National Institute Of Materials Physics - Romania

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.

This work concerns the preparation and characterisation of homogeneous bulk Co2Mn (Si, Ge, Ga, Sn Sb0.8Sn0.2) to be used as targets for pulsed laser deposition of magnetic contacts for spintronic devices. The samples have been prepared by direct synthesis from elemental powders followed by vertical gradient freeze. Characterisation of their structural properties and compositional homogeneity was performed by X-ray diffraction and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy. The results show one-phase for the Heusler alloys. The Curie temperatures of the samples have been measured by both thermogravimetric analysis and differential scanning calorimetry and are in a good agreement with the previously published values. The saturation magnetisation values are close to the values for single crystals. (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.

The temperature variation of the fine and hyperfine parameters of Mn2+ in single crystals of PbWO4 in the low temperature range reveals the presence of a resonant mode of frequency omega = 8.8 x 10(12) rad s(-1). Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T-2 variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn2+ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.

TiO2 multilayer films were deposited by the sol-gel method on glass and SiO2/glass substrates. 10% of Cu was added to the coating solution, to study its effect on the amorphous to anatase phase transition, with a view to improving the photocatalytic activity. Densification of the films was obtained by thermal treatment at 500 degrees C for 1 hour in oxidative and reductive conditions. In order to check the correlation between the influence of copper, substrate and annealing temperatures on the structural, morphological and optical properties, the samples were characterized by FTIR, XPS, SEM and fractal analysis. Transformation of CuO from as-prepared samples into Cu2O and metallic Cu, depending on the treatment conditions, was shown. The composition and stoichiometry of the oxide films in the first atomic layers were quantitatively determined as chemical state relative concentrations.

In this work, we present a detailed high frequency characterization of conventional and optimized Gunn-diodes containing a graded gap injector. It is shown that the Al-concentration in the hot electron injector influences the occupation of the Gamma- and L-valleys and hence the electron drift velocity in the active Gunn layer. Quantitative values for the occupation of the L-valley can be extracted from the S-parameter measurements. (C) 2004 Elsevier Ltd. All rights reserved.

The present paper investigates the interface trap density of a new high-kappa gate dielectric stack, La2Hf2O7/SiO2 on Silicon. Amorphous La2Hf2O7 thin films are deposited by metal evaporation in the presence of atomic oxygen beams on an ultra-thin SiO2 layer (1.5 nm) grown by rapid thermal oxidation on a p-type Si substrate. A combination of electrical (C-V) and cross sectional TEM measurements indicates a value of the dielectric constant kappa of about 19 +/- 2.2. The interface states density (D-it) was measured using the conductance method for different La2Hf2O7 thicknesses ranging from 3 nm to 14 nm. Dit ranges from 3.4 x 10(10) eV(-1) cm(-2) to 4.8 x 10(12) eV(-1) cm(-2) and shows a quasi-linear dependence on the La2Hf2O7 layer thickness. The interface state density increase with film thickness could have different explanations, such as oxygen de-passivation and/or defect generation at the Si-SiO2 interface, formation of a new Si-SiO2 interface by Si oxidation or alternatively, the SiO2 interfacial layer reduction by oxygen vacancies.

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 behavior of two semi-interpenetrating polymer networks (S-IPNs) based on polyurethane (PU) and epoxy maleate of bisphenol A (EMBA) under exposure to UV radiation for long period of times (up to 200 h) was investigated. The networks obtained by the specific mixture under certain thermal treatment included in their structures crosslinked EMBA and linear PU. Advanced photooxidative degradation took place for long time of UV irradiation (over 160 h, especially), with almost a total destruction of the raw structures. The important modification of both the surface and of the morphology of the tested samples with the time of irradiation was also analyzed. Simultaneously, as compared to results obtained for short time of UV radiations (up to 10 h), where the mechanical properties of the tested samples improved, long time of UV radiations led to deterioration of many of the mechanical properties. (C) 2004 Elsevier B.V. All rights reserved.

Effects of electronic charge disproportionation were observed by Fe-57 Mossbauer spectroscopy at low temperatures in substituted LaFeO3 perovskites as (La1-xCax)FeO3 and La(Fe1-xMgx)O-3 (xless than or equal to0.5). The presence of both Fe3+ and Fe5+ species was evidenced in both series of compounds. Direct evidence for disproportionation was supported by correlating the Mossbauer results with the chemical determination of tetravalent iron at RT. The evolution of hyperfine parameters vs. substitution degree was comparatively analysed for Fe3+ and Fe5+ ions in relation to their electronic microscopic origin. Electron delocalisation processes are discussed in terms of ion distribution and valence state. (C) 2004 Elsevier B.V. All rights reserved.