National Institute Of Materials Physics - Romania

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.

The characterization of nanocrystals embedded in alkali halide crystals concerning shape and size distribution has been performed. Optical absorption, transmission electron microscopy (TEM), x-ray diffraction (XRD) and electron spin resonance (RES) methods has been used. The experimental results are in good agreement with the theoretical considerations concerning absorption and scattering of light by the metal clusters.

Annealing studies of a proton irradiated Si test diode (1MeV- neutron equivalent fluence is phi (eq) = 1.82x10(13) cm(-2)) were performed. The macroscopic characterization shows that during the annealing experiment the type inversion effect happened. The changes in the microscopic features during the annealing treatment were investigated using Thermally Stimulated Current method.

New aspects of the influence of water vapour in the interaction of SnO2 gas sensors with different reducing gases are evidenced by an original experimental method. This consists in measuring the conductance transients during and after rapid transitions from dry into humid air containing identical concentrations of methane and CO, respectively. The results indicate that while methane and water compete in reacting with one type of oxygen ions on the surface, CO reacts mainly with another type of surface oxygen, almost insensitive to water. (C) 1999 Elsevier Science S.A. All rights reserved.

Polymer-coated quartz microbalances were produced and studied. They exhibited high sensitivities for heavy noncyclical hydrocarbons and CO2 concentration measurements. Detection limit of 1 ppm CO2 and 10 ppm Shellsol (Shell solvent containing: 28.7% C10H22; 66.4% C11H24; 5% C12H26) are obtained and reported. The time constants of the absorption and desorption processes were investigated also. A diffusion-free behaviour was observed and related to the porosity of the layer. Collateral AFM (atomic force microscopy) analysis of the polymer structure was performed. We explain big values of the absorption-desorption time constant for CO2 and small values for CnH2n+2 mixtures in connection with the peculiarities of the chemical reaction involved by the sensing mechanisms. (C) 1999 Elsevier Science S.A. All rights reserved.