F. Boscherini

X-ray optics of a dynamical sagittal-focusing monochromator on the GILDA beamline at the ESRF

The performance of a dynamical sagittal-focusing monochromator for hard X-rays is described. It consists of a flat first crystal and a diamond-shaped ribbed second crystal which is clamped by its central rib and dynamically bent by applying a force on its two apices. The system has proved to perform very well on the GILDA beamline at the ESRF. The horizontal acceptance varies with energy and with focusing geometry as predicted theoretically; the total available horizontal fan of radiation (3.6 mrad) is in fact collected in the 1:3 geometry. The system is routinely run in a dynamical focusing mode for XAFS spectroscopy in the energy range 5-30 keV with Si(311) crystals, with a constant spot size FWHM ~1 mm on the sample and without degradation of energy resolution or reproducibility. Using simple geometrical considerations we calculate the variations of the horizontal profile of the reflected beam during rocking-curve scans in different focusing geometries and find them in agreement with observed ones. Not only is this is a practical aid in alignment but it illustrates the X-ray optics of sagittal focusing in an elegant way.

Ge-Si intermixing in Ge quantum dots on Si(001) and Si(111)

Exploiting Ge K-edge x-ray absorption spectroscopy we provide direct evidence of Si–Ge intermixing in self-organized strained and unstrained Ge quantum dots on Si, and provide a quantitative measurement of the average composition. For Ge/Si(001) dots with equivalent thickness in the range 5.8–38 nm and morphology ranging from that typical of coherently strained to that associated with relaxed dots we find that the average Si composition is approximately 30%. For Ge/Si(111), we find that the wetting layer has a Si composition near 50%. We discuss these results in terms of the energetics of dot formation and argue that strain-enhanced diffusion of Si into Ge should be considered as an important factor in minimizing the strain energy of the system.

XAFS study of Ti-silicalite: structure of framework Ti(IV) in presence and in absence of reactive molecules (H2O, NH3)

X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting Si(IV), has been performed in order to study the effect of the presence or absence of ligands such as H2O and NH3 on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH3 has been studied and described in detail. It has been found that the Ti(FV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number under the action of adsorbates.

Ge-Si intermixing in Ge quantum dots on Si

We have provided direct evidence for the presence of considerable Si-Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ge was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski-Krastanov growth.

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...

Probing Long‐Lived Plasmonic‐Generated Charges in TiO2/Au by High‐Resolution X‐ray Absorption Spectroscopy

Exploiting plasmonic Au nanoparticles to sensitize TiO2 to visible light is a widely employed route to produce efficient photocatalysts. However, a description of the atomic and electronic structure of the semiconductor sites in which charges are injected is still not available. Such a description is of great importance in understanding the underlying physical mechanisms and to improve the design of catalysts with enhanced photoactivity. We investigated changes in the local electronic structure of Ti in pure and N-doped nanostructured TiO2 loaded with Au nanoparticles during continuous selective excitation of the Au localized surface plasmon resonance with X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Spectral variations strongly support the presence of long-lived charges localized on Ti states at the semiconductor surface, giving rise to new laser-induced low-coordinated Ti sites.

Treatment of EXAFS data taken in the fluorescence mode in non-linear conditions.

The aim of this work is to investigate the possibility of extracting correct structural parameters from fluorescence EXAFS data taken at high count rates with an energy-resolving detector. This situation is often encountered on third-generation synchrotron radiation sources which provide a high flux on the sample. Errors caused by pulse pile-up in the extraction of structural information have been quantified in a real experiment, and different approaches to the problem of data correction have been elaborated. The different approaches are discussed in a comparison of the ability of each kind of correction to recover the correct structural parameters. The result of our analysis is that it is possible to work in non-linear conditions and correct the data, if the response of the acquisition system is known. Reliable structural information can be obtained with data acquired up to a count rate equal to approximately 60% of the inverse of the dead time.

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.

Fe and Mn K-edge XANES study of ancient Roman glasses

Abstract: The X-ray Absorption Near Edge Structure technique has been successfully applied to the study of ancient glasses fromPatti Roman Villa (Messina, Sicily). From the chemical point of view, the samples are ‘ low-magnesia’ glasses, with a compositiontypicaloftheRomanperiod.FluorescencedetectedFeandMnK-edgeXANESspectraandadetailedpre-edgeanalysisconfirmthatthe color of the glass fragments depends on the oxidation state of iron and suggest the deliberate addition of manganese oxide asdecolorant during the melting procedure. This work shows that X-ray Absorption Spectroscopy is a potentially very useful non-destructive technique that can be applied in archaeological studies on many different materials. Key-words: XANES, archaeological glasses, iron, manganese. Introduction It is well known that the color exhibited by glasses can bedeterminedbythe oxidationstateandthe electronic config-uration of the metal ions in them (Doremus, 1973; Hender-son,1985;Freestone,1991).Theseareusuallyelementsbe-longingtothetransitionrowoftheperiodictable,whichab-sorb characteristic frequencies of the visible region as a re-sult of d-d electronic transitions. In particular, ancient glas-sesoftencontainironatlevelsthatcanimpartatypicalinci-dental green coloration. To minimize this problem, fromaround the middle of the first millennium BC, substanceswere added which tended to neutralize the colorant effectsof the iron. Before the Roman period, antimony was themaindecolorant,whilefromthesecondcenturyBCmanga-nese become important(SayreSSayre,1963;Newton, 1978;Freestone,1991).Sincethecharacterizationof colorant and decolorant components is important in un-derstanding the manufacturing technique of the ancientglasses, we have applied X-ray Absorption Spectroscopy(XAS) withsynchrotronradiationtothe study of theoxida-tion state of iron and manganese in a number of glass sam-ples of archaeological interest, characterized by differentcolors (from green to pale brown to uncolored).The samples are fragments of perfume bottles of the 2

Site preference and local geometry of Sc in garnets: Part II. The crystal-chemistry of octahedral Sc in the andradite-Ca3Sc2Si3O12 join

Abstract Investigation of scandium incorporation in garnets along the synthetic Ca3Fe23+Si3O12-Ca3Sc2Si3O12 (adr.CaSc) join, based on the same multi-technique approach used in the companion paper (Oberti et al. 2006a), shows that (1) Sc is incorporated exclusively at the Y octahedron; (2) the local coordination of Sc is slightly different in Sc-poor than in Sc-rich compositions (Sc-O = 2.06 Å in CaSc10 vs. 2.10 Å in CaSc30-90); (3) the local coordination of Ca is also slightly different in Sc-poor than in Sc-rich compositions [Ca1,2-O are 2.34(2) and 2.48(2) Å in CaSc10 and 2.36(2) and 2.50(2) Å in CaSc90, with Δ fixed at 0.14 Å in all the samples]; (4) the linear increase of the unit-cell edge along the join derives from multiple changes in the geometry of the different polyhedra and from the rotation of the tetrahedron around the 4̄ axis (α rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12-Ca3Fe23+Si3O12 (grs.adr) join. CaSc-rich garnets, where a large X dodecahedron coexists with a large Y octahedron and a Z tetrahedron occupied by Si, similar to pyrope-grossular garnets, have the highest α values observed to date in calcium silicate garnets. Slightly lower α values are observed in pyrope and almandine, but correspond to a different structural arrangement, where a small X dodecahedron coexists with a small Y octahedron. These results further confirm the efficiency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.