Vanadium Films Research Articles

Enhanced luminous transmittance of thermochromic VO2 thin film patterned by SiO2 nanospheres

In this study, an ordered SiO2 nanosphere array coated with vanadium dioxide (VO2) has been fabricated to enhance transmittance with the potential application as an energy-efficient coating in the field of smart windows. SiO2 arrays were formed using the methods of self-assembly, and VO2 thin films were prepared by rapid thermal annealing (RTA) of sputtered vanadium films. VO2@SiO2 arrays were characterized by scanning electron microscopy, X-ray diffraction, a four-point probe, and UV-vis-NIR spectrophotometry. Compared with the planar films, the films deposited on 300 nm diameter SiO2 nanospheres can offer approximately 18% enhancement of luminous transmission (Tlum) because the diameter is smaller than the given wavelength and the protuberance of the surface array behaves as a gradation of refractive index producing antireflection. The solar regulation efficiency was not much deteriorated.

Hydrogenation behavior on 30 nm vanadium thin films with two different adhesion conditions

The influence of a thin polycarbonate de-adhesion layer on the hydrogen concentration is studied on 30 nm vanadium films deposited on glass substrates, using electrochemical hydrogenography in an optical microscopy and scanning tunneling microscopy. It is shown that the optical reflection provides information about the de-adhesion morphology (buckles) while the optical transmission signal gives information about concentration and film thickness changes. Artificially patterned samples allow simultaneous studying adhered and de-adhered film parts, for similar mean concentrations. The optical data clearly show a different hydrogen behavior of the two parts. Data interpretation suggests higher local hydrogen content in the adhered film parts than for the detached films parts. Strong changes in the optical transmission of the adhered film parts can be attributed to strong morphological changes at the film surface. These changes are mainly attributed to grain sliding processes in the vanadium film.

Large spin Hall angle in vanadium film

We report a large spin Hall angle observed in vanadium films sputter-grown at room temperature, which have small grain size and consist of a mixture of body centered tetragonal (bct) and body centered cubic (bcc) structures. The spin Hall angle is as large as θV = −0.071 ± 0.003, comparable to that of platinum, θPt = 0.076 ± 0.007, and is much larger than that of bcc V film grown at 400 °C, θV_bcc = −0.012 ± 0.002. Similar to β-tantalum and β-tungsten, the sputter-grown V films also have a high resistivity of more than 200 μΩ∙cm. Surprisingly, the spin diffusion length is still long at 16.3 nm. This finding not only indicates that specific crystalline structure can lead to a large spin Hall effect but also suggests 3d light metals should not be ruled out in the search for materials with large spin Hall angle.

The influence of post annealing on the structure and optical properties of vanadium oxide thin films

Vanadium and vanadium oxide films were deposited onto glass and silicon substrates at room temperature by DC magnetron sputtering and reactive DC magnetron sputtering, respectively. The effect of post annealing in air atmosphere were explored. The as-deposited vanadium and vanadium oxide films were annealed under air atmosphere at different temperatures (100-500°C) and annealing times (6, 12 hours). The structural properties of the films were studied by X-ray diffraction and scanning electron microscopy. The optical properties of the films were studied by UV-Vis spectrophotometer. The results showed that the annealed vanadium films have a formation of polycrystalline V2O5 structure while the optical transmittance is low due to the imperfect thermal oxidation. For the annealed vanadium oxide films, the vanadium oxide with the phase of V2O5 is also presented while the transmittance decreased when the annealing temperature is increased. This is due to the light loss by the scattering at the rough surface. A comparison of the results with the annealed vanadium and annealed vanadium oxide films will be discussed. Moreover, the effect of the post annealing temperature and the annealing time on the structural and optical properties will also be discussed.

Concentration dependence of hydrogen diffusion in clamped vanadium (0 0 1) films

The chemical diffusion coefficient of hydrogen in a 50 nm thin film of vanadium (0 0 1) is measured as a function of concentration and temperature, well above the known phase boundaries. Arrhenius analysis of the tracer diffusion constants reveal large changes in the activation energy with concentration: from 0.10 at 0.05 in H V−1 to 0.5 eV at 0.2 in H V−1. The results are consistent with a change from tetrahedral to octahedral site occupancy, in that concentration range. The change in site occupancy is argued to be caused by the uniaxial expansion of the film originating from the combined hydrogen induced expansion and the clamping of the film to the substrate.

Novel high power impulse magnetron sputtering enhanced by an auxiliary electrical field.

The high power impulse magnetron sputtering (HIPIMS) technique is a novel highly ionized physical vapor deposition method with a high application potential. However, the electron utilization efficiency during sputtering is rather low and the metal particle ionization rate needs to be considerably improved to allow for a large-scale industrial application. Therefore, we enhanced the HIPIMS technique by simultaneously applying an electric field (EF-HIPIMS). The effect of the electric field on the discharge process was studied using a current sensor and an optical emission spectrometer. Furthermore, the spatial distribution of the electric potential and electric field during the EF-HIPIMS process was simulated using the ANSYS software. The results indicate that a higher electron utilization efficiency and a higher particle ionization rate could be achieved. The auxiliary anode obviously changed the distribution of the electric potential and the electric field in the discharge region, which increased the plasma density and enhanced the degree of ionization of the vanadium and argon gas. Vanadium films were deposited to further compare both techniques, and the morphology of the prepared films was investigated by scanning electron microscopy. The films showed a smaller crystal grain size and a denser growth structure when the electric field was applied during the discharge process.

Temperature dependence of electrical resistivity in oxidized vanadium films grown by the GLAD technique

Abstract Vanadium and vanadium oxide thin films are deposited by DC magnetron sputtering. A first series of pure vanadium films are prepared by glancing angle deposition (GLAD). The incident angle α of the particle flux is systematically changed from 0 to 85°. For the second series, the angle α is kept at 85° and oxygen gas is injected during the growth by means of the reactive gas pulsing process (RGPP). A constant pulsing period P = 16 s is used whereas the oxygen injection time t ON is varied from 0 to 6 s. After depositing, films are annealed in air following 11 incremental cycles from room temperature up to 550 °C. For both series, the DC electrical resistivity is systematically measured during the annealing treatment. Vanadium films sputter deposited by GLAD become sensitive to the temperature for incident angles α higher than 60°. The most significant annealing effect is observed for films prepared with α = 85° with a strong increase of resistivity from 2.6 × 10 − 5 to 4.9 × 10 − 3 Ωm. It is mainly assigned to the oxidation of GLAD vanadium films, which is favoured by the high porous morphology produced for the highest incident angles. The resistivity vs. temperature evolution is also measured and related to the occurrence of the VO 2 phase. By combining GLAD and RGPP processes, the reversible variation of resistivity associated to the VO 2 phase is even more pronounced. Oxygen pulsing during deposition and the voided structure produced for the highest incident angles enhance the oxidation of vanadium through the films thickness. The porous architecture by GLAD and the oxygen injection by RGPP have to be carefully controlled and optimized for the growth of vanadium oxide compounds, especially to favour the formation of the VO 2 phase.

Effect of Au on the Performance of Porous Silicon/V2O5 Nanorods Heterojunctions to NO2 Gas

A novel composite of Au-functionalized porous silicon (PS)/V2O5 nanorods (PS/V2O5:Au) was prepared to detect NO2 gas. PS/V2O5 nanorods were synthesized by a heating process of pure vanadium film on PS, and then the obtained PS/V2O5 nanorods were functionalized with dispersed Au nanoparticles. Various analytical techniques, such as field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), have been employed to investigate the properties of PS/V2O5:Au. Herein, the PS/V2O5:Au sample exhibited improved NO2-sensing performances in response, stability and selectivity at room temperature (25[Formula: see text]C), compared with the pure PS/V2O5 nanorods. These phenomena were closely related to not only the dispersed Au nanoparticles acting as a catalyst but also the p-n heterojunctions between PS and V2O5 nanorods. Whereas, more Au nanoparticles suppressed the improvement of response to NO2 gas.

Synthesis and gas-sensing properties of the silicon nanowires/vanadium oxide nanorods composite

As air pollution is becoming more and more serious in recent years, gas-sensing devices have attracted intensive attention. In particular, NO2 is one of the most toxic gases in the atmosphere, which tends to produce acid rain and photochemical smog. Thus, there is a strong demand of cheap, reliable and sensitive gas sensors targeting NO2. Gas sensors fabricated on silicon substrates with room-temperature operation are very promising in power saving, integrated circuit processing and portable detectors. More important, the silicon nanowires (SiNWs)-based devices are compatible with very large scale integration processes and complementary metal oxide semiconductor technologies. In the present work, the novel nanocomposite structure of (SiNWs)/vanadium oxide (V2O5) nanorods for NO2 detection is successfully synthesized. The SiNWs are fabricated by a combination of nanosphere lithography and metal-assisted chemical etching. Vanadium films are deposited on SiNWs by DC magnetron sputtering, and then V2O5 nanorods are synthesized with subsequent thermal annealing process for full oxidation in air. The morphology and crystal structure of product obtained are characterized by field-emission scanning electron microscopy and X-ray diffraction. The characterization results indicate that V2O5 nanorods are uniformly distributed on the surfaces of SiNWs. The increased specific surface area of SiNWs/V2O5 nanocomposite provides more adsorption sites and diffusion conduits for gas molecules. Therefore, the novel structure of the nanocomposite is conducive to gas-sensing. In addition, the sputtering time has an obvious influence on the morphology of vanadium oxide. With the increase of the sputtering time, the specific surface area and the number of p-n heterojunctions formed in the nanocomposite are both less than those of nanocomposite with appropriate sputtering time. The gas-sensing properties are examined by measuring the resistance change towards 0.5-4 ppm NO2 gas at room temperature by the static volumetric method. Results show that the nanocomposite with shorter deposition time has better gas-sensing properties to low-concentration NO2 gas than those of bare SiNWs and nanocomposite with longer deposition time. On the contrary, the responses of the nanocomposite to other high-concentration reducing gases are very low, indicating good selectivity. The enhancement in gas sensing properties may be attributed to the change in width of the space charge region, which is similar to the behavior of p-n junction under forward bias, in the high-density p-n heterojunction structure formed between SiNWs and V2O5 nanorods. In conclusion, these results demonstrate that the SiNWs/V2O5 nanocomposite has great potential for future NO2 gas detection applications with low consumption and good performance.

Micropore-Dominant Vanadium and Iron Co-Doped MnO2 Hybrid Film Electrodes for High-Performance Supercapacitors

To further improve the electrochemical performance of electrode materials for supercapacitors, we have achieved a significant increase of the micropore volume of a MnO2 hybrid film on IrO2 nano-wedges grown on a pre-treated Ti plate by the co-doping of vanadium and iron (V+Fe). X-ray diffraction and microstructural analyses demonstrate that the V+Fe co-doped MnO2 hybrid films consist of the lamellar structure that is composed of γ-MnO2 nanocrystallites jointed by disordered interface. Nitrogen sorption analysis confirms that the pore characteristics of the MnO2 film change from mesopore-dominant to micropore-dominant after the co-doping, accompanied by increases in the pore volume and specific surface area. The specific capacitance of the V+Fe co-doped MnO2 hybrid film electrode declines from 426 F g−1 to 314 F g−1 with a relatively limited loss of 26% when the galvanostatic (GV) charging-discharging rate is increased from 0.2 to 5 A g−1. In particular, the doped MnO2 film electrode has a loss of only 6% in the specific capacitance after 9000 cycles at a charge-discharge current density of 10 A g−1.

Thickness Measurement of V2O5Nanometric Thin Films Using a Portable XRF

Nanometric thin films have always been chiefly used for decoration; however they are now being widely used as the basis of high technology. Among the various physical qualities that characterize them, the thickness strongly influences their properties. Thus, a new procedure is hereby proposed and developed for determining the thickness of V2O5nanometric thin films deposited on the glass surface using Portable X-Ray Fluorescence (PXRF) equipment and the attenuation of the radiation intensity Kαof calcium present in the glass. It is shown through the present paper that the radiation intensity of calcium Kαrays is proportional to film thickness in nanometric films of vanadium deposited on the glass surface.

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

Vanadium-doped zinc oxide films for piezoelectric application

Nanocrystalline zinc oxide:vanadium (ZnO:V) films were deposited over thoroughly cleaned glass substrates using the reactive direct current magnetron sputtering technique. The deposited ZnO:V films were analyzed for their structural, morphological, optical and electrical properties using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), UV-visible spectrophotometry and the two-probe method, respectively. Microstructural parameters of the films varied with respect to substrate temperature, substrate bias and sputtering powers. FE-SEM micrographs of the films showed that the prepared films were porous when compared with pure ZnO films. ZnO:V films were highly transparent and the optical band gap value of the films varied between 3·1 and 3·37 eV. Piezoelectric generation of the ZnO:V films was tested by different testing methodologies. The lab-scale device fabricated in the present work generated 100 µV upon applying vibration. Scaling up this device would yield piezoelectricity of higher orders of voltages.

Structural characterization and electrochemical properties of cerium–vanadium (Ce–V) mixed oxide films synthesized by chemical route

Mixed oxide thin films of cerium and vanadium were obtained by chemical bath method from Ce(NO3)3 and 6H2O, NH4VO3 with distilled water as solvent in the presence of polyethylene glycol (PEG) as surfactant and dip coated on non-conducting quartz substrate. The optical, structural and morphological properties depend on post-deposition temperature. No remarkable effect was observed upon the addition of surfactant. The amorphous to polycrystalline transition of the films revealed that the inclusion of CeO2 was responsible for the widening of the energy bandgap 3.8eV. The supercapacitive behavior of the electrode was evaluated using cyclic voltammetry (CV) and galvanostatic charge discharge techniques in 1M LiClO4. The specific capacitance of 179F/g was achieved for the mixed oxide electrode at a scan rate of 20mV/s.

Evaluation of the nanomechanical properties of vanadium and native oxide vanadium thin films prepared by RF magnetron sputtering

Polycrystalline vanadium thin films of 50, 75, and 100nm thickness were deposited by magnetron sputtering of a vanadium metal target of 2 inch diameter with 99.9% purity on native oxide covered Si substrates. One set of the fabricated samples were kept in moisture free environment and the other set was exposed to ambient air at room temperature for a long period of time that resulted in formation of native oxide prior to testing. The crystal structure and phase purity of the vanadium and the oxidized vanadium thin films were characterized by X-ray diffraction (XRD). The XRD results yield a preferential (110), and (200) orientation of the polycrystalline V films and (004) vanadium oxide (V3O7). The vanadium films thickness were verified using field emission scanning electron microscopy and the films surface morphologies were inspected using atomic force microscopy (AFM). AFM images reveal surface roughness was observed to increase with increasing film thickness and also subsequent to oxidation at room temperature. The nanomechanical properties were measured by nanoindentation to evaluate the modulus and hardness of the vanadium and the oxidized vanadium thin films. The elastic modulus of the vanadium and the oxidized vanadium films was estimated as 150GPa at 30% film thickness and the elastic modulus of the bulk vanadium target is estimated as 135GPa. The measured hardness of the vanadium films at 30% film thickness varies between 9 and 14GPa for the 100 and 50nm films, respectively, exhibiting size effects, where the hardness increases as the film thickness decreases. The hardness of the oxidized films depicted less variation and is reported as ∼10GPa at 30% film thickness for the three oxides. The scanning electron microscopy (SEM) imaging depicted a gradual progression of pile up as the film thickness increased from 75 to 100nm. It is noticed that as the film thickness increases the films experience softening effect due to grain coarsening and the hardness values depict the hardness of the Si substrate at deep indents.

Surface decoration of Au–Pt bimetallic inorganic–organic hybrid nanocomposite modified carbon ceramic electrode with vanadium N-salicylidene-L-histidine–al-MCM-41 for electrooxidation of thiosulphate

This work describes the electrochemical behavior of vanadium N-salicylidene-L-histidine-Al-mobil composition of matter no. 41 (MCM-41) (V(His)/Al-MCM-41) film immobilized on the surface of bimetallic Au–Pt inorganic–organic hybrid nanocomposite carbon ceramic electrode and its electrocatalytic activity toward the oxidation of thiosulphate. The surface structure and composition of the sensor was characterized by scanning electron microscopy (SEM). Electrocatalytic oxidation of thiosulphate on the surface of modified electrode was investigated with cyclic voltammetry, chronoamperometry and hydrodynamic amperometric techniques and the results showed that the vanadium film displays excellent electrochemical catalytic activities towards thiosulphate oxidation. The modified electrode indicated reproducible behavior and high level of stability during the electrochemical experiments.

Preparation of paraboloid-like VO2@SiO2 nanostructured arrays for enhanced transmission

An uniform nanostructured array of VO2@SiO2 was grown via depositing VO2 film on SiO2 monolayer array. VO2 films were fabricated by rapid thermal annealing (RTA) of sputtered vanadium films, and SiO2 arrays with diameter of about 600nm were formed using direct assembly at the air-water interface. Much interestingly, the surfaces of SiO2 nanospheres could be regarded as double-sided surfaces with paraboloid-like arrays which offered a gradual refractive index between air and the surface of SiO2. This SiO2 array was used as biomimetic antireflective structure (ARS) for enhanced transmission. In addition, with the change of oxygen flow rate, VO2@SiO2 array exhibited adjustable preferred orientation and more uniform grains compared with planar film. More importantly, VO2 film deposited on ARS array showed enhanced transmittance which would be important for future applications in smart windows.

Tuning phase transition temperature of VO2 thin films by annealing atmosphere

A simple new way to tune the optical phase transition temperature of VO2 films was proposed by only controlling the pressure of oxygen during the annealing process. Vanadium films were deposited on glass by a large-scale magnetron sputtering coating system and then annealed in appropriate oxygen atmosphere to form the VO2 films. The infrared transmission change (at 2400 nm) is as high as 58% for the VO2 thin film on the glass substrate, which is very good for tuning infrared radiation and energy saving as smart windows. The phase transition temperature of the films can be easily tuned from an intrinsic temperature to 44.7 °C and 40.2 °C on glass and sapphire by annealing oxygen pressure, respectively. The mechanism is: V3+ ions form in the film when under anaerobic conditions, which can interrupt the V4+ chain and reduce the phase transition temperature. The existence of V3+ ions has been observed by x-ray photoelectron spectroscopy (XPS) experiments as proof.

The novel preparation method of high-performance thermochromic vanadium dioxide thin films by thermal oxidation of vanadium-stainless steel co-sputtered films

The preparation of high-performance thermochromic vanadium dioxide films and the assurance of its batch-to-batch uniformity is technically difficult since the oxidation number of vanadium ranges from +2 to +5 under various reducing or oxidizing atmospheres at elevated temperatures. In this work, the vanadium dioxide films were fabricated by thermal-oxidation of vanadium and stainless-steel (mainly containing Fe, Cr, Ni) co-sputtered thin films at elevated temperature under air atmosphere. The as-prepared vanadium oxide films were further characterized by four-point probe measurement, ellipsometry, grazing incidence X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analysis. Our experiments demonstrated that with the use of an SS target we could efficiently deposit the magnetic atoms (iron and nickel) and nonmagnetic Cr elements into vanadium films in stable plasma conditions. The shift of the main peak of XRD patterns to smaller angles showed that Fe, Cr, and Ni elements were successfully doped into vanadium oxide films. These dopants depressed the vanadium atoms from over-oxidization during the heat treatment. As a result, a noticeable amount of VO2(M) crystalline phase (higher V(+4)/V(+5) ratios) was obtained from the as-prepared films accompanied with the increment of the SS target voltage. Consequently, the formation of V6O13, V3O7, and V2O5 vanadium oxides crystallites was significantly minimized after the thermal-oxidation stage, which gave the as-prepared films outstanding thermochromic-responsive properties. The proposed scheme is proved to be a novel fabrication technique to prepare high-performance thermochromic vanadium dioxide films with the assurance of batch-to-batch uniformity.

Vanadium Nitride Thin Films and Nanoclusters: Growth and Electrochemical/XPS Characterization

Pseudocapacitive or electrochemically active materials can achieve capacitance values up to 10-100 times greater than those obtained by materials based only on an electric double layer (EDL) mechanism. The pseudocapacitive behavior of transition metal oxides has been extensively studied in the last decades and assigned to redox reactions occurring at the surface of the material. Very recently, transition metal nitrides such as MoxN, VN or TiN have emerged as promising electrode materials for electrochemical capacitors. These materials are inexpensive, have a high molar density good chemical resistance and, most importantly, in contrast to oxides they exhibit very high electronic conductivity value. Extremely high specific capacitance values of 1340 F g-1 was reported for nanostructured VN [1]. To shed some light on the mechanism of such huge capacitance we have carried out a comparative study of VN thin films and VN nanoclusters grown and characterized in-situ to relate electrochemical properties with the structure and composition of the surface of vanadium nitride. An UHV system (SPECS) equipped with tools for XPS, e-beam assisted evaporation of metals, high pressure cell and electrochemical cell for I-V cycling was used for in-situ synthesis and characterization. A recipe to grow VN thin films and nanoclusters via direct nitridation of thin vanadium film in atmosphere of 1 bar N2 at temperature 800 oC has been developped. Stoichiometry of the VNxOy compounds can be controoled by post oxidation of VN in oxygen atmosphere at elevated temperature. Growth of VN nanoclusters using the same procedure has been performed on HOPG surface. Electrochemical characterization of VN and VNxOy thin films demostrated impressive areal capacitance of ~2000 µFcm-2 in 1M KOH electrolyte at scan rate 1Vs-1. XPS characterizationwas applied to elucidate electrochemical reactions occurring on the surface of VN films. [1] D. Choi et al., Adv. Mater., 18, 1178 (2006)