Publications

The doping process of LiF with Pb has been described by using structural ab initio modeling and experimental results. The values of formation energy of several complexes involving Pb ions show that the presence of charge-balancing vacancies ease the metal incorporation in the crystal. The metal successively captures the vacancies to form a final stable complex Pb-Li(center dot) + V-Li('). Experimental data from X-ray absorption spectroscopy confirm the presence of Pb in LiF crystals in this site revealing both Pb-F bonds and collinear Pb-F-Li atomic configurations. A maximum solubility of Pb in LiF of the order of 3 x 10(20)/cm(3) can be estimated from these calculations. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793751]

The surface structure, interface reactivity, electron configuration and magnetic properties of Sm layers deposited on Si(0 0 1) at various temperatures are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and magneto-optical Kerr effect (MOKE). It is found that metal Sm is present on samples prepared at low temperature, with an interface layer containing SmSi2 and Sm4Si3. When samples are prepared at high temperature, much less metal Sm is found, with an increasing amount of SmSi2. Room temperature ferromagnetism is observed for all prepared layers, with a decrease of the saturation magnetization when samples are prepared at high temperature. It is found that ferromagnetism implies mostly a compound with approximate stoichiometry Sm4Si3. Also, the decrease in the intensity of the XAS 2p(3/2) -> 3d white lines with the corresponding increasing amount of SmSi2 may be explained by assuming a higher occupancy of Sm 5d orbitals (5d(2) configuration), most probably due to hybridation effects. (C) 2012 Elsevier B. V. All rights reserved.

Cotton fabrics were coated with arrays of ZnO hexagonal prisms using an electroless (catalytic/auto-catalytic) deposition process. A typical three step method, similar to those used for electroless deposition of metals on insulating substrates, consisting of pre-activation, activation and deposition steps was employed. The low-dimensional ZnO particles were grown from an aqueous solution containing zinc nitrate as source of zinc ions and dimethylamineborane as reducing agent. The as-obtained ZnO-coated cotton fabrics were characterized from the point of view of structure by X-ray diffraction (XRD) and morphology by scanning electron microscopy (SEM). The XRD studies demonstrate that the ZnO particles have a hexagonal wurtzite crystalline structure. The SEM observations prove that the cotton fibers are homogeneously covered by hexagonal prisms which have uniform base size of approximately 500 nm and height of 1 mu m. Optical spectroscopy measurements show that the functionalization with ZnO strongly decreases the transmittance in the UV vis region of the cotton fabrics. An important characteristic is that the ZnO-functionalized cotton fabrics exhibit superhydrophobicity, with water contact angles exceeding 150 degrees. The technique described is highly reproducible, easy scalable and cheap, allowing a wide range of applications. (C) 2012 Elsevier B.V. All rights reserved.

Highly textured ZnO thin films were implanted with Co ions at fluences of 1 x 10(16) and 1 x 10(17) ions cm(-2). Although their microstructure observed by analytical high-resolution electron microscopy was very different, SQUID magnetometer measurements showed qualitatively similar magnetic properties. In the low-fluence film, only small modifications, such as a slight amorphization, planar defects or very small (<1-1.5 nm) Co clusters, were hardly observable. Implantation at a higher fluence led to important changes in the film structure: heavy amorphization and metallic Co precipitates were identified by electron diffraction, energy dispersive x-ray spectroscopy (EDS), electron energy loss spectroscopy and scanning transmission electron microscopy spectrum imaging. EDS microanalysis additionally revealed the presence of similar to 2 at% Co atoms that were likely dissolved in the ZnO lattice at the atomic level. No Co oxides or other secondary oxide phases were detected. For both implanted samples, the dimensions of Co precipitates were below the superparamagnetic limit at room temperature. Thermo-magnetization curves and magnetic hysteresis loops measured in the temperature range from 5 to 300 K indicated two superimposed magnetic regimes, one predominant above 50 K and the other below 50 K. We analysed the possible mechanisms and quantitative contributions to explain the observed magnetic behaviour at low and room temperature.

A simple approach to fabricate periodic arrays of ring bumps, disk and convex bumps or holes on silicon and gallium arsenide substrates is reported. In the process, a mono or double layer of 700 nm diameter silica colloidal particles were self-assembled on the substrates and a single pulse from a 200 fs laser at 387 nm wavelength was applied. The surfaces were irradiated at normal incidence by immersing the substrates in a glass container filled with acetone, carbon tetrachloride and 1,1,2-trichlor-trifluorethan liquid precursors. The influence of the medium on the near-field interactions for both substrates is investigated. (C) 2012 Elsevier B.V. All rights reserved.

The influence of different TiO2 precursors (laboratory-prepared sol-gel powders, dried and thermally treated at 400 degrees C, and commercial powders such as anatase by Aldrich and P25 Aeroxide by Evonik) on the structure and morphology of the titanate nanotubes obtained by hydrothermal method was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. In all cases, titanate-based nanotubes were obtained, having similar structure and morphology. The effect of the thermal treatment on the structural stability was also studied by differential thermal analysis and thermogravimetry analysis (TG/DTA) and differetial scanning calorimetry (DSC) measurements up to 600 degrees C, to determine the transformation of titanate nanotubes into other phases. Complementarily, by XRD investigation, the phases that develop after the thermal treatment of the titanate nanotubes in 110-400 degrees C temperature range were identified. The TEM micrographs come to confirm the deterioration of morphology and transformation from nanotubes to particles. The thermal stability of the investigated nanotubes led to the conclusion that this property depends not only on the sodium content but also on the type of precursor. The highest thermal stability was noticed for the nanotubes synthesized starting with anatase (Aldrich) commercial powder.

Ferromagnetic FexGe1-x with x = 2%-9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 degrees C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 x 1) surface structure of Ge(001) with Fe deposition. X-ray photoelectron spectroscopy (XPS) at Ge 3d and Fe 2p core levels evidenced strong Fe diffusion into the Ge substrate and formation of Ge-rich compounds, from FeGe3 to approximately FeGe2, depending on the amount of Fe deposited. Room temperature magneto-optical Kerr effect (MOKE) evidenced ferromagnetic ordering at room temperature, with about 0.1 Bohr magnetons per Fe atom, and also a clear uniaxial magnetic anisotropy with the in-plane [110] easy magnetization axis. This compound is a good candidate for promising applications in the field of semiconductor spintronics.

Composites based on polydiphenylamine (PDPA) doped with heteropolyanions of H3PW12O40 and single-walled carbon nanotubes (SWNTs) were prepared by electrochemical polymerization of diphenylamine (DPA) on carbon nanotube films deposited onto Pt electrodes. HRTEM studies reveal that the electrochemical polymerization leads to the filling the spaces between tubes which compose the bundles, creating a monolithic film on the Pt electrode. The resulting composites were tested as active materials in supercapacitors. Resonant Raman scattering studies showed that the electropolymerization of DPA in the presence of H3PW12O40 and SWNTs leads to the covalent functionalization of SWNTs with doped PDPA. The covalent functionalization of SWNTs with PDPA doped with H3PW12O40 heteropolyanions was revealed by FTIR spectroscopy, based on the changes in the vibrational features of PDPA and H3PW12O40. These changes included i) a down-shift of the PDPA IR bands, which was attributed to the C-H bending vibrational mode of benzene (B), C-aromatic-N, C-C stretching (B) + C-H bending (B) and C-C stretching vibrations of the B ring, from 1174, 1321, 1495 and 1603 cm(-1) to 1165, 1313, 1487 and 1599 cm-1, respectively; ii) a change in the peak positions of IR bands associated with the W = O and P-O-W vibration modes of H3PW12O40; and iii) a down-shift of the IR band situated in the spectral range 650-725 cm(-1), which was assigned to the inter-ring deformation vibration mode. The characterization of symmetric solid-state supercapacitors was performed for electrodes prepared from i) SWNTs functionalized with PDPA doped with H3PW12O40 heteropolyanions, ii) SWNTs electrochemically decorated with H3PW12O40 heteropolyanions, and iii). PDPA doped with H3PW12O40 heteropolyanions. Preliminary results indicate high discharge capacitance values of up to 157.2 mF/cm(2) for SWNTs functionalized with PDPA doped with H3PW12O40 heteropolyanions. The discharge capacitance of this material is superior to those recorded for SWNTs electrochemically decorated with H3PW12O40 heteropolyanions (similar to 18.2 mF/cm(2)) and PDPA doped with H3PW12O40 heteropolyanions (similar to 62.1 mF/cm(2)). (c) 2012 Elsevier B.V. All rights reserved.

We have studied the one-step procedure for simultaneous synthesis and sintering of SiC-doped MgB2 by the spark plasma sintering technique. Two types of composition, one in which Mg is strongly deficient, with the atomic ratio , and one in which Mg content is slightly higher than the stoichiometric value, specifically , were investigated. The amount of SiC was 12 wt.% and 9 wt.%, respectively. For comparison we also studied the way the deficit of Mg can be compensated in a second process of sintering. The sample with Mg deficit shows that SiC is left almost unreacted but the results are spectacular: the highest critical temperature, 36.5 K, the highest upper critical field and the highest self-field critical current density 6.7x10(5) A/cm(2) at 10 K. In the sample with overstoichiometric Mg, SiC is decomposed, carbon diffuses within MgB2 but the critical temperature is only of 35.8 K and the zero-field critical current density is one order of magnitude lower. The compensation of the deficit of Mg in the two-step procedure is not efficient. The critical temperature is even lower, 35.8 K, the upper critical field is also lower despite SiC decomposition and C diffusion within MgB2 and the critical current density is slightly above 10(5) A/cm(2). However, at low temperatures and fields of order 7 T the sample with overstoichiometric Mg and the sample prepared by the two-step procedure have higher critical current density.

The influence of different ions NO3 (-) and SO4 (2-) on the carbon steel corrosion in ammonium chloride was investigated using mass loss measurements and potentiodynamic polarization. Corrosion products were analyzed using X-ray photoelectron spectroscopy (XPS) and simultaneous thermal and differential scanning calorimetry (TG/DSC). XPS analysis shows that the main product of corrosion is a non-stoichiometric Fe3+ oxyhydroxide, consisting of a mixture of FeO(OH) and FeO(OH) containing inclusions of these anions, species such as Fe3+O(OH,Cl-); Fe3+O(OH,SO4 (2-)); and Fe3+O(OH,NO3 (-)). TG/DSC confirms the decomposition of the rusty products formed by chemical corrosion, compounds like Fe3+ oxyhydroxides, with beta-FeOOH as the major phase, crystal structure of which may contain Cl-, NO3 (-), and SO4 (2-)-e.g., akaganeite [Fe3+O(OH,A)].