Publications

A series of CuGa1-xInxTe2 bulk quatemary compounds (x=0.17, 0.33, 0.50, 0.67, 0.83) has been synthesized from the corresponding stoichiometric melts by a rapid cooling procedure. The same procedure was used to obtain the ternary bulk compounds CuGaTe2 and CuInTe2 with the aim to compare, according the Vegard law, the match between molar fraction of the indium (x) and the lattice parameters for the aforementioned quatemary compounds. The nature of the crystalline phases, the local structure homogeneity and stoichiometry of these compounds have been investigated by x-ray diffraction (XRD) and scanning electron microscopy (SERI) techniques. The analysis revealed the presence of the chalcopyritic structure for all the samples.

Pb(ZrxTil .x)O-3 (x = 0 divided by 0.8) graded thin films were fabricated by sol-gel method on Pt/ SiO2/ Si substrates using metal alkoxide solution. The samples were crystallized by conventional thermal annealing in air for 30 min. at 700 degreesC. The ferroelectric properties of the graded structure were investigated Specific unusual behavior in the ferroelectric loops like large offset on the polarization axis was observed and disscused.

The morphology and structure of diamond films, deposited on cubic boron nitride (c-BN) crystals by microwave-plasma-enhanced chemical vapor deposition, is studied by high-resolution scanning electron microscopy and micro-Raman spectroscopy. The c-BN crystals, with sizes of 200 to 350 mu m and grown by a high-temperature/high-pressure technique, were embedded in a copper holder, and used as substrates in deposition runs of 15 min to 5 h. The nucleation centers for diamond appear as well-shaped cuboctahedral crystallites, having diameters of approximately 100 nm. With increasing deposition time the diamond crystallites grew larger, forming islands on the c-BN faces. In some cases, epitaxial growth was observed on the (111) c-BN faces where coalesced particles gave rise to very smooth regions. A number of diamond crystals with peculiar shapes are observed, such as a pseudo five-fold symmetry due to multiple twinning. Moreover, both randomly distributed carbon tubes, about 100 nn in diameter and 1 mu m in length, and spherically shaped features are observed in samples prepared under the typical conditions of diamond deposition, this effect being ascribed to the influence of plasma-sputtered copper contamination. Quite unusual diamond crystals with a deep, pyramidal-shaped hole in the middle grew on the copper substrate between the c-BN crystals. (C) 2000 Elsevier Science S.A. All rights reserved.

The PLSnZT powders were obtained from soluble peroxide precursors of Ti(IV) and Zr(IV) by precipitation with lead nitrate, lanthanum nitrate and tin chloride solutions. The precipitation of Pb(II), Ti(IV) and Zr(IV) from solution in pseudobinar systems was studied as a function of pH and temperature. The structural evolution of powders was studied using XRD. The powders were pressed, sintered, coated with silver paste and poled in silicon oil at high voltage. The density, contraction, electrical permitivity epsilon(33) and displacement constants d(31) were determined as a function of composition and thermal treatment temperature. The same values were measured for the material with the same composition obtained in the classical ceramic route by solid state reaction. The density (8.1 g/cm(-3)), relative electrical permitivity (500) and d(31) (16 x 10(-12) m.V-1) obtained by the sol-gel colloidal route at a sintering temperature of 1100 degrees C were 30-40% higher comparing with the same constants in the classical route at a sintering temperature of 1250-1300 degrees C.

Cadmium sulphide films were prepared by Chemical Bath Deposition (CBD) technique from CdCl2, CdSO4, Cd(CH3COO)(2) respectively, as cadmium precursor salts, and thiourea in ammoniacal medium. In this paper we study the changes in the morphology and properties of the films prepared from different cadmium salts, although the starting reaction parameters as concentration of the reactants, pH (denoted as pH(i)), temperature, and time of reaction for all precursor cadmium salts are identical. It is proposed that, during the deposition process, depending upon the anion type of cadmium salt the pH of reaction mixture decrease with different rates. The pH(i) decreases in the deposition bath could allow the change of the layers growth mechanism and therefore the change in the morphology and properties of the films. Above a certain value of starting pH (pH(i) >12.5), the pH decrease did not affect the growth mechanism and properties of the layers. The structure and morphology of the films were investigated by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray diffraction (XRD). Also the optical, electrical and photoelectrical characteristics of the layers and their dependence on films morphology were presented. These studies have allowed us to establish the optimum conditions for fabrication of quality CdS films using CdCl2, CdSO4, or Cd(CH3COO)(2) as cadmium precursor salts.

There is an increasing interest in Si-based optoelectronics using Si1-xGex nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for possibilities to increase the contribution of the radiative recombination to the emission. One possible approach involves self-organised growth of lattice-mismatched layers. In the present paper, p-i-n structures, using one layer with Ge islands and which emit in the near infrared up to room temperature were fabricated. The self-organised growth of Ge was performed at 700 degreesC with a small coverage (9 ML) so as to avoid plastic relaxation of the islands, but with a high growth rate (0.3 ML/s) which leads to the formation of a broad bimodal island distribution (small- and medium-sized islands). The diode structure including the Ge islands was deposited in the form of mesas using selective epitaxial growth by low-pressure chemical vapour deposition. The mesa areas were Varied with the aim of demonstrating the influence of size distribution of the islands on the light emission. At low current density the emission is dominated by islands with smaller band gap (larger valence band offset) while at higher currents emission from islands with larger band gap takes place. From the comparison of single diodes with arrays of small-area diodes with the same total area it is found that the arrays emit three times more light due to the lower total number of deep traps in each diode. (C) 2001 Published by Elsevier Science Ltd.

In this letter, we present a study of surface-enhanced Raman spectra of thin layers of phthalocyanine (Pc) complexes deposited on Ag and Au rough surfaces. The materials of this work are MgPc (monophthalocyanine) and the asymmetrical lutetium bisphthalocyanine (tBu)(4)PcLuPcCl8 (Pc'LuPc ") under three oxidation states. Enhancement of specific bands is observed depending on the surface and the oxidation state. SERS signatures of neutral, oxidised and reduced Pc'LuPc " are identified.

Heteropolynuclear complex compounds isolated in Cr2O3. xH(2)O-La(NO3)(3)-M(NO3)(2)-A-NH3, (A = CH3COO-,C2O42-; M = Cu, Ni) systems were used as precursors for LaCr1-xMxO3. The perovskite structure of doped lanthanum chromites was determined by XRD analysis. The unit cell volumes, specific surface areas and magnetic properties were investigated. The proposed methods offer new, simple, less expensive procedures for obtaining doped lanthanum chromites. (C) 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved.

The structural and morphological evolution of the NiZn ferrite during mechanical activation has been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM) and chemical analysis. During the milling process an important amount of metallic phases due to the vials and balls contaminates the ferrite powder and functions as co-participant to the mechanical activation. The XRD- spectra of the as-milled powders showed the presence of the metallic contribution up to 200 hours of milling. For longer intervals of time the extralines intensities due to the presence of metallic phases decrease and only a spinel phase is observed after 400 hours of total milling. The TEM image revealed, after 200 hours, spheroidal nanometric particles in the range 30-80 nm, alongside of some needle type particles. When the milling time was increased, some big plate monocrystals appear and grow. After 400 hour milling, polyhedral crystals 250 divided by 1000 nm size could be observed in the powder.

We measure the azimuthal anchoring strength and easy axis gliding of the nematic 5CB on weakly anisotropic substrate. We study the anchoring evolution at a time scale ranging from a few minutes up to a few months. We separate approximately the elastic and the viscous response and we obtain the gliding velocity as a function of the applied torque. We discuss the observed gliding times and nonlinear gliding viscosity.