National Institute Of Materials Physics - Romania

Pulse shaping introduces the method that makes possible the production of tunable arbitrary shaped pulses. We extend this method to control the prevalent growth of cubic SiC films on Si (10 0) substrates by pulsed laser deposition at temperatures around 973 K from a SiC target in vacuum. We used a laser system generating 200 fs pulses duration at 800 nm with 600 mu J at 1 kHz. The obtained structures are investigated by electron microscopy, X-ray diffraction and profilometry. We observed grains embedded in an amorphous texture, characteristic in our opinion to the depositions obtained with very short pulses. We present a comparison of deposited films with and without pulse shaping. Pulse shaping promotes increased crystallization and results in the deposition of thin structures of cubic SiC with a strongly reduced density of particulates, under similar deposition conditions. (c) 2005 Elsevier B.V. All rights reserved.

We achieved the growth of cubic silicon carbide (SiC) films on (100)Si substrates by pulsed laser deposition (PLD) at moderate temperatures such as 750 degrees C, from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The morphology of deposited films is dominated by columns nucleated from a thin nanostructured beta silicon carbide (beta-SiC) interface layer. The combined effects of columnar growth, tilted facets of the emerging columns and the presence of particulates on the film surface, lead to a rather rough surface of the films. (c) 2005 Elsevier B.V. All rights reserved.

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a beta-diketonate-alkoxide iron(III) dimer compound, [Fe-2(OCH3)(2)(dbM)(4), Hdbm = diberizoyhriethane] (Fe-2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). The diamagnetic Substitution method. was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga-Fe dimers(GaFe). The single-ion zero-field splitting(ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe-O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S = 1, 2, 3, which were analyzed using the full Spill Hamiltonian description of all exchange coupled pair of s = 5/2 spins. The anisotropic spin-spin interaction tensor of Fe2 possesses a principal axis close to the Fe-Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves ail rhombic component J(E) = (J(X) - J(Y))/2 approximate to 0.093 cm(-1) of magnitude comparable. to the dipolar interaction, and even to the rhombic part of the single-ion ZFS (E = 0.097 cm(-1)). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe-4 and Fe-8 families of single-molecule magnets and the anti Ferrornagnetic iron wheels. (c) 2005 Elsevier Inc. All rights reserved.

The transport properties of nanocrystalline silicon films are presented. The studies were performed on fresh and stabilized nanocrystalline porous silicon (nc-PS) and on Si/SiO2 nanocomposite films. Current-voltage characteristics and the temperature dependence of the dark current were measured on both systems. From the current-voltage characteristics of the oxidized nc-PS, the Si-SiO2 potential barrier was determined (2.2 eV). The Si/SiO2 measurements proved that the main conduction mechanism for the nanodot systems is the field-assisted tunneling under Coulomb blockade. The quantum confinement was evidenced by the activation energies observed in the current-temperature characteristics and it was modeled by means of an infinite quantum well. The trapping-detrapping processes were studied by using the Optical Charging Spectroscopy (OCS) method: the traps are filled by illumination with a suitable wavelength at low temperature and the detrapping is produced by heating at a constant rate. The experimental investigations evidenced six trapping levels in nc-PS, four located at the surface/interface and two deep ones located in the bulk. The deepest level was observed only in oxidized samples. The modeling of the OCS trapping-detrapping processes allows for the determination of the characteristic parameters that cannot be directly obtained from the measurements.

The properties of the electron paramagnetic resonance spectra of Gd3+ ions in PbWO4 single crystals have been investigated in the X-band microwave frequency region, in the 1.5 to 290 K temperature range. The observed S-4 symmetry of the local crystal field at the Gd3+ impurity ions strongly suggests that the Gd3+ ions substitute for the Pb2+ lattice cations, with charge compensation at a distance. The spin Hamiltonian parameters are comparable with those of the Gd3+ ions in other isomorphous tungstates. The temperature variation of the fine structure parameters B-2(0) and B-4(0) points to an energy transfer between the impurity ions and the host lattice, which takes place mainly through a local vibrational mode of frequency omega = 3.1 x 1013 rad s(-1).

EPR and TEM investigations on sublimated solid Alq(3), [tris(8-hydroxyquinoline)aluminum(Ill)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.

Unusual conductivity behavior, is observed in the semiconducting phase of granular manganites. It is characterized by, at a certain temperature, of a switching process between two activation energies. A core-shell model is developed to explain the presence of the conductivity of the two activation energies. The model suggests that inside the disordered thin outer layer the energy bands are split into isolated energy levels. Additional correlations between paramagnetic behavior and the existence of the two activation energies within the core-shell model are furnished by the ESR spectroscopic technique. (c) 2006 Published by Elsevier Ltd.

A microstructural investigation on polycrystalline electroluminescent Alq(3) obtained by train sublimation method is reported in the paper. The morphology and crystalline structure of the obtained Alq3 phases have been analyzed by transmission electron microscopy performed on samples collected from 3 different regions along the sublimation tube.

X- and Q-band electron paramagnetic resonance (EPR) investigation of different crystalline Alq(3) (tris(8-hydroxyquinoline)aluminium (III)) fractions formed by a train sublimation method are reported. Several paramagnetic defect centres corresponding to 1/2, 1, and 3/2 spin are observed at room temperature. Their intensity is dependent on the temperature, nature of the crystalline phase, and preparation conditions. Spectra simulation and analysis based on the spin Hamiltonian appropriate to a high spin system (S >=, 1) suggest the existence of randomly oriented triplets and quartets in annealed Alq(3) fractions. The crystalline Alq(3) phases responsible for the EPR powder spectra have been identified by transmission electron microscopy measurements performed on these sample fractions.

The modification of the polyester track membranes by treatment in radiofrequency plasma generated by capacitive coupled parallel plate discharge in air and ammonia was studied. The investigation of the surface properties (roughness, pore size distribution, contact angle) before and after the plasma treatments was performed. The treatment resulted in increase of the surface roughness and of the pores effective diameter and change of wettability. Membranes presenting lower or higher contact angles, as compared with the initial one can be obtained by selecting adequate values for the radiofrequency power and treatment time values. (c) 2005 Elsevier B.V. All rights reserved.