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.     

Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO3 and W0.95Ti0.05O3 thin films

We present a comparative microscopic and spectroscopic study of the morphology and composition of WO₃ and W_0.95Ti_0.05O₃ thin films, grown by radio-frequency magnetron reactive sputtering at substrate temperatures varied from room temperature to 500 °C, using atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). With increasing growth temperature, the AFM results show increase in the average crystallite size and in the surface roughness for both undoped and doped samples. The AFM data, along with the Raman results, clearly indicate that for the given set of experimental conditions, higher growth temperatures are required to obtain crystalline Ti-doped WO₃ films than for WO₃ films. Also, the Raman results suggest a potential phase transformation from a monoclinic WO₃ structure to an orthorhombic, but more probably a tetragonal, configuration in the W_0.95Ti_0.05O₃ thin films. This remark is based on the observed shifting, with Ti doping, to lower frequencies of the Raman peaks corresponding to W–O–W stretching modes of WO₃ at 806 and 711 cm⁻¹ to 793 and 690 cm⁻¹, respectively. XPS data indicate that the doped material has a reduced WO_3−x stoichiometry at the surface, with the presence of W⁶⁺ and W⁵⁺ oxidation states; this observation could also be related to the existence of a different structural phase of this material, corroborating with the Raman measurements.     

Investigations on niobium tungsten oxide thin films for optical modulation

The electrochromic parameters such as high color contrast, quick switching time, excellent modulation of percentage transmittance accompanied by long cyclic durability and stability are of utmost importance for researchers in the electrochromism. Till present, tungsten trioxide (WO₃) is the overwhelming candidate as an electrochromic material because of its excellent coloring efficiency, chemical stability and acceptable switching time. Despite, there are many possibilities to further improve the performance of tungsten oxide thin films based electrochromic devices. In this study, niobium (Nb) is doped into WO₃ lattice to further enhance its functionality and overall performance_. It is found that adding minimal concentrations of Nb (0.64%) in WO₃ results in instant coloring and bleaching times (~?11.5 and?~?2.9 s, respectively), which outwit the switching times of pristine WO₃. For Nb-doped WO₃ films, considering the percentage transmission difference between bleached and colored states, the ΔT value increased 10% from 50 to 60% at 700 nm and the maximum current density value of?~?19.4 mAcm^?2 has been achieved. In addition, an increase of 0.93 cm^?1 path length in optical density has been found that is higher than the values of 0.74 cm^?1 for only pristine WO₃ films. The corresponding current density values of the Nb-doped samples were reduced to 99.4% (16.17 mAcm^?2) and 98% (4.6 mAcm^?2), to achieve better stability and durability. Based on the band gap study, crystal structure and surface morphology of the Nb-doped samples, the electrochromic mechanism is discussed in details.

     

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.     

Electrochemical properties of tungsten oxysulphide thin films as positive electrodes for lithium microbatteries

Several WO_yS_z tungsten oxysulphide thin films were tested as positive electrodes for lithium microbatteries. The amorphous WO_1.05S₂ thin film was found very promising. A capacity decrease occurred during the first few cycles, after which the films were able to intercalate reversibly up to 11 lithium ion per formula unit under high regime (75 μA/cm²). They were tested for 250 charge-discharge cycles, between 30 V and 1.2 V. X-ray photoelectron spectroscopy measurements were performed on different compounds in both intercalated (Li₁WO_1.05S₂, Li_2.7WO_1.05S₂ and Li_3.8WO_1.05S₂) and partially deintercalated (Li₁WO_1.05S₂) states in order to understand the redox processes occurring during the first dischargecharge cycle. The analysis of both the W4f and the S2p peaks has shown that the redox processes involve not only the tungsten atoms but also sulphur atoms. At the beginning of the intercalation, W⁶⁺ was first partially reduced into W⁵⁺, and then into W⁴⁺, but the important stage was the reduction of W⁴⁺ into W⁰. In W⁰, the electron binding energy was very close to that of metallic tungsten. At the same time, S ₂ ^2- ions were partially reduced into S^2- ions. But only the reduction process of tungsten atoms appeared to be totally reversible.     

Formation of nanoscale tungsten oxide structures and colouration characteristics

In this work, pH dependent evolution of tungsten oxide (WO₃) nanostructures is being reported along with physical characteristics. The synthesis was carried out via an inexpensive solvothermal cum chemical reduction route, with sodium tungstate (Na₂WO₄) and cetyl trimethyl ammonium bromide (C₁₉H₄₂NBr) as main reactants. The X-ray diffraction, together with transmission electron microscopic studies have revealed formation of regular polyhedral nanocrystalline structures and fractals as one goes from higher pH (= 5·5) to lower pH (= 2) values. The average crystallite size, as calculated through Williamson–Hall plots, was varied within 2·8–6·8 nm for different pH samples. Fourier transform infrared spectroscopy reveals in-plane bending vibration δ (W–OH), observable at ∼1630 cm^− 1 and strong stretching ν (W–O–W) located at ∼814 cm^− 1. Raman spectroscopy has divulged WO₃ Raman active optical phonon modes positioned at ∼717 and 805 cm^− 1. The thermochromic and photochromic properties of the nanoscale WO₃ sample prepared at pH = 5·5, are also highlighted.     

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.     

Supercapacitive performance of hydrous ruthenium oxide (RuO<Subscript>2</Subscript>·<Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O) thin films deposited by SILAR method

The amorphous hydrous ruthenium oxide (RuO₂·nH₂O) thin films were deposited by a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method. These films were characterized for their structural, surface morphological, and compositional study by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDAX) techniques. The wettability test was carried out by measuring the water contact angle. The scanning electron microscopy study showed small RuO₂ particles are grouped together to form porous agglomerates. The FT-IR study confirmed the formation of hydrous ruthenium oxide films. The hydrophilic nature of ruthenium oxide (RuO₂·nH₂O) thin films was observed from water contact angle measurement. The presence of Ru and O in the film was confirmed by EDAX analysis. The supercapacitor behavior of these films studied in 0.5 M H₂SO₄ electrolyte showed maximum specific capacitance of 162 F g⁻¹ at 10 mV s⁻¹ scan rate. These films exhibit 80% cycling performance after 2,000 cycles. The charge–discharge studies carried at 1 mA cm⁻² current density revealed the specific power of 3.5 KW kg⁻¹ and specific energy of 29.7 W Kg⁻¹ with 93% coulombic efficiency.     

Solar water oxidation in sputter-deposited nanocrystalline WO3 photoanodes via tuning of Ar:O2 flow rate combinations

Thin film WO₃ photoanodes were prepared by reactive sputtering in Ar and O₂ gas mixtures of various flow rate combinations. Furnace annealed films were nanocrystalline monoclinic WO₃ with (002), (020) and (200) plane orientations. Water oxidation in 0.33 M H₂SO₄ electrolyte under simulated solar illumination showed that photoanodes deposited in highest Ar and O₂ flow rate combinations exhibited highest photocurrent of 4.1 mA cm⁻² (at 1.3 V vs Ag/AgCl) compared to 3–3.8 mA cm⁻² for photoanodes deposited in lower flow rate combinations. The higher photocurrents were ascribed to lower bulk resistivity and charge transfer resistance at the WO₃/electrolyte interface. These photoanodes consisted of randomly oriented (002), (020) and (200) planes in contrast to the preferentially orientated (002) and (200) planes of photoanodes which were highly resistive with poorer photocurrent responses. These results were interpreted in terms of the effects of Ar:O₂ flow rate combinations on the distribution of oxygen vacancies and formation of crystallographic shear planes in the sputtered films.     

The solution growth route and characterization of electrochromic tungsten oxide thin films

Electrochromic tungsten oxide thin films were prepared by using an aqueous solution of Na₂WO₄·2H₂O and dimethyl sulfate. Various techniques were used for the characterization of the films such as X-ray diffraction, cyclic voltammetry, SEM analysis and VIS-spectroscopy. The thin film durability was tested in an aqueous solution of LiClO₄ (0.1 mol/dm³) for about 7000 cycles followed by cyclic voltammetry. No significant changes in the cyclic voltammograms were found, thus proving the high durability of the films.The optical transmittance spectra of coloured and bleached states showed significant change in the transmittance, which makes these films favorable for electrochromic devices.     

Insertion of H, Li, Na and K into thin films prepared from silicotungstic acid—a photoelectron spectroscopy study

Electrochemical insertion by a set of different ions (H⁺, Li⁺, Na⁺ and K⁺) into a tungsten oxide thin film was studied by photoelectron spectroscopy. The tungsten oxide thin film incorporating Si atoms was produced from a silicotungstic acid (SiWA) solution. The insertion compounds were measured by core level photoelectron spectroscopy (W 4f) and the contributions from ions of different oxidation states could be monitored simultaneously. SiWA films having a W⁶⁺/W_tot ratio of 0.7 could be prepared for all cations investigated. At this ratio the W 4f core level electronic structure for H⁺ inserted SiWA films was found to be very similar to that of H⁺ inserted into crystalline monoclinic WO₃ in that both films show the presence of W⁴⁺, W⁵⁺ and W⁶⁺. The measurements on Li⁺ inserted SiWA films indicate an electronic structure very similar to that of the smaller (H⁺) ion. The K⁺ inserted film displays a similar behaviour although the existence of W⁴⁺ was difficult to ascertain. Interestingly, a different behaviour was observed for the Na⁺ inserted compound. In this case, the binding energy shift of the W 4f peak upon reduction is clearly different from that obtained for the other insertion materials.     

Ca<Subscript>3</Subscript>WO<Subscript>6</Subscript>: a novel microwave dielectric ceramic with complex perovskite structure

Present work introduces a novel Ca₃WO₆ microwave dielectric ceramic with a complex perovskite structure. The Ca₃WO₆ ceramic was prepared by solid state reaction method and can be well densified at above 1,260 °C for 2 h in air. All the XRD patterns can be fully indexed as a single-phase monoclinic structure (space group P2₁/n). The sharp Raman vibration mode at 810 cm⁻¹ suggests the long range order in the Ca₃WO₆ structure. The best microwave dielectric properties can be obtained in ceramic sample sintered at 1,275 °C for 2 h with a permittivity ~15.3, a Qf value ~29,200 GHz and a TCF value about −30 ppm/°C. Applying the oxide additivity rule, the calculated permittivity agrees well with the measured value. This kind of ceramic might have some potential value for microwave application for its good microwave dielectric behavior. The (Ca_1/2W_1/2) complex cations holding the site of Ti⁴⁺ in perovskite structure would introduce many new systems in complex perovskite compounds in the future.     

Structural,optical and microscopic properties of chemically deposited Mo<Subscript>0.5</Subscript>W<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> thin films

Mo_0.5W_0.5Se₂ thin films were obtained by using relative simple chemical route at room temperature. Various preparative conditions of the thin films are outlined. The films were characterized by X-ray diffraction, scanning electron microscope, optical and electrical properties. The grown films were found to be uniform, well adherent to substrate and brown in color. The X-ray diffraction pattern shows that thin films have a hexagonal phase. Optical properties show a direct band gap nature with band gap energy 1.44 eV and having specific electrical conductivity in the order of 10⁻⁵ (Ωcm)⁻¹.     

Synthesis and characterization of niobium doped hexagonal tungsten bronze in the systems,Cs<Subscript>x</Subscript>Nb<Subscript>y</Subscript>W<Subscript>1−y</Subscript>O<Subscript>3</Subscript>

Samples of nominal compositions, Cs_0.25Nb _y W_1−y O₃ and Cs_0.3Nb _y W_1−y O₃ with 0.0 ≤ y ≤ 0.25 and 0.0 ≤ y ≤ 0.3 were synthesized using appropriate amounts of Cs₂WO₄, WO₃ and WO₂ in evacuated and closed silica glass tubes at 800 °C. The polycrystalline products contain hexagonal shaped crystals of up to 15 μm diameter as long as y ≤ 0.15. X-ray powder patterns of the samples reveal the formation of hexagonal tungsten bronze (HTB-I) type phase with y < 0.1. A mixture of HTB-I and an analogous less reduced hexagonal tungsten bronze (HTB-II) type phase is seen when y ≥ 0.1. HTB-II content increases with increasing y, revealing close similarity to bronzoid type phases when y = x. Results of SEM/EDX analysis also support a partial substitution of tungsten by niobium in the HTB-I type phase. Infrared absorption and optical reflectivity data shows the effect of increasing amount of non-metallic phase for y > 0.1 and the effect of counterdoping by Nb⁵⁺/W⁵⁺ substitution in the metallic HTB-I type phase for y ≤ 0.1, respectively. Reinvestigations in the system Rb_0.3Nb _y W_1−y O₃ (0.0 ≤ y ≤ 0.175) show similar results with increasing content of HTB-II type phase related with y.     

Sol–gel derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript> thin films: effect of annealing on structural,morphological and optoelectronic properties

Nanocrystalline Co₃O₄ thin films were prepared on glass substrates by using sol–gel spin coating technique. The effect of annealing temperature (400–700 °C) on structural, morphological, electrical and optical properties of Co₃O₄ thin films were studied by X-ray diffraction (XRD), Scanning Electron Microscopy, Electrical conductivity and UV–visible Spectroscopy. XRD measurements show that all the films are nanocrystallized in the cubic spinel structure and present a random orientation. The crystallite size increases with increasing annealing temperature (53–69 nm). These modifications influence the optical properties. The morphology of the sol–gel derived Co₃O₄ shows nanocrystalline grains with some overgrown clusters and it varies with annealing temperature. The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 2.58 to 2.07 eV with increasing annealing temperature between 400 and 700 °C. These mean that the optical quality of Co₃O₄ films is improved by annealing. The dc electrical conductivity of Co₃O₄ thin films were increased from 10⁻⁴ to 10⁻² (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of Co₃O₄ films annealed at 400–700 °C were estimated to be of the order of 2.4–4.5 × 10¹⁹ cm⁻³ and 5.2–7.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ respectively. It is observed that Co₃O₄ thin film annealing at 700 °C after deposition provide a smooth and flat texture suited for optoelectronic applications.     

Interactions of gamma rays with tungsten-doped lead phosphate glasses

Undoped lead phosphate glass of the composition PbO 50 mol%, P₂O₅ 50 mol% together with samples of the same ratio doped with various WO₃ contents were prepared. UV–Visible spectroscopic studies were measured out in the range 200–1100 nm before and after successive gamma irradiation. Infrared and Raman spectroscopic measurements were carried out for the undoped and WO₃-doped samples. All the prepared samples are observed to absorb strongly in the UV region due to the combined contributions of absorption from trace iron impurities and sharing of lead Pb²⁺ ions. The bluish WO₃-doped lead phosphate samples reveal visible absorption bands which are attributed to the existence of pentavalent W⁵⁺ ions. ESR measurements support this assumption. Infrared and Raman spectra indicate the presence of metaphosphate chains as the structural main building units and the possible presence of appreciable pentavalent (W⁵⁺O₃) of W⁵⁺ units together with hexavalent WO₄ units. Gamma irradiation reveal the shielding behaviour of the studied tungsten-doped lead phosphate glasses due to the combined presence of heavy Pb²⁺ ions and tungsten ions.     

Structural and surface morphological properties of chemically deposited Mo<Subscript>0.5</Subscript>W<Subscript>0.5</Subscript>S<Subscript>2</Subscript> thin film

Preparation of layered type semiconductor Mo_0.5W_0.5S₂ ⁻ thin films has been successfully done by using chemical bath deposition method. Objective of the studies are related to structural, optical, morphological and electrical properties of the thin films. The preparation method is based on the reaction between tartarate complex of Mo and W with thiourea in an aqueous alkaline medium at 363 K. X-Ray diffraction reveals a polycrystalline film composed of both MoS₂ and WS₂ phases. The optical study shows that the band gap of the film is 1.6 eV. Electrical conductivity is high which is in the order of 10⁻³–10⁻² (Ώ cm)⁻¹.     

Deposition of AgGaS<Subscript>2</Subscript> thin films by double source thermal evaporation technique

In this study, polycrystalline AgGaS₂ thin films were deposited by the sequential evaporation of AgGaS₂ and Ag sources with thermal evaporation technique. Thermal treatment in nitrogen atmosphere for 5 min up to 700 °C was applied to the deposited thin films and that resulted in the mono phase AgGaS₂ thin films without the participation of any other minor phase. Structural and compositional analyses showed the structure of the films completely changes with annealing process. The measurements of transmittance and reflectance allowed us to calculate the band gap of films lying in 2.65 and 2.79 eV depending on annealing temperature. The changes in the structure with annealing process also modify the electrical properties of the films. The resistivity of the samples varied in between 2 × 10³ and 9 × 10⁶ (Ω-cm). The room temperature mobility depending on the increasing annealing temperature was in the range of 6.7–37 (cm² V⁻¹ s⁻¹) with the changes in carrier concentrations lying in 5.7 × 10¹³–2.5 × 10¹⁰ cm⁻³. Mobility-temperature dependence was also analyzed to determine the scattering mechanisms in the studied temperature range with annealing. The variations in the electrical parameters of the films were discussed in terms of their structural changes.     

Ferroelectric and dielectric properties of Bi4-xNdxTi3O12 thin films prepared by sol–gel method

Bi_4-xNd_xTi₃O₁₂ (BNT-x, x = 0, 0.25, 0.50, 0.75 and 1.0) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel method. The microstructure, ferroelectric and dielectric properties of BNT-x thin films were investigated. The single-phase BNT-x thin films were obtained. With increasing Nd content, the preferred orientation changed from random to (117) and surface morphologies changed from the mixture of rod- and plate-like grains to rod-like grains. The Nd substitution improved the ferroelectric and dielectric properties of BTO films. BNT-x films showed better electrical properties at x = 0.50—1.0. BNT-0.75 film exhibited the best electrical properties with remanent polarization (2P _r) of 26.6 μC/cm², dielectric constant (ε _r) of 366 (at 1 MHz), dielectric loss (tanδ) of 0.034 (at 1 MHz), leakage current density (J) of ±3.0 × 10⁻⁶ A/cm² (at ± 5 V) and fatigue-free characteristics.     

Thermal Conductivity and Interfacial Thermal Resistance: Measurements of Thermally Oxidized SiO2 Films on a Silicon Wafer Using a Thermo-Reflectance Technique

This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO₂ films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO₂ film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO₂ film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10⁻⁹ m²·K·W⁻¹) between the gold film and the thermally oxidized SiO₂ film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10⁻⁹ m²·K·W⁻¹). The apparent interfacial thermal resistances between the thermally oxidized SiO₂ film and the silicon wafer were found to scatter significantly (± 9 × 10⁻⁹ m²·K·W⁻¹) around a very small thermal resistance (less than ± 4.5 × 10⁻⁹ m²·K·W⁻¹).