Structural and transport properties of Li-intercalated vanadium pentoxide nanocrystalline films

The X-ray diffraction (XRD), transmission electron microscopy, density, electrical and thermoelectric power (TEP) properties of nanocrystalline Li _x V₂O₅ ? nH₂O xerogel films (0 ≤ x ≤ 22 mol.%) were investigated. The films were produced by the sol–gel technique (colloidal route), which was used to enable high-purity, uniform preparation. The relative intensity of the (002) XRD line increased with increasing Li content. The particle size was found to be about 6.0 nm. Electrical conductivity and thermoelectric power were measured parallel to the substrate surface in the temperature range 300–480 K for the as-prepared films. The electrical conductivity showed that all the samples were semiconductors and that conductivity increased with increasing Li content. The conductivity of the present system was primarily determined by hopping carrier mobility, which was found to vary from 6.81 × 10^?6 to 0.33 × 10^?6 cm² V^?1 s^?1 at 380 K. The carrier density was evaluated to be 8.73 × 10¹⁹–1.118 × 10²¹ cm^?3. The conduction was confirmed to obey non-adiabatic small polaron hopping. The thermoelectric power, or Seebeck effect, increased with increasing Li content. The results obtained indicate an n-type semiconducting behavior within the temperature range investigated.     

Characterization and transport properties of intercalated polypyrrole-vanadium pentoxide xerogel nanocomposite

Layered vanadium pentoxide (V₂O₅) xerogel is used as host material for the synthesis of conducting polypyrrole intercalated nanocomposites. Powder X-ray diffraction spectra show the increase of interlayer space of V₂O₅ xerogel indicating the formation of polypyrrole in the interlayer space of the host. Thermal stability of intercalated polypyrrole is superior compared to bulk polypyrrole. Dc conductivity of the xerogel follows Arrhenius type temperature dependence while the nanocomposites exhibit Mott's three-dimensional variable range hopping. Optical band gap decreases with intercalation of polypyrrole.     

Local structure study of vanadium pentoxide 1D-nanostructures

Vanadium pentoxide (V₂O₅·nH₂O) 1D-nanostructures as nanowires and nanorods have been obtained by decomposition of vanadium peroxide in hydrothermal conditions. Electron microscopy, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) were employed to characterize the morphology and the local structure of as-obtained samples. Scanning transmission electron microscopy (STEM) revealed that the diameter of the nanowires and nanorods were found to be 10–20 and 30–40 nm, respectively. The results demonstrated that a combination of Raman and XAS techniques allowed the accurate characterization of the local structure of V₂O₅ 1D-nanostructures which are related to different morphologies. Analyses of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra reveals that the local structure of V in the as-obtained samples is similar to the bulk V₂O₅ (in orthorhombic phase), except for a higher degree of local symmetry within the structure of the VO₅ square pyramid. Additionally, the nanostructures prepared by this technique present a single crystalline nature and could emit visible light at room temperature which is related to the local order of V atoms of the studied samples.     

Effect of vanadium pentoxide on the electrical, dielectric, and optical properties of poly(vinyl alcohol)/vanadium pentoxide nanocomposites

In this study, the effect of vanadium pentoxide (V₂O₅) on the electrical, dielectric, and optical properties of poly(vinyl alcohol) (PVA)/V₂O₅ nanocomposites was examined. The PVA/V₂O₅ nanocomposites were prepared by solution mixing, followed by film casting. The as-prepared films were characterized by X-ray diffraction, thermogravimetric analysis, as well as impedance spectroscopy. The obtained results indicated that, with increasing V₂O₅ content, the PVA composite exhibits considerable vestige of crystallization and the PVA peaks become broader. The electrical conductivity, σ _dc, increases as the temperature and the dopant level of V₂O₅ increase. The frequency dependence of alternating current conductivity was governed by a critical frequency, ω _p. The values of ω _p are found to be thermally activated. The loss tangent (tan δ) shows a peak with the frequency, which indicates a dielectric relaxation in these composites with relaxation time decreases with both temperature and V₂O₅ content increase. Optical absorption edge and direct and indirect energies for all mentioned samples were determined and discussed.     

Kinetic studies of laser-induced synthesis of thin vanadium pentoxide films

The kinetics of Ar⁺ laser-induced oxidation of 100 nm thick vanadium films on glass substrates is followed by measuring time-dependent changes in reflected and transmitted intensity of a He-Ne probe beam. The growth rate of the vanadium pentoxide layer increases with increasing laser powerP as = ₀ exp(-a/P). At power densities above kW/cm² vanadium pentoxide crystallizes from the melt.     

Electrical characterization and Raman spectroscopy of individual vanadium pentoxide nanowire

We measured I–V characteristics, electrical resistance, and Raman spectra in the temperature range from room temperature to above 600 K to obtain nanodevices. Measurements were taken on a single V₂O₅ nanowire deposited on a Si template, where two- and four-point metallic contacts were previously made using e-beam lithography. In both two- and four-point probe measurements, the I–V curves were clearly linear and symmetrical with respect to both axes. Drastic reduction in electrical resistance and deviation from single valued activation energy with increasing temperature indicated phase transitions taking place in the nanowire. From temperature-dependent HR-Micro Raman measurements, reductions from V₂O₅ to VO₂/V₂O₃ phases took place at a temperature as low as 500 K, when electrons were injected to the nanowire through electrical contacts.     

Phase transition kinetics of vanadium pentoxide II. Theoretical results

Experimental data on the phase transformation kinetics in vanadium pentoxide due to surface oxygen loss are analyzed theoretically. A model for the process as a one-dimensional problem with oxygen loss from the surface and coupled interface and diffusion controlled growth modes is described. This model appears to match well the experimental data with reasonable numbers for the surface loss rate and diffusion constant. In particular, the model reproduces changes in the number of phase fronts as a function of electron beam flux. In addition, the analysis confirms that the effective diffusion constant is electron beam flux dependent.     

Charge transport mechanism in pressed pellets of polymer proton conductors

The charge transport inside some polymer proton conductors, prepared by chemical doping of poly(propargyl alcohol) and poly(p-diethynylbenzene) with acids, or by electrochemical polymerisation of thionaphtheneindole, has been investigated. All the systems, studied as powder pressed pellets between two gold electrodes, show very low electrical conductivity due to electrons motion. In presence of environmental humidity the conductivity increases dramatically as a consequence of proton migration through the material. A mechanism for the charge transport through the materials, based on the proton conductivity of a thin layer of acid aqueous solution adherent to the polymer grains, has been hypothesised. Since the investigated doped polymers show a reversible conductivity variation as a function of water content, they are proposed as materials for humidity sensor construction.     

Charge transport near pressure-induced antiferromagnetic quantum critical point in Magnéli-phase vanadium oxides

Resistivity (ρ) measurements on Magnéli phases V₇O₁₃ and V₈O₁₅ were performed under high pressures up to 3.5GPa. We have achieved a pressure-induced transition from an antiferromagnetic metal to a paramagnetic metal (PM) at critical pressures P_c≈3.4 and 3.3 GPa for V₇O₁₃ and V₈O₁₅, respectively. The critical behavior of ρ(T) near P_c turned out to be quite unusual in that no noticeable precursor effect was observed. This strongly contrasts with the canonical quantum critical point behavior observed in chemically modified systems such as Ni(S,Se)₂ and V₂O₃. We propose that the presence of two distinct Fermi surface segments is responsible for the observed unusual behaviors.     

A biaxial nematic gel phase in aqueous vanadium pentoxide suspensions

Aqueous suspensions of V₂O₅ ribbons are one of the very few examples of mineral liquid crystals. In the concentrated regime, we show that these ribbons organize in a biaxial nematic gel phase. A Couette shear cell was used to produce a well oriented sample for in situ synchrotron X-ray scattering studies. We observed two perpendicular anisotropic sections of reciprocal space, which proves the biaxial symmetry of the nematic order. The thermodynamic and flow properties of the biaxial nematic are well described by hard-core theories. We suggest the use of a shear geometry to produce and investigate single domains of other biaxial nematics, reported but still questioned in the literature. Received 26 May 1999     

Proton and electron conduction in polymer intercalated vanadium pentoxide xerogel

A comprehensive proton and electron conduction in vanadium pentoxide xerogel-conducting polymer nanocomposites are analyzed by the impedance spectroscopy. The complex impedance plots are used to estimate the proton and electron conductivity. Both protonic and electronic conductivities follow Arrhenius type behavior with temperature. Protonic conductivity increases with the increase of intercalated conducting polypyrrole. The highly ordered conducting polypyrrole (PPY) molecule enhances the diffusion of H₃O⁺ ions into the host lattice. Broad dielectric loss peaks are found in a wide range of frequency domain (KHz–MHz). The intercalated water within the oxide network of vanadium pentoxide remains supercooled up to lowest measured temperature 137 K.     

X- and gamma-ray irradiation effects on vanadium pentoxide thin films

Thickness and composition of thin films can be measured with X- and gamma-rays. In this work, thickness and composition of vanadium pentoxide thin films are investigated by energy dispersive and wavelength dispersive X-ray fluorescence systems. Also, the surface analysis of vanadium pentoxide thin films irradiated with Rhodium Kα X-rays and 59.54?keV gamma-rays emitted from 100?mCi and 5?Ci Americium-241 radioactive sources is performed by scanning electron microscope. It is observed that X- and gamma-rays are destructive for vanadium pentoxide thin films. Also, the composition of vanadium pentoxide thin films changes by irradiation with X- and gamma-rays.     

Charge transport mechanism in metal-Tb2GeS5-Ge structure

Structures of the metal-amorphous film of an insulator (or semiconductor)-semiconductor type are being used more and more widely in electronics. Interest therefore attaches to an investigation into the characteristics of such structures based on new insulating and semiconducting materials. In the present paper, the charge transport mechanism in aluminum-amorphous film of Tb₂GeS₅-germanium structures, which can generate an alternating current in the rf range, is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1981.