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We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi[subscript 0.65]Fe[subscript 0.35]O[subscript 3-x] film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe[superscript 4+]) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The k[subscript chem] values obtained by OTR were significantly lower than the AC-IS derived k[subscript chem] values and k[subscript q] values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchemand kqvalues obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchemvalues. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films. Keywords: Thin films; perovskite; mixed ionic and electronic conductor; oxygen surface exchange; optical absorption; impedance spectroscopy, United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0002633)

The effect of an applied overpotential on oxygen isotope incorporation and diffusion in oxide thin film electrodes is investigated by a novel experimental approach. A special electrode geometry leads to in-plane electron flow, perpendicular oxide ion flow and a well-defined laterally varying driving force. This design allows one to obtain a series of tracer depth profiles induced by a range of overpotentials on one and the same thin film. The approach was applied to La0.8Sr0.2MnO3(LSM) thin films deposited by pulsed laser deposition (PLD) on yttria stabilized zirconia (YSZ) single crystals. Tracer depth profiles were measured by secondary ion mass spectrometry (SIMS). These depth profiles include examples of pronounced apparent uphill diffusion that can be explained by considering an interplay of polarization-induced changes in stoichiometry within the LSM grains combined with fast oxygen transport along the grain boundaries., United States. Department of Defense. Basic Energy Sciences (grant No. DE-SC0002633)

Both polycrystalline and single-crystal films of iron-substituted strontium titanate, Sr(Ti0.65Fe0.35)O3-delta, prepared by pulsed laser deposition, show room-temperature magnetism and Faraday rotation, with the polycrystalline films exhibiting higher saturation magnetization and Faraday rotation. The magnetic properties vary with the oxygen pressure at which the films are grown, showing a maximum at pressures of approximately 4 mu Torr at which the unit-cell volume is largest. The results are discussed in terms of the oxygen stoichiometry and corresponding Fe valence states, the structure and strain state, and the presence of small-volume fractions of metallic Fe in single-crystal films grown at the optimum deposition pressure. Integration of magneto-optical polycrystalline films on an optical-waveguide device demonstrates a nonreciprocal phase shift., JST PRESTO, JSPS Postdoctoral Fellowships for Research Abroad; NSF; FAME; DARPA; MARCO; Center for Development of Nanoscience and Nanotechnology, CEDENNA, Chile; Fondecyt; DGIIP USM, Chile; JSPS; Center for Materials Science and Engineering (CMSE)

Electrospun polyaniline (PAni) fibers doped with different levels of (+)-camphor-10-sulfonic acid (HCSA) are fabricated and evaluated as chemiresistive gas sensors. The experimental results, based on both sensitivity and response time, show that doped PAni fibers are excellent ammonia sensors and that undoped PAni fibers are excellent nitrogen dioxide sensors. The fibers exhibit changes in measured resistances up to 60-fold for ammonia sensing, and more than five orders of magnitude for nitrogen dioxide sensing, with characteristic response times on the order of one minute in both cases. A time-dependent reaction-diffusion model is used to extract physical parameters from fitting experimental sensor data. The model is then used to illustrate the selection of optimal material design parameters for gas sensing by nanofibers., Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract ARO W911NF-07-D-0004)

International audience; Densification and sintering of CeO2 nanoparticles and their electrical properties were simultaneously studied as a function of temperature in controlled atmosphere using a modified dilatometer. CeO2 nanoparticles simultaneously shrink and become more resistive upon initial heating, associated with desorption of water. The electrical conductance G at 300-550 A degrees C revealed a pO(2) dependence described by log (G) = A + n x log (pO(2)) with n similar to -1/6, consistent with n-type conduction. The results were analyzed with a defect equilibrium model based on the reduction of ceria and formation of doubly ionized oxygen vacancies and electrons. The activation energy was found equal to (1.3 A +/- 0.1) eV, which results in an enthalpy of reduction of (2.7 A +/- 0.4) eV, considerably lower than that for bulk ceria (similar to 4.5 eV). The coarsening of particles during heat treatment at 800 A degrees C were analysed assuming grain boundary diffusion-limited sintering. Although the coarsened powder shows a similar pO(2) dependence, the activation energy was considerably higher (1.9 A +/- 0.1) eV, leading to a reduction enthalpy of (4.5 A +/- 0.4) eV. The decrease in the enthalpy of reduction with decreasing particle size is consistent with the increasing fraction of oxide ions residing at the surface. Alternate interpretations based on space charge effects and surface adsorption/desorption were considered and found to be less consistent with the experimental results.

One of the top design priorities for semiconductor chemical sensors is developing simple, low-cost, sensitive and reliable sensors to be built in handheld devices. However, the need to implement heating elements in sensor devices, and the resulting high power consumption, remains a major obstacle for the realization of miniaturized and integrated chemoresistive thin film sensors based on metal oxides. Here we demonstrate structurally simple but extremely efficient all oxide chemoresistive sensors with ~90% transmittance at visible wavelengths. Highly effective self-activation in anisotropically self-assembled nanocolumnar tungsten oxide thin films on glass substrate with indium-tin oxide electrodes enables ultrahigh response to nitrogen dioxide and volatile organic compounds with detection limits down to parts per trillion levels and power consumption less than 0.2 microwatts. Beyond the sensing performance, high transparency at visible wavelengths creates opportunities for their use in transparent electronic circuitry and optoelectronic devices with avenues for further functional convergence., Korea (South). Ministry of Intelligence and Economy (Contract K0004114), Korea Institute of Science and Technology (Grant 2E22121), Korea (South). Ministry of Environment (Research Program)