National Institute Of Materials Physics - Romania

In this paper, we recall the main characteristics of Raman spectra of single-walled carbon nanotubes. We illustrate our studies by results obtained on HiPCO samples on one hand, and "arc electric" samples on the other hand by using two different excitation wavelengths. We focus on the peculiarity of observing separately semi-conducting and metallic tubes. We emphasize also the role played by the radial breathing modes whose frequency depends on the tube diameter and on the need to take into account the interactions between tubes when they are in bundles in order to determine accurately the tube diameter distribution. Finally we demonstrate briefly the evidence of using the SERB (surface enhanced Raman scattering) spectroscopy to study reactions at interfaces or between components in nanotubes/conjugated polymers composites.

We have synthesized polymer particles of the type alternated copolymers DVB-maleic anhydride. The dimensions of the obtained particles have been determined by electron microscopy and are in the range 1-2 mu m. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), in a concentration of 5% b.w. (polymer/LC).

The high exciton binding energy and band gap energy of ZnO thin films open the prospect of fabricating semiconductor lasers in the ultraviolet spectral range. A prerequisite for laser diode fabrication is highly p-doped ZnO which was not reproducibly obtained up to now. Without intentional doping ZnO exhibits n-type conduction. ZnO thin films have been obtained by radio-frequency assisted pulsed laser deposition. A metallic Zn target was used for ablation in an oxygen and nitrogen RF discharge. The electrical and morphological properties of the films grown on Si were studied by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), optical absorption and Hall Effect measurements for different ratios between the nitrogen and oxygen content. The AFM images of the as grown ZnO films reveal high quality surfaces with low values for the surface roughness and a sharp distribution of grains sizes as an effect of the RF discharge. The XRD patterns for all samples exhibit only (002) and (004) peaks indicating that the c-axis is always oriented normal to the substrate surface. The films present p-type conductivity with different carrier concentration and mobility depending on the nitrogen/oxygen ratio. (c) 2007 Elsevier Ltd. All rights reserved.

Synthesis of CaCO3 was performed in ultrasonic field in order to improve its properties. The evolution of the precipitation reaction in ultrasonic field provided a solid phase with aragonite morphology slightly impurified with vaterite. Particles dimensions were diminished, the solid phase presenting an important fraction of nanometer size particles.

A study was made to determine the effect of sample shape, compacting pressure and green density gradients on the final shape and density of sintered bodies. Experiments were carried out on a PZT type powder, pressed with a large range of shapes from long cylinders up to thin discs, at pressures ranging between 40 and 180 MPa. Important shape distorsion were found for both long cylinders and thin discs but surprisingly the final density was not essentially changed even if the green compacted density showed important changes within the samples. Differences in shrinkage of diameter and thickness brought about by sintering for samples with different shapes were found.

The refined Fe-57 Mossbauer spectra of some static-disordered crystalline media (with melilite and Ca-gallate structure) evidenced observable electric and magnetic crystal field dispersions [1-4]. The data of the crystalline hyperfine fields and their dispersion parameters were calculated using the modeling procedure given in [5]. The obtained values of the magnetic and quadrupole splitting parameters compared with the experimental data showed the possibility to predict the dispersion of the crystal fields.

Scanning electron microscopy (SEM) and energy dispersive X-ray measurements reveal inhomogeneities in the distribution of elements in W-, Li- doped PZT materials with the composition PbZr0.51 Ti0.49-2.5x W1.5xLixO3 (x = 0.03 and 0.05). Increasing the tungsten concentration leads to an increase of inhomegenities number in the PZT matrix. The high sintering temperature and Li+ concentration contribute to the vitreous aspect Of the materials. X-ray diffraction patterns revealed the Presence of both rhombohedral and tetragonal phases in the two materials. Considering the materials as composites with a 0-3 connectivity pattern, a mathematical approach is proposed. This was used to calculate the contribution of the agglomerations to the experimental dielectric constant.

The Rayleigh scattering of Mossbauer radiation (RSMR) is a well-established and successful method for dynamics and structural investigations of ordered and disordered systems. The recoilless emission and absorption of low-energy gamma rays, combined with the high selectivity of the nuclear resonance absorption phenomenon, permits an energy resolution of the order of 10(-8) eV for the 14.4 keV transition of Fe-57. The results of our studies by RSMR on pyrolytic graphite (C) and rubidium tetrachlorozincate (Rb(2)nCl(4)) are given. The contribution of the coherent inelastic scattering to the total intensity in C(002) is described like a consequence of the coupling motions of C atoms in the hexagonal plan. The normal-incommensurate phase transition in Rb2ZnCl4 is discussed in connection with the photon-phonon interaction.

We report here on pentacene based organic field effect transistors (OFETs) with a high-k HfO2 gate oxide. HfO2 layers were prepared by two different methods: anodic oxidation and sol-gel. A comparison of the two processes on the electrical properties of OFETs is given. Ultra thin nanoporous (20 nm) sol-gel deposited oxide films were obtained following an annealing at 450 degrees C. They lead to high mobility and stable devices (mu = 0.12 cm(2)/V S). On the other hand, devices with anodic HfO2 revealed a little bit more leaky and show some hysteresis. Anodization, however, presents the advantage of being a fully room temperature process, compatible with plastic substrates. Stability and response to a bias stress are also reported. (c) 2006 Elsevier Ltd. All rights reserved.