Mostrando recursos 61 - 80 de 106,577

  1. Cathodoluminescence of defects in sintered tin oxide

    Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier
    Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and...

  2. Cathodoluminescence study of GaN-infilled opal nanocomposites

    Cremades Rodríguez, Ana Isabel; Sanchez, M.; Piqueras de Noriega, Javier; Golubev, VG.
    In this study, three-dimensional ordered synthetic opals formed by spheres of 230 nm impregnated with GaN have been investigated. In some of the samples gold and platinum were also introduced. The potential applications of GaN-opal assemblies include use in large surface area (of the order of 10 m_2/cm_3 GaN-based light-emitting devices at a high density of elements. Also, the photonic crystal properties of perfect opal-GaN composites, characterized by the presence of a photonic band gap in the visible range, allow a substantial modification of the emitting properties of GaN-based nanodevices in this spectral region. The optical properties of GaN synthesized...

  3. Study of pinholes and nanotubes in AlInGaN films by cathodoluminescence and atomic force microscopy

    Herrera, M.; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier; Stutzmann, M.; Ambacher, O.
    Cathodoluminescence (CL) in the scanning electron microscope and atomic force microscopy (AFM) have been used to study the formation of pinholes in tensile and compressively strained AlInGaN films grown on Al2O3 substrates by plasma-induced molecular beam epitaxy. Nanotubes, pits, and V-shaped pinholes are observed in a tensile strained sample. CL images show an enhanced emission around the pits and a lower intensity at the V-shaped pinholes. Rounded pinholes appear in compressively strained samples in island-like regions with higher In concentration. The grain structure near the pinholes is resolved by AFM. (C) 2004 American Institute of Physics.

  4. Hybrid topological derivative-gradient based methods for nondestructive testing

    Carpio Rodríguez, Ana María; Rapun, M.L.
    This paper is devoted to the reconstruction of objects buried in a medium and their material properties by hybrid topological derivative-gradient based methods. After illustrating the techniques in time-harmonic acoustic problems with different boundary conditions and in electrical impedance tomography problems with continuous Neumann conditions, we extend the hybrid method for a realistic model in tomography where the boundary conditions are given at a discrete set of electrodes.

  5. Growth and luminescence properties of micro- and nanotubes in sintered tin oxide

    Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier
    Sintering SnO_2 under argon flow at temperatures in the range of 1350-1500 degreesC causes the formation of wires, rods, and tubes on the sample surface. At high temperatures of the mentioned range, microwires with lengths of hundreds of microns are formed. At lower temperatures the formation of micro- and nanorods as well as micro- and nanotubes takes place. The influence of ball milling of the starting powder on the formation of tubes is investigated. The local cathodoluminescence measurements show a different defect structure in the tubes than in the sample background.

  6. Growth and luminescence of elongated In2O3 micro- and nanostructures in thermally treated InN

    Magdas, D.A.; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier
    Indium oxide elongated micro- and nanostructures have been grown by thermal treatment of InN powder. Chains of nanopyramids connected by nanowires, forming a necklace-like structure, as well as cubes and arrow-like structures consisting of a long rod with a micron size pyramid on the top, grow at temperatures in the range 600-700 degrees C in a catalyst free process. The structures have been characterized by scanning electron microscopy and cathodoluminescence.

  7. Three dimensional nanowire networks and complex nanostructures of indium oxide

    Magdas, Dana A.; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier
    Elongated In2O3 micro- and nanostructures have been grown by two-step thermal treatments of compacted InN powder, at temperatures between 350 and 700 degrees C. Different thermal treatments have been found to induce the growth of structures with different sizes and morphologies as wires, rods, or arrows. The experimental conditions leading to the different structures, as well as the evolution of the structures during the treatments, have been investigated. Some treatments lead to large scale formation of three dimensional networks of the mentioned structures. The mechanism of network formation is discussed. The structures have been characterized by cathodoluminescence, scanning electron microscopy,...

  8. Renormalization scale-fixing for complex scattering amplitudes

    Llanes Estrada, Felipe J.; Brodsky, S. J
    We show how to fix the renormalization scale for hard scattering exclusive processes such as deeply virtual meson electroproduction by applying the BLM prescription to the imaginary part of the scattering amplitude and employing a fixed-t dispersion relation to obtain the scale fixed real part. In this way, we resolve the ambiguity in BLM renormalization scale-setting for complex scattering amplitudes. We illustrate this by computing the H generalized parton distribution at leading twist in an analytic quark-diquark model for the parton-proton scattering amplitude that can incorporate Regge exchange contributions characteristic of the deep inelastic structure functions.

  9. Thermal growth and structural and optical characterization of indium tin oxide nanopyramids, nanoislands, and tubes

    Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier; Gregorati, Luca
    In-doped SnO_2 microtubes as well as Sn-doped In_2O_3 (ITO) nano- and microislands have been grown by thermal treatment of compacted SnO_2-In_2O powders under argon flow at 1350 degrees C in a catalyst-free process. The SnO_2 tubes contain about 1 at. % of In, even when the In content in the starting mixture was as high as 52 at. %. However, the ITO nanoislands and nanopyramids, grown preferentially on the faces and edges of the tubes, present an In content up to six times higher than the tubes. Spatially resolved cathodoluminescence shows a higher emission from the Sn-rich structures, so that...

  10. Growth and luminescence of N doped TiO_2 nanowires

    Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Gregoratti, Luca; Piqueras de Noriega, Javier
    Titanium oxide nanowires have been grown by thermal treatment of pressed TiN powder under argon or N-2 flow. It has been found that two-step treatments at two different temperatures, 400 degrees C and 800 degrees C, lead to the growth of nanowires all over the sample surface. The nanowires are of single crystalline rutile structure. Energy dispersive X-ray spectroscopy and photoelectron spectroscopy measurements show that the oxide nanostructures contain N from the starting nitride. The mechanism of N incorporation into the oxide and its possible effect on the luminescence are discussed.

  11. Electrical properties of nanostructured tin oxide surfaces produced by thermal treatments

    Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Gregorati, Luca; Heiderhoff, R.; Balk, L.J.; Piqueras de Noriega, Javier
    Sintering SnO(2) powder in air or under an oxygen atmosphere at different temperatures, leads to polycrystalline samples with nanostructured surface as revealed by atomic force microscopy (AFM). The thermal treatments are also responsible for the variation of the surface electrical properties, as studied by scanning spreading resistance microscopy (SSRM) and scanning tunnelling microscopy and spectroscopy (STM-STS). The surface presents a p-conductance, contrary to the n-type characteristic of the bulk, and a band gap lower than the bulk band gap (3.6 eV). The electrical behaviour at the grain boundaries and the role of oxygen are discussed. X-ray photoelectron spectroscopy (XPS) results...

  12. Espectroscopía Raman Laser de substancias de interés biológico y nutricional. Laser raman spectroscopy in substances of biological and nutritional interest

    González Gonzálvez, Alicia

  13. Dynamically induced scalar quark confinement

    Llanes Estrada, Felipe J.; Alkofer, Reinhard; Fischer, Christian S
    We employ a functional approach to investigate the confinement problem in quenched Landau gauge QCD. We demonstrate analytically that a linear rising potential between massive quarks is generated by infrared singularities in the dressed quark-gluon vertex. The self consistent mechanism that generates these singularities is driven by the scalar Dirac amplitudes of the full vertex and the quark propagator. These can only be present when chiral symmetry is broken. We have thus uncovered a novel mechanism that directly links chiral symmetry breaking with confinement.

  14. On the non-attractive character of gravity in f(R) theories

    Albareti, F. D.; Ruiz Cembranos, José Alberto; Cruz Dombriz, Álvaro de la; Dobado González, Antonio
    Raychaudhuri equation is found provided that particular energy conditions are assumed and regardless the considered solution of the Einstein's equations. This fact is usually interpreted as a manifestation of the attractive character of gravity. Nevertheless, a positive contribution to Raychaudhuri equation from space-time geometry should occur since this is the case in an accelerated expanding Robertson-Walker model for congruences followed by fundamental observers. Modified gravity theories provide the possibility of a positive contribution although the standard energy conditions are assumed. We address this important issue in the context of f(R) theories, deriving explicit upper bounds for the contribution of space-time...

  15. Light-nuclei far from stability

    Dobado González, Antonio; Poves, A

  16. Squark photoproduction at hera

    Dobado González, Antonio; Herrero, M. J.

  17. Toponium stoponium mixing

    Dobado González, Antonio

  18. Production of leptoquarks from superstring models in e-p colliders

    Dobado González, Antonio; Herrero, M. J.; Muñoz, C

  19. Some implications of a top mass close to the W-boson mass

    Dobado González, Antonio; Herrero, M. J.

  20. Are scalar leptoquarks testable at the present CERN collider?

    Dobado González, Antonio; Herrero, M. J.; Muñoz, J

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