XANES Study of Structural Disorder in Amorphous Silicon

An investigation of the structure of several amorphous silicon (a-Si) films is presented. Samples were prepared by using the ion beam sputtering technique at different substrate deposition temperatures. X-ray absorption spectroscopy and multiple scattering formalism have been used to detect structural variations of the a-Si films. The analysis of the XANES (X-ray absorption near-edge structure) spectra shows that increasing the substrate deposition temperature leads to a structural change toward a higher-level short-range order.

Is phytoextraction a suitable green treatment for metal-contaminated sediments ?

International audienceThe cleaning of waterways by regular dredging generates great volumes of sediments and, owing to human activities, these sediments often contain large amounts of metals. These materials are usually spread on landfill sites. Phytoremediation could be a stategy for the reclamation of these polluted sediments. To our knowledge, phytoextraction with hyperaccumulating plants has been few tested on contaminated sediment. This work focuses on the mechanisms of Cd accumulation in Arabidopsis halleri, a Cd and Zn hyperaccumulator, and the effects of this species on a metal polluted sediment

Analysis of Si-K edge EXAFS in the low k domain

Studies of the silicon absorption spectrum above the K-edge in crystalline silicon carbide SiC and in pure crystalline silicon are reported. We show that for these systems, no high order scattering is needed to interpret the experimental results down to few eV above the edge. This behaviour can be explained assuming, below typically 4Å-1, a flattening of the phase factors versus k : all the data are understood in terms of single backscattering of the photoelectron by the neighbours and then can be analysed with a simple Fourier transform.Nous avons étudié le seuil K du silicium dans SiC et Si cristallins. Nous montrons que dans ces systèmes, l'EXAFS jusqu'aux très faibles énergies du photoélectron peut être interprété par un formalisme de diffusion simple. Les résultats obtenus nous conduisent à avancer l'hypothèse d'une invariance des déphasages en fonction de k pour k < 4 Å-1

Nanometric Si/C/N Powders : EXAFS Study at the Silicon K Edge

Preceramic nanosized Si/C/N powders with a variable C/N ratio have been investigated by X-ray absorption spectroscopy at the silicon K edge. By combining XANES and EXAFS analysis results, we have proposed a model for the local structure of the as-prepared powders. This structural amorphous model has been tested through multiple scattering calculations using the FEFF code calculations. The evolution during the pyrolysis process is followed, specially for intermediate C/N values for which a delay in the crystallisation temperature occurs. Before the formation of very small crystallites, one can observe a reorganisation of the short range atomic structure

ETUDE PAR DIFFUSION DES RAYONS X DES FLUCTUATIONS CORRELEES DANS LES VERRES METALLIQUES

Cette étude est effectuée par diffusion des rayons X dans un large domaine angulaire allant de la diffusion centrale au premier anneau de diffraction. On compare ici des alliages amorphes Cu-Ti élaborés soit par trempe du Liquide soit par pulvérisation cathodique, et l'on montre que pour la dernière méthode de préparation il se crée des fluctuations spatialement corrélées dans l'alliage avant sa cristallisation. Nos résultats montrent aussi que le pré-pic observé avec les rayons X doit être attribué à l'effet de taille.An X-ray scattering study is carried out with a specific camera which records the intensities from the small-angle region to the first diffraction halo of an amorphous material. Cu-Ti metallic glasses prepared either by sputtering or by liquid-quenching are compared. In the former case, space correlated fluctuations take place in the amorphous alloy before crystallization. Our results also show that the pre-peak which is revealed by X-rays must be attributed to the size effect

STRUCTURAL EXAFS STUDY OF COMPOSITE MATERIALS OF THE SiC TYPE

L'environnement du silicium dans les fibres composites type SiC a été étudié par EXAFS et comparé à celui trouvé dans la silice, et dans le carbure de silicium cristallin. Notre étude conclut à l'existence dans ces fibres de microcristaux de SiC noyés dans une matrice d'oxyde.The Silicon environment in composite SiC fibres has been investigated by EXAFS and compared to the SiO2 glass and quartz, crystalline SiC and pure Si. We concluded that these fibres are a microcrystalline structure of SiC embedded in an oxide matrix

Sodium coordination environments in silica films

The local structure centered on sodium after diffusion in silica (Na-SiO2 samples) has been determined by means of extended X-ray absorption fine structure (EXAFS) studies. The NaSiO2 samples are of particular interest because (i) their sodium content can be varied over a wide range of concentration and (ii) their local structure is representative of that of soda-silica glass. EXAFS analyses reveal the existence of a well-defined local structure involving oxygen, sodium, and silicon neighbors. The Na-O, Na-Na, and Na-Si bonds lengths, which amount to 0.23, 0.30, and 0.38 nm, respectively, do not depend on sodium concentration. This environment closely resembles that found in soda-silica glass. Moreover, it is compatible with the "target site" and "the site memory effect" suggested by recent theories of the ionic conductivity in oxide glasses