X-ray absorption spectroscopy of vanadium dioxide thin films across the phase-transition boundary

Abstract

X-ray absorption spectroscopy ͑XAS͒ and x-ray photoemission spectroscopy of the V L edge and O K edge were performed on VO 2 thin films rf sputtered at various conditions. The spectra give evidence of the changes in the electronic structure depending on the film quality. XAS of the O K edge shows a decrease of the spacing between 3d and 3d bands by 0.8 eV with concurrent broadening of both bands for the sample sputtered at lower substrate temperature and consequently having more polycrystalline and disordered character. 3d band position appears to be more sensitive to the sample quality, indicating that the cation-ligand interaction is mostly affected likely due to the distortion of the local O coordination surrounding a V ion. The observed variation of the spectra in films of different morphologies may reflect the changes of the density of states responsible for the considerable variation of the metal-insulator transition ͑MIT͒ properties reported for VO 2 thin films synthesized at different conditions. The study of the temperature dependence of the XAS spectra including repeated measurements across the MIT revealed both reversible and irreversible V Ledge and O K-edge changes. The thermal cycling of the VO 2 films through the MIT shows irreversible shifts of the conduction bands toward lower photon energies apparently caused by the sample deterioration due to the lattice transformations at the MIT. The signature of a phase transition in a VO 2 film at MIT temperature ͑T MIT ͒ is clearly seen in the XAS O K-edge spectra which show reversible switches of the 3d and 3d bandwidths by approximately 20% depending on the sample being above or below T MIT .

Optical properties of VO2 films at the phase transition: Influence of substrate and electronic correlations

“…1(d). In the insulating side the conductivity follows an activated behavior with ∆E = (256 ± 4) meV; these values are in excellent agreement with previous studies [22,27]. With a mobility of µ n = 0.1 cm 2 /Vs we can determine the charge carrier density N c = 5 × 10 23 cm −3 in accordance with Hall measurements [27].…”

“…on the physical properties has been subject of recent studies [21][22][23][24][25][26]. Here we measured the infrared optical properties of VO 2 films and compared them with numerical results from density functional theory.…”

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5/TiO2 catalyst

“…Only two distinct resonances are observed in the spectrum: the main resonance at 517.8 eV and well-distinguished shoulder at 515.9 eV. A similar line shape has been reported for bulk VO 2 [70,72,73]. Moreover, according to the correlation between the oxidation state and the L 3 -edge peak position of binary vanadium oxides, this resonance corresponds to the V 4+ state [74,75].…”

Resistance switching of epitaxial VO2/Al2O3 heterostructure at room temperature induced by organic liquids

“…38,[44][45][46] Figs. 5(c)-5(e) show the Lorentzian-Gaussian fits of V 2p peaks for the VO 2 film without polar liquids, with cyclohexane, and with ethylene glycol, respectively.…”