National Institute Of Materials Physics - Romania

Electrochemical deposition in nanoporous ion track membranes is used for the preparation of multisegment CdTe-homojunction diode nanowires. Our study is based on the fact that the deposition overpotential strongly influences the composition of the compound semiconductor nanowires. Therefore, the transport behavior of the nanowire devices can be tailored by appropriately choosing a certain sequence of electrodeposition potentials. The wires were characterized using scanning electron microscopy, energy dispersive x-ray analysis, optical spectroscopy and x-ray diffraction. The current-voltage characteristics measured prove that, by appropriately choosing the voltage pulse pattern, one can fabricate nanowires with ohmic or rectifying behavior. The semiconducting nanowires are sensitive to light, their spectral sensitivity being characteristic of CdTe. The preparation of functional nanostructures in such a simple approach provides, as a major advantage, an increase in the process reproducibility and opens a wide field of potential optoelectronic applications.

This paper presents a study of some thin monomer films deposited by MAPLE. These monomers have been synthesised from maleic anhydride and aniline derivatives and are characterized by a molecular structure adequate for the generation of optical non-linear phenomena. UV-VIS, FTIR, Raman and Photoluminescence spectroscopy have been used to investigate the influence of the experimental conditions on the properties of the films. X-Ray Diffraction has evidenced a certain degree of disorder in the thin films, which is correlated with the randomly orientated molecules and AFM images have revealed different roughness for different monomer layers and different substrates. Second harmonic measurements have shown a strong signal emitted by the thin film prepared from the monomer with one [-NH-NH-] donor and two [-NO(2)] acceptor groups. This thin film is characterized by good transparency and low photoluminescence emission at excitation with the wavelength of the second harmonic (lambda-400 nm).

Silicon wafers have been submitted to hydrogen RF-plasma treatment in various experimental conditions. Hydrogen RF-plasma treatment induced two kinds of effects on Si wafers, depending on the treatment conditions: surface corrugation and formation of structural defects below the free surface. Atomic force microscopy (AFM) investigations showed that the surface roughness significantly increased with the treatment duration, leading to the formation of pyramidal humps on the surface. The structural defects resulting after the plasma treatments were investigated by conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) techniques. The specificity of the induced extended defects due to hydrogen decoration was emphasized. Three types of extended defects were identified and characterized: planar defects in the {111} and {100} planes and nanometric voids. Point defects related to the hydrogenation process were investigated by electron paramagnetic resonance (EPR) in correlation with the electron microscopy results.

A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g(parallel to) and g_, are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g_-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S-4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce3+ ions with tetragonal symmetry S-4 in the PbWO4 scintillator. (c) 2009 Elsevier BM. All rights reserved.

Nanoparticles of iron oxide embedded in C(60) matrices are synthesized by co-evaporation of iron and C(60)-fullerene, and analyzed using X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy and MOKE magnetometry. C(60) molecules in the neighborhood of the iron oxide nanoclusters transfer charge to the oxide, and the charge deficit fully delocalizes onto the fullerene molecules. The composition of the nanoclusters of iron oxide is similar to both hematite and magnetite, in comparable proportions. One analyzes the amplitude of the pre-edge XANES peak, yielding the average ionization state of absorbing Fe atoms, which is confirmed by the simulations of XANES spectra.

Nanoxide based composites are suitable materials for advanced medical application. Fe(3)O(4), CoFe(2)O(4), and MnO(2) nanoparticles, synthesized by different soft chemistry routes (precipitation in the presence of soft template and solution combustion) were coated by a biocompatible polymer, sodium alginate. The oxides and corresponding composites were structural and morphological characterized. The saturation magnetization for Fe(3)O(4)-based composite is 10.1 emu.g(-1), resulting magnetic domains of 5.2 nm that suggests a superparamagnetic property. Radical scavenging capability of the composites was investigated by two different methods, Trolox Equivalent Antioxidant Capacity (TEAC) and peroxide-chronoamperometry.

Amorphous thin films of SnSe2 and Ge2Sb2Te5 doped by different amount of silver (0.1, 0.2, 0.5 and 1 Ag atoms per formula unit) have been prepared by pulsed laser deposition (PLD) starting from solid polycrystalline targets. The films were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The good gas sensing properties for CO, as well as the sensitivity for CH4 and NO of the Ag; doped SnSe2 films have been demonstrated in the composition SnSe2Ag0.2. The structural effect of silver introduced in Ge2Sb2Te5 matrix has been investigated. The freshly deposited thin films doped by various amount of Ag develop three phases: an amorphous one, and two crystalline phases consisting of a major fcc cubic phase of AgSbTe2 and a minor cubic phase of composition Ag2Te. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nonlinear materials Y1-xRxCa4O(BO3)(3) (R = Lu or Sc) with 0 < x <= 0.5 were synthesized for the first time. The room temperature X-ray diffraction studies show that Y1-xLuxCa4O(BO3)(3) and Y1-xScxCa4O(BO3)(3) compounds with x < 0.4 and x <= 0.2, respectively, have single phase monoclinic symmetry (space group Cm) and conform to the Vegard law, according to which the lattice parameters are linearly dependent on the impurity (R3+) concentration. The X-ray patterns of Y1-xRxCa4O(BO3)(3) materials with x >= 0.4 and x > 0.2, respectively, reveal the presence of extra phases which imply their non-congruent melting and difficulties in crystals growth.

Plasma polymerization has been carried out in a parallel plate RF discharge reactor in mixtures of argon with pentacyclic precursors, namely thiophene, furan and pyrrole. The plasma characterization showed the presence of emission bands peculiar to each monomer. The deposition of highly transparent films with partial conservation of the pentacyclic structure, inclusion of various oligomers, roughness and surface energy depending on the precursor nature was demonstrated by material investigations results. The material conductivity is significantly improved upon iodine doping in the case of polythiophene and polypyrrole-like films, while an opposite behavior was observed for the polyfuran-like films.

(Bi0.5Na0.5)TiO3 doped with 8 mol% BaTiO3 (BNT-BT0.08) powder was prepared by an acetate-alkoxide sol-gel method. The evolution of the structure and microstructure of the precursor gel, heated at various temperatures as well as, the piezoelectric and ferroelectric properties of BNT-BT0.08 ceramics were studied. Nanopowder of BNT-BT0.08 with average particles size of 30 nm and, crystallized on the perovskitic structure of (Bi0.5Na0.5)TiO3, were obtained from the precursor gel by heating at 600 degrees C, 3 h in air. The ceramics of composition 0.92(Bi0.5Na10.5)TiO3-0.08BaTiO(3), manufactured from the as-prepared nanostructured powder, present good dielectric properties (epsilon(r,max) = 4000-4500, tan delta = 0.02-0.03) and high mechanical quality factor (Q(m) similar to 500). (C) 2009 Elsevier B.V. All rights reserved.