Properties Of VO2 Films Research Articles

Compare the phase transition properties of VO2 films from infrared to terahertz range

ABSTRACTVO2 with reversible semiconductor–metal phase transition properties is particularly available for the application in smart opto-electrical devices. However, there are rare reports on comparing its phase transition properties at different ranges. In this study, the VO2 films are designed with the similar crystalline structure and stoichiometry, but different morphologies by inorganic and organic sol-gel methods, and their phase transition characteristics are compared both at infrared and terahertz range. The results indicate that the VO2 film prepared by inorganic sol-gel method shows more compact nanostructure. It results in larger resistivity change, infrared and terahertz switching ratio in the VO2 film. Moreover, it presents that the phase transition intensity of VO2 film in terahertz range is more sensitive to its microstructure. This work is helpful for understanding the susceptibility of terahertz switching properties of VO2 to its microstructure. And it can provide insights for the applications of VO2 in terahertz smart devices.

Effects of Zirconium Ions Doping on the Structural and Thermochromic Properties of VO2 Thin Films

As an inorganic functional material, VO2 thin films are expected to be used for smart windows. However, these films are not conducive to practical applications when the phase transition temperature (Tc) of the VO2 is approximately 68°C, which is greater than room temperature. To decrease Tc, the effect of Zr4+ doping on the structure and properties of VO2 films were investigated. The films were prepared using a sol–gel process, spin-coating on quartz glasses, and annealing at 525°C in a tube furnace within a nitrogen atmosphere. The results demonstrate that these films have a highly preferential crystalline orientation on the substrate; however, the primary two x-ray diffraction pattern peaks shift slightly toward the small angle, and when 7 at.% Zr4+ was doped in the VO2 film, the deviation angle was 0.16°, and the crystallite size was approximately 98 nm. A large number of grains were found on the surface of the pure VO2 films, but all the Zr4+ doped films had a layered structure, and the thickness of the films was approximately 200 nm. The Tc of these films was characterized using differential scanning calorimetry, and the luminous and solar transmittance was characterized using a UV–Vis–NIR spectrophotometer, which demonstrated that the effect of Zr4+ doping decreased Tc by approximately 1°C per 1 at.% on average, and the Tc of the 7 at.% film was slightly greater than that of the 5 at.% film, the phase transition temperature of which was 61.4°C. The transmittance decreased as the doping concentration increased, and 1 at.% Zr4+ doping increased the luminous regulation efficiency (ΔT lum) and solar modulation efficiency (ΔT sol). The ΔT lum of the pure VO2 film and the 1 at.% Zr4+-doped VO2 films was 10.9% and 11.2%, and ΔT sol was 14.4 and 15.2%, respectively.

The effects of niobium on the structure and properties of VO2 films

Although vanadium dioxide (VO2) is undergoing trials for use in various thermal and optical applications due to its unique characteristics, its phase transition temperature must be decreased significantly for it to be used in smart windows in buildings. Niobium (Nb)-doped vanadium dioxide films were prepared on quartz substrates with sol–gel method, spin coating technique, and subsequent annealing at 550 °C in a pure nitrogen atmosphere. The effects of niobium on the structure, morphology, phase transition temperature and optical transmittance of VO2 films were investigated. The results show that no new phase came into beings after Nb doping, and that all of the films were of high crystalline quality. All of the films were compact from their surface morphologies, but their granules’ shapes evolved from small spheres to larger rods and even blocks as the doping concentration increased. The main valences of the vanadium ions in the films were +4, and Nb5+ ions were well present in the films. Although the phase transition temperature of the thin VO2 film decreased from 67.6 to 28.1 °C, both its optical transmittance and its transmittance mutation decreased significantly as the Nb’s molar concentration increased from 0 to 20 %. The maximum infrared transmittance mutation of the film decreased from 67.02 to 32.52 %.

Studies on magnetron-sputtered V2O5/ZnO thin films

Crystalline vanadium oxide (V2O5) thin films were grown over zinc oxide (ZnO) buffer layer-coated glass substrates at room temperature by magnetron sputtering technique. Initially, ZnO thin films have been formed by oxidizing sputtered zinc films at different annealing temperatures (300–500°C) in ambient air for 1 h. The influence of ZnO buffer layer on the structural, morphological, optical and electrical properties of the V2O5 films was studied using the X-ray diffraction (XRD), field-emission scanning electron microscopy, energy-dispersive analysis of X-rays, ultraviolet–visible spectrophotometry and four-point probe method. The thickness values of the deposited films were measured using the stylus probe profilometer. XRD patterns of the V2O5 deposited over ZnO (VZO) showed reflection planes corresponding to V2O5 and ZnO buffer layer. The optical studies showed the enhancement on the optical properties of V2O5 films deposited over ZnO buffer layer. The electrical studies of the VZO films showed that the resistivity of VZO films varied from 400 to 60 Ω cm. The enhancement in the optical and electrical properties may be due to the improvement in the crystallinity of the film because of ZnO buffer layer.

Nanostructured V2O5 thin films deposited at low sputtering power

Vanadium pentoxide (V2O5) films were deposited on cleaned silicon (Si) and glass substrates by reactive dc magnetron sputtering at different sputtering powers (50–70W) at room temperature (RT). Microstructural, morphological, topological, optical and electrical properties of the films were studied by X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), atomic force microscopy (AFM), UV–vis spectroscopy and four point probe method (FPP) respectively to analyze the impact of the sputtering power on the properties of V2O5 films. The average grain size of the films increased from 77 to 90nm on increasing the sputtering power and also all the deposited films showed characteristic peaks in the range 920–970cm−1 in Fourier transform infrared spectroscopy (FT-IR) spectrum corresponding to the V2O5. The optical bandgap and electrical resistivity of the films were found to be decreased while increasing the sputtering power. Four probe electrical resistivity measurements show the typical semiconductor behavior of the deposited film. The film deposited at 70W sputtering power at RT exhibited a temperature coefficient of resistance (TCR) of −1.22%/°C and resistivity (Rs) of 22Ωm. The low sputtering power used and room temperature deposition of the V2O5 films in the present investigation is suitable for complementary metal oxide semiconductor (CMOS) fabrication.

Large epitaxial bi-axial strain induces a Mott-like phase transition in VO2

The metal insulator transition (MIT) in vanadium dioxide (VO2) has been an important topic for recent years. It has been generally agreed upon that the mechanism of the MIT in bulk VO2 is considered to be a collaborative Mott-Peierls transition, however, the effect of strain on the phase transition is much more complicated. In this study, the effect of the large strain on the properties of VO2 films was investigated. One remarkable result is that highly strained epitaxial VO2 thin films were rutile in the insulating state as well as in the metallic state. These highly strained VO2 films underwent an electronic phase transition without the concomitant Peierls transition. Our results also show that a very large tensile strain along the c-axis of rutile VO2 resulted in a phase transition temperature of ∼433 K, much higher than in any previous report. Our findings elicit that the metal insulator transition in VO2 can be driven by an electronic transition alone, rather the typical coupled electronic-structural transition.

Preparation and modification of VO2 thin film on R-sapphire substrate by rapid thermal process

The VO2 thin film with high performance of metal–insulator transition (MIT) is prepared on R-sapphire substrate for the first time by magnetron sputtering with rapid thermal process (RTP). The electrical characteristic and THz transmittance of MIT in VO2 film are studied by four-point probe method and THz time domain spectrum (THz-TDS). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and search engine marketing (SEM) are employed to analyze the crystalline structure, valence state, surface morphology of the film. Results indicate that the properties of VO2 film which is oxidized from the metal vanadium film in oxygen atmosphere are improved with a follow-up RTP modification in nitrogen atmosphere. The crystallization and components of VO2 film are improved and the film becomes compact and uniform. A better phase transition performance is shown that the resistance changes nearly 3 orders of magnitude with a 2-°C hysteresis width and the THz transmittances are reduced by 64% and 60% in thermal and optical excitation respectively.

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.

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.

Ab Initio Calculations and Measurements of Thermoelectric Properties of V2O5 Films

Density functional theory and the Boltzmann transport equation were used to calculate the thermoelectric transport coefficients for bulk V2O5 and MV2O5 (M = Cr, Ti, Na, Li). The structural relaxation for the given compounds based on the ABINIT code was observed. The temperature dependences of the Seebeck coefficients as well as electrical and thermal electrical conductivities of all relaxed structures displayed anisotropic behavior. Electrooptical measurements of thermoelectric properties were carried out on V2O5 thin films obtained by thermal evaporation with different post-annealing treatments. A Seebeck coefficient of −148 μV/K at T = 300 K was obtained in the in-plane direction for V2O5 thin films with thickness less than 100 nm.