Publications

Monte-Carlo estimations of the integrals of Bose-Einstein distribution function for magnons are performed, by avoiding number of approximations involved in the usual derivation of the Bloch T-3/2 law, describing the dependence of magnetization with temperature in the low temperature regime, where the main mechanism of magnetization decrease is due to spin waves. More precisely: (i) the magnon dispersion law is considered the exact one (depending on the crystal structure) and not the quadratic approximation epsilon(k) alpha k(2); (ii) the integrals are performed over the first Brillouin zone and the limit of the integral is not extended to infinity; (iii) the dimensionality of the system is taken into account: 1D, 2D and 3D systems are considered; (iv) finite size magnetic systems are treated by rigorous summation of the Bose-Einstein distribution function, without using the continuous limit. The results can be summarized as follows: (a) Infinite three-dimensional (3D) systems exhibit deviations from the Bloch T-3/2 law, namely the deviation from zero Kelvin magnetization vertical bar Delta M vertical bar/M(0) behaves as T-gamma, where gamma is sensibly (by some 10 %) larger than 3/2 and, also, depends on the crystal structure considered. (b) Finite 3D systems exhibit more drastic deviation from the Bloch law, in that the gamma exponent deviates seriously from 3/2, going up to 2.5 for small systems of about 1,000 atoms. (d) Finite 2D systems also show serious deviations from the Bloch-like T law. As described a few years ago [C.M. Teodorescu, Surf. Sci. 601, 4292 (2007)], finite temperature magnetization may be considered only in finite size 2D systems. Several recent examples from literature are analyzed in the framework of the actual theoretical results. It is shown that M(T) analysis in the low temperature regime allows one to extract the average nanoparticle size of the magnetic moieties, and as well the exchange interaction and average spin quantum number.

X-ray photoelectron spectroscopy (XPS) analyses of lead zirco-titanate Pb(Zr,Ti)O-3(001) single crystal thin layers as a function of the time spent between sample preparation by pulsed laser deposition and introduction in to a ultrahigh vacuum revealed the fact that freshly prepared samples showed a shift of the C 1s towards a higher binding energy, together with shifts of core levels originating from the substrate (particularly Ti 2p and O 1s) towards a lower binding energy. This behaviour is explained by considering that the molecules of contaminants (fatty acids, alcohols, esters) are adsorbed preferentially on areas exhibiting outwards polarization P(+). Thus, photoelectrons originating from contaminants will have larger binding energies because of the charge state of the P(+) areas, whereas the substrate XPS signals from these P(+) areas are attenuated by the contaminants, with the consequence of a prevalence of XPS substrate signals originating from the P(-) areas, shifted towards lower binding energies. Piezoresponse force microscopy confirmed the assumptions derived from XPS results and suggests the existence of an interplay between the adsorption of contaminants and the surface polarization state.

The inhibition ability of dissolved 5,10,15,20-tetrakis(4-pyridyl)-21H, 23H-porphine (TPyP) on the corrosion of carbon steelin H2SO4 5% was investigated by evaluating the mass loss from the volume of the evolved hydrogen and by direct weight measurements at different temperatures. Results indicated that the presence of TPyP was able to reduce the corrosion rate of steel with over 50% efficiency, depending on TPyP concentration and temperature. The surface state of steel was characterized by AFM before and after the steel sample was corroded in H2SO4 solution (5%), the smoothing and uniforming effect of porphyrin being obvious, especially at higher temperature.

The iron oxide nanoparticles were synthetized by an adapted coprecpitation method. Stuctural and morphological characterization of the obtained iron oxide nanoparticles were investigated by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Furthermore, in vitro and in vivo studies were investigated by cell viability assay and HeLa cells. For the analysis of iron oxide toxicity in vivo, several mice were treated with normal saline and iron oxide via intraperitoneal injection (IP). The XRD spectra showed the peaks associated to the spinel cubic lattice type with the lattice of 0.835 nm. By magnified TEM image, it could be observed that the samples have a uniform morphology with relatively spherical shape and nanometric size. Moreover, inverted fluorescence microscopy images of HeLa cells with normal phenotype and HeLa cells treated for 72 hourswith a suspension of gamma-Fe2O3 nanoparticles revealed the non-toxic character. The histophtological studies have demonstrated that at 72 hours after IP administration, the iron oxide nanoparticles are not accumulating in kidney and spleen, thus establishing their utility as drug delivery systems targeted to these organs.

The aim of this study was to synthetize new nanoparticles based on methyltrimethoxysilane coated hydroxyapatite (MTHAp) for lead removal in aqueous solutions. The morphological and compositional analysis of MTHAp was investigated by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Removal experiments of Pb2+ ions were carried out in aqueous solutions with controlled concentration of Pb2+ and at fixed pH of 5. After the removal experiment of Pb2+ ions from solutions, porous hydroxyapatite nanoparticles were transformed into PbMTHAp_5 via the adsorption of Pb2+ ions followed by a cation exchange reaction. Our results demonstrate that the porous hydroxyapatite nanoparticles can be used as an adsorbent for removing Pb2+ ions from aqueous solution.

ZnO nanofibers were obtained by calcination of electrospun Zn(Ac)(2)center dot 2H(2)O/PMMA composite fibers and characterized by X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy. The thermal treatment led to polymer burning and polycrystalline hexagonal ZnO phase formation. The average fiber diameters range between 450 and 600 nm before calcination and 200 - 300 nm after calcination. PL investigation revealed a strong dependence of ZnO fibers emission band on the calcination temperature. Furthermore, electrical contacts were fabricated by photolithography and electric characteristics were measured.

In this letter, we study the higher-order modes excited in a high-permittivity cylindrical dielectric resonator antenna (DRA), situated asymmetrically on top of a rectangular slot in the ground plane of a microstrip line. Besides the modes excited by a symmetric positioning of the DRA above the feeding slot, we identified five new modes: TEme(010), TEme(011), TEme(210), TEme(020), and TM212 respectively. The higher-order modes offer to the DRA antenna some interesting features: They have different radiation patterns for different operating modes; modes with close resonance frequency give wideband capabilities; and sometimes, when a mode has poor performance in one polarization, it is possible that another mode with close frequency can compensate it.

Finite size effects and interfacial interactions as well as their influence on the magnetic properties of nanosized systems are discussed by starting from very basic principles of magnetism. Some preparation and subsequent processing tools for a proper engineering of the properties of such magnetic nanosized systems are introduced together with specific characterization tools. A summary of the most important technological applications related to size effects and interfacial interactions, with exemplifications starting from bio-medical applications of magnetic fluids to magnetoresitive multilayers for sensor applications are also provided.

The peculiarities of vanadium and its oxides based nanocomposite structures fabrication are brought in this paper. The selected fabrication technological conditions allow creating a V2O5 based dielectric matrix in which conducting clusters are uniformly distributed. Some optical and electrical properties of such structures are presented. It was shown that the obtained layers possess high charge sensitivity.

The electroluminescence and current density versus applied voltage were measured for the OLED cell structure which contain IrQ(ppy)(2)-5Cl as emitting layer. Two parameters, emission quantum yield and charge transport behaviors, which influences the electroluminescence, were modeled mainly concerning the non-radiative rate constants, delocalization of the HOMO and LUMO states, internal reorganization energies and bandgap of IrQ(ppy)(2)-5Cl. The non-radiative constant rates were determined using the three zero-field splitting substates responsible for the phosphorescence model and shows an increasing of these rates together with a strong interaction with the polystyrene matrix. Delocalization of the HOMO and LUMO states on the quinoline ligand, together with the small reorganization energies and bandgap lowering indicates good charge transport properties for the IrQ(ppy)(2)-5Cl compound.