National Institute Of Materials Physics - Romania

Amorphous thin films of langasite and YAG have been prepared from crystals targets by pulsed laser deposition, at room temperature, in vacuum, on silicon wafers. Bulges of micrometer size are formed on the film surface. Larger size bulges (10-50 micrometers in diameter) are characteristic to the annealed langasite films. Annealing at high temperatures leads to the crystallization of the films. The annealed langasite films (850 degrees C) are polycrystalline and partially oriented with the plane (001) parallel to the surface of the silicon wafer. The bulges break easily in the heat treated films. Their empty structure was remarked on the electron microscope images. The YAG films annealed at 1100 degrees C shows a crystalline YAG phase depleted in Y and traces of Y2O3. An interesting feature is the presence of silver particles spread along the macrodefects (fracture lines) of the film.

Metal-doped benzil crystals have been grown by thermal gradient solidification in a vertical transparent growth configuration to investigate the effect of metallic guest on the ordered organic host. We have identified the conditions for growing homogeneous, optically good crystals of benzil doped with sodium and silver, limiting the effect of supercooling, low thermal conductivity and anisotropy of the growth speed (temperature gradient at the liquid-solid interface: 10-25 degrees C, moving speed of the growth interface 2.0 mm/h). The nature and concentration of the dopant are parameters affecting, through the growth process, the crystalline perfection and the optical properties of the organic matrix. Bulk optical characterisation, by spectrophotometrical methods, has offered details on some intrinsic properties of the system metal particles/benzil crystalline matrix. Analytical processing of the experimental data emphasised that benzil is a wide optical band gap organic semiconductor E-g = 2: 65 eV: We also have investigated the effect of sodium and silver on the properties of benzil crystal as potential transparent semiconductor matrix for (nano) composite metal/molecular organic material. With the increase of sodium concentration from c = 1 to 6 wt%, a small narrowing of the band gap has been remarked. The same behaviour has been found for benzil doped with silver (c = 2wt%) compared to pure benzil. (C) 2004 Elsevier B.V. All rights reserved.

A model for the structure of copper-tin liquid alloy has been developed using the standard reverse Monte Carlo method. The interstitial void structure (size distribution) was analysed. The effects of various kinds of voids (small size and large size) on the interference function and radial distribution function were investigated. Predictions related to the formation of some ternary alloys by filling the interstices of the basic alloy were advanced.

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

The photostimulated luminescence (PSL) from Eu-doped fluorobromo- and fluorochlorozirconate glass-ceramics is reviewed. The PSL is attributed to the characteristic emission of Eu2+ present in BaX2 (X = Br, Cl) crystallites which form in the glass upon thermal processing. Both the photoluminescence intensity and the PSL efficiency increase with the size of the crystallites in a way that is consistent with a PSL-inert surface layer surrounding the crystallites. The increasing crystallite sizes also result in decreased optical transparency at the stimulation wavelength due to scattering. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using Raman scattering and Fourier transform infrared (FTIR) spectroscopy, we show that the vibrational properties of single-walled carbon nanotubes (SWNTs) functionalized with conducting polymers (CPs), such as polyaniline (PANI) and poly (3, 4-ethylenedioxythiophene) (PEDOT), are different depending on the synthesis method. In the case of SWNTs electrochemically functionalized with PANT, the increase in intensity of the Raman band at 178 cm(-1), associated with radial breathing modes of bundled SWNTs, indicates that an additional roping of nanotubes with PANT takes place. Besides, the functionalization of SWNTs with PANT induces strong steric hindrance effects observed in FTIR spectra by the enhancement of bands at ca. 740-750 and 772 cm(-1) associated to the deformation vibrations of the benzoid and quinoid ring, respectively. Raman spectra recorded after the electropolymerization of 3, 4-ethylenedioxythiophene (EDOT) on the SWNTs film reveal that a side-wall functionalization takes place. (c) 2005 Elsevier B.V. All rights reserved.

Raman spectroscopy and cyclic voltammetry (CV) are used for the investigation of the oxidation-reduction processes of single-wall carbon nanotubes (SWNTs) films in an HCl 0.5 M solution. In the potential ranges (+100; +800) and (0; +1500) mV vs. saturated calomel electrode (SCE), the oxidation-reduction reactions of SWNT films, in both aqueous and semi-aqueous HCl 0.5 M solutions, are reversible and irreversible, respectively. At potentials higher than +1000 mV vs. SCE, SWNTs break in fragments of different sizes. A post-treatment with an alkaline solution results in a partial restoration of the SWNTs. The functionalization of SWNTs with polydipherylamine (PDPA) is presented as an application of the reversible oxidation-reduction reactions of nanotubes in an HCl 0.5 M solution. In order to differentiate between a deposition of successive layers of the polymer and a functionalization process, a spectroelectrochemical study of the deposition of polypyrrole (PPY) on SWNT films was carried out, too. (c) 2004 Elsevier B.V. All rights reserved.

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C-60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-1) is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C-60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-1), which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size. (C) 2004 Published by Elsevier Ltd.

We review our experimental data obtained by Raman spectroscopy on single-walled carbon nanotubes (SWNTs) using surface-enhanced Raman scattering (SERS). We focused on the study of the dependence of the SERS spectra of SWNTs on both the rough metallic substrate type and the film thickness. In particular, we show that degradation of the nanotubes, mainly metallic, occurs for very thin films with the formation of graphitic, fullerene-like or amorphous carbon particles. We show also that SWNTs reacting with H2SO4 form a salt of the hydrogensulfate type similar to that resulting from graphite-H2SO4 interaction. SWNTs exhibit a complicated spectroelectrochemical behavior in aqueous and non-aqueous H2SO4 solutions that depends also on the sweep potential range. We demonstrate that the oxidation-reduction processes have a quasi-reversible or irreversible character. Finally, SERS studies on compressed SWNTs in different host matrices, inorganic and organic, reveal nanotube breaking and that tube fragments may react or not with the host matrix used. Copyright (c) 2005 John Wiley & Sons, Ltd.

UV-VIS transmission and fluorescence spectroscopy have been used to study some intrinsic properties of crystalline films of pure metadinitrobenzene (m-DNB) and, pure and doped benzil. We have evaluated the band gap energy, E-g = 2.92 eV, emphasising the near classical semiconductor behaviour of m-DNB. The two bands split of the (n, pi*) level is assumed to be the origin the two edges absorption in benzil films with the energetic thresholds at E-g1 = 2.84 eV and E-g2 = 3.55 eV. The red shift of the emission peak in m-DNB films has been correlated with the process of self-absorption of the emitted radiation in the thicker film. The effect of the impurities on the shape and position of the emission peaks in benzil has been investigated and we have observed no significant shift of the absorption peak situated at 3.25 eV. We also have identified a three steps process for the relaxation of the excited crystalline lattice of benzil involving the geometrical change of the (S-1) excited state, the intersystem crossing and the radiative decay by phosphorescence with a peak at 2.30 eV. The small blue shift of this emission peak in benzil highly doped with sodium has been attributed to the modification of the molecular geometry of benzil as a result of the change in the trans-planar molecular configuration induced by the chemical reaction between the atoms of alkali metal and the carbonyl groups. (C) 2004 Elsevier B.V All rights reserved.