Publications

The electronic properties of two-dimensional toroidal surfaces of nanometer size have been investigated by approximately solving the time-independent Schrodinger equation. The effects of the quantum confinement on the electron population of these structures were shown by studying the average electron density on the surface of the torus. The unique electron spread that resulted from such computations encouraged further study on the field emission properties of this two-dimensional manifold. The Wentzel-Kramers-Brillouin approximate approach was used for a preliminary study of the field emission properties of the nanotori for various geometrical parameters. (c) 2011 American Vacuum Society. [DOI: 10.1116/1.3531935]

Graphene sheets were prepared using electrochemical and chemical steps followed graphite electrochemical oxidation, graphite chemical oxidation, ultrasound expholiation and chemical reduction of carbon material. Films of graphene were electrophoretically deposited from aqueous dispersion on FTO electrode. These films were tested as cathode for a dye-sensitized solar cell. The morphology of the prepared products was imaged by scanning electron microscopy; the intermediate compound, graphene oxide was characterized by IR spectroscopy.

Experimental data show that the normal state resistivity of superconducting Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi-2212) whiskers increases after a 6 h irradiation by a synchrotron mu-beam with 17 keV photons. We analyse this result on the basis of previously reported effects in Bi-2212 whiskers due to ageing or heating processes. A finite element model of the experimental setup clarifies that the heat load induced by the microbeam has to be excluded as a possible cause for the material changes. The knock on the interstitial, loosely bound, O species by secondary electrons is discussed as the most likely mechanism responsible for this effect.

We investigated the anisotropy dependence of the characteristic fields of the second peak magnetization, specifically, the onset field B-on and the peak field B-sp, in Y1-xPrxBa2Cu3O7-delta single crystals. The empirical dependence arising from data analysis suggests a logarithmic dependence of both these fields on the anisotropy factor gamma which disagrees with the universal scaling law gamma(2)(B) = F(t) proposed previously. We attribute the faster decrease of the two fields at high anisotropy to the dramatic changes in the magnetic state of the system which starts to display a noticeable paramagnetic contribution at high Pr-doping, hence high anisotropy. The incipient phase separation could also reduce the elastic properties of the flux line lattice which determines onset of the second peak magnetization. (C) 2010 Elsevier B.V. All rights reserved.

The capability of anti-Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4-ethylenedioxythiophene (EDOT) on a Au support covered with a single-walled carbon nanotube (SWNT) film immersed in a LiClO4/CH3CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4-ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti-Stokes Raman spectrum. The additional increase in Raman intensity in the anti-Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un-doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag+, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric C-C stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti-Stokes Raman effect. Copyright (C) 2010 John Wiley & Sons, Ltd.

Experimental studies on patterning hexagonal Ge nanostructures have been conducted on Si substrates through deposition of Ge with polystyrene spheres as a mask. The size distribution of the patterned Ge nanostructures is narrow with the full width at half maximum being less than 10% of the dot size. The two-dimensional patterned Ge nanostructures were further introduced in a Si matrix. Cross-section transmission electron microscopy reveals periodic dark stripes representing the deposited Ge dots in an a-Si matrix.

Barium titanate (BaTiO3) has been doped in situ with 5 mol% thulium by a sol-gel method. The as-prepared gel powder consists of nanosized grains (20-30 nm) and crystallizes on the cubic BaTiO3 lattice, at 700 degrees C. Ba0.95Tm0.05TiO3 ceramics derived from this powder have tetragonal perovskite structure and contain a small amount of Tm2Ti2O7 pyrochlore phase. These ceramics exhibit dielectric constants of 4282-3240 and dielectric loss (tan delta) of 0.1077-0.0161 at Curie temperature T-c=132 degrees C and at 10 Hz-100 kHz, respectively. For a drive voltage of 400 V, the hysteresis loop recorded at the frequency of 100 Hz shows a remnant polarization (P-r) value of 76 mu C/cm2 and a coercive field (E-c) of 124 V and for 1 kHz a remnant polarization (P-r) value of 58 mu C/cm2 and a coercive field (E-c) of 116 V.

The article reports the preparation and complex characterization of iron-containing phosphate glasses considered to be ecological materials, as they contain nontoxic compounds related to environment. The oxide system Li2O-MgO-(CaO)-Al2O3-P2O5-(FeO/Fe2O3) was investigated in respect to its structural changes caused by MgO replacement with CaO and by the iron addition. UV-vis-NIR (ultraviolet-visible-near infrared) spectroscopy as well as thermo-gravimetric (TG) measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD) analysis, electronic paramagnetic resonance (EPR), and Mossbauer (nuclear gamma resonance) spectroscopy have been used to investigate redox states and coordination symmetry of iron, together with vitreous network changes during the heat treatment up to 1000 degrees C. UV-vis-NIR transmission spectroscopy revealed no structural modifications when MgO was substituted by CaO, but noteworthy absorption bands attributed to Fe2+/Fe3+ species. TG analysis made in the 20-1000 degrees C range shows low weight loss accompanied by several thermal effects, as evidenced by DTA. XRD patterns for the glass samples heat treated at about 700 degrees C revealed the presence of different phosphate crystalline phases containing Mg, Al, and Fe ions. EPR spectroscopy revealed the presence of paramagnetic Fe3+ ions and the change of the first coordination symmetry, when the samples are heated below the vitreous transition temperature. Mossbauer spectroscopy has evidenced two paramagnetic species, Fe2+ and Fe3+, both in octahedral coordination symmetry and a clustering process supported by only Fe3+ ions.

Polydiphenylamine/single walled carbon nanotube (PDPA/SWNT) composites were synthesized electrochemically aiming at their application as active electrode materials for rechargeable lithium batteries. The electrochemical polymerization of diphenylamine (DPA) on a SWNT film immersed in a 1 M HCl solution was studied by cyclic voltammetry. Comparing cyclic voltammograms recorded on a blank Pt electrode with those obtained for a SWNT film deposited on Pt electrode one observes in the latter case a decrease of the DPA reduction potential. To elucidate electrochemical polymerization mechanism, photoluminescence studies on DPA/SWNT and PDPA/SWNT systems were carried out. Additional information concerning the functionalization process of SWNT with PDPA was obtained by Raman and IR spectroscopy. Using the PDPA/SWNT composite as active material for the positive electrode of a rechargeable lithium cell (LiPF(6) electrolyte), the charge-discharge tests show a specific discharge capacity of ca. 245 mA h g(-1), much higher than the 35 mA h g(-1) for pure PDPA. (C) 2010 Elsevier B.V. All rights reserved.