National Institute Of Materials Physics - Romania

Ba0.6Sr0.4TiO3 (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance-voltage (C-V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.

The magnetron sputtering method was used to obtain layers of indium tin oxide or hydroxyapatite on glass fiber woven and on reference glass plates. The layers were characterized by scanning microscopy, UV-vis spectroscopy and conductivity measurements. Hydrophilic properties of the woven samples were investigated by optical microscopy following the behavior of a water droplet upon the each of the samples. The change of the drop profile in time due to wetting of the fabric leads to the wetting kinetics. The most efficient treatments to improve the hydrophilic/wetting properties of the glass fiber woven were found.

Single-walled carbon nanotubes (SWNTs) may be used in technological applications as individual entities instead of bundled tubes. The Raman signature of these two forms is still a controversial subject. The profile, peak position and relative intensity of the Raman band associated with the tangential vibration modes (G band similar to 1600cm(-1)) and radial vibration modes (RBM similar to 100-200cm(-1)) are the main parameters which can reveal the dispersion state of nanotubes. Resonant Raman scattering and absorption spectroscopy on SWNTs furnish complementary information concerning the efficiency of the nanotube dispersion in individual entities. (C) 2008 Elsevier B.V. All rights reserved.

Giant optical fields locally generated in nanoscale metal structures through the local excitation of surface plasmons underlie a variety of nonlinear optical processes. Using silver and gold rough metallic films, reproducible enhancements of the Raman signal on various materials including carbon nano-tubes have been demonstrated. In addition, an abnormal anti-Stokes Raman emission, reminiscent of Coherent anti-Stokes Raman Scattering (CARS), is accurately observed on nano-metric thin films. We demonstrate that under a tight focusing of the excitation light, a CARS-type emission, resulting from a wave mixing process between the incident laser light and Stokes Raman light generated by a SERS mechanism, is produced. The results which corroborate the CARS emission are the following: a square relation between the CARS signal intensity and the film thickness; a square relationship between the CARS signal intensity and the exciting laser intensity; a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes signal; a polarization ratio in the anti-Stokes side always greater in comparison with that measured for an usual Stokes Raman signal. We demonstrate also that this mechanism may contribute to a clear identification of the state of the observed nanotubes, i.e. in isolated or bundled forms, even when tubes are in powders. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electrical memory elements based on Ge-Sb-Te pure and doped by Sn-Se have been obtained by pulsed laser deposition on special substrates covered by gold as well as on common glass. A set of electrical memory elements in a 4x4 matrix structure on the glass substrate has been produced. Devices with 3 and 10 memory elements have been constructed and tested for their memory properties. The special features of the voltage-current characteristics have been revealed.

Ceramics (Ba1-xSrx)TiO3, (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO3, SrCO3 and TiO2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0 wt.% MgO and 1 wt.% MnO2. The sintering treatment was performed in die temperature range 1200 to 1260 degrees C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: rho(x)=5.4(2)-0.8(8).x, (g/cm(3)). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure BST for x <= 0.25, whereas a uniform one was noticed for doped samples of x = 0.25. Well faceted grains of similar to 5 pm size, Could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1kHz, in the temperature range - 150 to + 150 degrees C, at a pace of similar to 2 degrees C /min. Sharp transitions, with higher peaks values of permittivity were noticed with the increase of the sintering temperature. A shift Of the Curie points to lower values was noticed with the increase of Sr content, according to the equation: Tc = 127.4-331.x. Measured at room temperature, in the frequency range of similar to 1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x <= 0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices. (C) 2008 Elsevier B.V. All rights reserved.

Transparent oxyfluoride glass ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing CaF2:Eu3+ nanocrystals were produced by annealing of the initial glasses slightly above the CaF2 phase crystallisation peak. The relative good transparency of these materials was due to the small 65 nm, nucleated CaF2 nanocrystallites. X-ray diffraction data have shown an increase of the cell parameters for CaF2:Eu3+ compared with CaF2 which was attributed to the substitution of Ca2+ ions with Eu3+ ions in the CaF2 nanoparticles. Optical investigations using UV-VIS-NIR absorption and luminescence spectroscopy have shown an increase of the splitting of the Eu3+-ion associated luminescences and small shifts of the Eu3+ ion characteristic IR absorption peaks. These effects have been attributed to the Eu3+ environment evolving from the glass to the crystalline-like one.

Semiconducting single-walled carbon nanotubes resonantly excited by the interband E-22(S) electronic transitions (at 1064 nm) display for the two components of the radial Raman band-one associated with the isolated tubes and the other associated with the bundled tubes-an anti-Stokes/Stokes Raman intensity ratio (I-aS/I-S) which deviates oppositely from the predictions of the Maxwell-Boltzmann formula. A cooling and heating vibration process, evidenced by an enhancement and diminishment of (I-aS/I-S), appears in the isolated and bundled nanotubes, respectively. Here we confirm a cooling process, observed only for semiconducting nanotubes, which emerges from the relaxation of the E-22(S) excited state by the electronic relaxation from E-22(S) to E-11(S) that precedes the spontaneous luminescence emission at E-11(S). Metallic nanotubes do not exhibit luminescence and no cooling effect is observed. Both semiconducting and metallic nanotubes show for the bundled component of the radial Raman band an enhancement of (I-aS/I-S) such as is frequently observed in a coherent anti-Stokes Raman scattering process.

Carbon Nanotubes are exotic low-dimensional systems, and due to their remarkable properties, are promising components for technological applications. The electronic and vibrational signatures raising from their existence as individual entities or associated in bundles, is still a controversed subject. A careful analysis of the G (similar to 1600 cm(-1)) and RBM (100-200 cm(-1)) band features as: profile, peak position and intensity in the Raman spectra may be a valuable indication about their dispersion state. The UV-VIS-NIR absorption spectrum comes to support this assumption. Observing the absorption spectra associated to resonant excitation of semiconducting and metallic nanotubes, may reveal complementary informations concerning the individualization procedure efficiency.

ZnO nanowires were electrodeposited by using a template approach. Ion track polycarbonate foils (30 mu m thick) were used as templates. Current-voltage (I-V) characteristics were recorded in the temperature range 40-300 K. I-V characteristics show a symmetric, non-linear shape, at temperatures greater than 240 K and voltages greater than 15 V. At lower temperatures, linear characteristics were recorded for the voltage range used. The temperature dependence of the electrical resistance is activated, with activation energy of 0.42 eV at temperatures greater than 240 K. (C) 2007 Elsevier B.V. All rights reserved.