National Institute Of Materials Physics - Romania

The intermediate, self-organized phase, in the As-Se system has been modeled on the basis of clusters with one or more selenium-selenium bridges. The X-ray diffraction patterns of the glass models at the limits of the 'floppy phase-stressed rigid phase' window have been calculated. The structural and optical anisotropy in a matrix of clusters containing Se-Se bonds is supposed to be a consequence of breaking and re-linking the broken clusters in the direction of the polarized light beam that acts upon the chalcogenide material. (c) 2006 Elsevier B.V. All rights reserved.

This paper presents the investigation of the optical properties of bisphenol A polycarbonate organic matrix/CdS clusters composite material in correlation with the method for the films' preparation using as precursors cadmium acetate (Cd(CH3COO)(2)), respectively ammonium thiocyanate (NH4CNS), and as general solvent for all the components, is dimethyformamide. The effect of the heat treatment temperature (90 degrees C or 150 degrees C) on the position and shape of the fundamental absorption edge was investigated for given substrate, drying conditions and duration (1.5 h or 0.5 h) of the heat treatment. A large blue shift of the absorption edge was evidenced in films prepared in vertical configuration and thermally treated at 90 degrees C for 1.5 h, which can be associated with a small dimension of the CdS crystallites and in consequence with quantum confinement effects. Using the effective mass theory we have deduced a crystallite dimension in the range 8.8-15 nm. We also have studied the interaction of the light with this composite material evaluating an optical band gap between 2.92 and 3.69 eV (depending on the preparation and annealing conditions) and an absorption mechanism in CdS clusters closer to an indirect transition and deviated from the normal direct absorption process in bulk CdS semiconductor.

This paper presents the optical absorption properties in crystalline meta-clinitrobenzene (m-DNB) and benzil. The optical band gap as an intrinsic property of these crystalline compounds has been studied using UV-VIS Spectroscopy on thin film. Analysing absorption near the fundamental absorption edge we have deduced the wide band gap semiconductor behaviour of these aromatic derivatives. Processing the experimental transmission data using a function obtained by the superposition of a linear function and a power function we have evaluated the band gap energy in m-DNB, E-g=2.90 eV and emphasised the same two edges patterns of the light absorption spectrum in benzil deposited on glass as for benzil deposited on quartz, with energetic thresholds slowly moved through lower energies, E-g1=2.79 eV and E-g2=3.49 eV. We have also remarked no significant change in the absorption spectrum in benzil induced by the dopant (organic and inorganic).

The electrochemical deposition in ion track nanoporous membranes was employed for ZnO micro and nanowires preparation. A ZnNO3 bath was employed for the deposition, performed at -800 mV (vs. SCE) and at temperatures around 90 degrees C. Scanning and transmission electron microscopy were used to characterize the wires from a morphological point of view. Electron and X-ray diffraction were employed for the characterization of materials from the point of view of structure. The technique allowed the fabrication of nanowire arrays with good morphological and structural quality.

Various closed nanocluster configurations based on the stoichiometric arsenic-chalcogen composition have been built and relaxed by a computer Monte Carlo procedure. They can be used in the construction of the glass network. A model with random packing of various large, closed nano-clusters has been devised and fairly good agreement with experimental data for As2S3 has been obtained.

A novel method based on thermo-ionic vacuum arc plasma was developed to grow metal-carbon zero stress films for MEMS applications. The method chosen to prepare metal-carbon films uses an electron beam emitted by an externally heated cathode accelerated by a high anodic voltage. Applying a high voltage (1-6 kV) between cathode and anode bright plasma was ignited in carbon atoms. In order to minimize the internal stress and to get films with increased adherence, interlayer and graded interlayer with Fe, Cr, Al and Ni was deposited. The prepared films were characterized by XPS, XRD, AFM and ellipsometry.

This paper presents a study of the nonhomogeneities in doped meta-dinitrobenzene (m-DNB) crystalline films. These growth defects lying in the film plane with dimensions smaller than the films thickness are correlated with the effect of the dopant on the crystallization regime in the dendritic growth conditions. To evaluate the dimension of the growth nonhomogeneities we have used a model based on the Kubelka-Munk theory, emphasising the general constraints imposed to the sample and the experimental set-up, and developing a more complex model through a step-by-step relaxation of these constraints, that ensures a better approximation for our experimental configuration. We have deduced higher scattering coefficients in m-DNB doped with 1,3 dihydroxybenzene (resorcinol) than in m-DNB doped with 8-hydroxyquinoline (oxine) films prepared by a rapid thermal solidification process. We also have emphasised that for the films of m-DNB doped with oxine the size of the scattering centers is larger than the wavelength of the radiation and in films of m-DNB doped with resorcinol the radiation scattering corresponds to a multiple scattering process leading to the superposition of different scattering mechanisms on centers with different dimensions.

We present some investigations on the electrical conductivity of two benzene substituted derivatives (m-DNB, benzil) emphasizing the correlation between the molecular structure, purity, structural defects and the particularities of the conduction mechanisms. The influence of inorganic/organic doping on the I-V plots of silicon/wide-gap organic semiconductor/silicon heterostructures has been analysed. The most significant increase in the conductance has been obtained for organic crystalline films of benzil doped with 3 wt.% m-DNB and m-DNB films doped with I wt.% oxine or 10 wt.% resorcinol. The influence of the silicon wafer's properties as resistivity, conduction type and surface processing on the carrier transport properties in these structures has been studied. We have remarked an increase in the conductance of the organic films of m-DNB doped with oxine or resorcinol in heterostructures realized with chemically polished, "n" type single crystal silicon wafers, compared to lapped ones. A special type of conduction mechanism given by a Poole-Frenkel dependence was evidenced in resorcinol doped m-DNB in the low voltages range. (c) 2005 Elsevier B.V. All rights reserved.

The main purpose of this paper is to investigate the optical properties of PTCDA and Alq3 films, prepared by two steps, vacuum evaporation and deposition processes on platelets of glass, quartz, and indium-tin-oxide (ITO) coated glass. We have emphasised the bands structure of the absorption spectra with peaks situated at 358 nm, 374 nm, 475 nm and 552 nm in PTCDA, respectively 232 nm, 261 nm and 380 nm in Alq3 that confirms the dominant presence of Alq3 meridianal molecular isomer. For PTCDA films deposited on glass coated with ITO, the structure of the weak double peak at low wavelength is partially modified, but the positions of the two important absorption peaks situated at 2.25 eV and 2.61 eV are unchanged. The two different luminescence emission peaks obtained in Alq3 for different excitation wavelengths (gimel=360 nm and gimel=520 nm) suggest the existence of the facial isomer beside the meridianal one. We have evidenced a significant Stocks shift in the spectra (Delta E-PTCDA=0.40 eV; Delta E-Alq3=0.9 eV) and a large Frank-Condon shift (0.40-2.3 eV), suggesting important effect associated respectively with the solid state structure and important conformational differences between the ground and excited state.

Vibrational properties of composites based on single-walled carbon nanotubes (SWNTs) and conducting polymers of the type polyaniline (PANI) and poly (3,4-ethylene dioxythiophene) (PEDOT) are reported. For PANI-functionalized SWNTs, the intensity increase of the Raman band at 178 cm(-1) , associated with radial breathing modes of SWNTs bundles, indicates an additional roping of nanotubes due to the presence of the polymer. The interaction of this composite with NH4OH solution involves an internal redox reaction between PANI and SWNTs. Thus, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. The functionalization of SWNT side walls with PEDOT is invoked as well.