Publications

Nanostructures of gold embedded in potassium chloride matrices were obtained after the doped crystals growth, by electrolytical colouring, and by thermal treatment. When the TEM images of the nanostructures were analyzed using fractal geometry it means was proved that the self assembling of the metallic structures is governed by the diffusion limited aggregation (DLA) mechanism. The values of the fractal dimension, which were in all cases between 1.7 and 1.9, calculated using two methods, sandbox and box-counting, proved the DLA mechanism.

The electron transfer processes induced by the ultraviolet exposure of the Fe and Sb doped polyvinyl alcohol (PVA) thin films were studied in comparison with those appeared in the correspondent mono (Fe or Sb) doped ones. Samples with different thickness and subjected to different exposures were analysed by optical absorption as well as by Fe-57 and Sb-121 Mossbauer spectroscopy. The influence of each element from the pair on the electronic mechanism activated by the ultraviolet exposure was investigated. Accordingly, the (Fe+Sb) doped PVA presents improved electronic characteristics, in respect to the specific behaviour of single metal doped polymer recording media for real time holography.

Two samples of large-grained commercial superabrasive cubic boron nitride crystalline powders in different shades of amber coloration have been investigated by low frequency X (9.4 GHz)-band electron paramagnetic resonance (EPR) spectroscopy at room and low (T < 15 K) temperatures. Both samples contain three spectra component lines, A1, A2 and A3, in the same proportion, and the total concentration of the corresponding paramagnetic centers being proportional to the intensity of the amber coloration. After in situ broad-band UV illumination at low temperature, the intensity of the A1 component line becomes dominant. The observed effects are attributed to electron trapping at EPR silent A1(+) precursor centers, which seem to coexist with the A1 centers and to the eventual ionization of the paramagnetic A3 centers responsible for the A3 component line.

This paper presents the optical absorption properties in crystalline meta-clinitrobenzene (m-DNB) and benzil. The optical band gap as an intrinsic property of these crystalline compounds has been studied using UV-VIS Spectroscopy on thin film. Analysing absorption near the fundamental absorption edge we have deduced the wide band gap semiconductor behaviour of these aromatic derivatives. Processing the experimental transmission data using a function obtained by the superposition of a linear function and a power function we have evaluated the band gap energy in m-DNB, E-g=2.90 eV and emphasised the same two edges patterns of the light absorption spectrum in benzil deposited on glass as for benzil deposited on quartz, with energetic thresholds slowly moved through lower energies, E-g1=2.79 eV and E-g2=3.49 eV. We have also remarked no significant change in the absorption spectrum in benzil induced by the dopant (organic and inorganic).

Thin films (50 nm) of Fe100-xPdx and Fe100-xMnx (x=20 and 50) were deposited by laser ablation on a Si wafer and studied by X-ray diffraction (XRD) and conversion electron Mossbauer spectroscopy (CEMS). In all cases, the XRD patterns of the target surface before and after ablation show a congruent transfer of ablated material. For the Fe80Pd20 thin, film, XRD spectra show the formation of FePd, iron, Pd oxide and traces of hematite in the ablated film. Complementary information obtained by CEMS indicates the presence of an upper 50-nm thick surface layer of hematite in the deposited film. Fe50Pd50 phase was observed by CEMS and a Pd oxide component was identified by XRD, along with traces of hematite. In the case of Fe80Mn20, the CEMS spectrum is consistent with the Occurrence of the anti ferromagnetic FeMn phase and the XRD pattern also shows the presence of Fe in the structure. For the system Fe50Mn50, we obtain by CEMS an antiferromagnetic phase of FeMn and by XRD we trace the presence of nanohematite and in-depth iron. For x=20 in both iron-rich systems investigated, we observe the segregation of iron with subsequent conversion to hematite. The study shows that the stoichiometry of the compounds plays a decisive role in determining their structural and magnetic properties. (c) 2006 Elsevier Ltd. All rights reserved.

We report experimental investigations and modeling of the electronic transport in Si-SiO2 nanocomposite films. The current-voltage characteristics measured at room temperature are interpreted as due to high field-assisted tunneling. The activation energies from the current-temperature curves,are given by the energy separations between quantum confinement electronic states, determined from a quantum well model. Consequently, the calculated mean diameter of a nanodot (5.2 nm) is in good agreement with the microstructure data (5 nm). Also, the potential barrier between nanocrystalline Si and amorphous SiO2, previously obtained for nanocrystalline oxidized porous Si (2.2 eV), is confirmed. (c) 2006 Elsevier B.V. All rights reserved.

Magnesium-rich Mg-Ni-Fe intermetallic compounds have been prepared by two different routes: (a) short time ball milling of ribbons obtained by melt spinning; (b) long time ball milling of a mixture of MgH2, Ni and Fe powders. The first type of samples displays an hydrogen desorption kinetics better than the second one. Pressure composition isotherm measurements exhibit for both type of samples two plateaux, the lower and wider corresponding to the MgH2 phase and the upper and shorter corresponding to the Mg2NiH4 phase. The presence of the two types of hydrides is confirmed by X-ray diffraction analysis. Mossbauer spectroscopy shows that in melt spun and subsequently milled samples iron is mainly in a disordered structure and segregates after hydrogenation, while in directly milled powders remains mainly unalloyed. After multiple hydrogen absorption/desorption cycles the main part of iron is in metallic state in samples of both types, those of first type preserving better hydrogen desorption kinetics. (c) 2005 Elsevier B.V. All rights reserved.

In this work, we present the fabrication and full characterization of stoichiometric SiO2 nanoisland arrays ( dots) on silicon, grown through an anodic porous alumina template. Atomic force and transmission electron microscopy (AFM, TEM) were used to characterize the morphology, size, size distribution and density of the dots as a function of the anodization time used. It was found that dot density is lower for very short anodization times, and it stabilizes after a certain time. The dot height increases rapidly after nucleation, reaching values of 8 - 10 nm. With prolonged oxidation the dots continue to nucleate to fill the available area on the silicon surface underneath the porous alumina, while the well developed dots grow in height and width, reaching saturation values at 14 and 55 nm respectively. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy ( EELS) were used to investigate the stoichiometry and surface coverage of the dots.

UB4 is a moderate heavy fermion compound which does not exhibit my magnetic order down to 0.3 K. In this work, the magnetic properties of UB4 are investigated in an extended temperature range, 2-800 K. on high-quality single crystals. A cross-over from a low temperature 5f itinerant behaviour to a high temperature 5f localized behaviour is observed, marked by broad peaks in the magnetic susceptibility measured along the main crystallographic axes. (c) 2006 Elsevier B.V. All rights reserved.

Ballistic electron transport, namely quantized conductance, bend resistance, and its polarity-dependent breakdown, is studied in an asymmetric nanoscale cross junction prepared from a high-mobility Si/SiGe hetero structure. The cross junction is composed of four orthogonal leads which merge into two wide and two narrow quantum point contacts (QPCs). For a wide-narrow combination of leads the two-terminal differential conductance as a function of finite DC bias voltage reveals oscillations and half-plateaus characteristic for quantized conductance. In four-terminal bend resistance configuration with current injection into orthogonal leads negative bend voltage develops between the probing leads. Above a given current threshold the negative bend resistance breaks down just in that bias current polarity where electrons enter the cross region from the narrow QPC. This breakdown is attributed to phonon emission by hot electrons. (c) 2006 Elsevier B.V. All rights reserved.