National Institute Of Materials Physics - Romania

Time-dependent transport through two capacitively coupled quantum dots is studied in the framework of the generalized master equation. The Coulomb interaction is included within the exact diagonalization method. Each dot is connected to two leads at different times, such that a steady state is established in one dot before the coupling of the other dot to its leads. By appropriately tuning the bias windows on each dot we find that in the final steady state the transport may be suppressed or enhanced. These two cases are explained by the redistribution of charge on the many-body states built on both dots. We also predict and analyze the transient mutual charge sensing of the dots.

The etherification of biomass-based alcohols with various linear alpha-olefins under solvent-free conditions was followed in a space- and time-resolved manner on 9 mu m large H-Beta zeolite crystals by confocal fluorescence microscopy. This allowed us to visualize the interaction with the substrate and distribution of the coke products into the catalyst at the level of an individual zeolite crystal during the etherification process. The spectroscopic information obtained on the micrometer-scale zeolite was in line with the results obtained with bulk characterization techniques and further confirmed by the catalytic results obtained both for micrometer-scale and nanoscale zeolites. This allowed us to explain the influence of the substrate type (glycerol, glycols, and alkenes) and zeolite properties (Si/Al ratio and particle size) on the etherification activity. The etherification of the biomass-based alcohols takes place mainly on the external surface of the zeolite particles. The gradual blockage of the external surface of the zeolite results in a partial or total loss of etherification activity. The deactivation could be attributed to olefin oligomerization. The high conversions obtained in the etherification of 1,2-propylene glycol with long linear alkenes (up to 80%) and the pronounced deactivation of the zeolite observed in the etherification of glycerol with long linear alkenes (max. 20% conversion) were explained by the spectroscopic measurements and is due to differences in the adsorption, i.e., in the center of the zeolite particle for glycerol and on the external surface in the case of glycols.

Cobalt-aluminate spinel, CoAl2O4, has been synthesized by two synthesis variants of the complexation method: tartarate and gluconate routes. The complex precursors have been characterized by IR, UV-vis spectroscopy and thermal analysis. The cobalt-aluminate spinet has been investigated by XRD, IR, UV-vis and luminescence spectroscopy and TEM. The results indicated that in both cases the single-phase nanocrystalline CoAl2O4 spinet was formed. The average crystallite size from TEM images was in the range 50-60 nm. The emission spectra of CoAl2O4 in the visible region confirmed the presence of tetrahedral coordinated Co2+ ion. (C) 2010 Elsevier B.V. All rights reserved.

We measured the critical current density J(c) of optimally doped YBa(2)Cu(3)O(7) (- delta) thin films obtained by dc sputtering on (100) oriented SrTiO(3) substrates and by pulsed laser deposition (PLD) on (100) oriented MgO substrates with SrTiO(3) buffers. J(c) at various temperatures T was determined from the magnetization curves registered with a vibrating sample magnetometer for an external magnetic field H (oriented along the c axis) up to 90 kOe. It was found that J(c) of the films obtained by PLD usually overcomes the J(c) values of the sputtered films in the whole investigated (H, T) domain.

Large and transparent LiF:Pb and LiF:TI crystals were successfully grown by using Kyropoulos method. They were characterized using spectroscopic measurements combined with differential thermal analysis and thermogravimetry (TG) analysis, in order to investigate the effect of dopants upon the optical properties of these crystals. The optical absorption peaks at 278 nm (for Pb) and 273 nm (for TI) have been assigned to the A-type transition of these ions. Photoluminescence spectra have shown bands at 350 nm (for TI) and 375 nm (for Pb), which have been associated to the dopant ions in different configurations in the lattice. (C) 2010 Elsevier B.V. All rights reserved.

The nonlinear optical (NLO) crystal Y0.6Lu0.4Ca4O(BO3)(3) (starting composition) of good quality has been grown from the melt by the Czochralski pulling method. X-ray diffraction experiments show that the structure of grown crystal belongs to the monoclinic system with space group Cm and its unit cell parameters have been measured to be a = 0.80711(2)nm, b = 1.60132(1)nm, c = 0.35230(8)nm and beta = 101.166(1)degrees. The crystal possesses a wide transmission range from UV to IR, which is advantageous for applications as new NLO material for frequency doubling of solid state lasers operating in the near infrared range.

Macroporous polyacrylonitrile/hydroxyapatite (PAN/HAp) composite bead was studied for cadmium removal in a fixed-bed column. The morphology of PAN/HAp composite bead was measured. The adsorption performance of PAN/HAp composite bead in the column was examined by varying bed height and HAp amount in PAN bead. The maximum adsorption capacity and the exhaustion time were determined from the breakthrough curves. Experimental data was described using the Adams-Bohart model. Bed height did not exert large influence on the maximum adsorption capacity and the exhaustion time. The kinetic rate constant and the adsorption capacity of the bed were found to be affected by the total HAp amount.

Due to their physical and chemical properties (such as suitable band gaps, large absorption coefficients and good chemical stability) CdTe thin films are interesting for electronic and optoelectronic devices, including particularly photovoltaic cells for space technology. For that specific application, it is of prime importance to study in this type of materials the influence of ionizing radiations on their physical (structural, electrical and optical) properties. In this paper, the photovoltaic cells based on CdS/CdTe thin films, produced by thermal vacuum sublimation, were irradiated with 3 MeV protons at room temperature. The effects of irradiation were studied by investigating the changes in the electrical and optical properties of the cells. It was found that proton irradiation in the above mentioned conditions results mainly in the introduction of defects at the CdS/CdTe interface. A discussion about the possible origin of those defects is given.

In the first part of this study it has shown that the interaction between collagenous polypeptides (collagen type I with native structure of triple helix and collagen hydrolyzates with no significant secondary structure) and ionic surfactants (sodium dodecyl sulfate and hexadecyl trimethylammonium bromide) involves the formation of micelle-like clusters of ionic surfactant adsorbed onto polypeptide backbone (polypeptide-surfactant complexes) at surfactant concentrations much lower than its cmc. The present investigation is focused on the influence of polypeptide-ionic surfactant complexation on thermal denaturation of the collagen type I extracted from calfskin and young horse tendons. On the basis of capillary viscometry, a good correlation between denaturation temperatures and activation energies of viscous flow for the dilute aqueous collagen-ionic surfactant solutions at pH 2.5 and 8.0 has been found. Beginning from that, it has established that the presence of ionic surfactant reduces thermal stability of the two collagens, with a more pronounced effect in the case of sodium dodecyl sulfate. Accordingly, it has been proposed a thermal denaturation mechanism of the collagen triple helix in the presence of ionic surfactants.

Langmuir films and Langmuir-Blodgett multilayers based on fatty acid (stearic acid, barium stearate) mixed with carbon nanotubes have been prepared and investigated by isothermal compression (for Langmuir films) and X-ray diffraction (for Langmuir-Blodgett multilayers). An irreversible modification has been observed in the case of Langmuir films doped by carbon nanotubes, due to reordering of the nanotubes in the film matrix.