Microstructure study of amorphous vanadium oxide thin films using raman spectroscopy (original) (raw)
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Surface and Interface Analysis, 2005
Vanadium pentoxide films were prepared by pulsed laser deposition (PLD) and their surface structure evolution in relation to the growth temperature and as a function of lithium intercalation was studied. The deposition was made onto various substrate materials over a wide substrate temperature range of 30–500 °C, keeping the oxygen partial pressure at 100 mTorr. The surface properties of V2O5 films were studied by Raman scattering spectroscopy in order to understand the effect of substrate temperature on the structure and growth behavior. The results indicated that the structure of PLD V2O5 films is highly dependent on the growth temperature. The Raman spectra provide additional insight into the nature of the structural changes of lithiated films, showing the appearance of the δ‐ and γ‐phases of LixV2O5. Copyright © 2005 John Wiley & Sons, Ltd.
Microstructure study of amorphous vanadium oxide films
Applied Surface Science, 1999
Conversion films of vanadium oxides are potentiodynamically generated on vanadium in acetate electrolyte systems at high voltages. The microstructure of the about 5 mm thin anodic films is investigated. X-ray diffraction and transmission Ž . electron microscopy indicate the films are complete amorphous. X-ray photoelectron spectroscopy XPS measurements Ž . show V 2p binding energies of mixed valance vanadium sub oxides. Electron spin resonance ESR experiments on 3r2 isolated films at 130 K point to paramagnetic V 4q centers in a disordered octahedral oxygen surrounding. q 1999 Elsevier Science B.V. All rights reserved.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2004
The VO 2 multiphases such as V 2 O 5 , VO 2 , and V 2 O 3 are confirmed and the correlations between structural characteristics and growth conditions was investigated using the scanning electron microscopy ͑SEM͒, transmission electron microscopy ͑TEM͒, x-ray diffraction ͑XRD͒, and x-ray photoelectron spectroscopy ͑XPS͒. Also, the electrical characteristics of VO 2-based three terminal devices, attributed to structural and phase changes, are discussed. The spectra of VO 2 have three peaks composed of VO 2 at binding energy (BE)ϭ516.2 eV, V 2 O 3 at BEϭ515.6 eV, and V 2 O 5 at BEϭ517.0 eV. With increase in the growth temperature, crystal quality of VO 2 films improves and approaches single phase of VO 2 , then the peak position shifts to the spectra of oxygen-poor phase (V 2 O 3). With increase in the O 2 flow, the peak position shifts to the spectra of oxygen-rich phase (V 2 O 5). VO 2 films grown at optimal growth conditions have a change in resistivity of the order of 10 2 near a critical temperature, T c ϭ340 K.
Electrochimica Acta, 2006
In order to produce thin films of crystalline V 2 O 5 , vanadium metal was thermally oxidised at 500 • C under oxygen pressures between 250 and 1000 mbar for 1-5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V 2 O 5 films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O 2 at 500 • C resulted in a surface oxide film composed of V 2 O 5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V 2 O 5 oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 A/cm 2 . The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10 −13 cm 2 s −1 , respectively. Beneath the V 2 O 5 layer, lower oxides (mainly VO 2 ) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V 4+ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V 2 O 5 and V 6 O 13 was found in the oxide film.
Microstructural changes of amorphous V 2 O 5 films with lithium intercalation are studied using Raman-scattering measurements. The Raman spectra of as-deposited films show two broad peaks around at 520 and 650 cm Ϫ1 , due to the stretching modes of the V 3 -O and V 2 -O bonds, respectively, and a relatively sharp peak at 1027 cm Ϫ1 due to the V 5ϩ vO stretching mode of terminal oxygen atoms. In addition, there is a peak at 932 cm Ϫ1 that we attribute to the V 4ϩ vO bonds. Comparison of the Raman spectra of V 2 O 5 films with different oxygen deficiencies confirms this assignment. This Raman peak due to the stretching mode of the V 4ϩ vO bonds develops and shifts toward lower frequencies with increasing lithium concentration. Comparison to results from gasochromic hydrogen insertion indicates that the 932 cm Ϫ1 Raman peak is not a result of vibrations which involve Li or H atoms. We propose that the V 4ϩ vO bonds are created by two different mechanisms: a direct conversion from V 5ϩ vO bonds and the breaking of the single oxygen bonds involving V 4ϩ ions.
Chemistry of Materials, 2008
Raman microspectrometry has been used to investigate the local structural changes induced by the electrochemical lithium intercalation reaction in crystalline sputtered V 2 O 5 thin films in a liquid electrolyte. Contrary to usual composite electrodes made of a mixture of active material and conductive and binding agents, the use of a pure V 2 O 5 thin film allows a homogeneous Li insertion in the material and a high quality of Raman signatures to be obtained. The Raman spectra of Li x V 2 O 5 compounds for 0 < x < 1 are examined as a function of the lithium content and discussed in relation with the X-ray diffraction data pertinent to these h00-oriented thin films and literature data. An assignment of all Raman bands is proposed, and the Raman fingerprint of the-type phase, whose interlayer distance continuously increases with x, is clearly evidenced all along the Li insertion process: lithium ions rapidly produce an orthorhombic phase characterized by a vanadyl stretching mode at 984 cm-1 for 0 < x < 0.5, and further Li accommodation induces a splitting into two stretching modes, the first one shifting from 984 to 975 cm-1 , the second from x) 0.7 located at a fixed wavenumber of 957 cm-1. Both modes are consistent with the local structure of the lithium-rich phase called ′ and reflect the existence of two different lithium sites. This work illustrates that the structural changes, in terms of long-range order and local structure, are strongly dependent on the microstructure and morphology of the material.
Electrical and optical properties of hydrated amorphous vanadium oxide
Journal of Physics D: Applied Physics, 2008
Electrical and optical properties of amorphous vanadium oxide thin films obtained by electrochemical anodic oxidation are studied. It is shown that under cathodic polarization the hydrogen insertion into vanadium oxide from an electrolyte occurs. Metal-insulator transition in amorphous HxVO2 is found to be preserved up to high concentration (x ~ 1.5) of hydrogen. Memory switching with the N-type negative differential resistance, associated with the H + ionic transfer, is observed in "V/hydrated amorphous vanadium oxide/Au" sandwich structures.
Influence of deposition temperature on the growth of vacuum evaporated V2O5 thin films
Materials Letters, 2003
V 2 O 5 films were deposited on silicon (111) substrates by vacuum evaporation technique at various deposition temperatures of 300, 473, 573, 623 and 673 K. X-ray characterization revealed that the films deposited at T s V 473 K are amorphous and the film deposited at T s z 573 K is polycrystalline. It is interesting to note that the film deposited at T s = 573 K is strongly oriented with (001) planes parallel to the substrate and the degree of preferred orientation towards (001) planes found to decrease with further increase in the deposition temperature. The influence of deposition temperature on the growth of the V 2 O 5 films has been studied by Raman scattering spectroscopy. The films deposited on the silicon substrates maintained at 573 K are found to have better structural quality. D
Keywords: V2O5 thin films Thermo-optical effect XRR GIXRD Optical properties Raman spectra a b s t r a c t Crystalline V 2 O 5 films of thicknesses in the range: 78–119 nm were prepared on glass sub-strates by thermal evaporation with in-situ substrate heating at 350 • C. XRD studies found the growth of highly oriented orthorhombic V 2 O 5 films with two peaks corresponding to (001) and (002) crystal planes. Raman studies also confirmed the formation of orthorhom-bic V 2 O 5 phase. Electrical studies by two-probe study showed an increase in conductivity with temperature indicating the semiconducting nature of the samples. Optical studies found that UV–vis transmittance is maximum and the reflectance is minimum in film with lowest thickness. The values of activation energy for conduction are 0.15 eV, 0.16 eV and 0.19 eV and those of optical band gap are 2.85 eV, 2.63 eV and 2.67 eV for films of thicknesses: 78 nm, 88 nm and 119 nm respectively. Grazing incidence X-ray reflectivity studies on the film samples showed that the surface roughness increases with increase in thickness and that the density of the samples is 70.6% of that of the bulk V 2 O 5. In-situ high temperature Raman studies up to 60 • C found that the Raman bands at 283 cm −1 and 305 cm −1 broaden considerably and decrease in intensity with increase in temperature; these bands however recover their original shape on cooling. Both peak broadening and recovery effects occur with time lag of ∼30 min and ∼20 min respectively. A decrease in average UV–vis-NIR transmission of ∼1% was observed with increase in temperature, and this thermo-optical effect is reversible.
Nanostructural modifications of V2O5 thin films during Li intercalation studied in situ by AFM
Electrochemistry Communications, 2007
EC-AFM was used to study in situ the surface nanostructure of V 2 O 5 thin films grown on vanadium metal and its changes during lithium electrochemical intercalation corresponding to the reversible a-to-d phase transition. The results evidence the lateral extension and contraction of the oxide nanograins and the flattening of the oxide film surface. The increase and decrease of the lateral dimensions of the grains show that the surface reflects the volume expansion and contraction resulting from the dimensional changes of the oxide structure when lithium is inserted and de-inserted. An increase of 1010% and 1015% with respect to pristine oxide film was observed after the formation of the e and d phases, respectively. The lateral extension of the grains ($7%) and surface flattening subsist after de-intercalation showing the non-fully reversible change of the oxide nanostructure at the interface with the electrolyte. Repeated cycling causes aging characterized by the amplification of the lateral extension of the grains ($17%) and flattening of the oxide surface with respect to the pristine oxide film. It also modifies the surface of the oxide grains by creating new planes indicative of surface reconstruction and/or the emergence of slip planes.