Vanadium Oxide Thin Films Research Articles

Preparation of Vanadium Oxide Thin Films under Different Annealing Conditions

A novel method was used to fabricate VXOY thin films by combining magnetron sputtering and vacuum annealing processes on Al2O3 ceramic substrates, which greatly improved the success rate of the experiments and reduced the experimental requirements for the instruments. The substrate was magnetron sputter coated further oxidized in a tube furnace to produce a uniform V2O5 surface, then annealed at high temperatures. XRD and SEM measurements were performed on the obtained films under different annealing conditions. Characterization results of the samples indicate that the highly reproducible VO2 thin films can be achieved by controlling the process parameters of the magnetron sputtering and vacuum annealing. The experimental results are of great guidance value in VO2 thin film preparation.

Electrochemically Triggered Metal–Insulator Transition between VO2 and V2O5

AbstractDistinct properties of multiple phases of vanadium oxide (VOx) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VOx are crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VOx thin films under a wide range of temperatures. Reversible phase transitions between two adjacent VOx phases, VO2 and V2O5, are obtained. Cathodic biases trigger the phase transition from V2O5 to VO2, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time‐dependent response of the X‐ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO2 and V2O5 demonstrates the ability to induce major changes in the electronic properties of VOx by spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.

Investigation on microstructures and phase transition characteristics of titanium/yttrium co-doped vanadium oxide thin films

Co-doping with two elements has recently attracted attention as a novel approach for tailoring the semiconductor-metal transition (SMT) characteristics of vanadium dioxide (VO2). Here, we investigate the influence of titanium/yttrium (Ti/Y) co-doping on the microstructure and SMT characteristics of VO2 thin films. At low Y concentration, Ti/Y co-doped VO2 thin films exhibit a monoclinic structure similar to VO2 but characterized by a smaller grain size, whereas, the VO2(B) phase appears as the Y concentration is increased. Interestingly, even a complete amorphization of the film is obtained for co-doped VO2 thin films. The strong dependence of the microstructure upon the co-doping level is ascribed to the conjugated influence from the simultaneous adjustment of VV chains and VO bonds due to the existence of Ti4+ and Y3+ ions in VO2 lattice, respectively. As a result, the SMT feature of VO2 thin films is strongly limited or even fully disappears.

Electrochemical energy storage of nanocrystalline vanadium oxide thin films prepared from various plating solutions for supercapacitors

Nanocrystalline V2O5 is electrochemically deposited onto an indium tin oxide substrate in VOSO4–based solution with various acetate additives, i.e., lithium acetate, sodium acetate, and potassium acetate. The deposition conditions including pH value and acetate additive are found to be crucial factors in influencing the deposition rate, crystallinity, and porous structure of V2O5 electrodes. The electrochemical capacitive behavior of the deposited V2O5 electrodes in KCl electrolyte is investigated by cyclic voltammetry at various scan rates, ranged from 5 to 200 mV s−1. The specific capacitance of V2O5 electrode prepared from the potassium acetate containing plating solution is up to 350 F g−1, indicating that the level of K+ occupancy reaches as high as 0.71. This suggests that the occupancy of K+ ions is in tetrahedral and eight coordinated sites in V2O5 crystals. The capacitance retention at 200 mV s−1 compared to that at 5 mV s−1 reaches to 75% for this electrode. The enhanced performance is mainly attributed to the highly porous structure which significantly increases the active sites, imparts oxide/electrolyte interfaces for energy storage, and subsequently enhances the rate of insertion/extraction of K+ ions. The V2O5 electrode is capable of delivering high energy density up to 48.6 Wh kg−1, demonstrating a significant potential for thin-film energy storage devices.

Evaluation of Piezoresistive Property of Vanadium Oxide Thin Film Deposited by Sputtering

This article shows the evaluation of piezpresistive property of a vanadium oxide (VOx) film. The VOx film is deposited by the reactive sputtering method and patterned as a piezoresistor on an edge of suspended silicon (Si) membrane. The VOx resistor, electrode pads connected to the resistor, and the suspended membrane structure are fabricated using conventional microfabrication techniques. The resistance of the VOx resistor changes as the membrane is deflected. The deflection of the membrane is caused by evacuating to each differential pressure (from -0.02 to -0.05 by -0.01 MPa) and measured using a white-light interferometer. The resistance decreases as the strain is increased. The gauge factor of the deposited VOx is 259, which is higher than general semiconductor materials such as Si. The feasibility of VOx as a focusing material for MEMS/NEMS application (e.g., strain gauge) is indicated by our experiment.

Quartz Crystal Microbalance Coated with Vanadium Oxide Thin Film for $$\hbox {CO}_{2}$$ CO 2 Gas Sensor at Room Temperature

A gas-sensing device based on quartz crystal microbalance (QCM) covered with vanadium oxide thin film has been elaborated to detect $$\hbox {CO}_{2}$$ gas at room temperature. Vanadium oxide thin films were deposited onto QCM substrates by vacuum thermal evaporation technique. The vanadium oxide-coated QCM was heated at $$200\,^{\circ }\hbox {C}$$ for different times. The influence of the annealing time on structural and morphological properties of the deposited films was investigated by scanning electron microscopy, atomic force microscopy, and X-ray diffraction. The elaborated QCM/vanadium oxide structures were tested for $$\hbox {CO}_{2}$$ gas sensing. The results show that the sensor sensitivity increases with the annealing time due to the increase in the roughness of the surface. Moreover, the structure heated at $$200\,^{\circ }\hbox {C}$$ for 3 h exhibited a high resonance frequency shift ( $$\Delta f$$ ) under a $$\hbox {CO}_{2}$$ pressure of 40 mbar, fast response time (57 s), short recovery time (43 s), good stability, linearity, reproducibility, and reversibility.

Vanadium oxide thin films with high midwave & longwave infrared thermo-optic coefficients and high temperature coefficients of resistance

The development of thermo-optic modulator elements for midwave infrared (MWIR) and longwave infrared (LWIR) electro-optic systems, and the development of resistive thermometer elements for infrared imaging microbolometers require materials with high thermo-optic coefficients (TOC) and high temperature coefficients of resistance (TCR), respectively. In this paper, we synthesize novel vanadium oxide (VxOy) thin film structures and measure their MWIR/LWIR thermo-optic coefficients (TOCs) and their temperature coefficients of resistance (TCRs). The VxOy thin film are synthesized by sputter depositing interchanging, 5 nm-thick, layers of vanadium sesquioxide (V2O3) and vanadium (V) reaching a final thin film thickness of 95 nm. The sputter deposited multilayer structures are then ex-situ annealed in N2 and O2 atmospheres at 300 °C for 30 min. Infrared spectroscopic ellipsometry was used to measure the optical constants of the thin films as a function of temperature across the MWIR and LWIR bands (3000–14000 nm). The synthesized VxOy thin films exhibited high TOCs and high TCRs when operated in their semiconducting phase. A high TOC was measured reaching a maximum of 0.0278 °C−1 and 0.119 °C−1 at λ = 4000 nm for N2 and O2 annealed VxOy thin films respectively; and reaching a maximum of 0.0634 °C−1 and 0.139 °C−1 at λ = 10,000 nm for N2 and O2 annealed VxOy thin films respectively. Moreover, sheet resistance versus temperature measurements were conducted revealing room temperature sheet resistances of 1.495 k Ω/sq. and 1.516 k Ω/sq. and TCRs of −3.54%/°C and −3.46%/°C for N2 and O2 annealed VxOy thin films respectively.

Vanadium Oxide Thin Film Formation on Graphene Oxide by Microexplosive Decomposition of Ammonium Peroxovanadate and Its Application as a Sodium Ion Battery Anode.

Formation of vanadium oxide nanofilm-coated graphene oxide (GO) is achieved by thermally induced explosive disintegration of a microcrystalline ammonium peroxovanadate-GO composite. GO sheets isolate the microcrystalline grains and capture and contain the microexplosion products, resulting in the deposition of the nanoscale products on the GO. Thermal treatment of the supported nanofilm yields a sequence of nanocrystalline phases of vanadium oxide (V3O7, VO2) as a function of temperature. This is the first demonstration of microexplosive disintegration of a crystalline peroxo compound to yield a nanocoating. The large number of recently reported peroxide-rich crystalline materials suggests that the process can be a useful general route for nanofilm formation. The V3O7@GO composite product was tested as a sodium ion battery anode and showed high charge capacity at high rate charge-discharge cycling (150 mAh g-1 at 3000 mA g-1 vs 300 mAh g-1 at 100 mA g-1) due to the nanomorphology of the vanadium oxide.

Reversible, repeatable and low phase transition behaviour of spin coated nanostructured vanadium oxide thin films with superior mechanical properties

Smooth, uniform and crystalline vanadium oxide thin films were deposited on quartz by spin coating technique with four different rpm i.e., 1000, 2000, 3000 and 4000 and subsequently post annealed at 350, 450 and 550 °C in vacuum. Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) techniques were utilized for microstructural characterizations and phase analysis, respectively, for vanadium oxide powder and deposited film. Nanorods were observed to be grown after vacuum annealing. X-ray photoelectron spectroscopy (XPS) technique was utilized to study the elemental oxidation state of deposited vanadium oxide films. Thermo-optical and electrical properties such as solar transmittance (τs), reflectance (ρs), absorptance (αs), infrared (IR) emittance (εir) and sheet resistance (Rs) of different thin films were evaluated. Based on the optical characteristics the optimized condition of the film processing was identified to be spin coated at 3000 rpm. Subsequently, the nanoindentation technique was utilized to measure hardness and Young's modulus of the optimized film. The measured nanomechanical properties were found to be superior to those reported for sputtered vanadium oxide films. Finally, temperature dependent phase transition characteristics of optimized vanadium oxide films were studied by differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition was found to occur in the range of 44–48 °C which was significantly lower than the phase transition temperature (i.e., 68 °C) of bulk VO2.

Modification on the properties of vanadium oxide films deposited from NH4F added HNO3 treated ammonium metavanadate precursor solution by spray pyrolysis technique

Pure and F doped vanadium oxide thin films were deposited on microslide glass substrates at 400 °C by spray pyrolysis technique using nitric acid treated ammonium metavanadate (NTA) aqua precursor solutions and ammonium fluoride added NTA precursor solutions respectively. X-ray diffraction study showed that the deposited films are in mixed phase of vanadium oxides (V2O5, β-V2O5, VO2 and V4O7) whereas V2O5 phase shows the preferential growth along (001) plane. Average visible transmittance of the vanadium oxide films estimated in 500–800 nm range is increased from ~73% to 82% for 20 wt% ammonium fluoride added NTA precursor solution. The wavelength at which the maximum transmittance occurred in vanadium oxide film is shifted to lower wavelength side and the band gap of vanadium oxide films is slightly modified due to addition of ammonium fluoride in the NTA solution. Surface morphology of the vanadium oxide films is influenced by the addition of various concentration of ammonium fluoride in the NTA precursor solution. The film deposited from 15 wt% of ammonium fluoride added NTA precursor solution possesses carrier concentration, mobility and conductivity of about 8.6 × 1013 cm−3, 30.1 cm2 V−1 s−1 and 4.1 × 10−4 Ω−1 cm−1 respectively which are relatively high when compared to other reports.

Different threshold and bipolar resistive switching mechanisms in reactively sputtered amorphous undoped and Cr-doped vanadium oxide thin films

This study investigates resistive switching in amorphous undoped and Cr-doped vanadium oxide thin films synthesized by sputtering deposition at low oxygen partial pressure. Two different volatile threshold switching characteristics can occur as well as a non-volatile bipolar switching mechanism, depending on device stack symmetry and Cr-doping. The two threshold switching types are associated with different crystalline phases in the conduction filament created during an initial forming step. The first kind of threshold switching, observed for undoped vanadium oxide films, was, by its temperature dependence, proven to be associated with a thermally triggered insulator-to-metal transition in a crystalline VO2 phase, whereas the threshold switch observed in chromium doped films is stable up to 90 °C and shows characteristics of an electronically induced Mott transition. This different behaviour for undoped versus doped films has been attributed to an increased stability of V3+ due to the Cr3+ doping (as evidenced by X-ray photoelectron spectroscopy analysis), probably favouring the creation of a crystalline Cr-doped V2O3 phase (rather than a Cr-doped VO2 phase) during the energetic forming step. The symmetric Pt/a-(VCr)Ox/Pt device showing high temperature stable threshold switching may find interesting applications as a possible new selector device for resistive switching memory (ReRAM) crossbar arrays.

Oxidation state control of solution-processed vanadium oxide thin-films and resistive switching of VO2 thin-film in a metastable state

The solution processing of crystalline VO2 thin films has been intensively investigated for both fundamental studies and various electronic applications. However, previous studies have reported that the inevitable oxidation of vanadium cations in conventional precursor solutions hinders facile and reliable fabrication of VO2 thin films. Here we develop a kinetically stabilized precursor solution for solution-based VO2 thin-film fabrication. The stabilized solution precursor showed increased resistance to uncontrolled vanadium oxidation compared to conventional precursors. The synthetic conditions necessary for reliable VO2 thin-film fabrication were investigated at various oxygen concentrations and various annealing atmospheres. The oxygen level of an ambient atmosphere controls which of the three vanadium oxidation states arise, including V3+, V4+, and V5+ at a given temperature. The unusual occurrence of V3+ can be obtained without any reducing reagents. Nano-crystalline VO2 thin films subjected to rapid thermal shock exhibit metal-to-insulator transition at 58.9°C and resistive switching in a metastable state at 53.5°C, which could be further applied to future electronic devices such as memristors, Mott transistors, and Mott memories.

Reversible and repeatable phase transition at a negative temperature regime for doped and co-doped spin coated mixed valence vanadium oxide thin films.

Smooth, uniform mixed valance vanadium oxide (VO) thin films are grown on flexible, transparent Kapton and opaque Al6061 substrates by the spin coating technique at a constant rpm of 3000. Various elements e.g., F, Ti, Mo and W are utilized for doping and co-doping of VO. All the spin coated films are heat treated in a vacuum. Other than the doping elements the existence of only V4+ and V5+ species is noticed in the present films. Transmittance as a function of wavelength and the optical band gap are also investigated for doped and co-doped VO thin films grown on a Kapton substrate. The highest transparency (∼75%) is observed for the Ti, Mo and F (i.e., Ti–Mo–FVO) co-doped VO system while the lowest transparency (∼35%) is observed for the F (i.e., FVO) doped VO system. Thus, the highest optical band gap is estimated as 2.73 eV for Ti–Mo–FVO and the lowest optical band gap (i.e., 2.59 eV) is found for the FVO system. The temperature dependent phase transition characteristics of doped and co-doped VO films on both Kapton and Al6061 are studied by the differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition is noticed in the range of −24 to −26.3 °C.

Effect of oxygen gas flow rate on phase transformation and morphology of vanadium oxide thin films

The vanadium oxide thin films were deposited on glass substrate using DC reactive magnetron sputtering method. The effect of O2flow rate on the phase transformation, microstructure and surface morphology of thin films was studied by X-ray diffractometer, Raman spectroscopy and Field emission scanning electron microscopy (FESEM), respectively. The obtained results indicated that the O2 flow rate affected the lattice structure integrity significantly. In addition, the surface morphology of vanadium oxidethin films were depends strongly on the O2 flow rates.

Modification of the Properties of Vanadium Oxide Thin Films by Plasma-Immersion Ion Implantation

The paper describes the effect of doping with hydrogen and tungsten by means of plasma-immersion ion implantation (PIII) on the properties of vanadium dioxide and hydrated vanadium pentoxide films. It is shown that the parameters of the metal-insulator phase transition in VO2 thin films depend on the hydrogen implantation dose. Next, we explore the effect of PIII on composition, optical properties, and the internal electrochromic effect (IECE) in V2O5·nH2O films. The variations in the composition and structure caused by the hydrogen insertion, as well as those caused by the electrochromic effect, are studied by nuclear magnetic resonance, thermogravimetry, Raman spectroscopy, and X-ray structural analysis. It is shown that the ion implantation-induced hydrogenation can substantially enhance the manifestation and performance of the IECE in V2O5 xerogel films. Finally, the effect of PIII-assisted doping with W on the parameters of electrical switching in Au/V2O5·nH2O/Au sandwich structures is examined. It is shown that implanting small tungsten doses improves the switching parameters after forming. When implanting large doses, switching is observed without electroforming, and if electroforming is applied, the switching effect, on the contrary, disappears.

Tuning of thermally induced first-order semiconductor-to-metal transition in pulsed laser deposited VO2 epitaxial thin films

Vanadium oxide (VO2) thin films have drawn significant research and development interest in recent years because of their intriguing physical origin and wide range of functionalities useful for many potential applications, including infrared imaging, smart windows, and energy and information technologies. However, the growth of highly epitaxial films of VO2, with a sharp and distinct controllable transition, has remained a challenge. Here, we report the structural and electronic properties of high quality and reproducible epitaxial thin films of VO2, grown on c-axis oriented sapphire substrates using pulsed laser deposition at different deposition pressures and temperatures, followed by various annealing schedules. Our results demonstrate that the annealing of epitaxial VO2 films significantly enhances the Semiconductor to Metal Transition (SMT) to that of bulk VO2 transition. The effect of oxygen partial pressure during the growth of VO2 films creates a significant modulation of the SMT from around room temperature to as high as the theoretical value of 68 °C. We obtained a bulk order transition ≥104 while reducing the transition temperature close to 60 °C, which is comparatively less than the theoretical value of 68 °C, demonstrating a clear and drastic improvement in the SMT switching characteristics. The results reported here will open the door to fundamental studies of VO2, along with tuning of the transition temperatures for potential applications for multifunctional devices.

Effects of the annealing process on the structure and valence state of vanadium oxide thin films

V2O5 was used to prepare vanadium oxide films via the sol-gel method. The “n” in V2O5∙nH2O was determined to be approximately 1.67. After annealing at 390 °C for 4 h, a pure VO2 (M) phase was obtained and denoted sample Ⅴ. The crystallite sizes of VO2 (M) in samples Ⅱ, Ⅲ, Ⅳ and Ⅴ were 42, 41, 35 and 24 nm, respectively. The surface morphology of sample Ⅰ was flat; however, many small, dense particles covered the surface of sample Ⅴ. Sample Ⅱ had a layered structure, and samples Ⅲ and Ⅳ consisted of compact grains. The binding energy (BE) differences between the O1s and the V5+2p3/2, V4+2p3/2, and V3+2p3/2 core levels were 12.75, 14.0 and 14.7 eV, respectively. For the sample Ⅴ film, the maximum transmittance change at 2500 nm was 39%, the phase transition temperature (Tc) was 64.3 °C, and the hysteresis loop width was 12.1 °C.

Synthesis and characterization of nanocrystalline vanadium oxide thin films: electrochemical behavior by annealing in different atmosphere

Nanocrystalline vanadium oxide thin films were prepared using sol–gel dip-coating technique. The effect of heat treatment in different environment including air, N2, Ar, and O2 gas on the structural, optical, electrical, and electrochemical properties of nanocrystalline vanadium oxide thin films were investigated. The results indicated that the calculated average crystallite size was reduced by annealing in Ar environment. Scanning electron microscopy (SEM) images showed layered morphology on the surface of the film annealed in air atmosphere, whereas the film annealed under Ar and N2 ambient revealed granular and wrinkle morphology, respectively. This morphology altered to rather smooth surface by annealing in O2 environment. The optical bandgap of the films were found to be 1.75, 1.84, 2.08, and 2.10 eV annealed in air, O2, N2, and Ar environment, respectively. It was observed that the films annealed under Ar and N2 ambient had low resistivity (~ 0.2 Ω cm) and high carrier concentration, while the film annealed in nitrogen environment showed higher mobility of charge carrier. The electrochemical measurements showed that annealing under N2 ambient improved the intercalation of Li ions, leading a higher interfacial capacitance of 19.18 mF Cm−2, and decreased the charge transfer resistance due to surface defects created by heat treatment in nitrogen environment.

A metal-insulator transition study of VO2 thin films grown on sapphire substrates

Vanadium thin films were deposited on sapphire substrates by DC magnetron sputtering and then oxidized in a tube furnace filled with oxygen under different temperatures and oxygen flow rates. The significant influence of the oxygen flow rate and oxidation temperature on the electrical and structural properties of the vanadium oxide thin films were investigated systematically. It shows the pure vanadium dioxide (VO2) state can only be obtained in a very narrow temperature and oxygen flow rate range. The resistivity change during the metal-insulator transition varies from 0.2 to 4 orders of magnitude depending on the oxidation condition. Large thermal hysteresis during the metal-insulator phase transition was observed during the transition compared to the results in literature. Proper oxidation conditions can significantly reduce the thermal hysteresis. The fabricated VO2 thin films showed the potential to be applied in the development of electrical sensors and other smart devices.

Structural, optical and electrochemical properties of F-doped vanadium oxide transparent semiconducting thin films

In this study, F-doped vanadium oxide thin films with doping levels up to 60 at % were prepared by spray pyrolysis method on glass substrates. To measure the electrochemical properties, some films were deposited on fluorine-tin oxide coated glass substrates. The effect of F-doping on the structural, electrical, optical and electrochemical properties of vanadium oxide samples was investigated. The X-ray diffractographs analysis has shown that all the samples grow in tetragonal β-V2O5 phase structure with the preferred orientation of [200]. The intensity of (200) peak belonging to β-V2O5 phase was strongest in the undoped vanadium oxide film. The scanning electron microscopy images show that the samples have nanorod- and nanobelt-shaped structure. The size of the nanobelts in the F-doped vanadium oxide films is smaller than that in the pure sample and the width of the nanobelts increases from 30 to 70 nm with F concentration. With increasing F-doping level from 10 to 60 at %, the resistivity, the transparency and the optical band gap decrease from 111 to 20 Ω cm, 70 to 50% and 2.4 to 2.36 eV, respectively. The cyclic voltammogram (CV) results show that the undoped sample has the most extensive CV and by increasing F-doping level from 20 to 60 at %, the area of the CV is expanded. The anodic and cathodic peaks in F-doped samples are stronger.