National Institute Of Materials Physics - Romania

X-band ESR spectra of the calamitic and discotic lyotropic nematic phases in sodium dodecyl sulphate (SDS)/decanol/water system, doped with paramagnetic nitroxide free radicals are analyzed in this paper. The studied temperature range was chosen to include phase transitions of the lyotropic systems. The variation of the surfactant hydrocarbon chains ordering versus temperature is determined by the hyperfine splitting tensor (,4) and the spectroscopic splitting tensor ((g) under bar) anisotropies. The temperature dependence of the molecular order parameters for both lyotropic systems and the corresponding angular fluctuations are presented and discussed.

For the single-walled carbon nanotubes, the higher values of anti-Stokes/Stokes Raman intensity ratio (I(as)/I(s)) then the predictions of the Maxwell-Bolzmann formula indicates a CARS emission resulting from a wave mixing process between the incident laser light (omega(l)) and Stokes Raman light (omega(s)) generated by a SERS mechanism. The weaker value of (I(as)/Is) observed at resonant excitation of semiconducting nanotubes in the E(22)(S) van Hove state, relates a quantum cooling process occurred during the relaxation of the E22S excited state in two steps-by phonon emission from E(22)(S) to E(11)(S) followed by a spontaneous luminescence emission at E(11)(S).

Leakage current measurements were performed on epitaxial, single-crystal quality Pb(Zr,Ti)O-3 films with thicknesses in the 50-300 nm range. It was found that the voltage behavior of the leakage current has a minor dependence on thickness, which rules out the space-charge limited currents as main leakage source. Temperature-dependent measurements were performed to obtain more information on the transport mechanism through the metal-ferroelectric-metal (MFM) structure. The results are analyzed in the frame of interface-controlled Schottky emission. A surprisingly low value of only 0.12-0.13 eV was obtained for the potential barrier, which is much smaller than the reported value of 0.87 eV [I. Stolichnov , Appl. Phys. Lett. 75, 1790 (1999)]. The result is explained by the effect of the ferroelectric polarization on the potential barrier height. The low value of the effective Richardson constant, of the order of 10(-7)-10(-6) A/cm(2) K-2, suggests that the pure thermionic emission is not the adequate conduction mechanism for epitaxial MFM structures. The true mechanism might be interface-controlled injection, followed by a low mobility drift through the film volume.

The iron oxide nanoparticles and iron oxide nanoparticles coated with dextrin have been synthesized using aqueous solution of ferric and ferrous ions and mixtures of dextrin with sodium salt. The size of the iron-oxide nanoparticles is controlled by the concentration of sodium salt in the medium. An average size of iron oxide and iron oxide coated with dextrin was found by transmission electron microscopy (TEM). The iron oxide nanoparticles are nearly spherical with an average diameter of about 8.0 +/- 1 nm. The iron oxide nanoparticles coated with dextrin appear as cluster-like aggregates. The average diameter of these nanoparticles is about 5.1 nm. The attachment of the dextrin on the particle surface was confirmed by FTIR spectroscopy and thermogravimetric analyses (TGA).

We model a small quantum dot with a magnetic impurity by the Anderson Hamiltonian with a supplementary exchange interaction term. The transport calculations are performed by means of the Green functions within the equation of motion scheme, in which two decoupling procedures are proposed, for high and low temperatures, respectively. The paper focuses on the charge fluctuations for such a system, an aspect not addressed before, as well as on the Kondo resonance. We show a specific role of the excited state, which can be observed in transport and in spin-spin correlations. Our studies show a many-body feature of the phase shift of transmitted electrons, which is manifested in a specific dip. In the Kondo regime, our calculations complement existing theoretical results. The system shows three Kondo peaks in the density of states: one at the Fermi energy and two side peaks, at a distance corresponding to the singlet-triplet level spacing. The existence of the central peak is conditioned by a degenerate state (the triplet) below the Fermi energy.

Two new hetero tri-nuclear oxalates (NH4)(8)[Fe2CO(C2O4)(8)]-6H(2)O and [Fe2Co(C2O4)(2)(OH)(4)]center dot 2H(2)O have been synthesized. The compound presented opposite behaviour in ac-susceptibility measurements, different frustration level and spin-orbit coupling in magnetization ones. The Mossbauer approach pointed to local interactions governed by metal-oxalate strongly coupled unit. The data were interpreted in terms of role of positive (NH4) or negative (OH) bridging ligands on general magnetic properties. (c) 2006 Published by Elsevier B.V.

We intend to prepare periodic multilayered structures for photonic applications. With this goal we have performed a study of some characteristics of SiO (x) -P2O5 films deposited by the sol-gel method on glass and ITO (InSnO (x) )-coated glass. The as prepared films were annealed to different temperatures (150 and 200 degrees C). The chemical composition of the samples was determined by X-ray Photoelectron Spectroscopy (XPS). The XPS results revealed the presence of P in the as-deposited films. The structural an optical properties were examined by Fourier Transform Infrared Spectroscopy (FTIR), Spectroellipsometry (SE) and UV Transmission Spectroscopy. IR spectra of the deposited films attest the interaction of an amorphous SiO2 with the H3PO4 used as a P-precursor. Refractive indices for individual SiO (x) -P2O5 determined from SE measurements show a densification of the layer structure with the increasing temperature in the thermal treatment. The UV transmission spectra revealed a lower transmission for the sol-gel SiO (x) -P2O5 films as compared to ITO/glass substrate. AFM images proved the densification of the films with annealing in agreement with the ellipsometric results.

The formation of the electron trapped Fe+(IV) centre, produced by X-ray irradiation at 80 K and further annealing at temperatures of up to 700 K in chlorinated SrCl2: Fe crystals, has been investigated by Electron Paramagnetic Resonance. Our studies report the transformation of the monoclinic Fe+(III) centre into the axial Fe+(IV) centre above 450 K. The formation of the Fe+(IV) centre is attributed to the presence and thermally activated movement of neighbouring interstitial chlorine and alkali impurity Ne ions. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The xZnO-(1-x)alpha-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x = 0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mossbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x = 0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x = 0.3, while for x = 0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mossbauer spectra corresponding to x = 0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mossbauer spectrum for 12 h of milling had to be fitted with two sextets. For x = 0.3 and 12 milling hours, the Mossbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe2O4. This doublet clearly dominates the Mossbauer spectrum for x = 0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications. (c) 2006 Elsevier Ltd. All rights reserved.

Using an acetate-alkoxide sol-gel route in which the precursors are barium acetate, yttriurn isopropoxide and titanium diisopropoxide bis-acetylacetonate, we prepared a ferroelectric material with the formula: Ba1-xYxTiO3, x = 0.005. SEM analysis showed a polymeric microstructure of the gel due to the chelated titanium alkoxide precursor used as starting materials. The evolution of the structure and microstructure of the precursor gel heated at temperatures up to 1000 degrees C was studied by various techniques. The powder obtained by heating the gel at 1100 degrees C presented a homogeneous structure consisting of submicronic particles (similar to 200 mm). XRD and SAED analyses revealed that Ba0.995Y0.005TiO3 nanocrystals of about 5-10 rim appeared at 600 degrees C, together with BaCO3. The presence of barium carbonate was identified also by IR spectroscopy and thermal analyses. The ceramics obtained from the as-prepared powder presented good dielectric properties (capacitance = 840 pF/dielectric constant = 3860 and dielectric loss (tan delta) 0.078 at Curie temperatures of 120-121 degrees C).