Browsing by Author "Cabrera, A. L."

Now showing 1 - 7 of 7
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    Band gap determination in multi-band-gap CuFeO2 delafossite epitaxial thin film by photoconductivity
    (2019) Vojkovic, S.; Fernandez, J.; Elgueta, S.; Vega, F. E.; Rojas, S. D.; Wheatley, R. A.; Seifert, B.; Wallentowitz, S.; Cabrera, A. L.
    The photoconductivity within a wavelength range of 450-1100 nm was determined for a sample of epitaxial delafossite CuFeO2 film grown by pulsed laser deposition. The film thickness was estimated to be 75 nm. The resistance of the films was determined with four-contact van der Pauw's method and using monochromatic illumination of the film. The most significant change in resistance resulted in three rapid lineal conductivity increases at photon energies of similar to 1.5 eV (gap-1), similar to 2.1 eV (gap-2) and similar to 2.5 eV (gap-3). The conductivity properties are well correlated with prior optical absorption results obtained in the NIR-VIS region using transmittance spectroscopy.
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    Hydrogen absorption absorption by metallic thin films detected by optical transmittance measurements
    (PERGAMON-ELSEVIER SCIENCE LTD, 2010) Cabrera, A. L.; Avila, J. I.; Lederman, David
    The hydrogen absorption by bilayers of Pd/Nb and Pd/Ti, grown on glass substrates, was studied by measuring changes in optical transmittance and reflectance in the visible range (wavelengths between 400 nm and 1000 nm) of the films at hydrogen pressures between 3.99 x 10(2) and 4.65 x 10(4) Pa. The electrical resistance of the films was also measured during absorption to correlate with the optical data. All the films were grown by a controlled sputtering technique in high vacuum. Pd films ranging in thickness between 4 nm and 45 nm were also characterized when the films were exposed to a hydrogen pressure. The resistance and transmittance of all the Pd samples increased with the uptake of hydrogen until saturation occurred. For Pd/Ti bilayers, fast uptake of hydrogen was deduced from a transmittance increase, indicating hydrogen absorption in the Ti layer. In the case of the Pd/Nb bilayer, a decrease in transmittance was observed, indicating that hydrogen was not absorbed in the Nb layer. The transmittance decrease could be explained by a reduction of Nb native oxide by the hydrogen at the surface. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
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    Hydrogen adsorption study on nanostructured Ag-Rh films grown by supersonic cluster beam deposition
    (2023) Vega, F.; Fernandez, J.; Elgueta, S.; Cavaliere, Emanuele; Gavioli, Luca; Cabrera, A. L.
    Following recent literature work, an interesting result on hydrogen sorption by Ag-Rh clusters was reported which do not follow the common understanding of hydrogen ab-sorption in bulk alloys. Thus, nanostructured Ag-Rh films with a composition of 75% Ag-25% Rh were grown by supersonic cluster beam deposition onto microbalance quartz crystals in order to measure hydrogen absorption. The films were exposed to H-2 gas to study hydrogen absorption at different partial pressure. The Ag-Rh films were then characterized by XPS and AFM. The Ag/Rh atomic ratio was found to be 75/25 by XPS. Surface roughness was determined by AFM to be 3 nm RMS in all the films. In disagreement with the quoted literature report, no hydrogen absorption was observed in our Ag-Rh films up to 5:3 x 10(3)Pa of hydrogen pressure. The response of the balance was checked and validated by measuring hydrogen absorption on 10 and 100 nm Pd films. The hydrogen absorption by Pd yielded an atomic H/Pd ratio of similar to 0.65 in agreement with our prior measurements.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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    Hysteresis in the rhombohedral-orthorhombic phase transition of KNbO3 under inhomogeneous strain
    (2013) Baier-Saip, J. A.; Gutierrez, M. H.; Cabrera, A. L.; Baier, P. A.
    The influence of the heating rate on the low temperature phase transition (PT) of the piezoelectric crystal potassium niobate (KNO) was studied by micro-Raman spectroscopy. It is found that crystallographic defects are more important than the heating rate for the onset of the PT. If the strain shifts the transition temperature (TT) from the rhombohedral to the orthorhombic phase to lower values, then it also shifts the reverse PT to higher temperatures. The PT on heating is more sensitive to the rate than the PT on cooling. (C) 2012 Elsevier Ltd. All rights reserved.
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    Intra-cavity laser-assisted solar-energy conversion
    (2023) Jimenez, I.; Wallentowitz, S.; Seifert, B.; Volkmann, U. G.; Diaz-droguett, D. E.; Cabrera, A. L.; Gence, L.
    It is shown how to efficiently convert solar into electrical energy, taking advantage of laser amplification and intracavity use of a low-efficiency converter. The latter may consist of a low-efficiency transparent photovoltaic cell or a thermoelectric cell integrated into a metallic laser-cavity mirror, constituting a minor intra-cavity loss for the laser operation. The overall power-conversion efficiency is derived and discussed for a variety of current solid-state laser materials. It is shown that power-conversion efficiencies comparable with commercial silicon photovoltaic cells are obtained with current laser materials. & COPY; 2023 Optica Publishing Group under the terms of the Optica Open Access
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    Modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O3 thin films induced by CO2 adsorption
    (2011) Ramos-Moore, E.; Lederman, D.; Cabrera, A. L.
    We report on the modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O-3 thin films under the adsorption of CO2 gas. The samples were exposed to the gas in ultra high vacuum while different voltages between the top (Ag) and bottom (Pt) electrodes were applied. After dosing, the samples were heated from room temperature at 1.8 K/s, and a mass spectrometer was used to detect the desorbed molecules. The effective molecular sticking was demonstrated by the appearance of the carbon Auger peak in the surface of the sample and by the presence of CO2 in the desorption curves. After +(-) 9.9 V were applied to the bottom electrode during dosing, we found a shift of 0.52 (+0.58) mu C/cm(2) in the remnant polarization. These evidences suggest the existence of a depolarizing field induced by molecular adsorption at the surface of the top electrode, and contribute to highlight the potential use of ferroelectric thin films as gas sensors. (C) 2011 Elsevier B.V. All rights reserved.
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    Modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O3 thin films induced by CO2 adsorption
    (2011) Ramos-Moore, E.; Lederman, D.; Cabrera, A. L.
    We report on the modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O(3) thin films under the adsorption of CO(2) gas. The samples were exposed to the gas in ultra high vacuum while different voltages between the top (Ag) and bottom (Pt) electrodes were applied. After dosing, the samples were heated from room temperature at 1.8 K/s, and a mass spectrometer was used to detect the desorbed molecules. The effective molecular sticking was demonstrated by the appearance of the carbon Auger peak in the surface of the sample and by the presence of CO(2) in the desorption curves. After +((-)) 9.9 V were applied to the bottom electrode during dosing, we found a shift of -0.52 (+0.58) mu C/cm(2) in the remnant polarization. These evidences suggest the existence of a depolarizing field induced by molecular adsorption at the surface of the top electrode, and contribute to highlight the potential use of ferroelectric thin films as gas sensors. (C) 2011 Elsevier B. V. All rights reserved.