Publications

Our fluorescence study gave information on the presence of chemical defects and irregularities in crystal structure, offering a good tool for quality evaluation of organic crystalline materials. We have analysed the correlation between the bulk or thin film samples' quality determined by experimental parameters (thermal gradient, moving speed of the growth interface, crucible configuration, slow or fast solidification rates) and the fluorescence characteristics for pure meta-dinitrobenzene and meta-dinitrobenzene doped with 8 hydroxyquinoline or 1,3 dihydroxybenzene. The comparative study of the position and shape of the fluorescence emissions for pure and controlled doped materials nave us information on the chemical defects.

Mechanisms for orientation selection in complex oxide thin films are discussed, focusing on two examples: (1) development of a preferential texture in sol-gel prepared polycrystalline PZT films on Pt electrode layers and (2) selection of oxides for epitaxial growth on Si (111) and (100). In sol-gel PZT, a direct link was found between the formation of a well crystallized transient fluorite phase, promoted by reducing pyrolysis conditions, and the emergence of a strongly preferred (111) texture after crystallization. Meanwhile, in the MBE growth of epitaxial oxides on silicon, matching of silicon and oxide surface lattices is not a sufficient condition to achieve layer-by-layer growth, needed for planar films. In fact, for planar growth on Si(111), fluorite-structure compounds are required, while rocksalt and perovskite structure oxides are suited for the growth of low-roughness films on Si(100). These examples illustrate the important role played by surface energy in orientation selection. (C) 2003 Elsevier Ltd. All rights reserved.

The phase composition, local structure and magnetic interactions were investigated in FexOx-SiO2 nanocomposites obtained by modern sol-gel methods. The very sensitive method of Mossbauer spectroscopy has evidenced the strong influence of the silica matrix precursor on the iron phase composition in the initial composites as well as on the size distribution of the hematite nanoparticles obtained by suitable thermal annealing treatments.

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178cm(-1) associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH4OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. (C) 2004 Elsevier Ltd. All rights reserved.

In this paper, we report some experimental results concerning new aspects in laser ablation process of the YBCO thin film. Both structural and superconducting properties of the studied films were analyzed by transmission electron microscopy, selected area electron diffraction, elastic recoil detection analysis, Raman spectra, atomic force microscopy images and the ac magnetic susceptibility. We have obtained the epitaxial films with relative large area of c-axis grown film and high critical temperature, without any post ablation annealing process.

Partial replacement of Bi by Pb and Sb produces materials of high-T-c pure phase, with antimony playing an important role in accelerating the formation of the phase with the higher superconductive transition in the Bi-Sr-Ca-Cu-O system. The influence of the antimony precursors on the superconducting phases mechanism formation was examined. Using a solid state reaction route, two nominal compositions Bi(1.6)Pb(0.3)Sb(0.1)Sr(2)Ca(2)Cu(3)Oy and Bi(1.6)Pb(0.4)Sb(0.1)Sr(2)Ca(2)Cu(4)Oy were investigated. The sources of antimony were Sb2O3 alphaSb(2)O(4) and a prepared mixture Bi1.9Sb0.1Ox. The mechanism formation of the phases upon annealing was investigated using X-ray diffraction and simultaneous thermal analysis. Electrical properties of the obtained materials were evaluated. The presence of the antimony ions determined a higher reactivity of the system as well as the promotion of the superconducting high-T-c phase enhancement. (C) 2003 Elsevier Ltd. All rights reserved.

The electron paramagnetic resonance (EPR) properties of the Mn2+ ions in PbWO4 single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T = 20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn2+ impurity ions is tetragonal, strongly suggesting that the Mn2+ ions substitute for the Pb2+ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn2+ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains. (C) 2003 Elsevier Ltd. All rights reserved.

A new type of dual mode filter is proposed for mobile communications systems in 900 MHz frequency bandwidth such as GSM or GPRS. The planar filters were developed on a 0.635 mm substrate with a 10.8 dielectric constant. Several methods of modes coupling were investigated. Each dual mode resonator has a square-shape with the edge 20.5 mm and contributes with two poles to the filter response.

Epitaxial Pb(Zr0.2Ti0.8)O-3 (PZT) thin films were grown on single crystalline Nb-doped SrTiO3 (STON) substrates by Pulsed Laser Deposition (PLD). The top electrode was SrRuO3. The investigated electric and ferroelectric properties have revealed a strong asymmetry of the two PZT-electrode interfaces. A qualitative explanation based on the possible existence of a n(+) - p junction at the STON-PZT interface is suggested.

The paper presents experimental results concerning the influence of additives from a solution of the PVA on the optical properties of the polymer. The refractive index of the poly (vinyl-alcohol) was modified through the addition of inorganic materials and showed values between 1.5 and 2 (depending on the compositions and the curing conditions of the polymer. The transmittance indicated values above 80%. The additives we used were metal oxides/inorganic salts. The composition and the deposition processes were optimized to allow a better control of the optical parameters (refractive index, transmittance), to reduce the processing temperatures and to improve photosensitivity. PVA with these new light-sensitive compositions can be easily spin coated onto variety of semiconductor substrate and directly patterned. A process was developed for channel waveguides fabrication.