Publications

Calcium phosphate ceramic powders were synthesized via sol-gel method. The powders were sintered at 600 and 1000 degrees C and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (SEM and TEM), elemental microanalysis (EDS), infrared spectroscopy (FTIR), and thermal analysis (TGA and DTA). The XRD analysis revealed a well crystallized hydroxyapatite (HAp) structure at all temperatures. At 1000 degrees C a small amount of CaO (about 1.2 %) was detected. The average crystallite size increased from 19 nm at 80 degrees C to about 125 nm at 1000 degrees C. FTIR spectra showed the presence of various PO43- and OH- groups present in the powders. In order to verify the biocompatibility of HAP powders with hFOB 1.19 osteoblasts cells, an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test was applied. It was noticed that the ceramics obtained at 1000 degrees C, increased the cell viability with 10%, 40% and 55% after 6, 12 respectively 24 h, whereas that obtained at 600 degrees C was less effective. Our results proved the high biocompatibility of calcium phosphate ceramic powders obtained by sol-gel and sintered at 1000 degrees C.

Tin chalcogenides SnX2 and SnX, where X = S, Se and Te present a particularly interest for their electronic properties and applications in gas sensors. The state of tin in these materials is important for understanding of the sensing effect and improvement of the sensor performances. Mossbauer spectroscopy is a widely used technique for the analysis of the local electronic structure or chemical bonding in solids. In this paper we applied Mossbauer technique for the investigation of bulk and thin films of SnSe2 chalcogenide. The films of SnSe2 chalcogenide were obtained by the methods: PLD ("Pulsed laser deposition") and PED ("Pulsed electron deposition"). Mossbauer measurements were performed by transmission (TMS), respectively conversion electron spectroscopy (CEMS). By CEMS spectroscopy surfaces, coatings and thin films containing Sn can be studied on substrates and to various depths up to 1000 nanometers.

The precipitation of calcium carbonate by liquid-liquid reaction usually provides a mixture of polymorphs: rhombohedra calcite, orthorhombic aragonite with whiskers like particles and hexagonal vaterite with crystallites agglomerated as spheres. Calcite is the stable polymorph at ambient conditions. Recently the interest for aragonite, the high pressure polymorph is increasing because new fields, like biomaterials, are developing. We investigated the possibility of developing an aragonite type calcium carbonate with uniform particle size and submicron dimensions by precipitation in liquid-liquid reaction using double-jet precipitation method in the presence of ethanol, at the temperature of 50 degrees C, with a flow rate of the reagents of 10 ml/min. Two agitation modes were used: magnetic stirrer and an ultrasonic device. The ultrasonic field was applied either at the beginning of the reaction period or at the end of it. Intensity of the ultrasounds was also varied from 50 % to 100 %. All samples were examined by FTIR spectroscopy, scanning electron microscopy, and particle size analyzer. FTIR spectra proved that calcium carbonate synthesized in these conditions is almost pure aragonite. As example, the SEM image shows uniform product structure with fine rods with 2 divided by 3 mu m lengths and submicron diameters.

We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.

The most promising media for rewritable applications are the phase change materials [ 1]. For the modelling of the phase transition processes in phase change materials it has been considered the switching unit called "commuton" [2]. A commuton is a minimum cluster of atoms that supports the reversible change from OFF to ON state under the influence of an external applied energy pulse. It was developed a bidimensional model based on the cellular automata approach [3] to predict switching behavior in a Ge2Sb2Te5 phase change chalcogenide film. We suppose that the formation of the percolation paths as a function of phase change induced in commutons can explain the switching phenomenon. The percolation thresholds of the switching were calculated for different sizes of the simulated system and the percolation kinetics was investigated.

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb L-III and L-I edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 degrees C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G(0)W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.

The transport properties of the gamma-irradiated CrO2-polymer composites were investigated. The resistance R is strongly current I dependent, except at high temperature where it displays a semiconductor-like temperature dependence for all currents. At low currents, I <= 10 mu A, as the temperature decreases far below the ferromagnetic transition, the resistance decreases first, then reaches a minimum, and at last increases again toward a second peak. At even lower temperatures, the second peak is followed by a metallic-like temperature dependence of R that ends at a cusp point marking the metal-insulator transition. The increase of the current shifts the cusp toward lower temperatures and the first minima toward higher temperatures. The resistance increases with the increase in current for all currents in the range 0.2 <= I = 50 mu A, the resistance increases monotonically with the decrease in temperature in the whole temperature range but obeys different laws at low and high temperatures. An explanation attempt in terms of spin transport, disorder, and thermal effects is proposed. (C) 2009 Elsevier B.V. All rights reserved.

Electron paramagnetic resonance spectra from substitutional Mn2+ ions in quantum dots of cubic ZnS with tight size distribution centred at 2 nm were recorded in the 9.8 GHz and 34 GHz frequency bands. Their quantitative analysis with line shape simulation and fitting computer programs accounting for both forbidden transitions and line broadening effects demonstrate the presence of a local axial distortion attributed to a neighbouring extended planar stacking defect. The presence of such extended lattice defects, confirmed from a high resolution transmission electron microscopy study on presently investigated cubic ZnS quantum dots, seems to be essential in the incorporation and localization of Mn2+ activating ions in other cubic II-VI semiconductor quantum dots as well.

Electrodeposition of manganese doped ZnO wires into polycarbonate membranes from an aqueous solution containing Zn(NO3)(2), Mn(NO3)(2) and lactic acid was investigated using linear sweep voltammetry and electrochemical impedance spectroscopy (EIS). The equivalent electrical circuits used for fitting experimental results were similar to those for characterization of metal/polymer coating systems. The precipitation of metal oxides in the pores of membrane used as template was observed at potentials between -0.9 and -1.15V/SCE. Morphology of the ZnO: Mn submicron wires arrays was observed by scanning electron microscopy and the Mn content of the obtained samples was measured by X-ray analysis. The submicron wires prepared at a potential of -1.1V/SCE have a composition expressed by the formula Zn0.97Mn0.03.