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Edge Structure (edge + structure)
Selected AbstractsOrganic matter from comet 81P/Wild 2, IDPs, and carbonaceous meteorites; similarities and differencesMETEORITICS & PLANETARY SCIENCE, Issue 10 2009S. Wirick Sections were analyzed using a scanning transmission X-ray microscope (SXTM) and carbon X-ray absorption near edge structure (XANES) spectra were collected. We compared the carbon XANES spectra of these Wild 2 samples with a database of spectra on thirty-four interplanetary dust particles (IDPs) and with several meteorites. Two of the particles analyzed are iron sulfides and there is evidence that an aliphatic compound associated with these particles can survive high temperatures. An iron sulfide from an IDP demonstrates the same phenomenon. Another, mostly carbon free containing particle radiation damaged, something we have not observed in any IDPs we have analyzed or any indigenous organic matter from the carbonaceous meteorites, Tagish Lake, Orgueil, Bells and Murchison. The carbonaceous material associated with this particle showed no mass loss during the initial analysis but chemically changed over a period of two months. The carbon XANES spectra of the other four particles varied more than spectra from IDPs and indigenous organic matter from meteorites. Comparison of the carbon XANES spectra from these particles with 1. the carbon XANES spectra from thirty-four IDPs (<15 micron in size) and 2. the carbon XANES spectra from carbonaceous material from the Tagish Lake, Orgueil, Bells, and Murchison meteorites show that 81P/Wild 2 carbon XANES spectra are more similar to IDP carbon XANES spectra then to the carbon XANES spectra of meteorites. [source] Iron oxidation state in impact glass from the K/T boundary at Beloc, Haiti, by high-resolution XANES spectroscopyMETEORITICS & PLANETARY SCIENCE, Issue 11 2005Gabriele Giuli The samples have been analyzed by Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre-edge peak of our high-resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four- and five-coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash-form tektites, followed by progressive oxidation. [source] The low-energy electronic properties of graphene ribbons in spatially modulated magnetic fieldsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2008J. Y. Wu Abstract The low energy magnetoelectronic structures for a nanographene ribbon under modulated magnetic fields are investigated through the Peierls tight-binding model. They are dominated by the field strength, period, phase, the ribbon width, and edge structure. The modulated magnetic field could add state degeneracy, modify energy dispersions, alter subband spacings, affect carrier-density distributions, create additional band-edge states, and extend the partial flat bands. The main features of energy bands are directly reflected in density of states, such as the position, the height, the structure, and the number of the prominent peaks. These results are compared with a zigzag ribbon under a uniform magnetic field. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spin-polarized XANES: theoretical analysis of the Ni K-edge of NiF2PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2005G. Smolentsev Abstract Theoretical interpretations of spin-dependent X-ray absorption near edge structure (XANES) spectra measured by selectively monitoring of the Ni K-beta emission while scanning the excitation energy through the Ni K absorption edge have been performed. Analysis is based on a combination of self-consistent spin-polarized calculation of muffin-tin potential and a full multiple scattering theory of X-ray absorption. This approach allows us to separate the influence of dipole transition matrix elements and the density of empty electronic states on spin-dependent XANES. It is found that the matrix elements affect splitting between spin-up and spin-down spectra only near the absorption threshold, while differences in densities of states slightly shift the spectra in the region 25,35 eV above the main edge. The effects of the multielectron-term-dependent broadening of spin-dependent XANES and mixing of purely spin-polarized spectra were taken into account. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Impact of Annealing on the Conductivity of Amorphous Carbon Films Incorporating Copper and Gold Nanoparticles Deposited by Pulsed Dual Cathodic ArcPLASMA PROCESSES AND POLYMERS, Issue S1 2009Jose Luis Endrino Abstract The influence of annealing in argon at 300,°C on the conductivity, phase stability and electronic structure of hydrogen-free amorphous carbon (a-C) films containing copper (a-C:Cu) and gold (a-C:Au) nanoclusters was investigated. The motivation of this work is twofold: (1) to study the thermal stability of a-C:Cu and a-C:Au films and (2) to point out the relevance of X-ray absorption near edge structure (XANES) technique to study the structural evolution of metal-doped a-C nanocomposites. The films were produced at room temperature using a selective-bias pulsed dual-cathode arc deposition technique. Compositional analysis was performed with secondary neutral mass spectroscopy whereas grazing incidence X-ray diffraction (GIXRD) was used to monitor phase transformation and identify the dispersion or agglomeration of the crystallites within the carbon matrix. XANES spectra at the C-K was used to investigate the effect of annealing in argon on the electronic structure of the a-C matrix, while Cu K and Au L-edges were investigated on a-C:Cu and a-C:Au samples, respectively, to study the nanocluster evolution. XANES showed that the a-C host matrix increased its graphitic character and that stress was relieved upon annealing. No relevant changes were observed in the Au arrangements in a-C:Au films. In the case of the a-C:Cu samples, the Cu-K XANES spectra indicated the formation of Cu2O crystals which correlated well with GIXRD spectra and the decrease in conductivity. [source] Natural and Prosthetic Heart Valve Calcification: Morphology and Chemical Composition CharacterizationARTIFICIAL ORGANS, Issue 4 2010Raquel F. Weska Abstract Calcification is the most common cause of damage and subsequent failure of heart valves. Although it is a common phenomenon, little is known about it, and less about the inorganic phase obtained from this type of calcification. This article describes the scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy and Ca K -edge X-ray absorption near edge structure (XANES) characterization performed in natural and bioprosthetic heart valves calcified in vivo (in comparison to in vitro-calcified valves). SEM micrographs indicated the presence of deposits of similar morphology, and XANES results indicate, at a molecular level, that the calcification mechanism of both types of valves are probably similar, resulting in formation of poorly crystalline hydroxyapatite deposits, with Ca/P ratios that increase with time, depending on the maturation state. These findings may contribute to the search for long-term efficient anticalcification treatments. [source] Spectroscopic Characteristics of Differently Produced Single-Walled Carbon NanotubesCHEMPHYSCHEM, Issue 13 2009Zhongrui Li Prof. Dr. Abstract Single-walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X-ray absorption near edge structure, along with X-ray photoemission by following the total valence bands and C 1s core-level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp2 hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence-band X-ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition. [source] Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferationJOURNAL OF MICROSCOPY, Issue 3 2008H. OGITA Summary In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin ,v,3, and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell,cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin ,v,3, which have interacted with Necl-5, then form a complex with nectin, which initiates cell,cell adhesion and recruits cadherin to the nectin-based cell,cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin ,v,3 caused by the trans -interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation. [source] | |||||||||||||