Biblioteca Digital

This work presents a study on the effects of the particle size, material concentration and radiation energy on the X-ray absorption. CuO nanoparticles and microparticles were incorporated separately into a polymeric resin in concentrations of 5%, 10% and 30% relative to the resin mass. X-ray absorption by these materials was analyzed with a CdTe detector. The X-ray absorption is higher for the nanostructured material compared to the microstructured one for low energy X-ray beams for all CuO concentrations. (c) 2011 Elsevier Ltd. All rights reserved.

Filmes finos magnéticos têm grande apelo em mídias de gravação com alta densidade de dados. As propriedades magnéticas desses filmes, que dependem da estrutura atômica do material, podem ser modificadas ou induzidas pela presença de interfaces internas. Para o entendimento e melhoramento dessas propriedades, torna-se necessário o uso de técnicas capazes de fornecer informações seletivamente em profundidade. Neste trabalho, acoplamos a espectroscopia de absorção de raios X (XAS) à uma geometria de incidência rasante, e assim usamos a variação da penetração dos raios X dentro do material em torno do ângulo crítico de reflexão total para obter informações resolvidas em profundidade sobre a ordem estrutural e magnética local. Desenvolvemos uma metodologia de medidas e de análise desta informação. Esta metodologia foi aplicada em filmes de FePt e CoPt que produzimos pela técnica de deposição catódica. Em filmes de FePt, uma análise quantitativa completa nos permitiu caracterizar a camada de oxidação da superfície. Em filmes de CoPt, observamos que a ordem química, responsável pela anisotropia perpendicular é parcialmente perdida em grandes profundidades além da superfície para filmes de espessura superior a 50 nm. A presença desta camada desordenada...

The combination of x-ray absorption spectroscopy (XAS) with UV-Vis and Raman spectroscopies or with Differential Scanning Calorimetry (DSC) has been recently carried out on the D44 beamline of DCI-LURE. The different set-ups used to perform such combinations are described and examples of combined investigations belonging to different field of materials science (coordination chemistry, sol-gel and catalysis) are presented. © Physica Scripta 2005.

The present paper is a comprehensive study concerning Fe K-edge X-ray absorption spectroscopy (XAS) measurements, which were performed to characterize the local structure of (1 - x)Pb(Fe2/3W1/3)O 3-xPbTiO3 samples as a function of temperature and PbTiO3 content. Results obtained by the fits of extended X-ray absorption fine structure consist with rhombohedral symmetry for Pb(Fe 2/3W1/3)O3 composition at temperatures lower than room temperature. This result is in apparent disagreement with X-ray and neutron diffraction characterization which have been reported. This apparent disagreement is related to the fact that XAS probes the short-range order, whereas X-ray diffraction provides structural information about the average structure. Moreover, as the PbTiO3 content increases, a disorder has been detected at local structure of the FeO6 octahedron. Analysis of X-ray absorption near edge structure spectra did not show modifications in intensity nor energy of transitions. © 2013 American Institute of Physics.

This paper reports on the structural characterization of Pb 1-xLaxZr0.40Ti0.60O3 (PLZT) ferroelectric ceramic compositions prepared by the conventional solid state reaction method. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were used to probe the local structure of PLZT samples that exhibits a normal and relaxor ferroelectric behavior. From the Zr K-edge and Pb LIII-edge EXAFS spectra, a considerable dissymmetry of Zr and Pb sites was observed in all samples, including those showing a long-range order cubic symmetry and a relaxor behavior. The Raman spectroscopy results confirmed the existence of a local disorder in all PLZT samples through the observation of Raman active vibrational modes. The variation in the intensity of the E(TO 3) mode in the PLZT relaxor samples indicates that the process of correlation between nanodomains stabilizes at temperatures lower than T m. © 2013 Elsevier B.V. All rights reserved.

A goethite-based catalyst was evaluated using in-situ X-ray absorption near-edge structure (XANES) spectroscopy during methane oxidation under increasing reaction temperature. Determination of rank by median absolute deviation (DRMAD), evolving factor analysis (EFA), and multivariate curve resolution (MCR) were used to detect the species present in the catalyst during the reaction and determine their concentration profiles and their pure spectra. The reactants and reaction products were monitored on-line by mass spectrometer. The goethite-based catalyst was active for methane oxidation, with CO(2) and H(2)O as the main products. DRMAD and EFA were useful to determine the number of chemical species present in the catalyst structure during reactions. The catalyst presented phase transition during the reaction from goethite to maghemite according to XANES spectra determined by MCR. On the other hand, it was verified that the catalyst presented phase transition from goethite to wüstite in the process in the absence of the oxidant (O(2)).

X-Ray Absorption Spectroscopy (XAS) is a well-established analytical technique used extensively for the characterization of semiconductors in solid or liquid, crystalline or amorphous, bulk or nanoscale form. With this chapter, we provide a brief introduction to XAS, covering both theory and experiment, while we refer to more comprehensive texts for greater detail about this continually evolving technique. The chapter thus is a starting point upon which subsequent chapters build as they demonstrate the broad-ranging applications of XAS to semiconductors materials.

Synchrotron radiation X-ray fluorescence (SR-XRF) was used to characterize As speciation within natural fluid inclusions from three deposits with different hydrogeochemical and geological settings. The studied samples represent different compositions of Au-bearing fluids: typical orogenic Au deposit (low-salinity, ~6 mol% CO2 ± CH4; Brusson, Western Italian Alps); brines from a Proterozoic (Fe)- Cu-Au deposit (Starra, Queensland, Australia); and an As-rich magmatic fluid with a bulk composition similar to that typical of orogenic gold (Muiane pegmatite, Mozambique). Arsenic K-edge X-ray absorption spectra (XAS) were obtained from fluid inclusions at temperatures ranging from 25 to 200°C, and compared with spectra of aqueous As(III) and As(V) solutions and minerals. X-ray absorption near edge structure (XANES) data show that initially the fluid inclusions from all three regions contain some As in reduced form [As(III) at Brusson and Muiane; As-sulfide or possibly As(0) at Starra]. However, this reduced As is readily oxidized under the beam to As(V). Therefore, extended X-ray absorption fine structure (EXAFS) spectra for the As(III) aqueous complex could be collected only on the sample from the Muiane pegmatite containing large fluid inclusions with high As concentrations (>>1000 ppm). Analysis of these EXAFS data shows that As(OH)3(aq) (coordination number of 3.0 ± 0.2 atoms...

Many inorganic species are now recognized as being essential for life, including many forms of sulfur, phosphate and numerous classes of metal ions. For example, recent progress in the fields of biochemistry and biology has pointed out the critical importance of sulfur in the biosynthesis of vital cofactors and active sites in proteins, and in the complex reaction mechanisms often involved. Special attention has also been drawn to the diverse roles of alkaline (Na+, K+) and alkaline earth (Mg2+, Ca2+) metal ions in mediating the activity of RNA, proteins and many processes in living cells. While the general effect of these ions in biology is mostly understood, information on their detailed role is deficient. Here the application of softer X-ray absorption spectroscopy (XAS) to probe the local structural and electronic environment of such ions within their biological complexes and during physiological reactions is discussed. In addition, the required experimental set-up and the difficulties associated with conducting softer XAS experiments on biological samples are presented.; B. Akabayov, C. J. Doonan, I. J. Pickering, G. N. George and I. Sagi

Selenium compounds exhibit chemopreventative properties at supranutritional doses, but the efficacy of selenium supplementation in cancer prevention is dependent on the chemical speciation of the selenium supplement and its metabolites. The uptake, speciation, and distribution of the common selenoamino acid supplements, selenomethionine (SeMet) and Se-methylselenocysteine (MeSeCys), in A549 human lung cancer cells were investigated using X-ray absorption and fluorescence spectroscopies. X-ray absorption spectroscopy of bulk cell pellets treated with the selenoamino acids for 24 h showed that while selenium was found exclusively in carbon-bound forms in SeMet-treated cells, a diselenide component was identified in MeSeCys-treated cells in addition to the carbon-bound selenium species. X-ray fluorescence microscopy of single cells showed that selenium accumulated with sulfur in the perinuclear region of SeMet-treated cells after 24 h, but microprobe selenium X-ray absorption near-edge spectroscopy in this region indicated that selenium was carbon-bound rather than sulfur-bound. X-ray absorption and X-ray fluorescence studies both showed that the selenium content of MeSeCys-treated cells was much lower than that of SeMet-treated cells. Selenium was distributed homogeneously throughout the MeSeCys-treated cells.; Claire M. Weekley...

Selenite is an inorganic form of selenium that has a cytotoxic effect against several human cancer cell lines: one or more selenite metabolites are considered to be responsible for its toxicity. X-ray absorption spectroscopy was used to monitor Se speciation in A549 human lung cancer cells incubated with selenite over 72 h. As anticipated, selenodiglutathione and elemental Se both comprised a large proportion of Se in the cells between 4 and 72 h after treatment, which is in accordance with the reductive metabolism of selenite in the presence of glutathione and glutathione reductase/NADPH system. Selenocystine was also present in the cells but was only detected as a significant component between 24 and 48 h concomitant with a decrease in the proportion of selenocysteine and the viability of the cells. The change in speciation from the selenol, selenocysteine, to the diselenide, selenocystine, is indicative of a change in the redox status of the cells to a more oxidizing environment, likely brought about by metabolites of selenite. X-ray fluorescence microscopy of single cells treated with selenite for 24 h revealed a punctate distribution of Se in the cytoplasm. The accumulation of Se was associated with a greater than 2-fold increase in Cu...

The selenoamino acids methylselenocysteine (MeSeCys) and selenomethionine (SeMet) have disparate efficacies as anticancer agents. Herein, we use X-ray absorption spectroscopy to determine the chemical form of selenium in human neuroblastoma cells. Cells treated with MeSeCys contain a significant diselenide component, which is absent from SeMet-treated cells and suggests that metabolites of MeSeCys are capable of altering the redox status of the cells. The differences in the speciation of Se in the selenoamino acid-treated cells may provide insight into the differing anticancer activities of MeSeCys and SeMet.; Claire M. Weekley, Jade B. Aitken, Ian F. Musgrave, and Hugh H. Harris

Novel copper-cobalt oxide thin films with different copper/cobalt molar ratios, namely, [Cu]/[Co] = 0.5, 1, and 2, have been successfully coated on aluminum substrates via a simple and cost-effective sol-gel dip-coating method. Coatings were characterized using high resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, in combination with nanomechanical testing and field emission scanning electron microscopy (FESEM). The surfaces of both [Cu]/[Co] = 0.5 and 1 samples consisted primarily of fine granular nanoparticles, whereas the [Cu]/[Co] = 2 has a smoother surface. The analyses reveal that the increase of copper concentration in the synthesis process tends to promote the formation of octahedral Cu²⁺ which minimizes the development of octahedral Cu⁺, and these octahedral Cu²⁺ ions substitute the Co²⁺ site in cobalt structure host. The local coordinations of Co, Cu and O are not substantially influenced by the change in the copper to cobalt concentration ratios except for the [Cu]/[Co] = 2 coating where the local coordination appears to slightly change due to the loss of octahedral Cu⁺. The present film coatings are expected to exhibit good wear resistance especially for the [Cu]/[Co] = 1.0 sample due to its high hardness/elastic modulus (H/E) ratio. Finite element modeling (FEM) indicated that...

We have used high-resolution near-edge X-ray absorption fine structure spectroscopy to study the N 1s → 1π* resonance of N2 trapped below the surface of several compound semiconductors. The vibrational fine structure, observed from all samples under co

10 pages, 2 tables, 8 figures.-- PACS number(s): 75.50.Cc, 75.30.Kz, 61.10.Ht, 87.64.Ni; We present an x-ray magnetic circular dichroism (XMCD) study performed at both the Co K edge and the Lu L2,3 edges on (YyLu1−y)(Co1−xAlx)2 systems. The XMCD spectra reflect the different magnetic character of these systems, allowing us to monitor the transition from weak to strong ferromagnetism. The XMCD at the Lu L2,3 edges indicates the existence of an ordered 5d moment at the lutetium sites that is coupled antiparallel to the Co moment. Estimates of the magnetic moment of Lu have been obtained by applying the XMCD sum rules. Our results show that there is a correlation between the Lu 5d–induced magnetic moment and the magnetic character of the (YyLu1−y)(Co1−xAlx)2 compounds. These results suggest that the developing of the Lu moment plays an important role in reinforcing the magnetic interactions and favoring the ferromagnetic character of the Lu-rich compounds.; This work was partially supported by Spanish Grants No. MAT2005-06806-C04-02 and No. MAT2005-06806-C04-04. The synchrotron radiation experiments were performed at SPring-8 (Proposals No. 2004A0020-NSc-np and No. 2006A1107).; Peer reviewed

In this work, an imaging tool to investigate optical inhomogeneities with site and chemical sensitivities has been integrated in a hard x-ray microprobe. Freestanding GaN and epitaxially grown GaN:Mn on -Al2O3 are used to exploit the unprecedented scanning x-ray excited luminescence technique. Optical images of the radiative recombination channels are reported for several impurities and defect centers in sapphire and GaN compounds. Within the experimental accuracy, a visible nonuniformity characterizes the Mn centers in good correlation with former x-ray fluorescence map. Expanding the microprobe versatility, x-ray absorption spectroscopy in both photon collection modes (x-ray excited luminescence and x-ray fluorescence) is finally presented from a freestanding GaN layer.; Special thanks are due to Martin Hermann, Martin Eickhoff, and Martin Stutzmann (Walter Schottky Institute) for providing the sample used in this study. The authors are also grateful to Remi Tucoulou as well as Sylvain Laboure for their useful help.; Peer reviewed

6 pages, 1 table, 3 fiures, 1 appendix.-- Edited by Peter Brzezinski.; Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c6 and membrane-embedded photosystem I involves subtle changes in the heme iron, as inferred by X-ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c6 is observed upon interaction with photosystem I. This work constitutes a novel application of XAS to the analysis of weak complexes in solution.; This work was supported by the European Synchrotron Radiation Facility (ESRF SC- 1366), the Spanish Ministry of Education and Science (AP2000-2937, BMC2003- 00458 and MAT02-01221) and the Andalusian Government (PAI, CVI-0198).; Peer reviewed

Elemental Cu nanocrystals were synthesized in thin film SiO2 by ion implantation and thermal annealing. The local atomic structure and nanocrystal size distribution were investigated by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. We quantify the bondlength contraction and increased structural disorder in the nanocrystals as compared to a bulk Cu reference. Both are proportional to the inverse of the nanocrystal diameter, which in turn is proportional to the surface-area-to-volume ratio. In particular we show that a simple liquid-drop model can explain the bondlength contraction and estimate the surface tension of nanocrystalline Cu to be 3.8 ± 0.4 J/m2.

Interaction of low-energy nitrogen ions with ZnO surface has been studied by photoemission spectroscopy (PES) around N 1s core-level and near-edge X-ray absorption fine structure (NEXAFS) around N K-edge. Nitrogen can break Zn-O bonds at the surface and form N-O, Zn-N or Zn-N-O bonds, characterised by specific chemical shifts in PES or absorption peaks in NEXAFS. A distinctive signal from molecular nitrogen has also been observed in ion-bombarded samples in both NEXAFS and PES.

We present experimental results for x-ray absorption and resonant emission at the L-edge of the transition metal in MnF2 and CoF2. The emission data are corrected for self-absorption. The data are compared with calculations in both the free-ion approximation and with the effect of the ligand field of D4h symmetry included. The results of the calculations take into account interference terms in the Kramers-Heisenberg expression. We obtain very good agreement between experiment and theory for both x-ray absorption and resonant emission in the two compounds. The inclusion of the ligand field is important to achieve such agreement. However, the results of the calculation that does not take into account the interference terms are in better agreement with experiment, indicating that the model used probably overestimates the importance of interference effects.