Properties Of V2O5 Thin Films Research Articles

Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering

The influence of sputtering power on the electrical and infrared properties of VO2 thin films was investigated. It was found that the controlling of sputtering power is very important in realizing the pure VO2 (M) thin film. The thin films grown at the sputtering powers of 350 W and above have a similar phase transition behavior to bulk VO2. The infrared transmittance and electrical resistance of the VO2 thin film also depend on the sputtering power, and the hysteresis width is controlled by the size effect. The film thickness and defect density affect the amplitudes of the phase transition and phase transition temperature.

Sapphire substrate-induced effects in VO2 thin films grown by oxygen plasma-assisted pulsed laser deposition

We investigate the structural and electronic properties of VO2 thin films on c-plane sapphire substrates with three different surface morphologies to control the strain at the substrate-film interface. Only non-annealed substrates with no discernible surface features (terraces) provided a suitable template for VO2 film growth with a semiconductor-metal transition (SMT), which was much lower than the bulk transition temperature. In addition to strain, oxygen vacancy concentration also affects the properties of VO2, which can be controlled through deposition conditions. Oxygen plasma-assisted pulsed laser deposition allows favorable conditions for VO2 film growth with SMTs that can be easily tailored for device applications.

Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films

Vanadium dioxide (VO2) thin films with controlled grain sizes are deposited on amorphous glass substrates by pulsed laser deposition. The grain boundaries (GBs) are found as the dominating defects in the thin films. The semiconductor to metal transition (SMT) properties of VO2 thin films are characterized and correlated to the GB density. The VO2 films with lower GB density exhibit a sharper SMT with a larger transition amplitude. A high resolution TEM study at GB area reveals the disordered atomic structures along the boundaries and the distorted crystal lattices near the boundaries. The VO2 SMT amplitude and sharpness could be directly related to these defects at and near the boundaries.

Electrical transport properties of V2O5 thin films obtained by thermal annealing of layers grown by RF magnetron sputtering at room temperature

The present study investigates the main electrical transport mechanism in V2O5 thin films deposited by RF magnetron sputtering on the basis of the Mott's small polaron hopping model. The material under test was obtained at room temperature from a V2O5 target and then oxidized at high temperature under air atmosphere to obtain the desired V2O5 phase. The dependence of the electrical conductivity of the V2O5 thin films with temperature was analyzed using the Mott's small polarons hopping transport model under the Schnakenberg form. Model results suggest a polaron binding energy WH=0.1682eV, with a structural disorder energy WD=0.2241eV and an optical phonon frequency ν0=0.468×1013s−1. These results are in agreement with data reported in literature for single crystal V2O5. However, the carrier mobility μ=1.5019×10−5cm2/Vs computed in the non-adiabatic regime is significantly smaller than that of the single crystal, suggesting a strong electron–phonon coupling in the V2O5 thin films obtained with the proposed deposition method.

Radio frequency substrate biasing effects on the insulator-metal transition behavior of reactively sputtered VO2 films on sapphire (001)

Comprehensive studies have been performed on the effects of radio frequency (rf) substrate biasing on insulator–metal transition (IMT) properties of VO2 thin films grown on Al2O3 (001) substrates. As the rf substrate bias power increased, the ion energy during deposition increased, inducing a strong in-plane stress conversion and a modification of the rutile c-axis (cR-axis) length in the VO2 films. The reduced cR-axis length significantly lowered the IMT temperature. Increased IMT sharpness via improved crystallinity was obtained with rf substrate biasing at an appropriate power. Temperature coefficient of resistance at room temperature was comparable with reported values for impurity-doped VO2 films. Thus, the rf substrate biasing during reactive sputtering has great potential to control stress in thin films, which could finally control the IMT in oriented VO2 films.

Influence of the conditions of pulsed laser deposition on the structural, electrical, and optical properties of VO2 thin films

The technique of pulse laser deposition with the separation of plume drops is used to produce VO2 thin films on sapphire(0001) and silicon(111) substrates. It is established that the energy density at the target and the oxygen pressure influence the structural and electrical properties of the films. All of the VO2 crystal films exhibit semiconductor-metal transitions with a substantial change in the resistance (by 2–5 orders of magnitude). The transmittance in the range 200–800 nm and reflectance in the range 400–700 nm are studied in the temperature range from 20 to 100°C. The transmittance of the films at wavelengths from 300 to 800 nm shows a jump and hysteresis upon heating and cooling. It is for the first time established that the changes in the transmittance of the film are different in character at different wavelengths and the shape of the temperature hysteresis loop for optical transmittance in the visible and near-ultraviolet regions does not in all areas replicate the shape of the hysteresis loop for the resistivity of the VO2 films. The difference in the behavior of the hysteresis curves for the transmittance and resistance is attributed to variations in the absorption of the films under variations in temperature.

Hydrogen-doping stabilized metallic VO2 (R) thin films and their application to suppress Fabry-Perot resonances in the terahertz regime

We demonstrate that catalyst-assisted hydrogen spillover doping of VO2 thin films significantly alters the metal-insulator transition characteristics and stabilizes the metallic rutile phase at room temperature. With hydrogen inserted into the VO2 lattice, high resolution X-ray diffraction reveals expansion of the V-V chain separation when compared to the VO2(R) phase. The donated free electrons, possibly from O-H bond formation, stabilize the VO2(R) to low temperatures. By controlling the amount of dopants to obtain mixed insulating and metallic phases, VO2 resistivity can be continuously tuned until a critical condition is achieved that suppresses Fabry-Perot resonances. Our results demonstrate that hydrogen spillover is an effective technique to tune the electrical and optical properties of VO2 thin films.

Comparison of Structural and Electrochemical Properties of V2O5 Thin Films Prepared by Organic/Inorganic Precursors

Vanadium pentoxide thin films were produced from organic and inorganic precursors by sol gel dip-coating method. Fourier transform infrared spectroscopy and UV Vis spectroscopy were made to figure out structural properties of the films. Electrochemical properties were investigated by cyclic voltammetry. The shape of the curves was in agreement with a typical diffusion controlled cyclic voltammograms of amorphous V2O5 films for a reversible lithium ion intercalation/deintercalation process showing yellow–green–blue multi-electrochromism. V2O5 films, synthesized from organic precursor, indicated lower band gap energy, higher charge capacity as well as homogeneous and low granule size compared to inorganic route.

Preparation and properties of VO2 thin films by a novel sol–gel process

Vanadium dioxide (VO2) thin films were fabricated using a simple and novel sol–gel process in which V2O5 was used as the vanadium source; oxalic acid was used as the reducing agent; and polyvinyl alcohol was used as the film former to control the viscosity of the VO2 precursor solution and bond vanadium ions. The microstructure and surface morphology of VO2 films were studied by X-ray diffraction and scanning electron microscopy, respectively. The results showed that using polyvinyl alcohol forms porous nanostructure of VO2 films with a uniform grain size of ~25 nm. The measured optical reflectance shows well-defined phase transition as observed by an increase of reflectance upon heating above the transition temperature from ~11 to ~30 % at 1,100 nm. Upon cooling, the expected hysteresis is observed.

Effect of Oxide Buffer Layer on the Thermochromic Properties of VO2 Thin Films

VO2 thin films were deposited on soda lime glass substrates with ZnO, TiO2, SnO2, and CeO2 thin films applied as buffer layers between the VO2 films and the substrates in order to investigate the effect of buffer layer on the formation and the thermochromic properties of VO2 film. Buffer layers with thicknesses over 50 nm were found to affect the formation of VO2 film, which was confirmed by XRD spectra. By using ZnO, TiO2, and SnO2 buffer layers, monoclinic VO2 (VO2(M)) film was successfully fabricated on soda lime glass at 370 °C. On the contrary, films of VO2(B), which is known to have no phase transition near room temperature, were formed rather than VO2(M) when the film was deposited on CeO2 buffer layer at the same film deposition temperature. The excellent thermochromic properties of the films deposited on ZnO, TiO2, and SnO2 buffer layers were confirmed from the temperature dependence of electrical resistivity from room temperature to 80 °C. Especially, due to the tendency of ZnO thin film to grow with a high degree of preferred orientation on soda lime glass at low temperature, the VO2 film deposited on ZnO buffer layer exhibits the best thermochromic properties compared to those on other buffer layer materials used in this study. These results suggest that deposition of VO2 films on soda lime glass at low temperature with excellent thermochromic properties can be achieved by considering the buffer layer material having structural similarity with VO2. Moreover, the degree of crystallization of buffer layer is also related with that of VO2 film, and thus ZnO can be one of the most effective buffer layer materials.

Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications

Results are reported on tuning the electrical and optical properties of sputter-deposited vanadium dioxide (VO2) thin films through control of substrate growth temperature (Ts). As Ts increases from 550 to 700 °C, the morphology changes from granular to smooth film and finally to rough film. X-ray diffraction shows the presence of VO2 along with additional weak features related to the presence of non-stoichiometric phases. Electrical measurements show the phase transition to change from abrupt to gradual as both the below- and above-transition resistivities vary with Ts. The transition and hysteresis dependences observed in electrical resistivity are similarly observed in infrared transmission. Terahertz transmission measurements show that high conductivity above the phase transition is more important in achieving high modulation depth than obtaining high resistivity below the transition. We attribute changes in the electrical and optical properties to the formation of V and O vacancies, which result in diverse valence states from the ideal V4+ of VO2. Low Ts produces material with V5+ states resulting in higher resistivity in both the insulating and metallic phases. Alternatively, high Ts introduces material with V3+ states leading to lower resistivity in the insulating phase but slightly higher resistivity in the metallic phase.

Thermoelectric properties of V2O5 thin films deposited by thermal evaporation

This work reports the structural, optical, electrical and thermoelectric properties of vanadium pentoxide (V2O5) thin films deposited at room temperature by thermal evaporation on Corning glass substrates. A post-deposition thermal treatment up to 973K under atmospheric conditions induces the crystallization of the as-deposited amorphous films with an orthorhombic V2O5 phase with grain sizes around 26nm. As the annealing temperature rises up to 773K the electrical conductivity increases. The films exhibit thermoelectric properties with a maximum Seebeck coefficient of −218μV/K and electrical conductivity of 5.5 (Ωm)−1. All the films show NIR-Vis optical transmittance above 60% and optical band gap of 2.8eV.

Terahertz Transmission Properties of VO<sub>2</sub> Thin Films Deposited on c-and m-Plane Sapphire Substrates by Pulsed Laser Deposition

Vanadium dioxide (VO2) films were grown on c-and m-plane sapphire substrates by pulsed laser deposition (PLD) technique with VO2ceramic target. The VO2films with preferred growth orientation and uniform dense distribution have been achieved on both substrates, as confirmed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The terahertz (THz) transmission properties of VO2thin films were studied by terahertz time-domain spectroscopy (THz-TDS). The results indicate that the THz transmission properties of VO2films are strongly influenced by the sapphire substrate orientation, suggesting that VO2films are ideal material candidates for THz modulation.

Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering

VO2 (M) thin films with good optical switching properties have been grown by reactive sputtering method. The influence of sputtering parameters on the structural and optical properties of the as-grown VO2 thin films was investigated, and the correlation between the microstructure and optical switching properties were studied. It was found that the phase transition temperature, hysteresis width, and the amplitude of the transition depend on the sputtering gas pressure, and the amplitude of the transition can reach as high as 70% with an approximately zero infrared transmission in metal state at a wavelength of 2.5 μm. The anomalous optical properties of the VO2 thin films were analyzed and discussed together with the studies of the refractive index and optical band gap.

Thermochromic properties of VO2 thin film on SiNx buffered glass substrate

VO2 thin films were deposited on soda lime glass substrates with silicon nitride sodium-diffusion barrier layer as diffusion barrier, in order to investigate the effect of sodium ion diffusion on the formation of VO2. SiNx layers with thicknesses over 30nm were found to successfully prevent sodium ion diffusion in VO2 thin film and also contribute to the formation of VO2 thin film, which was confirmed by XRD spectra and XPS measurements. The change of infrared transmittance at 2500nm wavelength with temperature change from room temperature to 80°C was increased significantly, and the optical hysteresis width of the sample decreased by almost 6K as well. The results suggest that applying diffusion barrier can improve the thermochromic properties of the VO2 films for energy-saving smart coatings, and silicon nitride can be one of the effective materials to prevent sodium ion diffusion.

Optical and thermal properties of V2O5 thin films with crystallization

In order to show the crystallization behavior for various substrate temperatures we grew V2O5 films at different temperatures using a radio frequency sputtering system. Spectroscopic ellipsometry and a thermo-reflectance method were used to investigate the optical and the thermal properties of V2O5 films with different crystallization structures. The transmission spectra and the n and the k spectra revealed changes over the entire measurement range. The optical properties of the V2O5 films were affected by variations in the crystalline structure. The thermal conductivities of the V2O5 films with amorphous and crystalline structures were 0.838 Wm−1K−1 and 1.749 Wm−1K−1, respectively, and the thermal conductivity depended on the crystallization of the film. Furthermore, the variation of the crystalline structure led to changes in the optical and the thermal properties.

Tailoring of electrochemical properties of V2O5 thin films grown on flexible substrates using plasma-assisted activated reactive evaporation

The vanadium pentoxide (V2O5) thin films have been deposited using home built activated reactive evaporation technique on indium tin oxide-coated flexible Kapton substrates and investigated their microstructural and electrochemical properties. X-ray diffraction pattern displayed predominant (001) orientation designating the orthorhombic structure of the films deposited at optimised growth conditions. The surface of the films is observed to be composed of vertical elliptical-shaped grains of size 98 nm distributed uniformly over the surface of the films provided with root mean square surface roughness of 9 nm as evidenced from atomic force microscopy studies. As-deposited V2O5 thin films demonstrated constant discharge capacity of about 60 μAh/(cm2−μm) for 10 cycles at room temperature in the potential window of 4.0–2.5 V. The influence of silver (Ag) interlayer on electrochemical properties of V2O5 films was investigated and observed appreciable improvement in electrochemical performance of ‘V2O5/Ag/V2O5’ films. The multilayered V2O5/Ag/V2O5 films exhibited a discharge capacity of about 75 μAh/(cm2−μm) provided with enhanced cycliability.

Effect of annealing and electrochemical properties of sol–gel dip coated nanocrystalline V2O5 thin films

Nanocrystalline vanadium pentoxide (V2O5) thin films were deposited on glass substrates by a simple and cost effective sol–gel dip coating method. The effect of annealing on microstructure and optical properties of V2O5 thin films were investigated. Formation of nanorods with the average diameter of 500–750nm after annealing is observed by scanning electron microscopy. X-ray diffractometry indicates that an orthorhombic structured thin film is transformed to β-V2O5 nanorods by subsequent annealing at 500°C. It was also confirmed that the growth of nanorods strongly correlates with annealing conditions; nanorod formation can be explained by surface diffusion phenomenon. The electrochemical performance of the V2O5 nanorods was investigated by cyclic voltammetry.

Thermoelectric transport in V2O5 thin films

The density functional theory and the Boltzmann transport equation were used to calculate the thermoelectric transport coefficients for bulk V2O5. We performed the structural relaxation for the given compound based on the ABINIT code. The temperature dependence of the Seebeck coefficient as well as of the electrical and thermal electron conductivities of the X-ray and relaxed structures displays an anisotropic behavior while the corresponding traces for both structures present similar values. The simulation results are compared with the electro-optical measurement of the thermoelectric properties of V2O5 thin films obtained by thermal evaporation with different post annealing treatments. A Seebeck coefficient of −218μV/K at T=573K was obtained in the in-plane direction for thin films with thickness less than 100nm.

Infrared thermochromic properties of VO2 thin films prepared through aqueous sol-gel process

The stoichiometric vanadium(IV) oxide thin films were obtained by controlling the temperature, time and pressure of annealing. The thermochromic phase transition and the IR thermochromic property of 400 nm and 900 nm VO2 thin films in the 7.5 μm-14 μm region were discussed. The derived VO2 thin film samples were characterized by Raman, XRD, XPS, AFM, SEM, and DSC. The resistance and infrared emissivity of VO2 thin films under different temperature were measured, and the thermal images of films were obtained using infrared imager. The results show that the VO2 thin film annealed at 550 °C for 10 hours through aqueous sol-gel process is pure and uniform. The 900 nm VO2 thin film exhibits better IR thermochromic property than the 400 nm VO2 thin film. The resistance of 900 nm VO2 film can change by 4 orders of magnitude and the emissivity can change by 0.6 during the phase transition, suggesting the outstanding IR thermochromic property. The derived VO2 thin film can control its infrared radiation intensity and lower its apparent temperature actively when the real temperature increases, which may be applied in the field of energy saving, thermal control and camouflage.