Publications

A kinetic analysis under non-isothermal conditions has been performed on europium-doped phosphate glasses with molar composition 20.42 Li2O-10.25 Al2O3-58.49 P2O5-7.23 BaO-1.44 La2O3-2.16 Eu2O3 and particle size 50 mu m. Differential scanning calorimetry measurements at four heating rates, from room temperature up to 900 degrees C using synthetic air have revealed that no significant mass change occurs during the heat treatment, as normally expected for glass samples. The step changes in heat flow signals are associated with the presence of three important effects: the first one, that occurs as a slope corresponds to the glass transition effect (T-g) is followed by two exothermic peaks, first of them more pronounced, associated with the first crystallization process (T-p1), while, the second one, lower in intensity corresponds to the second crystallization (T-p2) process. As expected, all peaks (T-g, T-p1 and T-p2) increase with the increasing of the heating rate. Both the activation energy of crystallization process and the crystallization mechanism were comparatively analyzed by two free-model estimations and using the formal theory of transformations for heterogeneous nucleation. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

We address theoretically the electronic transport through graphene quantum dots with the emphasis on the transmission phase. Analytical and numerical results are presented regarding the existence - or not - of a lapse of the transmittance phase (and, consequentially, a Fano zero in the transmittance) at the charge neutrality point. A simple universal criterium is found, the phase lapses being always present if the contact sites belong to the same sub-lattice. ((c) 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Tetragonal Sr2FeMoO6 powders were prepared by the sol gel and solid-state reaction methods. The structure, microstructure and magnetic characteristics of the double perovskite Sr2FeMoO6 in the form of as-prepared powders and classical sintered ceramics were analyzed and compared. Moreover, the magnetic properties of Sr2FeMoO6 ceramics obtained by sol gel and solid state reaction methods and sintered by the classical method were compared with those of Sr2FeMoO6 ceramics obtained by the same two methods, but sintered by the spark plasma sintering technique. The morphology of the powders obtained from the gel precursor and oxides mixture was characterized by homogeneity, as well as the grains shape and size. The powder prepared by the sol gel method was finer (particle average size of 0.6 mu m) than those obtained from oxides mixture (particle average size of 0.9 mu m). Sintered ceramics with tetragonal Sr2FeMoO6 phase were prepared from the two mentioned powders by classical sintering at 1200 degrees C, 2 h in 5%H-2/Ar. The pellets obtained from the gel powder exhibited higher magnetic characteristics than those derived from the oxides mixture. The saturation magnetization and the total magnetic moment of the samples prepared by the sol-gel method were M-sat=41.18 emu/g and mu(exp)/f.u.=3.26, respectively, while for the samples derived from the oxides mixture were M-sat=40.77 emu/g and mu(exp)/f.u=3.10. Also, Sr2FeMoO6 pellets sintered by the conventional method showed higher saturation magnetization values than those sintered by spark plasma sintering technique. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Using surface-enhanced Raman scattering (SERS) and photoluminescence (PL) studies, new data concerning the electrochemical oxidation both of poly[(2,5-bisoctyloxy)-1,4-phenylene-vinylene] (BO-PPV) and the BO-PPV/HIPCO single-walled carbon nanotubes (SWNTs) composite are presented in this paper. The SERS studies performed at the excitation wavelengths of 752 nm and 532 nm of the BO-PPV/SWNTs composite that ensure the resonant excitation of the semiconductor and metallic nanotubes, respectively, reveal that the mixing of the two constituents results, on the one hand, in an isolation of semiconducting nanotubes from the bundles containing both metallic and semiconducting tubes and, on the other hand, in a functionalization of metallic SWNTs with BO-PPV. The isolation of semiconducting SWNTs is demonstrated by the narrowing of the G band and the change of the ratio between the intensities of the Raman lines situated in the low-frequency range that are assigned to the radial breathing vibrational modes. An additional isolation of the semiconducting nanotubes from the SWNTs bundles is reported when an electro-oxidation of the BO-PPV/SWNTs composite was performed in the potential range of (0; +2) V vs. Ag/AgCl. According to the Fourier transformed infra-red (FTIR) spectra and the SERS studies performed at an excitation wavelength of 1064 nm, the chemical interaction of BO-PPV with HIPCO SWNTs reveals a charge transfer between the two constituents that leads to the formation of BO-PPV covalently functionalized SWNTs. The BO-PPV luminescence quenching effect induced by the presence of SWNTs that consists of a change in the relative intensities of the three PL bands of the polymer peaked at 1.73 eV, 1.9 eV and 2.07 eV is due to the metallic tubes. The appearance of a new emission band at 2.32 eV was regularly observed when electrochemical oxidation of the BO-PPV/SWNTs composite was performed in the potential range of (0; +2) V. The PL band at 2.32 eV corresponds to ClO4- ions, which compensates the positive charges of BO-PPV generated during the electrochemical doping process. (C) 2014 Elsevier B.V. All rights reserved.

Nanosized TiO2 layers containing polyethylene glycol (PEG) with 20,000 molecular weight were prepared by sol gel method and deposited on three different substrates: microscope standard slides, Fluorine doped Tin Oxide (FTO) glass and Silicon, followed by thermal treatment at 450 degrees C. The effects of the substrate on the microstructural, morphological, chemical and optical properties of the films have been studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM), X-ray Photo-electron Spectroscopy (XPS) and Spectroscopic Ellipsometry (SE). The XRD showed that the film crystallization depends very much on the substrate used, thus the films deposited on glass are amorphous even after 1 h thermal treatment at 450 degrees C, while the films deposited on Silicon and FTO substrates are crystallized in anatase phase, in good agreement with the TEM analysis. The XPS analysis showed that a rather high carbon relative concentration in the TiO2 film (without PEG) exists; this suggests that an unavoidable carbon contamination occurs and is not related on the presence or absence of PEG. The thickness and surface roughness of the films are clearly dependent on the substrate type. The flat surfaces of glass and Si lead to the formation of thinner TiO2 films (around 30 nm thicknesses), while the FTO substrate surface leads to thicker film (thickness similar to 100 nm). The porosity of TiO2 films as found by SEM and TEM analyzes is dependent of substrate type; small pore sizes up to 20 nm are characteristic for the films deposited on glass, while larger pore sizes up to 200 nm are found for the films deposited on FTO substrate. Not least, the film transmissions in the visible region are also substrate dependent and the highest values (80%) were obtained for the films deposited on glass. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

Magnetic and electrical properties of the nanostructured ZnO films are affected by the nonrandom distribution of impurities in the film due to segregation at grain boundaries (GBs) or extended defects. However, mapping the nature and distribution of the impurities in the film is not trivial. Here we demonstrate a simple, statistically relevant, and nondestructive procedure of quantitative determination of the paramagnetic impurities segregated at the GBs in nanostructured semiconducting and insulating films. From correlated electron paramagnetic resonance and transmission electron microscopy investigations, we determined the localization of trace amounts of Mn2+ ions, present as native impurities in a ZnO film deposited by magnetron sputtering at room temperature. In the as-deposited ZnO film, the Mn2+ ions were all localized in nanosized pockets of highly disordered ZnO dispersed between nanocrystalline columns. After the samples had been annealed in air at >400 degrees C, the size of the intercrystalline region decreased and the diffusion in GBs was activated, resulting in the localization of a portion of the Mn2+ ions in the peripheral atomic layers of the ZnO columns neighboring the GBs. The proportion of Mn2+ ions still localized at the GBs after annealing at 600 degrees C was 37%. The proposed method for the assessment of the presence and nature of impurities and the quantitative evaluation of their distribution in semiconducting and insulating nanostructures is expected to find direct applications in nanotechnology, in the synthesis and quality assurance of thin films for spintronics and opto- and nanoelectronics.