National Institute Of Materials Physics - Romania

Pbl(2) crystallite incorporated in optically transparent polyacrylamide and poly (vinyl alcohol) matrix have been studied by Raman scattering, photoluminescence (PL) at low temperature and Scanning Electron Microscopy (SEM). The small particles of Pbl(2) embedded in different host matrix were prepared by: i) cooling to the room temperature a boiling saturated aqueous Pbl(2) solution containing polyacrylamide or poly (vinyl alcohol) and ii) by chemical reaction of KI and Pb(NO3)2 in aqueous polyacrylamide solution. In all cases, were identified hexagonal platelets of Pbl(2) and rods like particles. The rods are featured by a similar Raman spectrum and a specific green emission (cca 550 nm). Modification of the Raman and PL spectra results from a compressing effect produced by the penetration between the Pbl(2) layers of different molecular species. By compression the hybridized electronic level situated at the top of valence band, formed by the contribution.of the 6s and 5p states of Pb2+ and l ions, undergo a deformation that induces a weakness of the interaction between the lead ion and iodine electron within a layer so that the photoluminescence of intercalated Pbl(2) acquires the characteristics of Pb2+ emission when it is dissolved in an alkali halide crystal.

Lithium metallic colloids were studied in ion-implanted LiF crystals, irradiated with high energy He+ particles. The presence of metallic particles, displaying absorption bands at 420 and 460 nm due to the scattering of light on these particles, overlaps the absorption of M centers. The luminescence excited in this band, which is a superposition between the absorption of F-2 and F-3(+) centers, is reduced due to the metallic lithium particles, formed on the surface of the irradiated and at the end of the tracks of He+ particles, reducing the luminescence of the formed color centers. Diminution of the M band together with the growing of the 420 nm band versus irradiation doses, suggest a transformation of the color centers in small Fn-aggregates during the irradiation process, followed by a metallic conversion of Li ions.

Plasma Enhanced Chemical Vapours Deposition (PECVD) is a versatile method which allows for the preparation of amorphous hydrogenated carbon films with different structures by varying the deposition parameters. We have prepared carbonic structures by PECVD technique onto quartz and silicon substrates in atmospheres of argon and methane mixtures. These films were characterized by the UV-VIS, FTIR, Raman, Ellipsometry and AFM methods. The total RF power, gas pressure (0.1 mbarr) and substrate temperatures (<100 degrees C) were kept constant for all the depositions. Diamond-like carbon structures were obtained for a wide range of methane dilutions between 100% + 20% and a narrow transition from diamond-like (DLC) to polymer-like carbon (PLC) structures between 20% + 10% dilutions was found.

Barium strontium titanate with molar formula Ba0.75Sr0.25 TiO3 (BST) was prepared from raw materials by solid-state reaction. Samples were sintered at 1230 degrees C and 1260 degrees C for 2 h. In order to improve the sintering process, 1 wt. % MgO and 1 wt. % MnO2 were used as sintering additives. The sinterability, structural, morphological and dielectric properties of pure and doped BST samples were investigated. Perovskite structure of polycrystalline BST ceramics was determined by Xray diffraction. SEM investigations revealed a grain and pore size distribution, which is shifted to higher values by a slight increase of the sintering temperature from 1230 degrees C to 1260 degrees C. In the presence of additives, a better sintering and grain refining could be observed, whereas their lack induce a strong effect of exaggerated grain growth, by forming of large faceted grains. The presence of Mn ions in the doped BST ceramics was revealed by the EDX analysis. On the other hand, Mg ions are not evidenced. Temperature dependence of dielectric constant measured at 1 kHz shows a diffuse ferroelectric-paraelectric transition, which can be attributed to the large size dispersion of grains. The increase of sintering temperature leads to a shift of the Curie temperature to higher values.

One-dimensional structures having uniform diameters were prepared by electrochemical deposition using polycarbonate membranes as templates. Lead chalcogenides and ZnO tubes and rods growth processes were presented comparatively. The tendency of PbTe and PbSe semiconductors to grow as tubes both in membrane micro and nanopores was explained based on the absorption of lead ions on the pore wall.

A new method for the preparation of PbS/PZT composite materials is proposed. The PZT matrix has been obtained using the sol-gel method on Pt-coated Si substrates. The PbS (nano) crystals were subsequently obtained by dipping the porous PZT matrix in Pb(NO3)(2) and Na2S solutions at room temperature. The XRD spectra show the presence of the tetragonal phase of PZT. The microstructure of the film was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The size of the PbS (nano)-particles at the sample's surface is about 25 nm, estimated from the AFM analysis. The roughness of the sample's surface is about 5 nm. The porosity of the composite, estimated from the SEM analysis in cross-section is smaller than for the reference PZT porous matrix. The equivalent dielectric constant of the composite increases by a factor of 4-5 in comparison with the porous PZT reference and does not depend on the thickness of the surface PbS (nano)-crystalline layer. The saturated polarization (Ps) value for the PZT-PbS composite is smaller than that corresponding to the bulk PZT but higher than for the reference porous PZT. We attribute these effects to the partial filling of the pores in the PZT matrix with PbS.

Systematic excitation-power-density dependent and time-resolved single-dot photoluminescence studies have been performed on type-I InP/Ga0.51In0.49P quantum dots. These dots are rather flat and therefore exhibit larger than normal single-dot ground-state transition energies ranging from 1.791 to 1.873 eV. As a result of their low height, the dots have a very high aspect ratio (ratio of width to height) of approximately 27:1. In general, even at high excitation power densities, the dots with ground-state transition energies above 1.82 eV exhibit only s-shell emission, while the larger dots exhibiting ground-state emission below 1.82 eV tend to exhibit emission from several (in some cases up to eight) shells. Calculations indicate that this change is due to the smaller dots having only one confined election level while the larger dots have two or more. Time-resolved investigations indicate the presence of fast carrier relaxation and recombination processes for both dot types, however, only the larger dots display clear interlevel relaxation effects as expected. The temporal behavior has been qualitatively simulated using a rate equation model. Also, in a more detailed analysis, the fast carrier relaxation is described on the basis of a quantum kinetic treatment of the carrier-phonon interaction. Finally, the dots display a clear single-photon emission signature in photon statistics measurements.

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.

We present magnetization data on superconducting strongly Pr-doped YBa2Cu3O7-6 showing the existence of irreversibility at temperatures much higher than superconducting critical temperature and Neel temperatures for both planar Cu and Pr systems. A model in terms of cluster glass is proposed.

Cobalt ferrite and magnetite nanoparticles were obtained by "the complexation method" belonging to soft chemistry, in which cholic acid sodium salt acts as complexation agent. The precursors were characterized by IR, UV-vis spectra and magnetic measurements. X-ray diffraction spectra and transmission electron inicroscopy proved the nanocrystalline structure of the spinelic ferrites. (c) 2006 Elsevier B.V. All rights reserved.