Formation of argentic clusters and small Ag nanoparticles in soda-lime silicate glass

The formation of argentic clusters and very small Ag nanoparticles of 0.5 to 2 nm size in commercial soda-lime glass silver-doped by Ag/Na ion exchange in a mixed nitrate melt has been studied by electron microscopy and EXAFS. Particles formation was induced already during the ion exchange procedure, or by subsequent ion irradiation with 1.5 MeV He⁺ or 3 MeV Au⁺. The presence of nanoparticles was also macroscopically revealed by their surface plasmon resonance. The structural characterization indicates that specific configurations of silver oxide-like structures, so-called argentic clusters, are involved in the initial stage of nanoparticles formation.     

Chord length distributions and small-angle scattering

Chord length distributions describe size, shape and spatial arrangement of geometrical objects (particles). The chord length distribution is in principle proportional to the second derivative of the correlation function of small-angle scattering. It is calculable from a relative measurement of the scattering intensity I(h). In structure research, the characterization of numerous particle systems can be achieved by comparing experimental chord distributions with theoretical ones, provided the latter are available with sufficiently high precision for a lot of fundamental, universal shapes. Both sides of this concept are exemplified: – the step from a relative measurement of the scattering intensity of an isotropic two-phase sample to the chord length distribution (errors in and in , limited h-interval, corresponding to the region (1-2) nm < r in real space, must be observed); as well as the geometric matter of calculation of chord distributions as fingerprints for basic geometric figures, including the non-convex case. Received 15 March 1999 and Received in final form 26 April 2000     

Structural characterization of Rh/pumice SMAD catalysts

The structure of Rh/pumice catalysts prepared by the SMAD (Solvated Metal Atoms Dispersion) technique at different metal loadings has been investigated by EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), SAXS (Small-Angle X-ray Scattering), WAXS (Wide-Angle X-ray Scattering) and TEM (Transmission Electron Microscopy). According to EXAFS and XPS, a fraction of the Rh atoms is oxidised, but a noticeable part is also present as Rh ⁰. The Rh oxidation is attributed to the interaction of the Rh atoms with the hydroxyls of the support; after the formation of the oxide, the nucleation of metallic rhodium becomes possible. The WAXS data do not show evidence of rhodium fcc crystallites; the metal-bearing particles are probably amorphous and/or very small, as results from the SAXS and TEM data analysis. The disagreement between the latter two techniques, resulting in a small-angle determination of the average size of the particles that is about half that of TEM in the catalyst with the higher Rh loading, is acknowledged and discussed. Preliminary catalytic tests are described, demonstrating the suitability of using a low surface area support for the preparation of SMAD catalysts. Received 2 February 1999     

Structure of metal nanowires in nanoporous alumina membranes studied by EXAFS and X-ray diffraction

The structure of nanowires of different metals grown within nanoporous alumina membranes has been studied by EXAFS, WAXS and high energy X-ray diffraction. Nanowires of gold, silver, copper and iron adopt the lattice structure and bond distances of the bulk metals. Cobalt nanowires on the other hand were composed of a mixture of hcp phase, stable at room temperature, and fcc phase, which in bulk cobalt is normally stable only at high temperatures, in a ratio depending on the pore size. The nanowires are non-continuous but are made of nanocrystallites whose shape and size was found to depend strongly on the metal. All the metals except gold showed the presence of a preferred orientation which was slight in the case of Ag and Cu but much stronger in the case of iron and cobalt nanowires. Received 30 November 2000     

Structural characterisation of Ni clusters in AlN via X-ray absorption, X-ray diffraction and transmission electron microscopy

Ni ions were implanted in bulk AlN with the goal to form embedded metallic clusters. Combining several characterisation techniques such as X-ray absorption spectroscopy, X-ray diffraction and high resolution transmission electron microscopy, we determined the lattice parameter of the Ni clusters that display a fcc crystalline structure. The average size increases when the ion fluence is increased or after a thermal treatment. Thanks to moiré fringes observed by high resolution transmission electron microscopy and to satellite peaks seen on the diffraction patterns, we concluded that the annealed Ni clusters orientate their (002) planes on the (101) of AlN. Moreover, the satellite positions allowed us to calculate Ni cluster average diameters, that are in agreement with average sizes deduced by X-ray absorption spectroscopy. Received 25 August 1999 and Received in final form 8 February 2000     

Atomic Rayleigh scattering cross-sections and the associated anomalous dispersion in the X-ray regions

Elastic scattering cross-sections for Pd, Ag, Cd, In, Sn, Sb, Pt, Au and Pb are measured at an angle of 90 in the X-ray region 5.41 keV. These energies fall between the high-energy side of the L- and M-shell absorption edges of the atoms considered. The present atomic region is significant for solid X-rays to assess the contribution of resonance and solid-state environmental effects. Also it is the anomalous scattering region for many of the atoms of the periodic table. Experimental results are compared with theoretical calculations based on form factor formalisms including the anomalous corrections and available recent S-matrix values. Based on the experimental evidence, the present results indicate the influence of solid-state environmental effects, the importance of anomalous corrections nearer to absorption edges, the correctness of revised high-energy limit values, the superiority of S-matrix predictions over form factor values on measured elastic scattering cross-sections in the X-ray regime and also show the resonance behavior around K, L and M absorption edges. Received: 27 January 1998 / Received in final form: 4 January 1999     

A small-angle X-ray scattering study of the isotropic and nematic phases of V2O5 suspensions

We report small-angle X-ray scattering experiments performed in both the isotropic and nematic phases of aqueous V₂O₅ suspensions. We show that the scattering in the isotropic phase can be well described in the whole accessible q-range by only considering the form factor of non-interacting ribbons. We investigate the influence of concentration and pH on the dimensions of V₂O₅ ribbons and show that these parameters do not have any significant effect, as long as the system stays well within the chemical stability domain of the ribbons. We then show that nematic single domains display an anisotropic small-angle scattering pattern, even at scattering vectors small compared to that at which a characteristic correlation peak is observed. This feature is expected for a nematic phase, but was rarely observed. We finally try to describe this scattering within the framework of theories developed for the structure factor of a nematic polymer, and we reach the conclusion that chain ends are certainly important to understand this pattern. Received 21 July 1999 and Received in final form 17 December 1999     

Local structure of Co and Ni in decagonal AlNiCo investigated by polarized EXAFS

The local structure of cobalt and nickel in single crystals of decagonal quasicrystals with composition Al_71.5Ni_15.5Co₁₃ and Al₇₅Ni_14.5Co_10.5 have been studied by polarized EXAFS. Significant differences between the Ni and Co local environments have been detected. The effective absence of the 4 ? periodicity along the decagonal axis in these QCs is confirmed and indications about its reasons are presented. Received 26 July 2000 and Received in final form 20 October 2000     

X-ray spectroscopy of the metal-insulator transition in YbGdTe and YbLaTe

We report extended X-ray absorption fine structure (EXAFS) of Yb_1-xGd_xTe and Yb_1-xLa_xTe, two solid solutions which show an insulator-metal transition (IMT) as a function of x for a high donor concentration . The results are correlated to transport experiments and X-ray diffraction data, to analyse the IMT in relation to the structure at both a local and a macroscopic scale. A bimodal distribution is found for rare earth-Te nearest distances in these compounds crystallized in the fcc structure. In particular, we find the less rigid Yb-Te bond can be used as a local probe for the wavefunction of the electrons introduced by La or Gd donors, hence a new insight on the metal-insulator transitions which are of a different nature in the two solid solutions. Received: 23 March 1998 / Revised and Accepted: 12 July 1998     

Cu-Ni alloy nanocluster formation by ion implantation in silicate glasses: Structure and optical properties

Sequential ion implantation (copper and nickel) in silica and soda-lime glasses has been performed. The formation of copper-nickel alloy nanocluster in the glass host has been evidenced by synchrotron radiation-based techniques, namely X-ray diffraction and absorption spectroscopy. The nanocrystals' lattice parameter value was estimated, indicating the formation of Cu₅₅Ni₄₅ alloy particles. Optical absorption spectra are also discussed. Received 6 May 1999 and Received in final form 22 September 1999     

Investigation of S-H bonds in biologically important compounds by sulfur K-edge X-ray absorption spectroscopy

X-ray Absorption Near Edge Structure (XANES) spectroscopy, often provides a direct correlation between observed resonances in the spectrum and molecular bonds in the sample. This can be used as a fingerprint for the presence of a given molecular environment of the absorber atom in a sample. As the white line is found at similar energy positions for S-C and S-H bonds, this approach is impossible when both types of bond are present simultaneously, as often in biological systems. To develop a criterium for the presence of S-H bonds in such samples, reduced glutathione, reduced coenzyme A, cysteine and their corresponding oxidized forms were investigated using sulfur K-edge XANES, revealing a unique feature at 2 475.8 eV in the respective difference spectra. To correlate this structure to S-H bonds, H₂S and H₂S₂ were measured, whose difference spectrum also shows a structure at this energy position, whereas it is not present throughout a variety of C-S-C/C-S-S-C environments. Theoretical investigations suggest its correlation to a Rydberg transition occurring in the case of a S-H bond. Using this criterium, the presence of S-H bonds is in the purple sulfur bacterium Allochromatium vinosum during oxidation of intracellular accumulated sulfur, is proved, as expected from biological considerations. Received 1st February 2002 / Received in final form 10 June 2002 Published online 13 September 2002     

Nanoparticle chain-like formation in electrical double-layered magnetic fluids evidenced by small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) was performed on a series of Electric Double-Layered Magnetic Fluids (EDL-MF) composed of ferrite type-- CoFe₂O₄, MnFe₂O₄, ZnFe₂O₄, NiFe₂O₄ and CuFe₂O₄--nanoparticles of different crystalline sizes ( D _XR ranging from 40 to 139?, as determined by X-ray diffraction). The information concerning the scattering objects was obtained through the analysis of the distance distribution function p(r) and of the size distribution function D(R), both retrieved from SAXS data. The results show that EDL-MF, in the absence of an applied magnetic field, are composed of small magnetic particle aggregates in solution. These agglomerates are elongated in one direction (chain-like) with the longest dimension varying from 240 to 330?. The cross-section size is of the order of D _XR. The data also demonstrate that the maximum dimension of these aggregates is independent of the ferrite type. On the other hand, the number of aggregated magnetic particles is nanoparticle-size-dependent. Accordingly, larger ferrite-type nanoparticles as those with D _XR = 139? form aggregates composed of 2-3 magnetic particles, whereas smaller ones with D _XR 40? form agglomerates of about 6 magnetic particles in solution. As the nanoparticle size is reduced, it might increase the particle surface defects. Such occurrence would affect the particle surface charge density, which could reduce the electrostatic screening, favoring the agglomeration phenomenon. Received 28 February 2000 and Received in final form 28 August 2000     

Haem conformation of amphibian nytrosylhaemoglobins detected by XANES spectroscopy

We investigated for the first time the haem stereochemistry in the nitrosylated derivative of two amphibian haemoglobins, Xenopus laevis and Ambystoma mexicanum, by means of X-ray absorption spectroscopy technique with the aim to explain the relationships between the active site structure and physiological function of these proteins, compared to that from humans. Our results show that while the Fe site local structure of human HbNO is modulated by an allosteric effector such as IHP shifting the T-R equilibrium towards the T-state, the Fe site local structure of amphibians HbNO is stabilized in a particularly tensed T-state also without IHP.     

Structural characterization of pumice-supported silver-palladium metal clusters by means of XAFS and AWAXS

Silver-palladium bimetallic clusters were synthesized on pumice by the reduction of aqueous solution of metal salts with different Pd:Ag ratios. Used as bimetallic catalysts, in hydrogenation reactions, in situations where molecules can undergo several different reactions, they eliminate undesired reactions and maximize the desired ones. To characterize the structure of these bimetallic catalysts and control if the two metallic elements do mix or not to each other to form an alloy, the structure of the Ag-Pd bimetallic clusters was investigated by means of X-ray-absorption fine-structure spectroscopy (XAFS) and anomalous wide angle X-ray scattering (AWAXS) experiments performed at the European Synchrotron Radiation Facility (ESRF) using the GILDA and the BM16 beamlines. A correlation between Ag and Pd was found but not a clear evidence of alloying. Received: 21 December 1998 / Received in final form: 19 March 1999     

Local coordination geometry around Cu and Cu ions in silicate glasses: an X-ray absorption near edge structure investigation

We present an X-ray absorption near edge structure (XANES) study on Cu⁺ and Cu²⁺ ions in silicate glasses at the Cu K-edge, aimed to determine the geometry of the local structure around the metal. This study is based on the comparison between experimental data and theoretical calculations made in the framework of multiple scattering theory. The XANES signals relative to several clusters are simulated on the basis of known crystalline structures involving Cu⁺ and Cu²⁺ ions in silicate matrices. Concerning the Cu²⁺ in glass, the simulations suggest the presence of a square coordination of oxygen atoms around the absorber, with a possible presence of metal ions in the second shell. As for the Cu⁺ ions, the metal clustering is excluded and a linear O-Cu-O coordination is evidenced. Received 30 April 1999     

Effect of structural distortions on the magnetism of doped spin-Peierls CuGeO3

The chemical selectivity and great sensitivity of the Extended X-ray Absorption Spectroscopy technique allowed the determination, in the paramagnetic phase, of the structural distortions induced by doping in the spin-Peierls CuGeO₃ compound. The distorted environments were analyzed as a function of concentration, magnetic nature of impurity and the substitution site (Ni, Mn and Zn impurities on the Cu site, Si impurity on the Ge site). This has led to estimate the variation of the angles and pair distances, and hence to evaluate the magnetic coupling along the Cu chains in the vicinity of the impurities. The antiferromagnetic interaction between Cu first neighbors in the pure sample is found to be weakened around Ni, almost cancelled in the case of Mn doping, and even to change sign, producing a ferromagnetic coupling for Si doping. More generally, the structural distortions on a local scale are shown to be key parameters for the understanding of the magnetic properties of doped spin-Peierls compounds.     

Spontaneous condensation in DNA-polystyrene- b-poly(l-lysine) polyelectrolyte block copolymer mixtures

We investigated the condensation of calf thymus DNA by amphiphilic polystyrene_m-b-poly(l-lysine)_n block copolymers ( PS_m-b- PLys_n, m, n = degree of polymerization), using small-angle X-ray scattering, polarized optical microscopy and laser scanning confocal microscopy. Microscopy studies showed that the DNA condenses in the form of fibrillar precipitates, with an irregular structure, due to electrostatic interactions between PLys and DNA. This is not modified by the presence of hydrophobic PS block. Scattering experiments show that the structure of the polyplexes corresponds to a local order of DNA rods which becomes more compact upon increasing n. It can be concluded that for DNA/ PS_m-b- PLys_n polyplexes, the balance between the PLys block length and the excess charge in the system plays an essential role in the formation of a liquid crystalline phase.     

Asymmetric local displacements in the Bi2Sr2CaCu2O8+β superconductor

Instantaneous in-plane Cu-O bond distribution in the Bi₂Sr₂CaCu₂O_8+β (Bi2212) superconductor has been investigated by high k-resolution Cu K-edge extended X-ray absorption fine structure (EXAFS) measured with polarized vector parallel to the two orthogonal Cu-O-Cu bonds of the CuO₂ square plane. The results show an anisotropic Cu-O distribution in the two directions and provide further information on the local atomic displacements in the lattice-charge stripes. Received 5 June 2000     

Dynamics and correlations in magnetic colloidal systems studied by X-ray photon correlation spectroscopy

We have studied the static and dynamic behaviour of magnetic colloidal systems (ferrofluids) by Small-Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS). The main features of the novel XPCS technique will be illustrated by data taken on a model system, colloidal silica spheres. We will then present the results obtained on an optically opaque suspension of magnetic colloids (maghemite) in the wave vector range from 10^-3 to 10^-2?^-1. Translational diffusion in zero field and anisotropic diffusion under external magnetic fields will be discussed. Received 9 August 1999 and Received in final form 26 July 2000     

Formation of sharp metal-organic semiconductor interfaces: Ag and Sn on CuPc

A detailed investigation of the chemistry and electronic structure during the formation of the interfaces between thin films of the archetypal organic molecular semiconductor copper phthalocyanine (CuPc) and Ag or Sn deposited on it was performed using photoemission and near-edge X-ray absorption spectroscopies with synchrotron light. Our study demonstrates the formation of sharp, abrupt interfaces, a behavior which is of particular importance for applications in organic devices. Moreover, for Ag on CuPc we demonstrate that this interface is free from any reaction, whereas there is slight interface reaction for Sn/CuPc.