Vanadium Oxide Thin Films Research Articles

Fabrication and characteristics of VO x microbolometer integrated into gold black absorbing layer

Using vanadium oxide (VO x ) thin film as active layer, microbolometer with gold black absorber was fabricated due to the high mid-infrared (8-14μm) absorption property of gold black. This paper describes the design and fabrication of microbolometer with micromachining surface technology. The characteristics of the device are investigated. The detector performance exhibits maximum detectivity of 1.96 × 108cmHz1/2/W at 17 Hz with 8.8μA.

Optical, electronic, and microstructural properties of mixed phase RF magnetron sputtered vanadium oxide thin films on quartz and aluminized quartz

Amorphous vanadium pentoxide (major phase) thin films of various thicknesses were grown on the substrates that possessed low (aluminized quartz) and high (quartz) infrared (IR) emittance property utilizing radio frequency (RF) magnetron sputtering technique. Microstructural characterizations are carried out by several techniques such as noncontact optical profilometry, energy dispersive X‐ray (EDX) spectroscopy, X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and X‐ray diffraction (XRD). The as‐grown amorphous films show stoichiometric vanadium oxide, where vanadium is in V5+ as a major proportion compared with V4+ states. Thermo‐optical properties, namely, solar transmittance (τs), reflectance (ρs), absorptance (αs) and IR emittance (εir) along with optical constant, for example, optical band gap, are evaluated. A modified Urbach equation was used to simulate the optical absorption data to understand the presence optical absorption below the band edge.

Properties and Biomedical Applications of Hydrothermal method Synthesized of Vanadium Oxide nanoparticles

Vanadium oxide thin films were successfully prepared hydrothermally and deposited on a substrate using the drop casting method. Analytical techniques such as XRD, AFM, (SEM) analyses, Ultraviolet-Visible (UV-Vis), PL, and FTIR measurements were used to confirm the characterization of the prepared vanadium oxide NPs. The XRD measurements show that the VO2 thin film was polycrystalline, with a (monoclinic) phase and crystallite size calculated using Scherrer’s equation, as well as dislocation density and microstrain. The optical properties show that the energy gap (4.49) eV, and depending on PL, a single sharp emission peak was found at location 350 nm (3.54 eV). The Antibacterial activity of the vanadium oxide nanoparticles were investigated, with inhibition zone Escherichia coli (17) mm, Staphylococcus aureus 20 mm, Bacillus subtilis 18 mm, and Klebsiella pneumonia 19 mm and Candida isolates 15 mm. The study confirms that the prepared VO2 samples can be used as an antibacterial agent. The results suggest that proper tuning can make them a good antimicrobial agent.

Dielectric function of vanadium oxide thin films by thermal annealing.

The dielectric function of ${{\rm{VO}}_x}$ and ${{\rm{V}}_2}{{\rm{O}}_5}$ thin films is determined with the use of a spectroscopic Mueller matrix ellipsometer from 1.5 to 5.0 eV. The complex dielectric function of the films is calculated using the measured Mueller matrices filtered with the Cloude decomposition. ${{\rm{VO}}_x}$ shows high absorption in the UV region, a Tauc-Lorentz gap around 2.4 eV, and non-vanishing absorption in the visible. ${{\rm{V}}_2}{{\rm{O}}_5}$ shows a high absorption band centered at 2.87 eV, an indirect optical band gap at 1.95 eV, and a direct optical band gap at 2.33 eV. The ellipsometric characterization is supported by Raman, x-ray photoelectron, and photoluminescence spectroscopy.

Reactive pulsed DC magnetron sputtering deposition of vanadium oxide thin films: Role of pulse frequency on the film growth and properties

In this study, vanadium oxide (VOx) thin films were prepared by reactive pulsed DC magnetron sputtering (PDMS). The influence of pulse frequency (Fp) on the deposition process and properties of VOx films was investigated. The obtained results indicate that with the increase of Fp, which not only can enhance the oxidation of vanadium but also can facilitate the formation of a denser microstructure and smoother surface morphology in the films. More importantly, films with composition from metal-rich to maximum stoichiometry can be accessed by tuning the Fp, without varying any other parameters, especially, the flow rate of reactive gas. And temperature dependent electrical properties of the VOx films can also be tailored via the Fp to fall in the desirable range for microbolometer applications. The impacts of Fp on the reaction between V and O, thin film growth and some dynamic processes occurring during reactive PDMS were analyzed. The corresponding underlying mechanisms were discussed in detail. Our present work may provide a practical and effective way to develop the reactive sputtering towards hysteresis-free, and also paves the way for optimizing the reactive sputtered VOx films with desirable properties via Fp.

Vanadium oxide thin films with extremely narrow thermal hysteresis loop width were obtained by Ta doping for THz modulation applications

Based on the application potential of VO2 thin film in THz modulation field, only keeping its modulation amplitude is no longer applicable, and the film also needs to have lower phase transition temperature and narrower loop width, etc. In order to solve this problem, and since the high valence ion doping can reduce the phase transition temperature of VO2 films, the effect of Ta5+ doping on the phase transition performance of VO2 films is studied and analyzed in this paper. In this work, based on DC reactive magnetron sputtering technology, un-doped and Ta-doped VO2 films were prepared on high-resistance silicon substrate by two-step method. The microstructure of VO2 films before and after Ta doping was characterized by XPS, XRD and SEM images to assist the study and analysis of the influence of Ta5+ ions on the crystal phase structure of VO2. Finally, when the doping concentration of Ta5+ is 0.71%, the phase transition temperature (TC) of VO2 film decreases to 37.5 °C, the loop width (ΔH) narrows to 1.03 °C, and the optical modulation amplitude is maintained at 64.2%, which makes it more suitable for THz modulator.

Effect of annealing and swift heavy ions irradiation on vanadium oxide thin films

In this work, thin films (∼200 nm) of vanadium oxide were deposited on Si (100) substrate by e-beam evaporation method with low-cost V2O5 powder. Some of the as-deposited samples were annealed in the argon atmosphere at 500°C for 1 h and cooled at a natural rate in the argon atmosphere. The as-deposited samples were also irradiated with 100 MeV Ag ions beam at different fluences. Grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy revealed that as-deposited films were composed of the V6O13 phase (mixed valance state). This V6O13 phase was transformed into polycrystalline V2O5 phase after post-annealing. Raman spectra of irradiated films had a new peak of the V6O13 phase, and no change was observed at higher fluence, and in the FTIR spectra, bands shifted toward lower wavenumber side at higher fluence. This band shifting toward the lower wavenumber side may be due to the weakening of vanadium oxygen bonds, which create oxygen vacancies in the film. The creation of defects in films was due to the pressure applied by the molten zone, which is formed by the passage of SHIs in the material.

Interface‐Driven Thermoelectric Switching Performance of VO+‐Diffused Soda‐Lime Glass

Strongly confined NaVO+ segregation and its thermoresponsive functionality at the interface between simple sputter‐deposited amorphous vanadium oxide thin films and soda‐lime glass is substantiated in this work by in situ temperature‐controlled time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS). The obtained ToF‐SIMS depth profiles provide unambiguous evidence for a reversible transformation that causes systematic switching of the NaVO+/Na+ and Na+/VO+ intensities upon cycling the temperature between 25 and 340 °C. Subsequently, NaVO complexes are found to be reversibly formed (at 300 °C) in vanadium oxide‐diffused glass, leading to thermoresponsive electrical behavior of the thin‐film glass system. This new segregation and diffusion‐dependent multifunctionality of NaVO+ point toward applications as an advanced material for thermoelectrical/optical switches, in smart windows or in thermal sensors.

Gossamer-like V<sub>2</sub>O<sub>5</sub> Thin Films for the Detection of Toxic Volatile Organic Compounds

Nanostructured vanadium pentoxide (V2O5) thin films are successfully deposited on an ultrasonically cleaned glass substrate with different deposition parameters via spray pyrolysis technique. The X-ray diffraction analysis showed that the V2O5 films are polycrystalline with an orthorhombic structure. SEM analysis illustrated that the gossamer-like morphology at the substrate temperature of 325 °C. The average crystallite size of the films changes from 14.29 to 23.81 nm, due to the deposition temperature enhancement. The film which is deposited at a substrate temperature of 325 °C has shown high transmittance. XPS studies validated the existence of V5+ oxidation form of vanadium in vanadium oxide thin film. Gas sensors are electronic devices designed to trace the concentration of different toxic gases existing in the environment. Gas sensing characterization has been performed using static liquid distribution technique towards different volatile organic compounds such as acetone, methanol, toluene and xylene. The thin film prepared at a substrate temperature of 325 °C has shown the maximum response towards 100 ppm of toluene at room temperature. The response and recovery times are determined using transient response curve, and the obtained values are 21sec and 31sec, respectively.

Control of Polymorphic Properties of Multivalent Vanadium Oxide Thin Films

In this study, VxOy thin films were deposited on borosilicate glass substrates using direct current (DC) magnetron sputtering. The optoelectronic thermochromic properties of the resulting multiphase vanadium oxide thin films were investigated. As-deposited (at 280 °C) films were annealed at 350, 450, and 500 °C in an oxygen atmosphere for 30 min in a tube furnace to improve the crystallinity. Structural analysis indicated the formation of a mixed-phase vanadium oxide film consisting of VO2(B), V4O9, and V2O5 phases on the amorphous substrate after annealing above 350 °C. The results showed that the semiconductor-to-metallic phase transition temperature of the vanadium oxide film increased from 48 to 63 °C with increasing annealing temperatures. The sample annealed at 450 °C exhibited the highest variation in the infrared (IR) transmittance (ΔTIR = 28.42%) and the resistivity switch decreased by two orders of magnitude (1.4 × 10–1–2.3 × 10–3 Ω/cm). The thermal treatment temperature affected the width of the thermal hysteresis loop (HLW) and slope stiffness. A narrower HLW of 1.9 °C and a sharp slope stiffness of 8.74 were obtained for the sample annealed at 500 °C. The slope stiffness plays an important role in the fabrication of ultrafast tunable energy-saving smart windows and IR switches.

Phase evolution and thermochromism of vanadium oxide thin films grown at low substrate temperatures during magnetron sputtering

Vanadium oxides (VOx) have been studied extensively for applications in thermochromic materials, electrochomics, and infrared detectors due to their unique phase transition characteristics. However, various vanadium oxide phases usually occur under different deposition conditions due to their particularly complex vanadium-oxygen system. In this research, V3O7, VO2(B), VO2(M), and V2O5 thin films were obtained as pure or mixed phases by controlling the substrate temperatures between 250 °C and 400 °C during magnetron sputtering. The microstructure and phase composition of vanadium oxide thin films were characterized and analyzed using X-ray diffraction and Raman spectroscopy. The phase evolution was dependent on the substrate temperature and could be clarified. Metastable V3O7 and VO2(B) phases were obtained at substrate temperatures of 250–300 °C, while stable VO2 and V2O5 phases were obtained at 350–400 °C. The surface morphology and optical properties of vanadium oxide thin films with different substrate temperatures were investigated in detail. Our results provide methods for transforming vanadium oxide phases under well controlled substrate temperatures.

Effective photoabsorption of two-way spin-coated metal oxides interfacial layers: Surface microstructural and optical studies

In this study, Molybdenum oxide (MoO3) and Vanadium oxide (V2O5) thin films were grown separately on pre-deposited cobalt oxide (Co3O4) thin film to form two different bilayer structures. The films were synthesized by spin-coating deposition technique. The precursors were prepared from common chemical reagents. Sample's surface microstructure, elemental composition and optical properties were investigated. The microstructural studies of the deposited bilayers show that the Co3O4 underlayer comprises crystalline grains with some overgrown clusters. Nanorod arrays of MoO3 adhering to the underlying Co3O4 layer with no chemical interaction are observed. In the second sample, the V2O5 top layer completely laminated the proximate Co3O4 film. Optical properties of the samples were examined from the UV/Vis spectra data. Energy band gap and band tail width changed with the adherence of top layer. Optical skin depths were also analyzed to reaffirm the suitability of the structures as potential window in thin film photovoltaics. The study demonstrated cheap process of developing interfacial layers of some transition metal oxide materials with high photoresponse. It also showed that the layers can offer the advantage of enhancing light harvesting, charge transport and storage.

Spontaneous fluctuations in a plasma ion assisted deposition – correlation between deposition conditions and vanadium oxide thin film growth

In this work correlations between thin film crystallinity of plasma ion assisted electron beam evaporated vanadium oxide (VOx) and fluctuations of the deposition parameters during the growth process could be observed by in situ monitoring deposition conditions and electron microscopy studies. In the presented case, unintentional fluctuations in the gas flow at the plasma source caused by inhomogeneous melting of the target material lead to an increase in discharge current and therefore a decrease of the oxygen flow in the plasma source, resulting in the formation of highly crystalline bands due to a temporary increase in energy flux. The major part of the VOx thin film consists of a large number of nanocrystals embedded in an amorphous phase. In-depth structural analysis confirms a mixture of V2O5, in different modifications, VO2, as well as the mixed-valence oxides V4O9 and V6O13, for nanocrystalline parts and crystalline bands. These differ mainly in the degree of crystallinity being influenced by variations in discharge current, and partly in the amount of higher oxidized vanadium oxides. In future, precisely controlled variation of plasma source conditions will open up pathways to control and tailor crystallinity of electron beam evaporated thin films, allowing for production methods for patterned thin films or layers with graduated crystallinity. This may give rise to a new class of coatings of nanohybrids combining amorphous VOx with low electrical conductivity and crystalline domains providing a higher electrical conductivity which is useful for electrochromic displays, smart windows, and solar cells.

Communication—Electrochemical Reduction of N2 to Ammonia by Vanadium Oxide Thin Films at Neutral pH: Oxophilicity and the NRR Reaction

Electroreduction of N2 to NH3 is an energy- and environmentally-friendly alternative to the Haber-Bosch process. Little is known, however, about reactive sites for electrochemical nitrogen reduction reaction (NRR) at Earth-abundant oxide or oxynitride surfaces. Here, we report N-free VIII/IV-oxide films, created by O2 plasma oxidation of polycrystalline vanadium, exhibiting N2 reduction at neutral pH with an onset potential of −0.16 V vs Ag/AgCl. DFT calculations indicate that N2 scission from O-supported V-centers is energetically favorable by ∼18 kcal mol−1 compared to N-supported sites. Theory and experiment yield fundamental insights concerning the effect of metal oxophilicity towards design of earth-abundant NRR electrocatalysts.

Highly crystalline V2O5 and V6O13 thin films by PLD and a study on morphology transition of V2O5 by post annealing

Vanadium oxide thin films were deposited on stainless steel substrates at different temperatures using the Pulsed laser deposition technique. Post annealing effect on the better crystalline V2O5 film was studied and found that the annealing had a positive impact on the crystallization. The structural and morphological properties were investigated by XRD, Raman, and SEM analysis. The presented work opens up the changes that occur in phase orientations and the tunable morphological growth in the Vanadium oxide thin films. The prepared thin films have potential applications in energy storage devices.

Titanium Vanadium Oxide Thin Films Prepared by Thermal Oxidation of Titanium-Vanadium Alloy as Photocatalyst for Methylene Blue Degradation under Visible Light

In this work, titanium-vanadium oxide composite thin films were prepared by thermal oxidation of vanadium and titanium metallic alloys, at 500 °C under air atmosphere. The metallic alloys, with different vanadium contents, were obtained by co-sputtering of vanadium and titanium targets on glass and silicon substrates. The films were characterized by means of scanning electron microscopy (SEM), x-ray diffraction, Raman spectroscopy, infrared spectroscopy, x-ray photoelectron spectroscopy, and UV–Vis spectroscopy. In addition, photodegradation of methylene blue (MB) in aqueous solution, on these composite films, under visible light, was also evaluated. SEM images show nanostructured films with nanospherical or nanobaranched surface morphologies. Furthermore, the results reveal that at low vanadium (V) content, the films consist of one or mixture of amorphous phases (TiVOx or TiOx plus VOx, or both). In the case of films with higher V content, two separated phases, namely V2O5 and amorphous TiOx are obtained. The UV–Vis analyses demonstrate a redshift of optical absorption over a broad band of optical frequencies for films with high V content. Furthermore, these films manifested a good catalytic activity toward MB degradation with degradation rate of 78% after 120 min of visible light irradiation. Such composite thin films can be promising not only for photocatalytic applications, but also for photonic and sensing devices.

Correlation of band electronic structure with efficiency in perovskite solar cells with vanadium (Ⅳ) oxide thin film buffers

Perovskite solar cells have been studied extensively in the area of perovskite solar cells because they have a comparatively free hysteresis. Through fabrication of a perovskite solar cell based on a vanadium oxide buffer, this study clarified the mechanism of electron and hole transport in the laminated layer upon irradiation with light. The power conversion efficiency (PCE) of the Vanadium (Ⅳ) oxide (VO2) sputtering process device was approximately 13% and with the spin-coating process was 8.5%. To investigate the physicochemical origin of such PCE differences depending on the process type, comprehensive band alignment and band structure analyses of the actual cell stacks were performed using X-ray photoelectron spectroscopy depth measurements. Accordingly, it was found that the inconsistent valence band offset between the perovskite absorption layer and V2O5 layer as a function of the VO2 process type caused a difference in the hole transport, resulting in the difference in the efficiency.

Electron stimulated desorption of vanadyl-groups from vanadium oxide thin films on Ru(0001) probed with STM.

Low-temperature scanning tunnelling microscopy (STM) is employed to study electron-stimulated desorption of vanadyl groups from an ultrathin vanadium oxide film. The vanadia patches are prepared by reactive vapour deposition of V onto a Ru(0001) surface and comprise a highly ordered network of six and twelve membered V-O rings, some of them terminated by upright V[double bond, length as m-dash]O groups. The vanadyl units can be desorbed via electron injection from the STM tip in a reliable fashion. From hundreds of individual experiments, desorption rates are determined as a function of bias voltage and tunnelling current. Data analysis reveals a distinct threshold behaviour with bias onsets at +3.3 V and -2.6 V for positive and negative polarity, respectively. The desorption rate varies quadratically (cubically) with the tunnelling current at positive (negative) sample bias, indicating that V[double bond, length as m-dash]O desorption is a many-electron process. Based on our findings, a mechanism for desorption is proposed that includes resonant tunnelling into anti-bonding or out of bonding orbitals, followed by vibrational ladder climbing in the binding potential of the V[double bond, length as m-dash]O ad-system. The underlying electronic states can be identified directly in the STM conductance spectra taken on the oxide surface.

Characterization of Sn doped vanadium oxide thin films for nonlinear optical applications

Tin doped vanadium oxide thin films were deposited on the glass substrate by thermal evaporation technique with 3%, 6%, 9% and 12% by weight of SnO2. The deposited films were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectrometer, atomic force microscopy (AFM) and UV-Vis spectroscopy. The amorphous nature of all deposited films was confirmed by the X-ray diffraction method. The elemental analysis of the films was confirmed by EDX analysis. UV–Vis spectroscopy is used for optical analysis such as optical transmission, absorption and band gap analysis of the thin films in the range 300–800 nm. The nonlinear optical parameters of the deposited thin films were estimated by empirical relations. Based on all these characterizations, the films are practical for linear and nonlinear applications.

Influence of Reactive Oxygen Gas on Sputtered-Vanadium Oxide Films under Post-Annealing in Vacuum

Vanadium oxide thin films were deposited on glass substrates by O2 reactive-RF magnetron sputtering from a vanadium (V) target without substrate-heating. The percentages of O2 gas were 10%, 7.5%, 6.0%, 5.0% and 2.5%. The total gas flow rate (O2/Ar) was kept at 25 sccm. As-deposited films were experienced post-annealing process at different temperatures and times. The crystallinity and chemical bonding states of films were examined by X-ray diffraction and Raman spectroscopy. The condition in annealing to active crystallinity depended on an earlier composition of the films. As O2-gas percentages were 10% and 7.5%, after annealing, the as-deposited VxOy films were transformed into crystalline V2O5 films. With decreasing in O2 percentage to 5.0% and 2.5%, the films were transformed into V2O3 and VO films, respectively. The films deposited with 6.0% O2 were crystallized to VO2 with phase B after annealing with 500 °C 15 h. By applying a longer time to 30 h at the high temperature 500 °C in annealing, VO2 films revealed only phase M formation.