National Institute Of Materials Physics - Romania

We report some spectroscopic arguments showing the guest-host interactions in the case of the composites containing Jacobsen salen complex and nanoporous molecular sieves of different types: SBA-15, MCM-48, MCM-41. The complex was embedded by impregnation, but care was given for washing out the weakly anchored part. Siliceous nanoporous materials lead to a weaker interaction than A1 containing forms. Thermal analysis has shown that at least in the case of the composite containing MCM-41 the embedded complex still contain chloride ions, therefore the bonding to the surface was apical. In situ experiments indicate that the thermal stability of the embedded complex is higher than in the free state. The differences among the composites coming from the structure of the molecular sieves are discussed.

Arsenic is the main component of many chalcogenide glasses. Due to its triple co-ordination arsenic has the tendency to form two-dimensional networks in both crystalline and amorphous state. Besides the layered form of arsenic it exists molecular arsenic. Small molecules are characteristic to this form of arsenic. Large arsenic clusters have been not identified up to now in amorphous films. A modelling study was carried out in order to demonstrate the possibility to find or to prepare fullerene-like configurations based on arsenic as well as nanotubes (or nanowires) of various diameters.

Single-walled carbon nanotubes (SWNTs) under a moderate non-hydrostatic pressure, of 0.58 GPa, undergo a non-reversible transformation. Due to the plastic deformations, structural defects and carbon nanotube fragments of different size are produced. Short fragments of spherical or ellipsoidal form, behaving as closed-shell fullerenes are observed both by Raman spectroscopy and photoluminescence (PL). The main vibrational indicative of the fragments of shorter length is the band at similar to 1458 cm(-1) that is regularly observed in the Raman spectra of fullerene. Similar to fullerenes self assemblies, the interaction between the nanotube fragments is noticed in the Raman spectrum by the band at similar to 94 cm(-1) that reveals an inter-particle vibration mode. When the nanotubes are dispersed into host matrix, as aromatic hydrocarbons (AHs) with isolated or condensed phenyl rings, supplementary mechanico-chemical reactions take place. For AHs with isolated phenyl rings, like biphenyl or p-terphenyl, a chemical functionalization of SWNT fragments is demonstrated by the appearance of a new Raman band at 1160 cm(-1). In PL spectra, the interaction of SWNTs with AHs, is noticed by a detailed vibronic structure appearing in the high energy side of the emission spectrum, that increase with the weight of SWNTs in the AHs/SWNTs mixtures.

The structure of a glassy sample of Ge30As4S66 has been measured by means of synchrotron radiation. X - ray diffraction data were processed and the atomo-electronic radial function (AERDF) was calculated. The high resolution AERDF curve allowed for an accurate determination of the first order distances between the atoms and the coordination numbers. A model of spatial structure, based on the concept of raft configuration was tested. An other model of the structure based on the continuous random network concept (CRN) was tried. The general conclusion drawn from this study supports the idea that raft and disordered configurations interlinked so that the dangling bonds in the structure be minimized, is the best model for such type of glass.

Correlated studies of Surface Enhanced Raman Scattering (SERS) spectroscopy, photoluminescence (PL) and viscosity on aggregation of C-60 fullerene in N-methyl-2-pyrrolidinone (NMP) solutions were performed. The aggregation is a result of storing of the solutions at the room temperature for different periods of time. The modified phonon spectrum in the aggregate phase is a consequence of the intermolecular interactions that lead to a large reduction in parent I-h C-60 symmetry. The Raman range below 800 cm(-1) is the most diagnostic for aggregate assignment. Substitution of the original of H, (1) band from similar to 269 cm(-1) with a new one at similar to 255 cm(-1) relates the formation of (C-60, NMP) complexes. In the second stage of aggregation, this band shifts at similar to 246 cm(-1) and a new band, associate to the interball interaction, is formed at similar to 94 cm(-1). PL emission of the C-60 solutes in NMP presents two components, one in red (1.92-2.00 eV) and another in green (2.18-2.3 eV), which originate in two successive processes. Their weights depend on C-60 concentration and on storage time. In the beginning are formed quickly molecular complexes (C-60, NMP) by charge transfer, which on associate slowly in stable aggregates. A dependence of specific viscosity on C-60 solution concentration is observed too.

Alignment properties of rubbed films of polyvinyl imidazole were studied by observing with the polarizing microscope the features of the nematic liquid crystal (LC) droplets onto these layers. It was found that the observing of the droplets can reveal the direction of easy axis on the alignment substrate not only in the case of LC having the director normal to the LC/air interface but also in the case of the director tangent to this interface, when no disclination line could be seen. We observed that some of the LCs are oriented parallel to the rubbing direction, but the other LCs are perpendicularly oriented. This peculiar behavior is at variance to that found on rubbed layers of polyvinyl alcohol. The role of the end-groups in the structure of the LCs on the alignment properties is discussed.

Zinc oxide thin films were obtained by laser ablation of a Zn target in oxygen reactive atmosphere, the oxygen being supplied either by a standard gas inlet valve or from a radio-frequency (rf) oxygen plasma. Pt-coated silicon and MgO were used as substrates. The influence of the deposition parameters as laser wavelength (266, 355, 1064 nm), laser fluence (1.5-20 J/cm(2)) and oxygen pressure (1-60 Pa) was studied. The influence of the rf plasma beam addition on the morphological proprieties of zinc oxide films was particularly investigated, simultaneously with several configurations of the direction of the ablation plasma, the rf plasma beam and the substrate. The obtained films, with thicknesses in the range of 50 nm to 1 mu m have been characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM). (c) 2005 Elsevier B.V. All rights reserved.

Thin films of zirconium tin titanate, (Zr0.8Sn0.2)TiO4 (ZST) were deposited using a pulsed electron beam source based on a channel-spark discharge for target ablation. An advanced degree of crystallization was obtained for the films deposited on alumina substrate post-annealed at 1000 degrees C. The crystalline lattice constants of the films are very close to those of the target material, which confirms the same stoichiometry in ZST films and in the bulk. (c) 2005 Elsevier B.V. All rights reserved.

Surface enhanced Raman scattering with 676.4 and 1064 nm excitations was used to investigate single-walled carbon nanotubes (SWNTs) compressed non-hydro statically at 0.58 GPa, alone or dispersed into chemical reactive and non-reactive host matrices. By compression, SWNTs break in fragments of different size. Between SWNTs fragments and the host matrix various mechanico-chemical reactions take place. In matrices such KI and Ag, donor-acceptor complexes are formed. In aromatic hydrocarbons like biphenyl or p-terphenyl, an ionic and covalent functionalization of SWNTs fragments is demonstrated by new bands at 1160 and 1458 cm(-1), the latter being associated to an A(g)(2) pentagonal pinch mode. (c) 2005 Elsevier B.V. All rights reserved.

Smooth a-axis oriented ZnO thin films are grown on (001) MgO substrates by pulsed laser deposition assisted by a very directional radio frequency oxygen plasma. The structure of the film is examined by X-ray diffraction, electron diffraction and high resolution electron microscopy. The film grows with vertical columns and is highly textured. Two preferential orientation relations between the ZnO film and the MgO substrate are observed: (10010)(ZnO) vertical bar vertical bar (002)(MgO); [0002](ZnO)vertical bar vertical bar[2 0 0](MgO) and (1010)(ZnO) vertical bar (002)(ZnO) vertical bar vertical bar [200](MgO) [1210](MgO). The film/substrate interface is flat and strained because of the rather large lattice mismatch between the (1010) plane of ZnO and the (002) plane of MgO. (c) 2005 Elsevier B.V. All rights reserved.