films were characterized by red shifted absorption. Interestingly 45 nm thick Sb films exhibit a broad volume plasmon resonance peak around 500 nm with a width of 200 nm. Progressive iodization of 25 nm thick film reveals two absorption bands at 381.7 nm (A₂) and 458.3 nm (B₃) with photon energies 3.25 and 2.70 eV, respectively, due to the development of SbI₃ valence band structure. Last members of hydrogen-like series of absorption levels A₂ and B₃ due to halogen doublet (3/2, 1/2) splitting have been observed at room temperature.     

Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO_3 + δ) thin films with over-stoichiometric oxygen content (δ), using LiClO₄ with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing δ. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO_3 + δ films with an optimized δ value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of ~ 213 cm²/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO_3 + δ films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li⁺W⁺⁵ states.     

Initial stages of WO3 growth on silicon substrates

Ultrathin films (5 nm, 10 nm and 20 nm effective thickness) of WO₃ have been deposited in high vacuum (10^− 6 Torr) onto single crystal Si(100) substrates and studied with X-ray diffraction, atomic force microscopy, scanning tunneling microscopy and spectroscopy. The experiments have been carried out on “as-deposited” thin films or after 1 h post-deposition annealing at various temperatures (ranging from 300 °C to 500 °C). A size induced increase of the amorphous to crystalline (monoclinic) phase transition has been observed for the 5 nm and 10 nm films, with a critical crystallite size of 25 ± 5 nm and a critical temperature of 345 ± 5 °C. All the experimental evidences show that, upon annealing, there is a diffusion limited aggregation growth of WO₃ that forms large flat two-dimensional islands composed by aggregates of individual crystallites approximately uniform in size and shape. These islands are isolated in the 5 nm thin films, are connected in the 10 nm case and form a uniform patchwork in the 20 nm thin films. Scanning tunneling spectroscopy shows the opening of a large surface band gap (2.7 eV) in the 500 °C annealed films and the significant presence of in gap states for thin films prepared with a lower (below 400 °C) annealing temperature. These findings are discussed in view of the optimization of the best morphological, structural and electronic parameters to fabricate WO₃ gas sensing devices at the sub-micrometer length scale.     

Nitrogen-incorporation induced changes in the microstructure of nanocrystalline WO3 thin films

Nitrogen incorporated tungsten oxide (WO₃) films were grown by reactive magnetron sputter-deposition by varying the nitrogen content in the reactive gas mixture keeping the deposition temperature fixed at 400 °C. The crystal structure, surface morphology, chemical composition, and electrical resistivity of nitrogen doped WO₃ films were evaluated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrical conductivity measurements. The results indicate that the nitrogen-incorporation induced changes in the microstructure and electrical properties of WO₃ films are significant. XRD measurements coupled with SEM analysis indicate that the increasing nitrogen content decreases the grain size and crystal quality. The nitrogen concentration increases from 0 at.% to 1.35 at.% with increasing nitrogen flow rate from 0 to 20 sccm. The corresponding dc electrical conductivity of the films had shown a decreasing trend with increasing nitrogen content.     

Influence of spray pyrolysis deposition parameters on the optoelectronic properties of WO3 thin films

The electrochromic (EC) properties of tungsten oxide (WO₃), such as coloration efficiency, cyclic durability and reversibility strongly depend on the structural and morphological properties, which are influenced by the deposition method and parameters.This paper presents the steps for optimizing the deposition parameters (substrate temperature, air flow pressure and precursor solution molarity) for improving the optical and electrical properties of WO₃ thin films for EC applications. WO₃ thin films were deposited by spray pyrolysis using tungsten hexachloride (WCl₆) dissolved in ethanol as precursor solution. The EC properties of optimized films were tested in two different electrolytes (H₂SO₄ 1 M and acetic acid/sodium acetate buffer with pH = 4) and changes in structure, composition and morphology of the films after coloration/bleaching cycles were discussed.The deposition temperature, carrier gas pressure and solution molarity were optimized at 250 °C, 120 kPa and 0.14 M respectively. Under these condition a dense, uniform film, with homogenous distribution of particles, good adhesion to the substrate, low roughness (9.02 nm), high transparency (> 70% in the 500-1100 nm range) and conductivity was obtained. Transmission modulation is higher for the sample cycled in H₂SO₄ 1 M (64% at 630 nm) compared to that cycled in the buffer (21% at 630 nm), whereas opposite results were obtained for coloration efficiencies 28 cm² C^− 1 (at 630 nm) and 35 cm² C^− 1 (at 630 nm), respectively. Changes in surface chemistry and morphology of the optimized sample were observed after cycling in H₂SO₄.     

Growth of primary and secondary amine films from polyatomic ion deposition

Amine-containing organic films are deposited onto silicon substrates from mass-selected beams of 5-200 eV Si₂NC₈H₁₉⁺ (silazane) and 15-100 eV C₃H₆N⁺ (allylamine) ions produced by electron impact ionization of 1,3-divinyltetramethyldisilazane and allylamine. Silazane films are also deposited onto aluminum substrates. These ion-deposited films are analysed directly and/or after air exposure by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Chemical derivatization prior to XPS analysis is utilized to distinguish primary and secondary amine groups in the films from non-amine nitrogen. Primary amines are absent in silazane films. Secondary amine containing films form at low silazane ion energies whereas the higher ion energies lead to formation of more inorganic, silico-carbo-nitride-like films. The ion energy trend is similar for films from allylamine ions, except for the fact that both primary and secondary amines are detected. The primary amines from allylamine ions survive film ageing in air for periods of several days. Ion-induced film-substrate reactions are observed for silazane films.     

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

We present the synthesis of tungsten oxide (WO_3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO_3−x, WO₃/W and WO₃/W/WO_3−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.