Publications

Pure TiO2 and S-doped TiO2 sol-gel nanopowders were prepared by controlled hydrolysis-condensation of titanium alkoxides. The influence of different Ti-alkoxides (tetraethyl-, tetraisopropyl- and tetrabutyl-orthotitanate) used in obtaining TiO2 porous materials in similar conditions (water/alkoxide ratio, solvent/alkoxide ratio, pH and temperature of reaction) has been investigated. The relationship between the synthesis conditions and the properties of titania nanosized powders, such as thermal stability, phase composition, crystallinity, morphology and size of particles, BET surface area and the influence of dopant was investigated. The nature of the alkyl group strongly influences the main characteristics of the obtained oxide powders, fact which is pointed out by thermal analysis, X-ray diffraction, TEM and BET surface area measurements.

First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of alpha-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47 degrees and 94 degrees. The growth mechanism is of vapor-solid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo-O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite fibers), on which the MoO3 belts are growing, are important and can be used for the directional growth control of the belts and formation of hierarchical structures. The vapor transport method allows the formation of beta-MoO3 belts that grow in the b-axis direction.

Thermal behaviour of CuS (covellite) obtained from the Cu(CH3COO)(2)center dot H2O and Na2S2O3 center dot 5H(2)O system, working at different molar ratio (1:6 and 1:4) in presence/absence of NH4VO3, was studied. It was established that the presence of vanadium in the system induces a densification of CuS nodules, but do not change the hexagonal CuS structure. It has an important influence in thermal behaviour of copper sulfide CuS obtained also. The morphological characteristics of CuS play an important role in the thermal stability and the stoichiometry of the thermal decompositions. Also, the possibility to obtain copper sulfides with greater cooper content was investigated.

The superconducting properties of iodine-intercalated high-temperature superconducting Bi2Sr2Ca2Cu3O10+x phase (Bi-2223) were systematically studied. It was found that for samples containing a significant amount of Bi2Sr2CaCu2O8+x , iodine intercalation results in the dramatic decrease of the inter-granular critical current density, as well as a significant decrease of the critical temperature (T-c), the critical current density in the grains (J(cg)), and of the amount of Bi-2223. For samples with a large amount of Bi-2223, T-c changes insignificantly, whereas J(cg) can even increase. We argue that the different behavior of the superconducting parameters is the result of various oxygen concentrations, and we explain the effect of iodine intercalation based on the parabolic dependence between T-c and the number of holes per CuO2 layer. The H(T) curves (determined from the peak position in the loss signal of ac susceptibility) for intercalated samples deviate significantly from the quasi 2D-like behavior, pointing toward an enhancement of the 3D fluctuations of vortices. For the change in the values and dimensionality of the flux pinning in the process of the intercalation, we attempted a qualitative explanation based on the models proposed in literature.

In this contribution, an extensive analysis of the primary defects in silicon: vacancy, interstitial and Si-FFCD is performed. Irradiation studies are a useful tool to study production, characteristics and annealing of defects. The experimental results obtained after high proton fluence irradiation of silicon detectors are used in this paper to understand aspects related to the existence and proprieties of primary defects. Investigations on possible differences induced by irradiation in the lattice of silicon, using transmission electron microscopy analysis, have been started and some first preliminary results are presented.

We present a study of the effect of the crystallites composition and size on the optical properties of vitreous aluminophosphate matrix doped with CdSxSe1-x crystallites. The influences of the glass matrix composition and, heat treatment temperature and duration on the stoichiometry of the crystalline phase have been investigated by optical methods. We have studied the effect of the sodium oxide contained in the matrix, annealing temperature variation from 425 degrees C to 475 degrees C and duration variation from 2 h to 6 h on the light absorption process, evaluating an optical band gap energy between E-g=1.74 eV and E-g=2.29 eV associated with changes in the composition of the crystallites. The blue shift of the absorption edge observed in samples containing Na2O annealed at 425 degrees C for 4 h or at 450 degrees C for 2 h was attributed to a variation in the crystallites' composition because their size is not favourable to the quantum confinement. We have also investigated the effect of individual CdS and CdSe clusters on the absorption properties of the aluminophosphate glass and on the process of zinc incorporation in the crystalline phase. We have made some assumptions concerning the effect of the aluminophosphate glass matrix on the direct band to band light absorption mechanism characterising the II-IV bulk semiconductor.

Photovoltaic properties of the metal-ferroelectric-metal structures, having SrRuO3 metal oxide electrodes and Pb(Zr,Ti)O-3 (PZT) as ferroelectric layer, are investigated by the short-circuit photocurrent (SC-PHC) in the 200-800 nm wavelength domain. The band-gap dependence on the Zr content was determined from the spectral distribution of the SC-PHC signal. It was found that the band-gap value increases linearly with the Zr content, from about 3.9 eV to about 4.4 eV. It is shown that the sign and the magnitude of the signal depend on the internal bias and on the spontaneous polarization direction and value. The photocurrent describes a hysteresis loop similar to that of the ferroelectric polarization and can be used as a nondestructive readout of the nonvolatile memories based on PZT films. The existence of a significant SC-PHC signal at wavelengths corresponding to subgap energies is attributed to the presence of charged, deep levels in the forbidden band. It is also shown that the epitaxial PZT films have the potential for solid-state UV detectors, with current responsivity as high as 1 mA/W. The results are not entirely consistent with a bulk photovoltaic effect and are discussed in the frame of a Schottky barrier model for the metal-ferroelectric interface. (c) 2007 American Institute of Physics.

The formation of ZrSnTiO thin films by pulsed-laser deposition has been studied. By the complementary use of Rutherford backscattering spectrometry, x-ray diffraction and scanning electron microscopy, the composition, crystalline structure and surface morphology of the films were investigated. The optimum conditions for the growth of dense, stoichiometric and crystalline films were determined, i.e. substrate temperature of 700 degrees C and oxygen pressure in the 10(-3)-0.1 mbar range. Such ZrSnTiO films present the classical orthorhombic structure for growth on Si substrates, while the use of MgO single crystal substrates leads to the formation of epitaxial ZrSnTiO films with a tetragonal structure. This previously unobserved ZrSnTiO crystalline phase obtained in thin film form is the result of an epitaxy stabilization effect, and presents improved optical properties as shown by the results of spectroscopic ellipsometry measurements. Moreover, the films grown at 700 degrees C under low oxygen pressure (< 10(-5) mbar) were composed of a Sn rich surface layer superimposed on an understoichiometric zirconium titanate oxide film near the substrate. This last layer, which crystallizes in a cubic structure and has never been reported in bulk material, could be related to the presence of oxygen vacancies in the films grown in these conditions.

In this contribution, the structural modifications of the material and the degradation of devices is modelled and compared with the experimental data for more resistivities, temperatures, crystal orientations and oxygen concentrations, considering the existence of the new primary fourfold coordinated defect, besides the vacancy and the interstitial. Some estimations of the behaviour of detectors in specific environments at the next generations of high-energy physics experiments as LHC, SLHC, VLHC, or ULHC are done. (c) 2006 Elsevier B.V. All rights reserved.

The paper deals with optical and electronic properties of the aluminophosphate glasses containing Fe-Mn and Fe-Cr ion pairs in different concentration. The influence of the mixed alkali ions over the electronic properties has been investigated. The optical behavior (optical transmission) of the glass samples has been studied by UV-VIS spectroscopy and the refractive index dependency on wavelength has been discussed. The transmission spectra show features specific for the doping transition ions (TM), revealing different oxidation states of iron (Fe2+/Fe3+), manganese (Mn2+/Mn3+) and chromium (Cr3+/Cr6+) in the vitreous network. Mossbauer spectroscopy offers information regarding the TM oxidation states, redox processes and the iron coordination symmetry in the vitreous network. In the case of Fe-Mn doped glasses, the percentage of Fe2+ is about 40% and a doubled iron content leads to an increasing of Fe2+ percentage up to 53%. The replacing of lithium ions by natrium ions (mixed alkali effect) provides an increasing of the Fe2+ percentage up to 56%. The occurrence of the tetrahedral or octahedral symmetry of Fe(2+)supercript stop ions bonded by O2- ions depends on the transition ion nature and Li+/Na+ ratio. Infrared absorption spectra of the pair transition ions-doped aluminophosphate glasses reveal optical phonons specific for the phosphate glass matrix.