Publications

We use a generalized master equation (GME) to describe the nonequilibrium magnetotransport of interacting electrons through a broad finite quantum wire with an embedded ring structure. The finite quantum wire is weakly coupled to two broad leads acting as reservoirs of electrons. The mutual Coulomb interaction of the electrons is described using a configuration interaction method for the many-electron states of the central system. We report some nontrivial interaction effects both at the level of time-dependent filling of states and on the time-dependent transport. We find that the Coulomb interaction in this nontrivial geometry can enhance the correlation of electronic states in the system and facilitate it's charging in certain circumstances in the weak coupling limit appropriate for the GME. In addition, we find oscillations in the current in the leads due to the correlations oscillations caused by the switched-on lead-system coupling. The oscillations are influenced and can be enhanced by the external magnetic field and the Coulomb interaction.

Matrix assisted pulsed laser evaporation (MAPLE) was used for growing urease thin films designed for biosensor applications in clinical diagnostics. The targets exposed to laser radiation were made from a frozen composite manufactured by dissolving biomaterials in distilled water. We used a UV KrF* (lambda = 248 nm, tau(FWHM) congruent to 30 ns, nu = 10 Hz) excimer source for multipulse laser irradiation of the frozen targets cooled with Peltier elements. The laser source was operated at an incident fluence of 0.4 J/cm(2). Urease activity and kinetics were assayed by the Worthington method that monitors urea hydrolysis by coupling ammonia production to a glutamate dehydrogenase reaction. A decrease in absorbance was measured at 340 nm and correlated with the enzymatic activity of urease. We show that the urease films obtained by MAPLE techniques remain active up to three months after deposition. (C) 2010 Elsevier B.V. All rights reserved.

A theoretical description for a single quantum-dot emitter in a microcavity is developed. We analyze for increasing steady-state pump rate the transition from the strong-coupling regime with photon antibunching to the weak-coupling regime with coherent emission. It is demonstrated how Coulomb interaction of excited carriers and excitation-induced dephasing can strongly modify the emission properties. Our theoretical investigations are based on a direct solution of the Liouville-von Neumann equation for the coupled carrier-photon system. We include multiple carrier excitations in the quantum dot, their Coulomb interaction, as well as excitation-induced dephasing and screening. Similarities and differences to atomic systems are discussed and results in the regime of recent experiments are interpreted. (C) 2010 Optical Society of America

Zinc oxide (ZnO) is a semiconductor with a wurtzite-type structure, useful for a variety of optical, optoelectronic and piezoelectric applications. We report here on the post deposition treatment (Rapid Thermal Annealing at 400 and 550 degrees C) impact on the microstructural properties of N-doped ZnO (ZnO:N) thin films grown on silicon substrate by r.f. magnetron sputtering of ZnN target. The ZnO:N films have been characterized by X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning (SEM) and Transmission (TEM) Electron Microscopy coupled with Energy Dispersive X-ray (EDX) and Selected Area Electron Diffraction (SAED) respectively and Fourier Transform Infrared (FTIR) Spectroscopy. XRD confirms that ZnO:N films are polycrystalline in the as deposited state. SEM, TEM and XRD measurements revealed a polycrystalline film with preferentially oriented columnar crystals. AFM studies reveal a drastic change of ZnO surface morphology upon RTA and the existence of areas with different roughness in the sample thermally treated at 400 degrees C. FTIR, XRD and TEM results shows that disorder associated mainly with grain boundary defects decreases as the annealing temperature increases.

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 A degrees C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 A degrees C for 3 h is proposed for synthesis of dense Ba(Ti0.87Sn0.13)O-3 ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300-500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti0.87Sn0.13)O-3 ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of similar to 2 mu m. The apparent densities of the sintered pellets at temperature of 1400 A degrees C were similar to 92% of the theoretical value of Ba(Ti0.87Sn0.13)O-3. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan delta) = 0.078 at Curie temperature of 63 A degrees C and 10 Hz.

Lithium borohydride (LiBH4) is a promising material for hydrogen storage, featuring a high gravimetric storage density (above 13 wt. % in the first decomposition step). However, studies of re-hydrogenation of the decomposition products have proven less successful, leading us to the requirement of modified-LiBH4 materials with better absorption-desorption behavior. A Mo-doped mesoporous material was obtained starting from commercial MSU-H and ammonium molybdate. This mesoporous material (MSU-H-type functionalized with molybdenum salt) was used as support in order to impregnate LiBH4. Even though the thermodynamics of LiBH4 hydrogen absorption-desorption is not modified, the desorption kinetics of supported LiBH4 is improved compared to that of pure LiBH4. Moreover, after re-hydrogenation at 450 degrees C only 3.2 wt. % H-2 of storage capacity is recovered for LiBH4, while for supported LiBH4 we achieved almost 5.2 wt. % H-2.

Porous nanoarchitectures based on TiO2 aerogel and colloidal gold particles have been obtained by different synthesis routes: impregnation of TiO2 aerogel with Au colloid, impregnation of TiO2 gel with Au colloid and preparation of TiO2 gel in the presence of Au colloid. The as prepared composites have been thermal treated at 500 degrees C and their structural and morphological properties were investigated. The Raman spectra and XRD data demonstrated the anatase structure of the composite obtained by impregnation of TiO2 aerogel with Au colloid and the mixed anatase-brookite structure of those obtained by impregnation of TiO2 gel with Au colloid and by preparation of TiO2 gel in the presence of Au colloid, respectively. TEM analyses suggest that the Au particles on the TiO2 surface as individual particles, as well as clusters are present. The photocatalytic performances of the synthesized composites were tested in the salicylic acid photodecomposition process and it was found that they are strongly influenced by both anatase - brookite TiO2 structure and Au nanoparticles dispersion on the TiO2 surface.

The to study of the total oxide (SiO2+SiOx) thickness, SiO2 and SiOx (e.g. Si2O, SiO, Si2O3) thicknesses on Si(100) crystalline substrate with take-off angles ranging from 30 degrees to 80 degrees has been carried out by spectrometric method. The d(s) X-ray Photoelectron Spectroscopy (XPS) thicknesses were compared with d(EL) thicknesses obtained by fitting the Spectroscopic Ellipsometry (SE) spectra. A qualitatively good correlation is revealed. However, from these estimations of film thicknesses it results that ellipsometry analysis cannot be as accurate as in XPS evaluation. This is due to uncertainty of used optical constants as well due to very thin oxide films used in this work.

This paper is focused on preparation and microstructure and electrochemical characterization of carbon-TiO2/Pt catalyst used as electrode in fuel cells applications. The platinum deposition was comparatively studied on graphitized carbon substrate and carbon substrate covered with titanium oxide. A strong influence on the platinum deposition, depending of the TiO2 presence, was found. The platinum nanoparticles about 5 nm in size are dispersed in aggregates of 50 to 100 nm on the catalyst surface. This dispersion is strongly affected by the titanium oxide and the substrate porosity. The Pt/TiO2/G and Pt/G electrodes were tested for their catalytic activity in the oxidation of ethanol. The prepared Pt/TiO2/G electrode show higher active surface area than for the Pt/G electrode, this increase in the surface area could be explained to be due to a spreading of wetting or the formation of new active sites at the Pt/oxide interface.