High Resolution X-ray Techniques Research Articles

Petrological Meaning of Ethnostratigraphic Units: Laminated Limestone of the Crato Formation, Araripe Basin, NE Brazil

Crato Formation, NE Brazil, consists of laminated limestones deposited in a lacustrine environment during the Early Cretaceous. The Nova Olinda Member, the lowest unit of the Crato Formation, is world-famous for their exceptionally well-preserved fossils. The Nova Olinda limestones have been economically exploited for centuries. Today, this commercial excavation still represents the main economic activity in the region. During the mining process, the quarry workers have developed an informal stratigraphic code to individualize the different beds in the quarries. Upward, the local workers recognize the succession as “lajão dos sete cortes”, “matracão", “lajão branco", “pão de milho”, “veio da paiba”, "veio doidão”, “lajão amarelo” and “veio do besouro”. Here, we used principles of ethnostratigraphy to verify the origin and validity of these informal stratigraphic nomenclatures by classifying them as ethnostra. For this, we applied a detailed petrographical analysis with thin sections and high-resolution X-ray techniques to characterize each ethnostratum. Our results suggest possible linkages between informal and formal stratigraphic nomenclatures in Nova Olinda Member. Finally, this study showed that ethnostratigraphy can be applied as an auxiliary tool in stratigraphic studies, in both, as an initial exploratory approach and as enhancing the relationship between the geologists and the local community.

Permeation of Nanoparticles into Pulmonary Surfactant Monolayer: In Situ X-ray Standing Wave Studies.

High-resolution X-ray techniques were applied to examine the effects of gold nanoparticles (size <5 nm) on natural pulmonary surfactant and pure DPPC monolayers preliminarily formed on water subphase in a Langmuir trough. Hydrophobic and hydrophilic nanoparticles were delivered from nanoaerosol using electrodeposition method. Grazing incidence diffraction, X-ray reflectivity, and X-ray standing wave measurements allow to monitor the changes in molecular organization of lipid monolayer and to locate the position of gold nanoparticles. X-ray experiments were performed over a period of 9-14 h. The obtained results evidenced that, on a long time scale, the deposition of nanoparticles, even at low doses, can induce pronounced alterations in lipid monolayer. The presented data can help to elucidate the mechanism of pulmonary translocation of inhaled nanoparticles that is of special interest for biomedical investigations of potential risk of nanoaerosols for human health.

High-Resolution X-Ray Techniques as New Tool to Investigate the 3D Vascularization of Engineered-Bone Tissue.

The understanding of structure–function relationships in normal and pathologic mammalian tissues is at the basis of a tissue engineering (TE) approach for the development of biological substitutes to restore or improve tissue function. In this framework, it is interesting to investigate engineered bone tissue, formed when porous ceramic constructs are loaded with bone marrow stromal cells (BMSC) and implanted in vivo. To monitor the relation between bone formation and vascularization, it is important to achieve a detailed imaging and a quantitative description of the complete three-dimensional vascular network in such constructs. Here, we used synchrotron X-ray phase-contrast micro-tomography to visualize and analyze the three-dimensional micro-vascular networks in bone-engineered constructs, in an ectopic bone formation mouse-model. We compared samples seeded and not seeded with BMSC, as well as samples differently stained or unstained. Thanks to the high quality of the images, we investigated the 3D distribution of both vessels and collagen matrix and we obtained quantitative information for all different samples. We propose our approach as a tool for quantitative studies of angiogenesis in TE and for any pre-clinical investigation where a quantitative analysis of the vascular network is required.

Measurement of layer width uniformity in quantum well infrared photodetectors by high resolution X-ray techniques

The present work reports the first measurement, via glancing incidence X-ray reflectivity (GIXR) and reciprocal space mapping, of the statistics (mean and standard deviation) of the epitaxial layer widths comprising quantum well infrared photodetectors. Both an optimization procedure and GIXR calculations for various hypothetical structures are used to determine that multiple quantum well structure which most likely yields the measured GIXR. We find that the absolute size of the layer width variation is different for different growths, but that the fractional layer width variation is about 2% for three samples studied. An important observation is that this layer width variation is consistent with an effusion cell temperature variation of 1°C during growth. A second result of this work was the derivation of an analytical formula for finding from a GIXR measurement the standard deviation of the epilayer widths about their mean value. Important in this derivation is the fact that at X-ray wavelengths the semiconductor refractive index is less than unity, and is a significant effect on the analysis for small (glancing) incident angles.

Lattice strain in oxidized Si nanostructure arrays from X-ray measurements

Although lattice strains in oxidized wafers can be detected by optical and X-ray methods the measurement of strains in non-planar structures such as occur in Si devices is more difficult. We describe a novel technique for measuring such strains and report results for oxidized Si. In our work we have fabricated regular arrays of Si pillars on a Si(001) surface and measured the strain in the individual pillars induced by oxidation. The pillars have diameters of the order 100 nm and spacing of ~300 nm. With high resolution X-ray techniques this spacing is sufficiently small to allow the ‘superlattice’ diffracted beams from the fabricated structure to be well resolved. We measure the strain from the displacement of this superlattice diffraction pattern relative to the normal allowed bulk reflections. We report results for different combinations of Si pillar diameter and oxide thickness. Differential thermal contraction also contributes to the strain. The measured strains are of the order of 10 −4. Our results are in good agreement with simulations based on current models of Si oxidation.

Determining period variations in a distributed Bragg reflector through high resolution X-ray analysis

We studied the structural properties of a GaAs/AlAs distributed Bragg reflector using high resolution X-ray techniques. We evaluated diffraction and reflection scans by considering several possible structural defects including period and individual thickness variations, compositionally graded interface layers, and interfacial roughness. Diffraction scans provided information on layer thicknesses and crystalline quality. Both X-ray diffraction and reflectivity curves were compared to calculated curves using dynamical theory and Fresnel multilayer theory, respectively, X-ray reflectivity measurements and triple axis reciprocal space measurements showed the interfaces to be abrupt, and the superlattice to deviate from perfect periodicity. Together, these techniques provided a detailed analysis of superlattice parameters. The standard deviation of layer variations was shown to be 1.5–2.0% for the quarter-wave stacks. We also present for the first time, the sensitivity of X-ray reflectivity measurements to period deviation, and use reciprocal space maps to distinguish period dispersion from interface roughness. The results indicate the accuracy of high resolution X-ray techniques for quantitative superlattice analysis.

The Relationship between InGaAs Channel Layer Thickness and Device Performance in High Electron Mobility Transistors

ABSTRACTThe performance of InGaAs/GaAs pseudomorphic high electron mobility transistors is anticipated to improve with increased channel thickness due to reduced effects of quantum confinement. However, greater channel thicknesses increase the probability of forming misfit dislocations which have been reported to impair device properties. We characterized the composition and thickness of the active layer in Al0.25Ga0.75As / In0.21Ga0.79As structures with different channel thicknesses (75 Å - 300 Å) to within ± 0.005 and ± 8 Å using high resolution x-ray techniques. We determined, using Hall and rf measurements, that the device properties of these structures improved with increasing thickness up to about 185-205 Å; degraded properties were observed for thicker channel layers. Cathodoluminescence results indicate that the mosaic spread observed in x-ray triple axis rocking curves of these device structures is due to the presence of misfit dislocations. Thus, even though misfit dislocations are present, the device structure performs best with a channel thickness of ∼185 Å. These results demonstrate that one can fabricate functional devices in excess of critical thickness considerations, and that these x-ray techniques provide an effective means to evaluate structural properties prior to device processing.

Elektronendichte-verteilungen in metallorganischen verbindungen. Wechselnde struktur- und bindungsverhältnisse in dimeren metallorganischen nickel-hydriden mit ionenpaar-beziehungen zu den hauptgruppen-metallen natrium und lithium

The compounds [[(C 2H 4) 2Ni] 2H][Na(TMEDA) 2] (I) and [[(C 2H 4) 2Ni] 2H][Li(CH 3 2NCH 2CH 2N(CH 3) CH 2CH 2N(CH 3 2] (II) have been investigated by high resolution X-ray techniques. Compound I is found to exist as distinct ions which are well-separated in the unit cell, while II shows an ion pair contact between the lithium ion part and one nickel atom, one ethene group, and the hydride in the anion. This latter interaction has little effect on the geometry of the anion, which is similar in both compounds. Each nickel atom is surrounded by a trigonal-planar coordination geometry comprising two ethene groups and the bridging hydrige. The NiHNi bridge is non-linear and the angle at the hydrogen atom is 131 and 111°, respectively. The unusual geometry adopted by the anion is explained in terms of an interaction between the two nickel atoms involving the empty p-orbital on one metal with the filled d-orbitals on the other. The bonding situation obtained from semi-empirical molecular orbital calculations is in accord with the observed electron density distributions.