National Institute Of Materials Physics - Romania

TiO2 thin films prepared by sol-gel method and by RF magnetron sputtering deposited on p-Si(111), n-GaAs(100), and glass have been investigated by XRD, XPS and AFM techniques. The characteristics of the film: colour, adherence and composition are presented in the case of thermal treatments (500 degrees C and 800 degrees C) for sol-gel deposited films as well as for RF sputtering. The TiO2 plasma deposited film is uniform, homogeneous and stoichiometric.

Manganese and copper doped ZnO films and nanowire arrays were prepared by cathodic electrodeposition from aqueous solution. The electrochemical process was examined by linear voltammetry and the morphology and the composition of obtained semiconductor were observed by SEM and EDX measurements. Optical absorption spectra indicated the presence of copper in ZnO: Mn: Cu nanowire arrays as Cu2+ ions. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The Ba1-xPbxNd2Ti5O14 (BNT: Pb) ceramics are appropriate for microwave devices due to their high dielectric constant and low loss in the microwave domain. Samples with x Pb content in the range 0 + 0.5 were prepared by solid-state reaction and sintered at temperatures T-s = 1230 + 1260 degrees C. XRD and SEM analysis indicate very good crystalline structure and polyhedral well formed grains. Controlled temperature coefficient of resonant frequency alpha is essential for dielectric resonators. The temperature coefficient of the resonant frequency alpha takes values from - 15 ppm/degrees C to + 70 ppm/degrees C. Highdensity values and adequate microwave parameters were obtained for x = 0.5 Pb content. The BNT: Pb ceramics sintered at 1250 degrees C and 1260 degrees C for 2 hours are suitable for dielectric resonators for frequencies up to 5 GHz. epsilon r = 76 + 87 and Q x f similar to 5000 GHz).

Thin films in the SiO2-P2O5 system are intensely studied due to their applications in microelectronics, sensing, nanophotonics, optoelectronics and as ionic conductors. Sol-gel is the most used method for preparation of such films. Previous studies established the very low reactivity of the phosphorous alkoxides and the high tendency of the phosphorous oxide to volatilise at thermal treatment. In order to identify the most appropriate precursor for obtaining layers with desired composition and properties a systematic study of the sol-gel film preparation using different phosphorous precursors was carried out. The films were deposited on ITO/SiO2 coated glass substrates at room temperature. To check the influence of the type of precursors on the layers thermal stability and properties a post deposition annealing was performed at temperatures of 150 and 200 degrees C. The film characterization was carried out using various techniques as Spectroscopic Ellipsometry (SE), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR).

We report in this contribution on the photosensitization effect of ZnO nanostructured thin films with copper-phthalocyanine (CuPc). ZnO films were prepared by pulsed laser deposition (PLD), on optical glass substrates, covered with 300 nm thick ITO films. The structure and morphology of ZnO films were optimized for photovoltaic-cell applications by controlling the growing conditions. ZnO films (wurtzite type) were (001) preferentially oriented in the growth direction. Organic dyes were vacuum sublimated on ZnO films, and subsequently a 300 nm thick copper film was vacuum sublimated on top of the structure, acting as an ohmic contact. Action spectra of ITO/ZnO/CuPc/Cu photovoltaic structures were recorded and a well defined response was observed in the range of absorption bands of organic dyes. The dark IN characteristics were also recorded, and electrical characterizations of the photovoltaic structures are given. The measured fourth quadrant IN characteristics under integral illumination (photoelement regime), allowed us to extract the typical photovoltaic cells parameters.

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.