S. Mobilio

STRUCTURAL AND OPTICAL INVESTIGATION OF INASXP1-X/INP STRAINED SUPERLATTICES

We report a complete characterization of InAsxP1−x/InP (0.05<x<0.59) superlattices epitaxially grown by low pressure metalorganic chemical vapor deposition and by chemical beam epitaxy. Samples were obtained by both conventional growth procedures and by periodically exposing the just-grown InP surface to an AsH3 flux. Using the latter procedure, very thin InAsxP1−x/InP layers (10–20 A) are obtained by P↔As substitutions effects. Arsenic composition of the so obtained layers depends both on AsH3 flux intensity and exposure times. Samples have been characterized by means of high resolution x-ray diffraction, high resolution transmission electron microscopy, 4 K photoluminescence, and extended x ray absorption fine structure spectroscopy. The combined use of high resolution x-ray diffraction and of 4 K photoluminescence, with related simulations, allows us to predict both InAsP composition and width, which are qualitatively confirmed by electron microscopy. Our study indicates that the effect of the formatio...

Local atomic environment of Cu ions in ion-exchanged silicate glass waveguides: An x-ray absorption spectroscopy study

We present an x-ray absorption spectroscopy study at the Cu K edge on Cu-doped soda-lime and BK7 glass waveguides obtained by ion exchange under various preparation conditions. The total electron yield and x-ray fluorescence yield collection modes were used to evidence the chemical behavior of surfacial and deep Cu ions. In all the samples copper is found preferentially as Cu+ throughout the whole exchanged region. The first coordination shell is formed by two to three oxygen atoms at 1.86 A, much shorter than the Na–O bond (2.3 A) in soda-lime glasses.

EXAFS studies of silver ion coordination in silver borate glasses

EXAFS measurements at the K and L 3 edges of silver have been performed on the binary ion conducting glasses (Ag 2 O · n B 2 O 3 ), with n = 2, 3, 4, 6. The first coordination shell of silver is characterized by a mean Ag-O distance r = 2.27 A and a very low coordination number ( N 2). This result indicates the presence of a local structure in the environment of the silver cation, almost independent of the Ag 2 O content. This structure is different from those found in crystalline silver-borate compounds and in sodium-borate glasses. For the vitreous Ag 2 O · 2B 2 O 3 a further coordination Ag-O with r = 1.99 A is singled out, confirming the possible bond between silver ions and non-bridging oxygens at high Ag 2 O content.

Structural study of a-Si1-xCx:H by exafs and x-ray scattering

We present a comprehensive study of short range order in a-Si 1-x C x :H using X-ray scattering and Si K-edge EXAFS. The X-ray scattering probes the total radial distribution function; by detailed fitting in R-space we measure short range order parameters around C and Si for the first and second shell. The C-C distances in first and second shell indicate that both carbidic and graphitic configurations are present. The coordination numbers indicate that there is a tendency to chemical order; at high C concentrations there is evidence for chemical order with phase separation. The EXAFS measurements probe local order in the Si-C alloy phase; there is clear evidence that even this phase is chemically ordered.

X-ray absorption near edge structure (XANES) determination of calcium sites of troponin C and parvalbumin

Using synchrotron radiation at the Frascati storage ring ADONE, the X-ray Absorption Near Edge Structure (XANES) has been applied to determine homologies and modifications of the local structure of the calcium binding sites of troponin C. In all four calcium binding sites, Ca2+ appears to be co-ordinated to carboxyl and carbonyl groups in a characteristic configuration. No structural difference has been found between high and low-affinity sites. A distortion of the Ca2+ site geometry by binding of Mg2+ has been observed. The XANES of parvalbumin has been measured and found to be different from troponin C. A tentative identification of the characteristic XANES spectra of the two different Ca2+ sites in this protein is reported.

Time-resolved X-ray powder diffraction on a three-way catalyst at the GILDA beamline.

Time-resolved X-ray diffraction experiments carried out at the beamline BM08-GILDA of ESRF allowed a study of the structural modifications taking place in a Pt/ceria-zirconia catalyst while the CO oxidation reaction was in progress. The capillary tube in which the sample is stored acts effectively as a chemical microreactor that ensures homogeneity of the sample treatments and minimization of diffusion effects. During the flowing of the reactant CO/He mixture, the investigated catalyst undergoes a fast Ce(IV)-Ce(III) partial reduction that involves the release of one O atom for every two reduced Ce cations. Because Ce(III) has a larger ionic radius than Ce(IV), the structural modification produces an increase of the lattice constant of the ceria-zirconia mixed oxide, and this increase is monitored by the translating imaging-plate device implemented at GILDA. The CO(2) resulting from the oxidation of the fluxed CO is monitored by a quadrupole mass spectrometer during the recording of the time-resolved X-ray diffraction pattern. The chemical and structural information was combined to show that the CO(2) yield is nearly constant until the catalytic system can provide oxygen for the reaction, while the structural rearrangement of the catalyst is delayed with respect to the switching on of the CO/He flux. After this induction time, during which CO(2) is produced with no structural modification of the catalyst, a fast increase of the lattice constant takes place.

EXAFS study on Ag-doped silicate glasses irradiated with low-mass ions

Abstract We present an EXAFS study on Ag-doped soda-lime glass waveguides obtained by ion-exchange and treated by low-mass ion-irradiation to promote the formation of metallic clusters. For differently prepared samples we have determined the Ag local order before and after the treatment. In the as-exchanged glasses Ag is coordinated with O at a distance significantly shorter than the NaO bond length in the unexchanged glass. In irradiated samples we found the formation of metal aggregates. Structural parameters of clusters in high-temperature irradiated sample are compatible with a compressed fcc structure while in room-temperature irradiated specimen a more complicated structure comes out.

Thermal oxidation of Si (001) single crystal implanted with Ge ions

The thermal oxidation of Ge-implanted Si single crystals has been investigated for different Ge doses (3×1015 cm−2 and 3×1016 cm−2) and different oxidation processes (in wet ambient at 920 °C for 30, 60, and 120 min, or dry ambient at 1100 °C for 30 min). The oxide roughness, the oxidation rate, the Ge diffusion, precipitation, and clustering, have been monitored by several experimental techniques: atomic force microscopy, transmission electron microscopy, Rutherford backscattering spectrometry, and x-ray absorption spectroscopy. We found that the surface roughness is related to the segregation of Ge at the oxide/substrate interface, occurring when the oxidation rate is faster than the Ge diffusion, in particular at the higher implanted dose (3×1016 cm−2) when processed in a wet ambient. For these conditions, we also observed an oxidation rate enhancement with respect to pure Si, and a strong indication that pure Ge clusters were formed. When a critical Ge concentration at the interface is reached, the ox...

Temperature modification of the Nb oxidation at the Nb/Al interface studied by reflEXAFS

Abstract We report on the modifications of the Nb oxidation in Nb/Al interfaces due to different thermal treatments. The samples were prepared by radio frequency (RF) ion sputtering and studied by X-ray absorption detected in total reflection mode (reflEXAFS). This technique combines the specific atomic sensitivity property of the extended X-ray absorption fine structure (EXAFS) with a surface sensitivity, due to the total reflection mode of the incident beam. We studied the niobium oxide revealed at the Nb/Al interface after different thermal treatments and on bare thick Nb film. Up to 200°C the increasing temperature does not modify appreciably the oxide layer, while higher temperature treatments (600°C) causes a further oxide formation. The study of the Nb K-edge absorption leads to the conclusion that no single phase Nb oxide is formed at the interfacial zone and no presence of NbAl alloy is observed.

Role of multielectron excitations in the L3 XANES of Pd

Abstract High resolution L3 and L2 XANES (X-ray absorption near edge structure) spectra of Pd metal have been measured at the Frascati synchrotron radiation facility. The L3 XANES shows an intense 2p → 4d “white line” while at the L2 a weaker structure appears. A good agreement between L3 XANES and the one-electron theory of Muller et al. has been found. All the possible final state effects due to the relaxation of the many body system are discussed. No many body effects have been seen on the “white line” at threshold. The intensity ratio between L3 and L2 absorption is 2.1 in the high energy range of XANES and it is 2.7 at the white line maximum. A weak broad band extending up to 13 eV and centered at ∼ 6 eV above the Fermi level, appearing in the L3-L2 difference spectrum, could be tentatively assigned either to the only possible very weak multielectron excitation present in XANES of Pd i.e. the “two hole-two electron” excited state or to the prevalence of the j = 5 2 total angular momentum for electrons in the 5sp band, hybridized with the 4d band, up to 3 eV above the Fermi level.