Synthesis and optical properties of vanadium dioxide nanoparticles in nanoporous glasses

A method for the synthesis of vanadium dioxide (VO₂) nanoparticles in nanoporous silicate glass matrices with a pore size of 17 and 7 nm has been developed. According to this, vanadium pentoxide (V₂O₅) nanoparticles are initially grown in the pores, and then V₂O₅ is reduced to VO₂ in hydrogen. The optical transmission spectra of 1-mm-thick VO₂-modified glass samples have been measured. The temperature dependence of the transmission coefficient has been studied in the course of the semiconductor-metal phase transition in VO₂ nanoparticles.     

Synthesis,characterization, and thermodynamic parameters of vanadium dioxide

A novel process was developed for synthesizing pure thermochromic vanadium dioxide (VO₂) by thermal reduction of vanadium pentoxide (V₂O₅) in ammonia gas. The process of thermal reduction of V₂O₅ was optimized by both experiments and modeling of thermodynamic parameters. The product VO₂ was characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The experimental results indicated that pure thermochromic VO₂ crystal particles were successfully synthesized. The phase transition temperature of the VO₂ is approximately 342.6 K and the enthalpy of phase transition is 44.90 J/g.     

Enhanced hydrophilicity of the Si substrate for deposition of VO2 film by sol–gel method

We have illustrated the role of hydrophilic nature of Si substrate played in the improvement of the contact performance between the vanadium dioxide (VO₂) film and Si substrate. The VO₂ films were fabricated by sol–gel method on single crystal Si substrate, which was pre-treated with hydrophilic solution and obtained a quite improved hydrophilicity. The bonding of Si substrate with precursor V₂O₅ gel was interpreted. The morphology and crystalline structure of the films were investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction. It is shown that the surface of the film on Si substrate with enhanced hydrophilicity is quite homogeneous and uniform. The film exhibits the formation of VO₂ phase with (011) preferred orientation. Moreover, the optical pump induced phase transition property of the film was studied by terahertz time-domain spectroscopy, which revealed around 70% reduction of transmission at 0.1–1.5?THz in the VO₂ film across the phase transition.     

Microstructures and thermochromic properties of tungsten doped vanadium oxide film prepared by using VOX-W-VOX sandwich structure

Tungsten doped vanadium oxide (VO_X) thin films were prepared by oxygen annealing VO_X-W-VO_X sandwich layers. X-ray photoelectron spectroscopy, X-ray diffraction and field emission scanning electron microscope were employed to characterize the compositions, crystal structures and surface morphologies, respectively. It was demonstrated that sandwich structure suppressed the crystallization of VO_X, and that V⁵⁺ was reduced by diffused W atom to V⁴⁺. The results of surface morphologies indicated that the grain arrangement of W doped vanadium dioxide film exhibited some regular patterns compared with the random grain distribution of undoped film. Electrical measurements showed that the square resistance of V₂O₅ film and semiconductor-metal transition temperature of VO₂-V₂O₅ film decreased obviously after W doping. In addition, thermal hysteresis loop was observed in W doped V₂O₅ film with thick W middle layer. The investigation of optical properties indicated that the optical band gap of W doped V₂O₅ film decreased with the increase of thickness of W middle layer, and the optical switching performance in the near-infrared range of VO₂-V₂O₅ slightly weakened after W doping.     

Synthesis of vanadium oxide nanotubes from V2O5 sols

Vanadium oxide nanotubes (VO_x-NTs) were prepared by using V₂O₅ sols as precursor for the first time with hexadecylamine (HDA) as structure-directing template via a modified sol–gel method followed by hydrothermal treatment. Structure and morphology of the nanotubes were investigated by XRD, SEM, HRTEM, TGA, FT-IR and XPS. The obtained nanotubes were found having outer diameters from 30 to 100 nm and inner diameters from 10 to 30 nm with length up to several micrometers. It is also confirmed that water molecules enter vanadium oxide layers together with structure-directing template and the low valence vanadium occurs in the VO_x-NTs.     

Controlled synthesis and electrochemical properties of vanadium oxides with different nanostructures

Vanadium oxides (V₃O₇·H₂O and VO₂) with different morphologies have been selectively synthesized by a facile hydrothermal approach using glucose as the reducing and structure-directing reagent. The as-obtained V₃O₇·H₂O nanobelts have a length up to several tens of micrometers, width of about 60?C150?nm and thickness of about 5?C10?nm, while the as-prepared VO ₂ (B) nanobelts have a length of about 1·0?C2·7???m, width, 80?C140?nm and thickness, 2?C8?nm. It was found that the quantity of glucose, the reaction temperature and the reaction time had significant influence on the compositions and morphologies of final products. Vanadium oxides with different morphologies were easily synthesized by controlling the concentration of glucose. The formation mechanism was also briefly discussed, indicating that glucose played different roles in synthesizing various vanadium oxides. The phase transition from VO₂(B) to VO₂(M) were investigated and the phase transition temperature of the VO₂(M) appeared at around 68 °C. Furthermore, the electrochemical properties of V₃O₇·H₂O nanobelts, VO₂(B) nanobelts and VO₂(B) nanosheets were investigated and they exhibited a high initial discharge capacity of 296, 247 and 227 mAh/g, respectively.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

Soft chemistry synthesis and crystal structure of MgxCu3−xV2O6(OH)4·2H2O

Mg_xCu_3−xV₂O₆(OH)₄·2H₂O (x ∼ 1), with similar crystal structure as volborthite Cu₃V₂O₇(OH)₂·2H₂O, was successfully prepared by a soft chemistry technique. The method consists of mixing magnesium nitrate and copper nitrate with a boiling solution of vanadium oxide (obtained by reacting V₂O₅ with few mL of 30 vol.% H₂O₂ followed by addition of distilled water). When ammonium hydroxide NH₄OH 10% was added (pH 7.8), a green yellowish precipitate was obtained. Using X-ray powder diffraction data, its crystal structure has been determined by Rietveld refinement. Compared to volborthite, the vanadium coordination changes from tetrahedral VO₄ to trigonal bipyramidal VO₅, and magnesium replaces copper, preferably, in the less distorted octahedron. At 300 °C, the phase formed is similar to the high pressure (HP) monoclinic Cu₃V₂O₈ phase. However at higher temperature, 600 °C, the phase obtained is different from known Cu₃V₂O₈ phases.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.     

Studies on the high-pressure reaction of Er2O3 and VO2

The reaction of erbium sesquioxide (Er₂O₃) and vanadium dioxide (VO₂) at 50 kbar and 30 kbar pressures and 1400°C were studied. In the course of the reaction, tetravalent vanadium ions were reduced to the trivalent state and erbium vanadites (ErVO₃) were obtained. The crystal structures of the products were examined by an x-ray diffraction analysis. At 50 kbar, a vaterite-type (μ-CaCO₃) compound isostructural with ErBO₃, which belonged to a hexagonal system, and at 30 kbar, a calcite-type vanadite belonging to a rhombohedral (pseudo-hexagonal) system were obtained. In the case of the reaction of erbium sesquioxide and vanadium trioxide (V₂O₃) at high pressure, a perovskite-type erbium vanadite was obtained. The magnetic properties of the compounds were studied.     

Polycrystalline films of vanadium oxides of controllable composition

The conditions necessary for preparing polycrystalline stoichiometric vanadium oxide films by the chemical transport of oxygen in a closed system were studied. Films of V₂O₃, V₃O₅ and VO₂ were synthesized on ceramic and sapphire substrates at 500 and 700 °C. An electron diffraction technique was used to control the phase composition. The equilibrium oxygen pressure above the films was shown to be higher than that above powder of the same composition.     

Microstructure and microhardness of vanadium oxides in the range VO<Subscript>0.57</Subscript>-VO<Subscript>1.29</Subscript>

X-ray diffraction and optical microscopy data are presented which demonstrate that substoichiometric vanadium oxide (VO_0.57-VO_0.97) consists of a cubic phase with the B1 structure (sp. gr. Fm \(\bar 3\) m) and an ordered monoclinic phase of composition V₁₄O₆ (sp. gr. C2/m). The content of the latter phase decreases with increasing oxygen content. The superstoichiometric vanadium oxide VO_1.29 is shown to contain trace amounts of V₅₂O₆₄. Vickers microhardness data for nonstoichiometric vanadium oxides in the range VO_0.57-VO_1.29 show that, with increasing oxygen content, their H _V has a tendency to decrease, from 18 to 12 GPa. Their microhardness is shown for the first time to have a maximum near the stoichiometric composition VO_1.00.     

Optical properties of vanadium oxides-an analysis

In this study, the optical properties of bulk and thin films of VO₂, V₂O₃, and V₂O₅, deposited on Al₂O₃ substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12 eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (ε ₂–R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R–E) spectra are explained in terms of the Penn gap (E _p). E _p values for VO₂ and V₂O₅ are close to the average of energies corresponding to the peaks ( $ \bar{E} $ E ¯ ) while, their values are even closer in V₂O₃, reflecting the degree of anisotropy in the order of V₂O₃ < VO₂ < V₂O₅. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V–O systems are studied as an effect of temperature change. The effective number of electrons, n _eff, participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in n _eff with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the ε ₂–E spectra.     

Phase evolution during synthesis of vanadium carbide (V8C7) nanopowders by thermal processing of the precursor

The phase and structure evolution during synthesis of vanadium carbide (V₈C₇) nanopowders by thermal processing of the precursor were investigated using X-ray diffraction (XRD). The morphology of the reactant was characterized by transmission electron microscopy (TEM). Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) were made on the precursor. The results indicate phase evolution sequences are NH₄VO₃→V₂O₅→VO₂→V₅O₉+V₄O₇→V₂O₃→VC_1−X→V₈C₇. The single phase V₈C₇ powders can be prepared at ∼1100 °C for 1 h, and the powders show good dispersion and are mainly composed of uniformly sized spherical particles with the majority diameters of 20-50 nm.     

Hydrothermal synthesis of nanocrystalline VO2 from poly(diallyldimethylammonium) chloride and V2O5

Novel vanadium dioxide nanorods were fabricated from V₂O₅ in the presence of a reducing agent, the poly(diallyldimethylammonium chloride) (PDDA) via a hydrothermal method at 180 °C for 48 h. The samples produced were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (FTIR), nitrogen adsorption (BET) and thermogravimetry (TG/DTG). The nanorods obtained are approximately 50 nm wide and from 300 to 500 nm long and presents high surface area (42 m² g⁻¹). The nanocrystalline B phase VO₂ is not produced by hydrothermal treatment in the absence of the PDDA polyelectrolyte.     

Optical properties of thin films of amorphous vanadium oxides

The results of an experimental investigation of the optical properties of anodic vanadium oxide films are presented. It is shown that films of different phase composition (VO₂, V₂O₅, or a mixture of two phases) can be obtained, depending on the oxidation regime, and that the absorption and transmission spectra are modified significantly in accordance. The optical properties of the oxides, whose composition is close to stoichiometric vanadium dioxide, demonstrate the occurrence of a metal-semiconductor phase transition in the amorphous films. The results presented are important both from the standpoint of technical applications of thin film systems based on anodic vanadium oxides and for more detailed understanding of the physical mechanism of the metal-semiconductor phase transition and the influence of structural disorder on the transition. Pis’ma Zh. Tekh. Fiz. 25, 81–87 (April 26, 1999)     

Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation

We present the structural and physical characterization of vanadium dioxide (VO₂) thin films prepared by reactive electron beam evaporation from a vanadium target under oxygen atmosphere. We correlate the experimental parameters (substrate temperature, oxygen flow) with the films structural properties under a radiofrequency incident power fixed to 50 W. Most of the obtained layers exhibit monocrystalline structures matching that of the monoclinic VO₂ phase. The temperature dependence of the electrical resistivity and optical transmission for the obtained films show that they present thermoelectric and thermochromic properties, with a phase transition temperature around 68 °C. The results show that for specific experimental conditions the VO₂ layers exhibit sharp changes in electrical and optical properties across the phase transition.     

Synthesis and thermal stability of W-doped VO2 nanocrystals

Pure and W-doped vanadium dioxide nanocrystals have been synthesized by using V₂O₅ and oxalic acid as precursors via a thermolysis method. The VO₂ nanocrystals have a nearly spherical morphology with size ranging from 50 to 100 nm. The metal-insulator transition (MIT) temperature of the nanocrystals decreases with increasing W-doping content. The successive heat-induced fatigue character of the MIT in W-doped VO₂ nanocrystals was investigated by DSC analysis together with structural study, and a high stability upon heating–cooling cycles was found with respect to MIT temperature, peak temperature and latent heat of the phase transition.     

The current–voltage characteristics of V2O5/n-Si Schottky diodes formed with different metals

In this work, we reported the effect of different metal contacts on performance of metal–oxide–semiconductor (MOS)-structured Schottky diodes formed with the vanadium pentoxide thin film (V₂O₅) interfacial layer. V₂O₅ thin films were deposited by radio frequency (RF) magnetron sputtering on n-type silicon (n-Si) and Corning glass (CG) substrates at room temperature. Then, the obtained films were annealed at 300 °C and 500 °C. The effects of annealing temperature on physical properties of the films were investigated by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, atomic force microscopy, UV–Vis spectroscopy, and photoluminescence. The MOS-structured Al/V₂O₅/n-Si, Ti/V₂O₅/n-Si and Au/V₂O₅/n-Si Schottky barrier diodes (SBDs) performance was analyzed with I–V measurements at room temperature. The Schottky diodes were compared with each other according to three methods (Classic, Norde and Cheung). The experimental results indicated that the Schottky diode produced with Al contact had better performance than the others.

     

Oxidation potential control of VO2 thin films by metal oxide co-sputtering

For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO₂) phase is essential. In VO₂ films sputter-deposited on a quartz substrate from a V₂O₅ target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO₂. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO₂ is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO₂. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO₂ thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V₂O₅ + TiO₂) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO₂ deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened.