1,721,046 research outputs found
The structure of nanocrystalline materials analyzed by X-ray Absorption Fine Structure: the case of porous silicon
No full textX-ray absorption fine Spectroscopy has been established as a suitable tool for the determination of the atomica scale structure of nanocrystalline materials. The present work reviews the potentialities and limits of the XAFS techniques when applied to the study of nanocrystalline systems
XRD and EXAFS studies on the structure of Er3+-doped SiO2-HfO2 glass-ceramic waveguides: Er3+-activated HfO2 nanocrystals
No full textX-ray absorption spectroscopy studies of glass structure
No full textIn the last few decades, X-ray Absorption Spectroscopy (XAS) has recorded a remarkable amount of interest in the study of glass structure for its sensitivity to the Short Range Order (SRO) around a selected atomic species. XAS greatest evolution started in the seventies of the last century thanks to an adequate and structurally useful theoretical interpretation of the Extended X-ray Absorption Fine Structure (EXAFS) beyond the absorption edge and to the contemporaneous advent of synchrotron radiation facilities. EXAFS probes the local structure of liquid, vitreous, amorphous, and crystalline materials; it is particularly important for noncrystalline systems since they do not give rise to the Bragg diffraction. EXAFS is also used to determine the local environment of impurities contained in glasses, even if they are very diluted. Today, a reliable theoretical description of glassy system structure through modern modelling techniques should be carried out on the basis of x-ray and/or neutron scattering patterns, along with a careful use of the information derived from SRO techniques like EXAFS, NMR, etc. The use of EXAFS in the case of nanocrystalline glass-ceramic systems is very important, since the obtained information are complemental to the ones determined by the Bragg diffraction. The paper gives a brief description of EXAFS theory and analysis methods in the case of disordered systems. Examples of state-of-the-art studies in different glassy systems will be presented along with a critical assessment of EXAFS potential for such systems
Ordered and disordered models of local structure around Ag cations in silver borate glasses based on x-ray absorption near-edge structure spectroscopy
No full textThe local coordination of Ag cations in silver borate glasses Ag2O.nB(2)O(3) has been studied by comparing the experimental x-ray absorption near edge structure (XANES) at the Ag K edge with results of theoretical simulations. We demonstrate that simple models which describe the local structure around Ag with a single geometric configuration cannot be reconciled with experimental XANES spectra. In order to obtain a satisfactory agreement between theory and experiment, it is necessary to include the disorder also at the short range. Structural information is extracted from XANES data using an empirical approach, which is based on the presence of a multiplicity of atomic structural configurations around photoabsorbing atoms. This approach is particularly suited for describing the local environment of atomic species belonging to glass network modifiers
X-ray absorption spectroscopy of strongly disordered glasses: Local structure around Ag ions in g-Ag2O center dot nB(2)O(3)
No full textThe local structure around Ag ions in silver borate glasses g-Ag2O center dot nB(2)O(3) (n=2,4) was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about 2.3-2.4 A and 2.5-3.4 A, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone
Connection between spectral features of BK edge XANES of minerals and the local structure
No full textThe potential of XANES spectroscopy for studying the structure of boron-containing compounds is assessed by theoretical modelling of B K edge spectra of minerals. Spectra arising from three-fold coordinated boron sites differ considerably from spectra arising from four-fold coordinated sites. The characteristic shape of the spectra is governed first of all by short range order and, to a lesser extent, by medium range order. Areas of main spectral peaks depend only on the coordination of the photoabsorbing site and can thus be used for quantifying the portion of boron atoms in BO3 and in BO4 units, in minerals as well as in glasses. The width of the main peak of spectrum at a four-fold coordinated boron site generally increases with the the spread of the B-O distances but deviations from this correlation may be substantial. In contrast to spectra of minerals, XANES of BPO4 crystal is governed by long range order and is thus not a suitable model for studying XANES of borate glasses
Transmission electron microscopy study of Ni-Si nanocomposite films
No full textNickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 degrees C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi2 and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi2 and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni-Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 degrees C leading to crystallization of a-Si at the silicide-silicon interface. (C) 2012 Elsevier B.V. All rights reserved
Borate versus silicate glasses: why are they so different?
No full textA comparative review is presented of a range of physical and chemical properties of borate and silicate glasses. Most attempts to explain the difference between the properties of these glasses, in terms of the so-called borate (or boron oxide) anomaly, have focussed on the change in co-ordination number of some of the boron atoms from three to four. The origin of this co-ordination number change is discussed at a fundamental level, together with the important role played, both by superstructural units and by network-modifying cations, in determining borate glass properties. In addition, the Krogh-Moe Griscom superstructural unit model for the structure of borate glasses is updated in line with more recent experimental data, and used to account for the way in which the fraction of 4-fold co-ordinated boron atoms in lithium, sodium and silver borate glasses varies with composition. It is concluded that the "anomalous" properties of vitreous B2O3 and binary M2O-B2O3 glasses are entirely consistent with respect to the electron configuration and bonding of the boron atoms and to the presence of superstructural units. Extrema in the properties of the binary glasses, when plotted as a function of composition, arise due to the interplay between two (or more) opposing factors. The major remaining question concerns the origin and relative magnitudes of the stabilisation energies for the various superstructural unit species
Size and surface effects in porous silicon studied by X-ray absorption spectroscopy
No full textX-ray absorption spectroscopy (XAS) at the Si-K absorption edge has been applied to study the influence of size and chemical-physical surface effects on the optical properties of porous silicon (PS). The comparison between XAS spectra obtained by monitoring both total electron yield (TEY) and photoluminescence yield (PLY) has allowed us to discriminate between light emitting and non-emitting sites in PS. Thanks to partial PLY-XAS measurements and their site selectivity, we have been able to detect the presence of energy levels inside the band gap of the Si nanocrystallites. The origin of these radiative states is related to the surface passivation, whose influence becomes particularly significant at decreasing sizes
- …
