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Publications (5)16.42 Total impact

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    ABSTRACT: Substoichiometric tungsten oxide films of approximately 10 nm thickness deposited with pulsed laser ablation on single-crystal TiO 2 substrates with (001) and (110) orientation show defect states near the Fermi energy in the valence-band X-ray photoelectron spectroscopy (XPS) spectra. The spectral weight of the defect states is particularly strong for the film grown on the (001) surface. In situ XPS under an oxygen pressure of 100 mTorr shows that the spectral weight of the defect states decreases significantly at 500 K for the film on the (110) substrate, whereas that of the film grown on the (001) substrate remains the same at a temperature up to 673 K. Furthermore, diffusion of titanium from the substrate to the film surface is observed on the (110) substrate, as is evidenced by the sudden appearance of the Ti 2p core level signature above 623 K and below 673 K. The film grown on the (001) surface does not show such an interdiffusion effect, which suggests that the orientation of the substrate can have a significant influence on the high-temperature integrity of the tungsten oxide films. Quantitative analysis of the O 1s core level XPS spectra shows that chemisorbed water from sample storage under ambient conditions is desorbed during heating under oxygen exposure. ■ INTRODUCTION The function of modern electric devices depends more and more on heterostructured architectures of materials and components. 1 For example, in multijunction solar cell electro-des, various layers of semiconductors are stacked in order to enhance their efficiency. 2 On top of the general layout of such heterostructures, the processing steps can have a significant, sometimes even decisive, influence on the functionality of the components and devices. 3 It has been highly regarded that the electronic structure is the key to relevant properties for materials such as electrodes, current collectors, and solid electrolytes or barrier layers. We demonstrate in this work how defect states in the electronic structure of tungsten oxide films grown on TiO 2 single crystals with two different crystallo-graphic orientations behave upon oxidative thermal treatment. TiO 2 is an ultraviolet-absorbing wide-band-gap semiconductor
    Chemistry of Materials 08/2012; · 8.24 Impact Factor
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    ABSTRACT: Anodization of α-Fe(2)O(3) (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core-level X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies, in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra, however, are shifted towards the opposite direction, that is, towards the Fermi energy, suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential.
    ChemPhysChem 06/2012; 13(12):2937-44. · 3.35 Impact Factor
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    ABSTRACT: The BaCe0.8Y0.2O3-{\delta} proton conductor under hydration and under compressive strain has been analyzed with high pressure Raman spectroscopy and high pressure x-ray diffraction. The pressure dependent variation of the Ag and B2g bending modes from the O-Ce-O unit is suppressed when the proton conductor is hydrated, affecting directly the proton transfer by locally changing the electron density of the oxygen ions. Compressive strain causes a hardening of the Ce-O stretching bond. The activation barrier for proton conductivity is raised, in line with recent findings using high pressure and high temperature impedance spectroscopy. The increasing Raman frequency of the B1g and B3g modes thus implies that the phonons become hardened and increase the vibration energy in the a-c crystal plane upon compressive strain, whereas phonons are relaxed in the b-axis, and thus reveal softening of the Ag and B2g modes. Lattice toughening in the a-c crystal plane raises therefore a higher activation barrier for proton transfer and thus anisotropic conductivity. The experimental findings of the interaction of protons with the ceramic host lattice under external strain may provide a general guideline for yet to develop epitaxial strained proton conducting thin film systems with high proton mobility and low activation energy.
    11/2011;
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    ABSTRACT: INTRODUCTION The structure and physicochemical properties of tungsten oxide have sparked interest by scientists and engineers for a long time because of its relevance for technological applications as photoelectrochemical cell anodes, gas sensors, and photochro-mic materials, for example. 1,2 On the basis of a study of defect-free and altered WO 3 surfaces with photoelectron spectroscopy, it has been speculated that the properties of WO 3 depend critically on the defect states in the bulk and on the surface, rather than on its intrinsic nature. 3 An early bulk and surface specific electronic structure study of WO 3 is presented in ref 4, where in particular it was found that the perfect (100) surface has no defect states in the band gap. WO 3 belongs to a class of materials that can tolerate deviations from stoichiometry by adjustment of oxygen octahedral using shear mechanisms, without formation of oxygen vacancies. 3 This was already earlier suggested by Matthias' observation that WO 3 single crystals were remarkably soft. 5 In the oxygen octaheder, the e g orbitals (d x 2 Ày 2, d z 2) of W orient directly to the oxygen atoms, whereas the t 2g orbitals (d xy , d yz , d zx) point between the coordinating ligands. 4 The corresponding calculated density of states is illustrated in Figure 3 in ref 4. The defect free (100) surface has a barely noticeable small density of states in the pÀd band gap, but surface states can be induced by creating oxygen Received: March 13, 2011 Revised:
    The Journal of Physical Chemistry C 07/2011; 115:16411-16417. · 4.84 Impact Factor
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    ABSTRACT: An approximately 125 nm thick pulsed laser deposited blue, nonstoichiometric WO3−δ film grows on TiO2 (110) in the [220] direction. Oxidative treatment at 400 °C turns the film color from blue to yellow and improves the film quality considerably, as shown by improvement of the Kiessig oscillations in the X-ray reflectometry curves. Detailed analysis of resonant valence band photoemission spectra of the as-deposited nonstoichiometric blue film and oxidized yellow film suggests that a transition near the Fermi energy originates from the nonstoichiometry, i.e., oxygen deficiency, and insofar poses electronic defect states that partially can be eliminated by heat treatment in oxygen. The defects of the as-deposited blue film seem to be located throughout the film, except for the top surface due to exposure to oxygen in ambient air. Thermal after-treatment under oxygen heals the defects in the bulk, whereas residual defect states appear to remain near the film–substrate interface. Potential strain at the substrate–film interface due to lattice mismatch may be one origin for the remanence of the defect states in the bulk.
    Journal of Physical Chemistry C. 01/2011; 115(33):16411-16417.