In situ study of the endotaxial growth of hexagonal CoSi2 nanoplatelets in Si(001)

This investigation aims at studying-by in situ grazing-incidence small-angle x-ray scattering-the process of growth of hexagonal CoSi2 nanoplatelets endotaxially buried in a Si(001) wafer. The early formation of spherical Co nanoparticles with bimodal size distribution in the deposited silica thin film during a pretreatment at 500 °C and their subsequent growth at 700 °C were also characterized. Isothermal annealing at 700 °C promotes a drastic reduction in the number of the smallest Co nanoparticles and a continuous decrease in their volume fraction in the silica thin film. At the same time, Co atoms diffuse across the SiO2/Si(001) interface into the silicon wafer, react with Si, and build up thin hexagonal CoSi2 nanoplatelets, all of them with their main surfaces parallel to Si{111} crystallographic planes. The observed progressive growths in thickness and lateral size of the hexagonal CoSi2 nanoplatelets occur at the expense of the dissolution of the small Co nanoparticles that are formed during the pretreatment at 500 °C and become unstable at the annealing temperature (700 °C). The kinetics of growth of the volume fraction of hexagonal platelets is well described by the classical Avrami equation.Fil: Costa, Daniel Da Silva. Universidade Federal do Paraná; BrasilFil: Huck Iriart, Cristián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Kellermann, Guinther. Universidade Federal do Paraná; BrasilFil: Giovanetti, Lisandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Craievich, Aldo F.. Universidade de Sao Paulo; BrasilFil: Requejo, Felix Gregorio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin

Size-dependent phase transitions in nanostructured zirconia-scandia solid solutions.

Size effects on phase stability and phase transitions in technologically relevant materials have received growing attention. Several works reported that metastable phases can be retained at room temperature in nanomaterials, these phases generally corresponding to the high-temperature polymorph of the same material in bulk state. Additionally, size-dependent shifts in solubility limits and/or in the transition temperatures for on heating or on cooling cycles have been observed. ZrO2-Sc2O3 (zirconia-scandia) solid solutions are known to exhibit very high oxygen ion conductivity provided their structure is composed of cubic and/or pseudocubic tetragonal phases. Unfortunately, for solid zirconia-scandia polycrystalline samples with typical micrometrical average crystal sizes, the high-conductivity cubic phase is only stable above 600°C. Depending on composition, three low-conductivity rhombo-hedral phases (β, γ and δ) are stable below 600°C down to room temperature, within the compositional range of interest for SOFCs. In previous investigations, we showed that the rhombohedral phases can be avoided in nanopowders with average crystallite size lower than 35 nm.LNLSCNPqANPCyTCLA

Reactive sputter magnetron reactor for preparation of thin films and simultaneous in-situ structural study by X-ray diffraction.

Reactive Sputter Magnetron (RSM) is a widely used technique to thin films growing\ud of compounds both, in research laboratories and in industrial processes. The nature\ud of the deposited compound will depend then on the nature of the magnetron target\ud and the nature of the ions generated in the plasma. One important aspect of the\ud problem is the knowledge of the evolution of the film during the process of growing\ud itself. In this work, we present the design, construction of a chamber to be installed\ud in the Huber goniometer in the XRD2 line of LNLS in Campinas, which allows in\ud situ growing kinetic studies of thin films.ANPCy

Crystal structure, cobalt and iron speciation and oxygen non-stoichiometry of La0.6Sr0.4Co1-yFeyO3-δ nanorods for IT-SOFC cathodes

Single-phased La0.6Sr0.4Co1-yFeyO3-δ (y = 0.2, 0.5, 0.8) nanorods exhibiting the rhombohedral perovskite-type phase were synthesized by a pore-wetting technique. We studied their chemical composition, crystal and electronic structures, morphology and hyperfine properties as a function of the Co/Fe content of the samples. Our results demonstrate that Co cations exhibit a slightly lower oxidation state than Fe ones, resulting in a higher oxygen non-stoichiometry δ for Co-rich samples. In addition, the values of δ determined in this work for nanostructured samples are much higher than those reported in the literature for bulk materials. This can be attributed to the high number of defects in nanomaterials and is probably one important factor in the high electrochemical performance for the oxygen reduction reaction of nanostructured La0.6Sr0.4Co1-yFeyO3-δ IT-SOFC cathodes, which have been reported in a previous work.Fil: Mejía Gómez, Augusto Enrique. Pontificia Universidad Javeriana; ColombiaFil: Sacanell, Joaquin Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Huck Iriart, Cristián. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ramos, Cinthia Paula. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Soldati, Analía Leticia. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Figueroa, Santiago Jose Alejandro. Centro Nacional de Pesquisa em Energia e Materiais; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Tabacniks, Manfredo H.. Universidade de Sao Paulo; BrasilFil: Fantini, Márcia C.. Universidade de Sao Paulo; BrasilFil: Craievich, Aldo Felix. Universidade de Sao Paulo; BrasilFil: Lamas, Diego Germán. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Low resolution structures of the retinoid X receptor DNA-binding and ligand-binding domains revealed by synchrotron x-ray solution scattering

Nuclear receptors are ligand-inducible transcription factors that share structurally related DNA-binding (DBD) and ligand-binding (LBD) domains. Biochemical and structural studies have revealed the modular nature of DBD and LBD. Nevertheless, the domains function in concert in vivo. While high-resolution crystal structures of nuclear receptor DBDs and LBDs are available, there are no x-ray structural studies of nuclear receptor proteins containing multiple domains. We report the solution structures of the human retinoid X receptor DBD-LBD (hRXR AB) region. We obtained ab initio shapes of hRXR AB dimer and tetramer to 3.3 and 1.7 nm resolutions, respectively, and established the position and orientation of the DBD and LBD by fitting atomic coordinates of hRXR DBD and LBD. The dimer is U-shaped with DBDs spaced at 2 nm in a head to head orientation forming an angle of about 10° with respect to each other and with an extensive interface area provided by the LBD. The tetramer is a more elongated X-shaped molecule formed by two dimers in head to head arrangement in which the DBDs are extended from the structure and spaced at about 6 nm. The close proximity of DBDs in dimers may facilitate homodimer formation on DNA; however, for the homodimer to bind to a DNA element containing two directly repeated halfsites, one of the DBDs would need to rotate with respect to the other element. By contrast, the separation of DBDs in the tetramers may account for their decreased ability to recognize DNA

Melting and freezing of spherical bismuth nanoparticles confined in a homogeneous sodium borate glass

The melting temperature and the crystallization temperature of Bi nanoclusters confined in a sodium borate glass were experimentally determined as functions of the cluster radius. The results indicate that, on cooling, liquid Bi nanodroplets exhibit a strong undercooling effect for a wide range of radii. The difference between the melting temperature and the freezing temperature decreases for decreasing radius and vanishes for Bi nanoparticles with a critical radius R = 1.9 nm. The magnitude of the variation in density across the melting and freezing transitions for Bi nanoparticles with R = 2 nm is 40% smaller than for bulk Bi. These experimental results support a basic core-shell model for the structure of Bi nanocrystals consisting of a central crystalline volume surrounded by a structurally disordered shell. The volume fraction of the crystalline core decreases for decreasing nanoparticle radius and vanishes for R = 1.9 nm. Thus, on cooling, the liquid nanodroplets with R < 1.9 nm preserve, across the liquid-to-solid transformation, their homogeneous and disordered structure without crystalline core.LNLS - Brazilian Synchrotron Light LaboratoryPRONEX/CNPqFAPES

Growth and Melting of Metallic Nanoclusters in Glass: A Review of Recent Investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-an-le X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi- or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Waoner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T(m) on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T(m) is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established

Crystallite size-dependent phases in nanocrystalline ZrO2–Sc2O3

ZrO2-10, 12 and 14 mol% Sc2O3 nanopowders were prepared by using a nitrate-lysine gel-combustion synthesis. These materials were studied by synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy after calcination at different temperatures from 650 to 1200 1C, which led to samples with different average crystallite sizes, up to about 100 nm. The results from SXPD and Raman analyses indicate that, depending on Sc2O3 content, the metastable t0 0-form of the tetragonal phase or the cubic phase are fully retained at room temperature in nanocrystalline powders, provided an average crystallite sizes lower than B30 nm. By contrast, powders with larger average crystallite sizes exhibit the stable rhombohedral, b and g, phases and do not retain or very partially retain the metastable t0 0 and cubic ones.Fil: Abdala, Paula Macarena. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas de las Fuerzas Armadas. Centro de Investigaciones en Sólidos; ArgentinaFil: Fantini, Márcia C. A.. Universidade do Sao Paulo. Instituto de Fisica de Sao Carlos; BrasilFil: Craievich, Aldo Felix. Universidade do Sao Paulo. Instituto de Fisica de Sao Carlos; BrasilFil: Lamas, Diego Germán. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas de las Fuerzas Armadas. Centro de Investigaciones en Sólidos; Argentin

SAXS study of crotapotin at low pH

The structure of crotapotin, a protein extracted from the venom of the Crotalus durissus terrificus, in solution at pH = 1.5, was studied by SAXS. The experimental results yield structural parameter values of the molecular radius of gyration R(g) = 13.6 angstrom, volume v = 16.2 x 10(3) angstrom3 and maximal dimension D(max) = 46 angstrom. The distance distribution function deduced from the scattering measurements is consistent with an overall molecular shape of an oblate ellipsoid of revolution with assymetry parameter nu = 0.45