Showing papers in "Il Nuovo Cimento D in 1997"

Regimes of X-ray phase-contrast imaging with perfect crystals

Abstract: Perfect crystals have recently been used as X-ray wavefront analysers to help produce phase-contrast images of non-periodic objects. Such images are essentially the maps of the phase gradients introduced in a plane X-ray wave upon passage through a weakly absorbing object. We show that the nature of the contrast in the images is determined by the ratio between the local wavefront curvature and the width of the crystal rocking curve. Depending on this ratio being small or large, two quite distinct regimes for image formation can be identified, namely the differential phase-contrast mode and the refractometric mode. We derive simple analytical formulae which can be used for the analysis of X-ray images of phase objects obtained in these two regimes.

Hydroxyl groups on oxide surfaces

Abstract: The hydroxylation of a-Al2O3(000l) and MgO surfaces is studied in ultra-high vacuum by high-resolution electron energy loss spectroscopy (adsorbate and phonon losses being distinguished via resonance scattering) and X-ray photoemission spectroscopy. Hydroxyl groups which arise from the dissociative adsorption of H2O on a-Al2O3(0001) surfaces and low-coordinated MgO surface sites are characterized by chemical shifts and stretching frequencies.

Adsorption of isolated Cu, Ni and Pd atoms on various sites of MgO(001): Density functional studies

Abstract: Results of gradient-corrected density functional calculations are presented for the adsorption of single Cu, Ni and Pd atoms on a regular MgO(001) surface. The surface has been represented by stoichiometric clusters embedded in large arrays of point charges to simulate the Madelung potential. Three adsorption sites have been compared: on-top adsorption on the oxide anion and on the magnesium cation as well as bridge adsorption over two adjacent oxide anions. The weakly adsorbed Cu atom is slightly more stabilized atop magnesium cations than in the alternative positions; however, the energy differences are small and an easy diffusion of Cu on the surface is predicted. For the significantly stronger adsorbed Ni and Pd atoms the calculations yield a clear preference to occupy the place atop oxygen anions. This finding does not support previous experimental assignments that atoms of small palladium clusters deposited on MgO(001) occupy positions on top of Mg cations.

Quantitative analysis of screw dislocations in 6H−SiC single crystals

Abstract: Screw dislocations along the [0001] axis in 6H−SiC single crystals have been studied extensively by Synchrotron White-Beam X-ray Topography (SWBXT), Scanning Electron Microscopy (SEM), and Nomarski Optical Microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: 1) the diameter of dislocation images in back-reflection topographs; 2) the width of bimodal dislocation images in transmission topographs; 3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and 4) the magnitude of the tilt of lattice planes in section topographs. The four methods show good agreement. SEM results reveal that micropipes in the form of hollow tubes run through the crystal emerging as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice constant along the [0001] axis). There isno were fitted to Frank’s prediction for hollow-core screw dislocations:D =μb2/4π 2 γ, where μ is the shear modulus, and γ is the specific surface energy. Statistical analysis of the relationship betweenD and b2 shows that it is approximately linear, and the constant, γ/μ, obtained from the slope, ranges from 1.1×10−3 to 1.6×10−3 nm.

Spectroscopic tools for probing the isolated titanium centres in MCM41 mesoporous catalysts

Abstract: Catalytically active Ti(IV) centres anchored to the inner walls of MCM-41 mesoporous silicas have been investigated using a combination of FT-IR, diffuse reflectance UV-visible and luminescence spectroscopy supplemented by the use of H2O and NH3 as molecular probes of the Ti coordination. The results re-affirm and extend the conclusions reported earlier for the Ti-MCM41 novel epoxidation catalysts, seeMaschmeyer T.,Rey F.,Sankar G. andThomas J. M.,Nature, 378 (1995) 159, who, usingin situ EXAFS studies, showed that the Ti(IV) centres exist largely as isolated active sites.

Some optimization criteria for biological systems in linear irreversible thermodynamics (

Abstract: Summary. — In the present work, some optimum working regimes for linear energy converters are analyzed from a thermodynamical point of view. The regimes studied are minimum entropy production, maximum power output and another one which represents a good compromise between high power output and low entropy production. The analysis is made within the domain of linear irreversible thermodynamics. Finally, the possibility that some biological systems satisfy these criteria is discussed. PACS 87.10 ‐ General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodyamics, information theory, cybernetics, and bionics).

Wave-optical description of X-ray phase contrast images of weakly absorbing non-crystalline objects

Abstract: The work deals with the theoretical wave-optical diffraction description of X-ray phase image peculiarities of non-crystalline objects in the Phase Dispersion Imaging technique. The equation for diffracted beam arnplitudeE h (x) coincides with the ones obtained by other authors. The equation for transmitted beam amplitudeE 0(x) contains the additional term with the field derivative∂E 0/∂x. The comparison of computer simulation of the X-ray intensity distribution in the image plane of the test object with the experimental results demonstrates that the theory is satisfactory.

Imaging of biological objects in the plane-wave diffraction scheme

Abstract: The X-ray plane-wave diffraction scheme was used for experimental testing of biological objects—blood vessels and laboratory animals. Two successively installed asymmetrically cut monochromators were used for the pseudoplane-wave preparation. The images were formed by analyzer fixed both in Laue and Bragg positions. Possibility of vessel system observation both in a live organism and cadavers was demonstrated in a number of experiments. The method has proved to be very promising in the non-invasive X-ray diagnostics.

Contributions of multipole terms to the photoelectric yield in X-ray standing-wave measurements

Abstract: The influence of higher-order multipole terms in the photoelectron yield on the result of an X-ray Standing Wave (XSW) measurement is discussed. The contributions of dipole, quadrupole and octupole term are analyzed for the simple model of a single atom within the XSW field. It is shown that each multiple term shows a different dependence on the polarization vectors and wavevectors of the direct and diffracted beam. The model is generalized for the case of a monolayer of atoms adsorbed on the surface of a crystal. We outline how multipole contributions can be determined by an XSW measurement.

The use of vibrational spectroscopy in the surface characterization of microcrystalline oxides

Abstract: A systematic investigation of the surface morphology and of the vibrational properties of CO adsorbed on several simple oxides with regular crystalline habit and exposing thermodynamically stable and neutral faces, is briefly reviewed with the aim to elucidate the spectroscopic manifestations of CO adsorbed on well-defined crystallographic positions; the cases of MgO and a-Cr2O3 are then discussed in more details. In order to have a better insight on the role of surface defectivity in catalytic reactions, the activity towards CO of perfect, low-index faces and of more defective situations (like those associated with edges, steps, corners) is also discussed. Finally, the experimental C-O stretching frequencies are compared with those calculated on the basis of the local electric field strengths at the adsorption sites.

Boles of low-coordinated surface ions in adsorption of gases on MgO

Abstract: MgO is a typical base catalyst and its basicity has been thought to arise from surface O2-lc (LC: low coordination). On the other hand, the roles of Mg2lc in LC state in catalysis and adsorption have been often underestimated. In this study simple gases such as H2, CH4, CO, and CO2 were used as test molecules to examine the roles of both ions in adsorption, especially at low temperatures. Infrared and TPD spectroscopic observations and, partly, MO calculations for these adsorption systems indicated that Mg2+Lc plays rather more important roles than O2-lc in the adsorption of these gases. Dynamic behavior of adsorbed species at low temperatures is also found.

Study of residual strains in wafer crystals by means of lattice tilt mapping (

Abstract: A high-resolution X-ray diffractometer equipped with an accurateX-Y translation stage has been used to obtain Bragg angle maps of large-area crystal wafers. It is found that the Bragg angle shifts observed by this method in conventional reflection geometries are mainly due to the local lattice tilts rather than to the lattice parameter changes. The formula to calculate from the lattice tilt maps the displacement of the crystal lattice sites with respect to the ideal unperturbed lattice is obtained. It is shown that such lattice displacement is quantitatively correlated to the wafer dislocation density and to the distribution of the Burgers vectors of the dislocations in the crystal. Semi-insulating Czochraslki-grown GaAs wafers with dislocation densities ranging from 104 to 105cm−2 have been analysed by this technique and by double-crystal X-ray topography. From the lattice tilt maps the dislocation density evaluated appeared similar to those determined by topography and etch pit density in this material. It is concluded that the majority of dislocations in the wafer have the same Burgers-vector component along the (100) growth axis. Finally the method is proposed as a new tool for characterizing large-area wafer crystals.

Theoretical study of the adsorption of acids and bases on TiO2 and MgO surfaces

Abstract: The adsorption of small molecules on TiO2 and MgO is investigated byab initio periodic Hartree-Fock calculations. The adsorption may be molecular or dissociative. This depends on their acidic and basic properties in gas phase. For the molecular adsorption, the molecules are adsorbed as bases on the metal sites and the adsorption energies correlate with the proton affinities. This also true on MgO, even if CO2 is ordinary considered as an acidic probe molecule. HSBA theory allows to compare the relative adsorption of molecules on TiO2 and MgO. The dissociations on the surface correlate with the gas phase cleavages: basic cleavage for MeOH and acidic cleavage for MeSH. Another important factor is the adsorbate-adsorbate interaction: at high coverage, stabilization through H-bonds favors the vicinity of adsorbates on the surface (islands of adsorbates and coadsorption effects). Electrostatic interactions explain the change of orientation (CO2 ) with the coverage.

Enhanced possibilities of section topography at a third-generation synchrotron radiation facility

Abstract: We show the new possibilities of section topography techniques at a third-generation synchrotron radiation facility, taking advantage of the high performances of this machine. Examples of the 1) so-called multiple sections, 2) visibility of weakly misoriented regions, 3) study of thick samples, 4) monochromatic and 5) realtime sections are presented.

In situ characterization of highly dispersed catalysts included within zeolite cavities and their photocatalytic reactivities

Abstract: A detailed characterization of catalysts prepared within zeolite cavities and their photocatalytic reactivities have been investigated usingin situ photo-luminescence, diffuse reflectance absorption, XAFS (XANES and FT-EXAFS) and ESR techniques. With the Ag+/ZSM-5 catalyst, it was found that Ag+ ions are highly dispersed in an isolated state within the zeolite cavities. UV irradiation of the Ag+/ZSM-5 catalyst in the presence of NO leads to the decomposition of NO into N2, O2 and N2O at 275 K. Dynamic studies show that the electron transfer from the excited state of the Ag+ ions (4d95s1) plays a significant role in initiating the decomposition of NO. Titanium oxide catalysts prepared within the Y-zeolite cavities via an ion-exchange method exhibit high and unique photocatalytic reactivities for the decomposition of NO into N2 and O2 as well as the reduction of CO2 with H2O showing a high selectivity for the formation of CH3OH. It was also found that the charge transfer excited state of the titanium oxide species, (T3+ -O-)*, plays a vital role in these unique photocatalytic reactions.

X-ray reflectivity reciprocal space mapping of strained SiGe/Si superlattices

Abstract: Small-angle diffuse X-ray reflection from a SiGe/Si multilayer has been studied theoretically and experimentally. The scattering process has been described by means of the DWBA method. For the description of the rough interfaces, the results of the Markov chain theory have been used. It has been demonstrated that the distribution of the diffusely scattered intensity in reciprocal plane gives some information on both the in-plane and inter-plane correlation properties of the multilayer roughness. The sample of SiGe/Si multilayer grown on a slightly miscut substrate has been investigated. The most suitable structure model describing the form of the interfaces is a combination of the two-level model with the staircase model.

Stabilization of mercury cuprates by thallium, superconductivity in Hg1-xTlxBa2Can-1CunO2n+2+d

Abstract: An intriguing way to stabilize the Hg-based cuprates is related to the partial substitution of mercury by foreign cations with higher valence. In this report, the substitution of thallium for mercury has been studied. A series of superconducting Hg1-xTlxBa2Can1CunO2n+2+d (with n=1, 2 , 3 and 0≤x≤1) has been synthesized in sealed tubes. The characterization of these samples was made by using X-rays diffraction, SEM, and electrical resistivity and magnetic susceptibility measurements. Our results show that the critical temperature Tc does not significantly vary by changing the ratio of Tl to Hg.

Structure and dynamics of nitrite sodalite. Anab initio study

Abstract: We present a first-principles study of nitrite sodalite, a synthetic zeolite. The NO-2 anions are hosted in octahedral cavities typical of such crystals, and each anion is surrounded by four Na cations in a tetrahedral geometry. We found that NO-2 is in a rotational state inside the zeolite’s cages. This rotational state explains the orientational disorder experimentally detected in nitrite sodalite.

Synchrotron X-ray topographic study of strain in silicon wafers with integrated circuits

Abstract: Section topographs made with synchrotron radiation show the strain field below the surface of silicon wafers which have gone through a process for integrated circuits. The contrast observed is a series of curved lines starting at one edge of an oxide layer and ending at its other edge. The strain is also calculated using the finite-element method. Electrical measurements such as the threshold voltage of a transistor are made in order to find the influence of the strain on the device performance and yield.

Reciprocal space mapping for semiconductor substrates and device heterostructures

Abstract: We produced wafer maps of triple-axis X-ray diffraction omega scans (omega or ω-map) to determine the location of high and low crystalline perfection in both the substrate and the 15–20 nm channel region in device-quality GaAs and InP-based pseudomorphic high electron mobility transistors. The triple-axis maps are more sensitive to different types of crystallographic defects than are double-axis measurements. A map showing full width at half-maximum variations provides information on variations in crystallographic tilt; a map showing full width at five thousandths maximum shows polishing damage variations across the wafer. Monitoring the detector position determines lattice parameter variations across the wafer as well, although we did not observe significant lattice parameter variations in a given wafer. The crystallographic perfection of the channel layer replicates that of the underlying substrate and electrical measurements taken at the different regions show that the lower crystalline quality conforms with reduced electrical performance. Omega maps are also used to assess the influence of different growth parameters and post-growth annealing treatments on substrate crystallographic perfection. The non-destructive nature of this technique makes it ideal for studying structure/performance relationships in semiconductor heterostructures.

Application of X-ray diffraction in Laue geometry to imperfect near-surface layers

Abstract: The paper presents the X-ray diffractometry analysis of thin-layer structures made by exploiting the combination of two diffraction geometries—the Bragg one and the Laue one. For this purpose a three-crystal setting was applied. Since the structural parameters of thin layers vary primarily along the surface normal, directing the diffraction vector along this normal, one can succeed in the assessment of the quality in whole, but not in the extraction of different components of lattice damage. The paper shows that, in order to evaluate the contributions, the application of the Laue geometry is promising; it substantially facilitates the study of relaxation, the determination of compositionally dependent properties and the evaluation of macro- as well as micro-strain components. The objects under study were GaAs/Si and SiGe/Si heterostructures as well as (CaF2+CdF2)/Si superlattices.

X-ray scattering from thin organic films and multilayers

Abstract: The real structure of LB-multilayers prepared with fatty-acid salts is dominated by finite-sized scattering aggregates. Their different length scales become visible using AFM. It shows that not the whole substrate is wetted by the film. The molecular order is restricted into domains. These micrometer domains are not homogeneous. They contain mesoscopic subdomains of different heights which vary in steps of double layers. Finally high-resolution AFM-maps display a nearly hexagonal arrangement of molecules within subgrains with a diameter of several 10 nm. This domain structure has to be taken into account when interpreting X-ray diffraction data. The size of the crystalline aggregates is obtained by means of X-ray grazing incidence diffraction. On the mesoscopic scale the domain size is determined by X-ray diffuse scattering experiments. Because Sinha’s model fails for the present kind of multilayers, we used another approach for data analysis. The lateral correlation length caused by height fluctuations is estimated without knowledge of a definite correlation function. Additionally the mosaicity of the domain orientation can be taken into account.

Configuration interaction and pairing in superconducting cuprates

Abstract: We propose a quantum-mechanical effect specific of the symmetry of the Cu-O plane of cuprate superconductors. The mechanism considers hole pairs in single-particle degenerate states which, because of the configuration interaction, form singlet states with zero on-site repulsion (W = 0 pairs). In the many-body problem the effective interaction due to the spin exchange with the holes ofb1 symmetry is attractive. The exact diagonalization of the extended Hubbard Hamiltonian with 4 holes using well-established parameters for several clusters shows that the ground state has1B2(xy) symmetry and that binding energies are of the order of 10 meV. Unlike other electronic mechanisms proposed so far, this effect is due to the symmetry of the system and does not rely on off-site interactions; when the symmetry is broken the repulsion wins. Besides the correct symmetry of the pair, we obtain good agreement with experiment for the binding energies and the singlet-triplet energy separation of the quasiparticles.

Characterization of microdefects in GaAs crystals with high-resolution X-ray diffractometry (*)

Abstract: The aim of the present work was to correlate the electric characteristics of Te-doped gallium arsenide grown by LEC technique with the defect contents of the samples. Measurements for GaAs samples, doped with Si, Ge and Ge+Te as well as undoped ones, grown by various methods (HB, VGF and LEC), were also performed. In order to characterize the defects High Resolution X-Ray Diffractometry was used. Measurements of diffuse scattering intensity maps around the reciprocal lattice points and plane wave reflection topography were employed. For most cases in the range of free-electron concentrations between 1·1017 cm−3 and 6·1018 cm−3 (as well as for SI samples) the isointensity contours in the maps exhibit generally the same character. For higher doping level the intensity of X-ray diffuse scattering increases and it can be further increased by the annealing which causes a decrease of free-electron concentration. An analysis of the reciprocal-space maps is presented. Several types of defects should be considered in order to describe isointensity contours.

Development of low-losses current leads based on multilayered Bi2223/Ag conductors

Abstract: A theoretical study enabling the design of 1 kA-class low-losses HTS current leads based on multilayered Bi-2223/Ag conductors is reported. The current lead has been designed with variable cross-section to match the calculated temperature profile with the experimental Ic(T) curve of the Bi-2223/Ag conductors to be assembled. The heat leak results 30% lower compared with an optimized copper lead.

High-resolution X-ray diffraction investigation of crystal perfection and relaxation of GaAs/InGaAs/GaAs quantum wells depending on MOVPE growth conditions

Abstract: The influence of growth conditions and the concentration of indium in the vapour phase on the crystal perfection of strained InGaAs/GaAs quantum wells grown by metalorganic vapour phase epitaxy is studied by high-resolution X-ray diffraction, cathodoluminescence, photoluminescence and transmission electron microscopy. A strong dependence of the misfit dislocation density on the growth temperature is found. With increasing In concentration in the vapour phase a transition to the 3-dimensional growth mode leading to a different type of defect formation is observed.

UV-Vis diffuse reflectance and FT-IR spectroscopic investigation of V2O5 /SiO2 and MoO3 /SiO2 catalysts for partial oxidation of methane to formaldehyde

Abstract: The structure of the supported phase of differently loaded V2O5 /SiO2 and MoO3 /SiO2 catalysts and the interaction of the surface species with the carrier were studied by DR UV-Vis and FT-IR spectroscopy In the case of the V2O5 /SiO2 systems, an evolution from isolated to clusterized tetrahedral (Td) V51 containing species was observed passing from low to high loading, whereas complex polymeric structures containing Mo6+ ions in octahedral coordination were found to be overwhelmingly present in MoO3 catalysts at any loading The isolated V5+ (Td ) species are suggested to be responsible for the high activity of low-loaded V2O5 /SiO2 samples, while the poor catalytic performances of MoO3 /SiO2 indicate that polymolybdate species are almost inactive in the partial oxidation of methane to formaldehyde

Phase-contrast hard X-ray microtomography by Bragg-Fresnel optics

Abstract: Recently it was found that transparent in hard X-ray (10–20 keV) range native and synthetic microobjects (10–20 microns size) mounted in a X-ray beam give an image contrast in spite of very small refraction and negligible absorption. The information obtained from registered diffraction patterns is deficient in the correct solution of the problem of phase-contrast computed microtomography (determination of the shape, density variation, inhomogeneity, defects in microobjects). But it is possible to obtain additional quantitative information using the so-called double-crystal diffraction technique where the X-ray wave formed by the Bragg-Fresnel lens and transmitted through the transparent microobject is the incident wave for the crystal analyzer.