Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Correlation between optical, electrical and structural properties of vanadium dioxide thin films

VO₂ films have been prepared on normal microscope glass slides by reactive rf magnetron sputtering of vanadium target in a mixture of argon and oxygen. Optical properties of the films were investigated by the UV/Vis/NIR Perkin–Elmer Lamda 9. Transmission electron microscope and atomic force microscope were used to investigate the structure of the films. Correlation between structural and optical properties of VO₂ thin films is investigated with respect to the dependence of both to substrate temperature.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.     

Preparation of Epitaxial MgO Films Deposited by RF Magnetic Sputtering on the IBAD-MgO Substrate

We deposited epi-MgO films on the textured ion beam assisted deposition (IBAD)-MgO substrates by RF magnetic sputtering at different substrate temperatures (600–850 °C), RF powers (110–224 W) and oxygen partial pressures (19.5–58.6 mTorr). The microstructure and surface morphology of epi-MgO films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). It was found that epi-MgO films with c-axis orientation could be easily fabricated for broad parameter ranges. But the in-plane full width half maximum (FWHM) of the epi-MgO film was dependent on the parameters, and the epi-MgO film with the smallest FWHM value of 5.22° was obtained at the optimum parameters. What’s more, the GdBa₂Cu₃O₇ films deposited on the epi-MgO/IBAD-MgO substrate by RF magnetic sputtering showed c-axis orientation.     

Effect of Ga doping concentrations,substrate temperatures and working pressures on the electrical and optical properties of ZnO thin films

We fabricated Ga-doped ZnO (GZO) thin films on glass substrate by RF magnetron sputtering method with different conditions of Ga₂O₃ concentration, substrate temperature and working pressure. Next we investigated the electrical, optical and structural properties of the GZO thin films. At a substrate temperature of 300 °C, a working pressure of 1 mTorr, and a Ga₂O₃ concentration of 3 wt%, the GZO thin films showed the lowest resistivity of 3.16 × 10^?4 Ω cm, a carrier concentration of 7.64 × 10²⁰ cm^?3 and a Hall mobility of 25.8 cm²/Vs. Moreover, the GZO thin films exhibited the highest (002) orientation under the same conditions and the full width at half maximum of X-ray peak was 0.34°. All GZO thin films showed the optical transmittance of more than 80 % in the visible range regardless of working conditions. The Burstein–Moss effect was observed by the change of doping concentration of Ga₂O₃. The GZO thin films were fabricated to have the good electrical and optical properties through optimizing doping concentration of Ga₂O₃, substrate temperature, working pressure. Therefore, we confirmed the possibility of application of GZO thin film as transparent conductive oxide used in flat panel display and solar cell.     

Advantages of inductively coupled plasma-assisted sputtering for preparation of stoichiometric VO2 films with metal-insulator transition

Inductively coupled plasma (ICP)-assisted sputtering with an internal coil enabled deposition of stoichiometric crystalline vanadium dioxide (VO₂) films on a sapphire (Al₂O₃) (001) substrate under widely various deposition conditions. The films showed a metal-insulator (M-I) transition around temperatures of 68 °C with several orders of change in resistivity. Particularly, low-temperature (250 °C) growth of VO₂ film with two orders transition decade was achieved in ICP-assisted sputtering, in contrast with conventional sputtering, which required 400 °C for VO₂ growth. Rutherford back scattering (RBS) measurements revealed that the VO₂ film prepared by ICP-assisted sputtering was exactly stoichiometric, containing no impurity atoms from the inserted coil material. The ICP-assisted sputtering was examined in comparison to conventional sputtering in view of plasma characteristics.     

Large phase-transition hysteresis for nanostructured VOx film prepared on ITO conductive glass by DC reactive magnetron sputtering

Nanostructured vanadium oxide (nano-VO_x) films were prepared on indium-tin oxide (ITO) glass substrate at low temperature by means of direct current (DC) reactive magnetron sputtering from pure vanadium target in Ar + O₂ atmosphere. Field emission scanning electron microscope (SEM) reveals that the VO_x film is composed of spheroidal nanoparticles whose diameters are in the range of 20–40 nm. This nano-VO_x film shows a broad hysteresis loop whose width is as large as 41.6 °C. Moreover, the metal-insulator transition (MIT) can also be triggered by Joule heat produced by electrical current through the ITO sublayer. Compared to traditional heating of the sample by heating plate, this Joule heating is more efficient and convenient, which enables potential applications of this nano-VO_x film on ITO conductive glass in compact storage devices.     

Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation

We present the structural and physical characterization of vanadium dioxide (VO₂) thin films prepared by reactive electron beam evaporation from a vanadium target under oxygen atmosphere. We correlate the experimental parameters (substrate temperature, oxygen flow) with the films structural properties under a radiofrequency incident power fixed to 50 W. Most of the obtained layers exhibit monocrystalline structures matching that of the monoclinic VO₂ phase. The temperature dependence of the electrical resistivity and optical transmission for the obtained films show that they present thermoelectric and thermochromic properties, with a phase transition temperature around 68 °C. The results show that for specific experimental conditions the VO₂ layers exhibit sharp changes in electrical and optical properties across the phase transition.     

Effects of post-rapid thermal annealing on structural, electrical and optical properties of hydrogenated aluminum doped zinc oxide thin films

In this study, hydrogenated aluminum doped zinc oxide (HAZO) thin films were prepared by DC magnetron sputtering in different H₂/(Ar+H₂) volume ratio atmosphere. The effects of post-rapid thermal annealing (RTA) in Ar+8 % H₂ atmosphere on the structural, optical, and electrical properties of the thin films were investigated systematically. Results showed that the RTA treatment effectively improved the electrical conductivity of the HAZO thin films with small hydrogen content, due to the increase of the Hall mobility and the carrier concentration. The lowest resistivity of the HAZO thin film deposited in 8 % H₂ ratio atmosphere reached 6.3 × 10^?4 Ω cm after RTA. The improved electrical properties of the RTA-treated HAZO films were ascribed to the activation of Al dopants, the increase of oxygen vacancies and the desorption of negative charged oxygen species at the grain. These results implied that RTA process might be useful to fabricate high quality HAZO films with a low thermal budget.     

Preparation of homogeneous VOX thin films by ion beam sputtering and annealing process

Polycrystalline VO_x thin films that were prepared for thermal-sensitive material of far infrared sensor had been deposited on Si substrates by ion beam sputtering deposition. Scanning electron microscopy images indicated that VO_x thin films (oxygen pressure of 1.5×10⁻³ Pa) were grown into compact and ultra-fine grains (?50 nm), the film surfaces seemed smooth and uniform. Four-point probe measurements showed that the homogeneity of the films was better than 98% in a size of 30×30 mm². The four-point probe measurement on hot plate presented the sheet resistance and the temperature coefficient of resistance of the VO_x thin film that were 50 kΩ/square and −0.021 K⁻¹ at 28°C, respectively. In addition, some samples annealed in Ar atmosphere had their resistance decreased. Thus, vanadium oxide films containing more amount VO₂ were obtained.     

In situ investigation of magnetron sputtering plasma used for the deposition of multiferroic BiFeO<Subscript>3</Subscript> thin films

Somrani et al. (J. Mater. Sci. 26:3316–3323, 2015) have recently investigated the BiFeO₃ (BFO) thin films growth by RF magnetron sputtering. The role of the processing parameters especially the oxygen flow, deposition temperature and annealing on the microstructure and optical properties of the deposited films has been extensively studied. In this work a detailed investigation of the plasma deposition dynamics of BFO films is presented. A plasma sampling mass spectrometer was inserted into the magnetron sputtering reactor to analyze the nature and energy distribution of each ion species impinging onto the substrate surface. It was find that at very low pressure (<5 mTorr), the ion energy spectra of the sputtered species showed a wide, bimodal distribution with a low energy component corresponding to the sheath potential and a much higher energy component arising from particles ejected from the target and only slightly thermalized in the gas phase. At higher pressure, all energy distributions became narrower due to a nearly complete thermalization. The Bi⁺-to-Fe⁺ ratio was also found to decrease with increasing pressure, going from about 4 at 3 to 1 at 30 mTorr. A similar feature was observed for the O⁺-to-O₂ ⁺ ratio. The plasma dynamic results have been correlated to the Rutherford backscattering spectroscopy characterization of BFO thin films.     

Polycrystalline films of vanadium oxides of controllable composition

The conditions necessary for preparing polycrystalline stoichiometric vanadium oxide films by the chemical transport of oxygen in a closed system were studied. Films of V₂O₃, V₃O₅ and VO₂ were synthesized on ceramic and sapphire substrates at 500 and 700 °C. An electron diffraction technique was used to control the phase composition. The equilibrium oxygen pressure above the films was shown to be higher than that above powder of the same composition.     

Synthesis and thermal stability of W-doped VO2 nanocrystals

Pure and W-doped vanadium dioxide nanocrystals have been synthesized by using V₂O₅ and oxalic acid as precursors via a thermolysis method. The VO₂ nanocrystals have a nearly spherical morphology with size ranging from 50 to 100 nm. The metal-insulator transition (MIT) temperature of the nanocrystals decreases with increasing W-doping content. The successive heat-induced fatigue character of the MIT in W-doped VO₂ nanocrystals was investigated by DSC analysis together with structural study, and a high stability upon heating–cooling cycles was found with respect to MIT temperature, peak temperature and latent heat of the phase transition.     

Chemical Modulation of Metal–Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal–insulator transition (MIT) of vanadium dioxide (VO₂) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO₂, endowing VO₂ with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO₂ nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO₂ and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO₂ MIT are also provided.     

Vanadium oxide films with improved characteristics for ir microbolometric matrices

Vanadium oxide (VO_x) films intended for use in uncooled IR microbolometric matrices were deposited by reactive magnetron sputtering on silicon substrates. Optimum deposition conditions were determined, which provide for the obtaining of films possessing a current 1/f noise level 3–10 times lower, extended dynamic range, and increased working temperature interval. It was found that the 1/f noise level of the VO_x films depends on the VO₂ phase content and grain size. It is suggested that the observed 1/f noise is caused by the martensite transformation characteristic of the semiconductor-metal phase transition in VO₂.     

Influence of oxygen pressure on the structural, electrical and optical properties of VO2 thin films deposited on ZnO/glass substrates by pulsed laser deposition

The influence of oxygen pressure on the structural and electrical properties of vanadium oxide thin films deposited on glass substrates by pulsed laser deposition, via a 5-nm thick ZnO buffer, was investigated. For the purposes of comparison, VO₂ thin films were also deposited on c-cut sapphire and glass substrates. During laser ablation of the V metal target, the oxygen pressure was varied between 1.33 and 6.67 Pa at 500 °C, and the interaction and reaction of the VO₂ and the ZnO buffer were studied. X-ray diffraction studies showed that the VO₂ thin film deposited on a c-axis oriented ZnO buffer layer under 1.33 Pa oxygen had (020) preferential orientation. However, VO₂ thin films deposited under 5.33 and 6.67 Pa were randomly oriented and showed (011) peaks. Crystalline orientation controlled VO₂ thin films were prepared without such expensive single crystal substrates as c-cut sapphire. The metal-insulator transition properties of the VO₂/ZnO/glass samples were investigated in terms of electrical conductivity and infrared reflectance with varying temperatures, and the surface composition was investigated by X-ray photoelectron spectroscopy.     

The large modification of phase transition characteristics of VO2 films on SiO2/Si substrates

The vanadium oxide (VO₂) films have been prepared on SiO₂/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar⁺ and H⁺ ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO₂ films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO₂ films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO₂ films depend on the quantity and location of argon atoms in the film matrix.     

Microstructures and thermochromic properties of tungsten doped vanadium oxide film prepared by using VOX-W-VOX sandwich structure

Tungsten doped vanadium oxide (VO_X) thin films were prepared by oxygen annealing VO_X-W-VO_X sandwich layers. X-ray photoelectron spectroscopy, X-ray diffraction and field emission scanning electron microscope were employed to characterize the compositions, crystal structures and surface morphologies, respectively. It was demonstrated that sandwich structure suppressed the crystallization of VO_X, and that V⁵⁺ was reduced by diffused W atom to V⁴⁺. The results of surface morphologies indicated that the grain arrangement of W doped vanadium dioxide film exhibited some regular patterns compared with the random grain distribution of undoped film. Electrical measurements showed that the square resistance of V₂O₅ film and semiconductor-metal transition temperature of VO₂-V₂O₅ film decreased obviously after W doping. In addition, thermal hysteresis loop was observed in W doped V₂O₅ film with thick W middle layer. The investigation of optical properties indicated that the optical band gap of W doped V₂O₅ film decreased with the increase of thickness of W middle layer, and the optical switching performance in the near-infrared range of VO₂-V₂O₅ slightly weakened after W doping.     

Structural characterization of delithiated LiVO2

Structural characteristics of delithiated LiVO₂ prepared by chemical and electrochemical methods have been examined by powder X-ray diffraction techniques. Electrochemical data and structural analyses of Li_1?xVO₂ samples show that, for x ? 0.3, removal of lithium from the layered LiVO₂ structure (space $\text{group = R}\text{3}\text{?}\text{m}$) results in a rearrangement of the vanadium ions in the cubic-close-packed oxygen lattice. In Li_0.22VO₂ approximately one-third of the vanadium ions are located in octahedral sites left vacant in the lithium layer. This results in a stabilized structure with near-cubic lattice constants.