Structure, Oxygen Content and Electric Properties of Titanium Nitride Electrodes in TiNx/La:HfO2/TiNx Stacks Grown by PEALD on SiO2/Si.

This work reports experimental results of the quantitative determination of oxygen and band gap measurement in the TiNx electrodes in planar TiNx top/La:HfO2/TiNx bottom MIM stacks obtained by plasma enhanced atomic layer deposition on SiO2. Methodological aspects of extracting structural and chemical information from (scanning) transmission electron microscopy imaging (bright field and high angular annular dark field), energy dispersive X-ray spectrometry and electron energy loss spectroscopy are thoroughly considered. The study shows that the oxygen concentration is higher in the TiNxOy bottom electrode (about 14.2 ± 0.1 at. %) compared to the TiNxOy top electrode (about 11.4 ± 0.5 at. %). The following average stoichiometric formulas are TiN0.52O0.20 top and TiN0.54O0.26 bottom for top and bottom electrodes, respectively. The amount of oxygen incorporated into TiNx during PEALD because of oxygen impurities in the plasma is minor compared to that because of diffusion from SiO2 and HfO2. This asymmetry, together with results on a sample grown on a Si substrate, shows that incorporating oxygen impurity from the plasma itself is a minor part compared to diffusion from the SiO2 substrate and HfO2 dielectric during the PEALD growth. We observe the presence of TiO2 at the interface between the Hf oxide layer and the Ti nitride electrodes as well as at the SiO2 interface. EELS analysis led to a band gap ranging from 2.2 to 2.5 eV for the bottom TiNxOy and 1.7-2.2 eV for the top TiNxOy, which is in fair agreement with results obtained on the top TiNx electrode (1.6 ± 01 eV) using optical absorption spectra. Measurement of sheet resistance, resistivity and temperature coefficient of resistance by a four-point probe on the top TiNxOy electrode from 20 to 100 °C corresponds to the typical values for semiconductors.

Combination of thresholding and fitting methods for measuring nanoparticle sizes and size distributions in (S)TEM.

The practical need for a simple and reliable tool for routine size analysis of nanoparticles with diameters down to a few nm embedded in a polymer matrix motivated the development of a new approach. The idea underlying the method proposed in this work is to combine intensity thresholding and contrast fitting procedures in the same software for particle recognition and measurements of sizes and size distributions of nanoparticles in transmission and scanning transmission electron microscopy images. Particle recognition in images is performed in an interactive process of manual setting the numerical threshold level after image preprocessing. We show that fitting the calculated gray level distribution to the real images is able to provide a maximum accuracy in measurements of the particle diameters in contrast to thresholding approaches. The fitting procedure is applied in the vicinity of nanoparticle images with the mass-thickness, diffraction, and chemical contrast. The grayscale function associated to the nanoparticle thickness is described using polynomial gt=g0+g1t+g2t2+g3t3… with degree ⩾ 2 and undetermined coefficients. The program for particle detection and size measurement-Analyzer of Nanoparticles (AnNa)-has been written and is described here. It was successfully tested on systems containing Ag nanoparticles grown and stabilized in aqueous solutions of different polymers for biomedical use and is available from the authors.

SEM and TEM for structure and properties characterization of bacterial cellulose/hydroxyapatite composites.

Preparation of composites with different properties and gradient of components is aimed at better performance of materials for bone substitution. Bacterial cellulose-hydroxyapatite (BC-HAP) composites with various mass ratio of the components (BC-25HAP, BC-4HAP, and BC-HAP) were prepared by a novel method of growing HAP nanocrystals (the linear size ≤30 nm) in water solutions in the presence of the BC gel-film micro-fragments. Varying the BC-HAP ratios leads to a gradual change of the physical properties of the materials. It was found that an increase in the BC content results in a decrease of the HAP crystal length and specific surface area, porosity, and pore volume while the values of density and Young's modulus values increase. SCANNING 38:757-765, 2016. © 2016 Wiley Periodicals, Inc.

Speciation and reactivity of uranium products formed during in situ bioremediation in a shallow alluvial aquifer.

In this study, we report the results of in situ U(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U(VI), and acetate to stimulate the growth of indigenous microorganisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy, and chemical extractions. Circa 90% of the total uranium was reduced to U(IV) in each reactor. Noncrystalline U(IV) comprised about two-thirds of the U(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U(IV) species are more suceptible to remobilization and reoxidation than crystalline U(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U(IV) formation across a wide range of aquifer parameters when designing in situ remediation plans.

Quantitative separation of monomeric U(IV) from UO2 in products of U(VI) reduction.

The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and tested using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

Non-uraninite products of microbial U(VI) reduction.

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

U(VI) reduction by spores of Clostridium acetobutylicum.

Vegetative cells of Clostridium acetobutylicum are known to reduce hexavalent uranium (U(VI)). We investigated the ability of spores of this organism to drive the same reaction. We found that spores were able to remove U(VI) from solution when H(2) was provided as an electron donor and to form a U(IV) precipitate. We tested several environmental conditions and found that spent vegetative cell growth medium was required for the process. Electron microscopy showed the product of reduction to accumulate outside the exosporium. Our results point towards a novel U(VI) reduction mechanism, driven by spores, that is distinct from the thoroughly studied reactions in metal-reducing Proteobacteria.

Structural similarities between biogenic uraninites produced by phylogenetically and metabolically diverse bacteria.

While the product of microbial uranium reduction is often reported to be "UO(2)", a comprehensive characterization including stoichiometry and unit cell determination is available for only one Shewanella species. Here, we compare the products of batch uranyl reduction by a collection of dissimilatory metal- and sulfate-reducing bacteria of the genera Shewanella, Geobacter, Anaeromyxobacter, and Desulfovibrio under similar laboratory conditions. Our results demonstrate that U(VI) bioreduction by this assortment of commonly studied, environmentally relevant bacteria leads to the precipitation of uraninite with an approximate composition of UO(2.0), regardless of phylogenetic or metabolic diversity. Coupled analyses, including electron microscopy, X-ray absorption spectroscopy, and powder diffraction, confirm that structurally and chemically analogous uraninite solids are produced. These biogenic uraninites have particle diameters of about 2-3 nm and lattice constants consistent with UO(2.0) and exhibit a high degree of intermediate-range order. Results indicate that phylogenetic and metabolic variability within delta- and gamma-proteobacteria has little effect on biouraninite structure or crystal size under the investigated conditions.

Effect of Mn(II) on the structure and reactivity of biogenic uraninite.

The efficacy of a site remediation strategy involving the stimulaton of microbial U(VI) reduction hinges in part upon the long-term stability of the product, biogenic uraninite, toward environmental oxidants. Geological sedimentary uraninites (nominal formula UO2) reportedly contain abundant cation impurities that enhance their resistance to oxidation. By analogy, incorporation of common groundwater solutes into biogenic uraninite could also impart stability-enhancing properties. Mn(II) is a common groundwater cation, which has a favorable ionic radiusfor substitution reactions. The structure and reactivity of Mn(II)-reacted biogenic uraninite are investigated in this study. Up to 4.4 weight percent Mn(II) was found to be structurally bound in biogenic uraninite. This Mn(II) incorporation was associated with decreasing uraninite particle size and structural order. Importantly, the equilibrium solubility of Mn-reacted uraninite was halved relative to unreacted uraninite, demonstrating changes in thermodynamic properties, while the dissolution rate was up to 38-fold lower than that of unreacted biogenic uraninite. We conclude that structuralincorporation of Mn(II) into uraninite has an important stabilizing effect leading to the prediction that other groundwater solutes may similarly stabilize biogenic uraninite.

Transmission electron microscopy of Ca oxide nano- and microcrystals in alpha-tricalcium phosphate prepared by sintering of beta-tricalcium phosphate.

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to study the porous and non-porous alpha-tricalcium phosphate (alpha-Ca3(PO4)2, alpha-TCP) prepared through a sintering procedure at 1200-1400 degrees C of beta-tricalcium phosphate (beta-Ca3(PO4)2, beta-TCP). The interpretation of experimental and calculated X-ray and electron diffraction patterns showed that the final product at 1400 degrees C was primarily alpha-TCP but roughly 3.0-8.0 wt.% of the starting beta-TCP phase and up to 8.0 wt.% of CaO were in the final product. TEM images and electron diffraction patterns showed that the CaO phase--formed by decomposition of TCP--exists as micron-sized areas of various oriented nanocrystals embedded into the bulk alpha-TCP material and also as self-standing spherulite particles of a few microns in size. Surprisingly, formation of CaO from TCP decomposition occurred at temperatures below those predicted from the phase diagram of the CaO-P2O5 system.

Scanning and transmission electron microscopy for evaluation of order/disorder in bone structure.

A comparative characterization of the structure of normal and abnormal (osteoporotic) human lumbar and thoracic vertebrae samples was carried out to reveal the type of possible disorder. Samples from the bone fragments extracted during the surgery due to vertebra fractures were examined by scanning electron microscopy (SEM), conventional and high resolution transmission electron microscopy (TEM and HRTEM), and X-ray energy dispersive spectroscopy (EDS). Contrary to what might be expected in accordance with possible processes of dissolution, formation and remineralization of hard tissues, no changes in phase composition of mineral part, crystal sizes (length, width, and thickness), and arrangement of crystals on collagen fibers were detected in abnormal bones compared to the normal ones. The following sizes were determined by HRTEM for all bone samples:

Pathological mineralization of cardiac valves: causes and mechanism.

OBJECTIVE: Mechanism of calcification of cardiac valves was investigated through a comparative characterization of structure, morphology, and size of hydroxyapatite (HAP) crystals formed in mineral deposit on cardiac valves, bone tissue, and crystals synthesized from aqueous solutions under definite conditions. METHODS: All deposits on cardiac valves and bone samples were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDS) in a Philips XL30 FEG microscope to evaluate their overall view and structure, to estimate the sizes of particles, and to carry out the chemical analysis. High resolution transmission electron microscopy (HRTEM) and electron micro diffraction was done for precise phase identification of individual crystals and measurements of their sizes. RESULTS: Mineral deposit on cardiac valves contained hydroxyapatite crystals (HAP) crystals with lengths from a few nanometers to a few hundred nanometers. Similar to the HAP precipitation in aqueous solutions, crystals in deposit were randomly oriented relative to each other and without the substrate effect on their orientation. Octacalcium phosphate (OCP) phase was also detected in the form of large (up to a few microns) crystals. The quantity of the OCP crystals was quite low in comparison with the amount of the HAP crystals. HAP crystals in bone samples were no more than 20 nm in length and textured in the HAP [0001] direction along collagen fibers. The HAP crystals from cardiac valves and bones were of uniform thickness comparable with the crystallographic unit cell. CONCLUSIONS: Mass crystallization model and hemodynamics in heart and arteries determine the mechanism of pathological calcification through the mediation of hydroxyapatite nanocrystals perpetually circulating with the bloodstream.

Model of the mechanism of Ca loss by bones under microgravity and earth conditions.

A comparative characterization of crystal structure, morphology, sizes, and orientation in Ca phosphate precipitation from aqueous solutions, the mineral phase in bones, and mineral deposits on cardiac valves has been performed by high-resolution transmission electron microscopy to model possible mechanisms of Ca loss by bones. Physiological changes occurring in organisms can lead to deep perturbations of the natural calcium phosphate supersaturation and its local distribution, which in turn influences the phase composition, morphology, and organization of the mineral phase. Formation of crystals with larger size or of two distinct phases instead of the single hydroxyapatite one can result in the deterioration of the Ca balance in bone and tissue destruction as well as the possible misorientation (or spread of orientation) between HAP crystals newly formed in the bone.