bluuebunny/crossref_metadata_2025 huggingface dataset:
Transition metal oxides revisited: ELNES of metal L2,3 and oxygen K edges
Authors: Krivanek Ondrej L. , Paterson James H. , Poppa Helmut R. , Rez P.
Published: June 2020

When examined at 0.5 eV or better energy resolution by electron energy loss spectroscopy (EELS), many inner shell loss edges begin to show new fine structures. Recently, we have been able to acquire inner shell loss spectra routinely at about 0.4 eV resolution, using the Gatan PEELS™ on the VG HB501 STEM. We have therefore decided to reinvestigate the energy-loss near-edge structure (ELNES) of oxygen K and metal L2,3 edges in first row transition metal oxides.

Figure 1 shows the metal L2,3 edges from vanadium oxide, chromium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, and copper oxide. Except for the chromium oxide sample, which was prepared by crushing and dispersing grains of crystalline Cr2O3, samples were made by vapor-depositing a thin metal film onto rock salt, heating it in air at about 400° C while still on the substrate, and subsequently floating it off. The resultant thin oxide films were from the same sample batches as those used for the EELS Atlas, where they were identified as VOx, MnO2, Fe2O3, CoO, NiO, and CuO.
Effects of accelerating voltage and thickness on the signal-to-noise ratio in Electron Energy Loss Spectroscopy (EELS)
Authors: Botton Gianluigi , L′Espérance Gilles
Published: June 2020

The advantages of medium accelerating voltages (200-400KV) for EELS have been considered to be an improved jump ratio of edges and an improved signal to background ratio for an increased range of thiknesses. However, very little experimental data of the Signal to Noise Ratio (SNR) as a function of incident electron energy (Eo) can be found in the literature. The interest in performing such measurements is to investigate the effect of accelerating voltage on the detectability limits in EELS since SNR can be related to the minimum detectable fraction. In this paper, measurements made on NiO samples show an improved SNR with increasing Eo at a constant thickness value. Changes in the angular distribution of scattering are considered to be responsible for the improved SNR.

The NiO sample studied was prepared by evaporation on a copper supporting grid. Uniform areas of different thicknesses (t) caused by the superposition of the film were analysed. Values of mean free path (λ) for NiO were found in the literature. All analyses were carried out with a Philips CM30 and a GATAN 666 Parallel EEL spectrometer (PEELS) coupled to a Link AN10000 MCA. Spectra were transfered on to an external computer for SNR calculations.
Structure determination of a new polymorph of TiO2 by high-resolution electron microscopy and computer simulations
Authors: Banfield Jillian F. , Veblen David R. , Smith David J.
Published: June 2020

A new, naturally occurring polymorph of TiO2 has been identified. This mineral forms lamellae generally only a few nanometers wide in anatase from two localities near Bintal Valais, Switzerland. The abundance of this mineral in anatase is too low to allow investigation by X-ray diffraction. The unit cell determined by electron diffraction is triclinic, with a = 0.754 nm, b = 0.448 nm, c = 0.616 nm, α = 78.90°, β = 124.55°, γ = 96.54°. The coherently intergrown lamellae are oriented with b parallel to a of anatase; the interface is parallel to (103) anatase.
In situ Transmission Electron Microscope observations of mesotaxial silicide formation in the CoSi2/Si system
Authors: Hull R. , White A.E. , Short K.T. , Yalisove S.M. , Loretto D.
Published: June 2020

A new technique for synthesis of buried epitaxial metal silicide layers in Si (“mesotaxy”) by high-dose implantation of Co and Ni into Si surfaces has been developed. Subsequent to implantation at energies in the few hundred keV range and doses in the 1017Cm−2 regime, thermal annealing at temperatures up to 1000°C results in the formation of well-defined and relatively high quality Si/metal disilicide/Si structures.

The exact implantation and processing conditions are crucial in determining the structure and quality of the buried silicide layer. In this work, we describe transmission electron microscope experiments which illuminate the silicide formation process both by static studies of as-implanted and annealed structures, and dynamical in-situ experiments where as-implanted structures are annealed inside the microscope to mimic the ex-situ annealing conditions. The structure geometry in these materials turns out to be close to ideal for such in-situ experimentation: typical implantation conditions for formation of a contiguous silicide layer result in tlqe metal layers being of the order a few hundred to a thousand Å and buried about 600-1000 Å below the Si surface. In-situ annealing in the plan-view geometry inhibits surface diffusion across the interfaces, which would be expected in the cross-sectional geometry (5). The typical penetration depths attainable in Si with 200 keV electrons, say ~ 1 micron, allow a significant thickness, hsubthin of Si substrate below the metal layer, thickness hm, to be retained during the in-situ experiment such that hm ≪hsubthin. This is important, as it ensures that the film stress condition (which arises because of the difference in bulk lattice parameters between the Si and metal silicide layers) is reasonably representative of the stress conditions relevant for the case of annealing on the unthinned substrate.
Crystal structure determination by convergent-beam electron diffraction
Authors: Vincent R. , Exelby D. J.
Published: June 2020

In recent years, significant progress has been made towards a solution for the general problem of crystal structure determination by convergent beam electron diffraction (CBED). Even if we consider only perfectly ordered, periodic crystals defined by one of the conventional space groups, diffraction methods based on a focussed sub-micron beam of electrons are applicable to several related sets of structural problems that are not accessible to conventional X-ray or neutron diffraction techniques. We assume here that the space group either is known or has been determined from CBED patterns and that phases and amplitudes for some subset of the structure factors are required. Two limiting cases have been explored in some detail. For crystals where the atomic parameters and Debye-Waller factors are known accurately from high quality X-ray data, information on the charge redistribution for bonding electrons is available from precise measurements of the low order structure factors. Following the original research of Kambe, some recent work has demonstrated that accurate structure amplitudes and three-beam phase invariants can be extracted from the dynamical intensity distribution in CBED reflections. In principle, this approach is completely general but considerable labour would be required to extract sufficient data to solve the structure of an unknown crystal, whereas a large set of kinematic intensities is acquired from a single X-ray pattern.
A new look at absorption in high-energy electron diffraction
Authors: Bird D.M. , King Q.A.
Published: June 2020

People seldom take absorption very seriously in electron diffraction calculations. Since the work of Hashimoto et al. it has been common practice to set the absorbative part of the potential coefficient V′g to be one tenth of the elastic part Vg. Although this may not be a bad approximation in simple metals, there is no reason why it should apply to more complicated structures. Here, Vg may be small because of interference between different atoms in the unit cell, but the same cancellation might not occur in V′g because the individual atomic contributions to Vg and V′g need not scale in the same way. In fact, the absorbative contribution increases more rapidly with atomic number than the elastic part. This effect is particularly marked in non-centrosymmetric crystals, where V′g and Vg need not have the same phase. Both V′g and Vg are now complex numbers which we write as

(1)

The squared modulus of the total potential coefficient now becomes

(2)

If g → −g, ϕ and ϕ′ change sign, so if the phase difference is non-zero the overall magnitude of the +g and −g potential coefficients are not equal. The standard assumption is that |V′g| = 0.1|Vg| and ϕg − ϕ′g = 0, which ignores corrections to the magnitude of Vg of order | V′g/Vg|.
Microdiffraction from very small crystals
Authors: Cowley J.M. , Pan M.
Published: June 2020

The possibility of using a dedicated STEM instrument to obtain diffraction patterns from regions 10Å or less in diameter has been known for many years and has been the basis for many investigations of the structures of small particles, surfaces of crystals and crystal defects. With some of the newer TEM instruments it is also possible to obtain diffraction patterns from regions 10Å in diameter and applications of these instruments for the study of small particles will undoubtedly follow, especially when field emission guns are added to provide intensities in the microbeams that are comparable with those of the dedicated STEM instruments. Many of the characteristic features of microdiffraction patterns from the dedicated STEMs arise from the fact that, to a good approximation the beam incident on the specimen may be assumed to be completely coherent. For the TEM instruments only partial coherence may be achieved because of the relatively large source size and because, even with a field emission source, the microprobe may be broadened by the effects of mechanical and electrical instabilities. Even with partial coherence, however, the interference effects seen with the fully coherent beams become evident to some extent when the total beam diameter is comparable with the dimensions of the diffraction object and so must be understood if the diffraction patterns are to be properly interpreted.
GexSi1-x/Si superlattices grown on (100)-, (111)-, and (110)-oriented Si substrates
Authors: Kuan T. S. , Iyer S. S. , Yeo E. M.
Published: June 2020

GexSi1-x/Si heterostructures have been studied extensively because of their potential device applications. Previous GexSi1-x/Si heteroepitaxy studies have been mostly confined to growth on (100) surfaces and have focused on how growth temperature and GexSi1-x layer thickness affect growth morphology and defect generation. In this work we compare the quality of GexSi1-x/Si superlattices grown on (100)-, (111)-, and (110)-oriented Si substrates. We find that these three growth directions give rise to different growth morphologies and defect structures. Strained GexSi1-x layers in a (100) GexSi1-x/Si superlattice annealed at 450 – 500°C have recently been reported to exhibit a CuPt-type long-range order. Thus, another objective of this work is to explore whether growth along the <110> and/or <111> directions can promote or suppress the onset of ordering.

Superlattices consisting of 18 layers of alternating Ge0.5Si0.5 (5 nm) and Si (20 nm) were grown by molecular beam epitaxy (MBE). Before growing the superlattice, a 3D-nm-thick Si buffer layer was grown on the Si substrate.
ALCHEMI studies on the location of zinc in GaAs
Authors: Christenson K. K. , Eades J.A.
Published: June 2020

The diffusion of Zn in GaAs is anomalous in that the diffusion coefficient D is proportional to the Zn concentration squared. Further, the diffusion rate of the column III species in III-V layer structures (ie. Ga and Al in GaAs-GaAlAs) can be increased by 105 with the addition of doping levels of Zn3. The column V sites are not affected. As an aid to understanding the diffusion process we have located the position of the Zn in the GaAs lattice.

There are four high symmetry positions in the zincblende structure that the Zn could occupy: Ga, As, T (Tetrahedral interstitial, located at 1/2 1/2 1/2 with four nearest neighbors at 0.433 a0) and H (Hexagonal interstitial, located at 5/8 5/8 5/8 with six nearest neighbors at 0.415 a0). Interstitial diffusion involves hopping between alternating T and H sites with the energy barrier to diffusion being equivalent to the difference in the potential energy of the two sites. Figure 1 indicates the possible low-index orientations for ALCHEMI studies which can differentiate between these sites.
Applications of high-resolution Electron Microscopy to semiconductor interfaces
Authors: Tsai H. L.
Published: June 2020

Practical applications of high-resolution electron microscopy (HREM) to semiconductor interfaces have been conducted in various areas. Among these are the use of the HREM as a diagnostic technique for interfacial quality, understanding of interfacial interactions, and analysis of misfit dislocations in heterostructures. These studies indicate that the HREM is a useful technique to provide in-depth understanding of interfaces for the process and defect controls.

The quality of an interface often plays an important role in the film quality or the performance of a semiconductor device. Figure 1a shows one example illustrating the generation of threading dislocations from particles located at the lattice-mismatched In0.15Ga0.85As/GaAs interface. The HREM image shown in Fig. 1b indicates a particle to be located at a misfit dislocation. This observation suggests that a particle at the heterointerface can promote the nucleation of a dislocation and/or the formation of the misfit dislocation.
Identification of superfine precipitates and crystalline defects in silicon using TEM phase-contrast imaging
Authors: Kantian V. C. , Green M. L. , Brasen D. , Luftman H. S. , Ha N. T.
Published: June 2020

Very large (≥1000Å in size) precipitates, and crystalline defects, such as dislocation loops or twins, can be identified in a straightforward manner using the conventional electron diffraction contrast in bright field/dark field (BF/DF) mode in a transmission electron microscope (TEM) On the other hand, superfine precipitates and crystallographic defects, whose sizes are ≤ 100Å, can be difficult to identify by the BF/DF technique. For example, the distinction between a small and a large dislocation loop (similar arguments hold for precipitates, too) is the way the elastic stress is distributed around the dislocation. The elastic stresses around a large loop have the same general form as that for a straight dislocation line. Hence, the BF and DF images of a large loop have the same general characteristics of the image of a straight dislocation line.[1] However, the image characteristics of a small dislocation loop are derived using the infinitesimal loop approximation of a dislocation line to describe the stress field of the small loop.[2] Hence, the image characteristics of a small loop can be complex with black/white or black/black dot contrast. In this article, we report a technique for distinguishing superfine precipitates and defects using the phase contrast imaging technique in the TEM without going through the time consuming and sometimes ambiguous BF/DF technique. This technique was applied to identify interfacial defects in homoepitaxial silicon films deposited by rapid thermal chemical vapor deposition (RTCVD).
Electron-stimulated damage processes in oxides under ultra-high vacuum (UHV) conditions
Authors: Buckett M.I. , Singh S.R. , Fan H. , Wagner T. , Marks L. D.
Published: June 2020

HREM has recently entered the surface science arena as a complementary technique for studies of electron-stimulated damage of oxide surfaces. The primary advantage offered is the ability to observe in-situ structural changes of the surface, as has been exemplified in the literature for NiO, WO3, V2O5, and TiO2, among other oxide crystals. One major drawback has been the inability to perform experiments under UHV conditions comparable to those used in conventional surface science techniques. Previous HREM reports have not considered the effect of the microscope environment in surface radiation damage.

In this study, we compared the surface radiation damage effects of oxides under UHV conditions (l0-10 Torr in a Hitachi UHV-H9000 instrument) to damage effects observed in conventional instruments (operating at approximately 10-7 Torr). A number of oxides were investigated, including NiO, V2O5 , WO3 , MoO3 , TiO2, and Ta2O5. Our results indicate that although the same basic damage mechanisms are operative under both vacuum conditions, subsequent surface alteration due to reoxidation or reaction with surface contamination occurs under non-UHV conditions. WO3, for example, underwent surface-initiated reduction to metallic W under UHV conditions (Fig. 1, 2 ).
In situ High-Resolution Electron Microscopy
Authors: Sinclair Robert
Published: June 2020

The strength of in situ electron microscopy lies in its ability to observe directly material changes which are pertinent to bulk processes. The most rigorous experiments employ a purpose-built specimen holder to simulate specific testing conditions (e.g. heating, cooling, straining, environment), with the associated microstructural changes deduced from appropriate micrographs, diffraction patterns or video-recordings. The influence of the electron microscope itself must always be taken into account (e.g. thin-foil specimen, large electron flux, high electron energy). However when artifacts are overcome, remarkable insight into the mechanisms of the behavior of solids can be achieved. The purpose of this article is to review our work in extending this capability into the high-resolution regime, so that atomic reactions can be followed. In the right circumstances this allows straightforward interpretation of atomic-scale phenomena.

Our studies have employed a Philips EM430ST, 300 kV transmission electron microscope (TEM) with about 0.2 nm resolution. To-date we have carried out only heating experiments, utilizing the standard side-entry, single-tilt specimen holder (model number PW 6592), mainly on reactions at semiconductor interfaces. The zone axis orientation necessary for high-resolution TEM is obtained by careful sectioning of the substrate crystals and judicious positioning in the heating holder. The imaging conditions (including drift) can be optimized at a temperature slightly below that required for the changes of interest, so that recording can be initiated immediately on ramping up.
Structural rearrangements in small gold particles
Authors: Lewis James , Smith David J.
Published: June 2020

A knowledge of the structure of small metal particles is fundamental to understanding their role in heterogeneous catalysis. It is therefore of much topical interest that structural rearrangements have been found to take place as a result of intense electron irradiation within the electron microscope. Initial observations were of gold particles on supports of carbon and silica-covered silicon, but further studies confirmed that qualitatively similar behaviour also took place for particles of Pt, Ru and Rh. Some of the factors which influence the motion have been noted, including the particle size, the nature of the substrate and, obviously, the current density of the electron beam. Considerable debate has been generated by these observations. It has been suggested, for example, that the structural fluctuations should be dampened by an electrically conducting substrate which would prevent the particles from becoming charged. It was also proposed that the “quasimelting” of gold particles was triggered by Auger decay, in effect heating the particles electronically rather than thermally. The energetics of these fluctuations have also been considered.

In this paper, we report our initial efforts to quantify some of the experimental factors involved in this quasimelting process. Three sets of samples were prepared: one was a normal holey carbon support film, the second was a holey silicon support film, and the third was a carbon film upon which graphitised carbon had been deposited.
Electron and ion irradiation-induced dissolution of mechanical twins in the intermetalic U3Si: An in situ HVEM study
Authors: Allen Charles W. , Birtcher Robert C.
Published: June 2020

The uranium silicides, including U3Si, are under study as candidate low enrichment nuclear fuels. Ion beam simulations of the in-reactor behavior of such materials are performed because a similar damage structure can be produced in hours by energetic heavy ions which requires years in actual reactor tests. This contribution treats one aspect of the microstructural behavior of U3Si under high energy electron irradiation and low dose energetic heavy ion irradiation and is based on in situ experiments, performed at the HVEM-Tandem User Facility at Argonne National Laboratory. This Facility interfaces a 2 MV Tandem ion accelerator and a 0.6 MV ion implanter to a 1.2 MeV AEI high voltage electron microscope, which allows a wide variety of in situ ion beam experiments to be performed with simultaneous irradiation and electron microscopy or diffraction.

At elevated temperatures, U3Si exhibits the ordered AuCu3 structure. On cooling below 1058 K, the intermetallic transforms, evidently martensitically, to a body-centered tetragonal structure (alternatively, the structure may be described as face-centered tetragonal, which would be fcc except for a 1 pet tetragonal distortion). Mechanical twinning accompanies the transformation; however, diferences between electron diffraction patterns from twinned and non-twinned martensite plates could not be distinguished.
In situ observations of the amorphous-to-gamma transformation in ion-implanted Al2O3
Authors: Sklad P. S.
Published: June 2020

Transmission electron microscopy (TEM) was used in an investigation of the recrystallization behavior of amorphous surface layers on single crystal [0001] α-Al2O3. Conventional imaging and diffraction techniques were used to characterize the microstructures produced by annealing bulk specimens in the temperature range from 800°C to 1200°C. It was found that the amorphous Al2O3 first transforms to γ-Al2O3, a cubic transitional form, which in turn transforms to α-Al2O3. The γ to α transformation begins at the original crystalline-amorphous interface and proceeds toward the specimen surface. By measuring the position of the γ-α interface as a function of annealing temperature and time it was possible to determine the temperature-dependent velocity of the γ-α transformation front. Assuming a thermally activated process, the data are consistent with an activation energy of 3.6 eV for the transformation. However, no amorphous material remained, after even the shortest anneals (15 min) at the lowest temperature (800°C). These results indicate that the kinetics of the amorphous to γ transformation are faster than those of the γ to α transformation. Therefore a series of in-situ experiments was carried out to investigate the transformation of amorphous Al2O3 to γ-Al2O3. Amorphous Al2O3 surface layers, free of implanted impurities, were produced by implantation at −185°C with 2 × 1016 Al/cm2 at 90 keV and 3 × 1016 O/cm2 at 55 keV. Cross-sectioned specimens were prepared following established techniques.2 The specimens were annealed in-situ in a Philips CM12 operating at 120 kV with a Gatan Model 628 single tilt heating holder. Progress of the transformations was monitored with a Gatan model 622 video system.
The derivation of Bloch waves in the reflection case: Analytical solutions and numerical calculations
Authors: Gjønnes K. , Peng L.-M. , Gjønnes J.
Published: June 2020

The Bloch wave description of electron diffraction was originally derived with reflection experiments in mind, but has since been applied mainly to the transmission case, where the neglect of back scattering leads to an eigenvalue equation for the anpassung, γ, and eigenvectors (Bloch waves). In the reflection case such an approximation can not be made, γ will appear in second order. A general solution was formulated in terms of 2N Bloch waves which are obtained as non-orthogonal eigenvectors of a non-Hermitian matrix by Colella and Moon. Due to this added complexity there has been little discussion of Bloch wave solutions and properties pertaining to reflection electron diffraction – most calculations are based on integration procedures normal to or parallel to the surface.

We have studied the derivation of Bloch wave solutions in the reflection case by starting from the equation for scattering ,using one greens function operators for forward scattering (Z>Z’)
Elemental mapping with a parallel-detection Electron Energy-Loss Spectrometer
Authors: Krivanek Ondrej L. , Meyer Chris E. , Tencé Marcel
Published: June 2020

Elemental maps, that is images showing the concentration of different elements in a sample, can be obtained in an electron microscope equipped with an electron energy-loss spectrometer (EELS) by acquiring and processing data in three dimensions: spatial coordinates x and y, and the energy loss ΔE. Since the electron detector is necessarily at most a two-dimensional one, acquiring all the required data at the same time is not possible. Instead, one can either use an imaging electron spectrometer and acquire a series of whole images at one energy at a time, or use a small probe in a scanning-transmission electron microscope (STEM), and acquire the data image-point by image-point. With a serial-detection spectrometer the data at each image-point must be recorded sequentially, while with a parallel-detection spectrometer a whole spectrum can be recorded at the same time.

The two approaches are illustrated schematically in figure 1. The individual sampling points in the three- dimensional volume have been called voxels (by analogy with two-dimensional pixels).
The microstructure of a TiB2/Ti-47 Al composite material
Authors: Popoola O. , Heuer A.H. , Pirouz P.
Published: June 2020

The addition of fibres or particles (TiB2, SiC etc.) into TiAl intermetallic alloys could increase their toughness without compromising their good high temperature mechanical and chemical properties. This paper briefly discribes the microstructure developed by a TiAl/TiB2 composite material fabricated with the XD™ process and forged at 960°C.

The specimens for transmission electron microscopy (TEM) were prepared in the usual way (i.e. diamond polishing and argon ion beam thinning) and examined on a JEOL 4000EX for microstucture and on a Philips 400T equipped with a SiLi detector for microanalyses.

The matrix was predominantly γ (TiAl with L10 structure) and α2(TisAl with DO 19 structure) phases with various morphologies shown in figure 1.
High-resolution transmission electron microscopic study of highly oxygen doped silicon layer
Authors: Takaoka H. , Tomita M. , Hayashi T.
Published: June 2020

High resolution transmission electron microscopy (HRTEM) is the effective technique for characterization of detailed structure of semiconductor materials. Oxygen is one of the important impurities in semiconductors. Detailed structure of highly oxygen doped silicon has not clearly investigated yet. This report describes detailed structure of highly oxygen doped silicon observed by HRTEM. Both samples prepared by Molecular beam epitaxy (MBE) and ion implantation were observed to investigate effects of oxygen concentration and doping methods to the crystal structure.

The observed oxygen doped samples were prepared by MBE method in oxygen environment on (111) substrates. Oxygen concentration was about 1021 atoms/cm3. Another sample was silicon of (100) orientation implanted with oxygen ions at an energy of 180 keV. Oxygen concentration of this sample was about 1020 atoms/cm3 Cross-sectional specimens of (011) orientation were prepared by argon ion thinning and were observed by TEM at an accelerating voltage of 400 kV.
Morphological studies on poly(ethylene oxide)/hydroxypropyl cellulose blends
Authors: Gerroir P. J. , Sundararajan P. R. , Bluhm T. L. , Piché Y.
Published: June 2020

The studies of polymer blends for industrial applications is widespread owing to the possible diversity of such blends. In addition to binary blends in which both components are amorphous, blends that consist of a crystalline and an amorphous polymer have received particular attention. One such system studied in detail in the literature is that of blends of poly(ethylene oxide) (PEO) and poly(methylmethacrylate) (PMMA). These two are vinyl-type polymers having similar skeletal chain cross-sections. The subject of the present study is the physical characterization of blends of PEO and hydroxypropyl cellulose (HPC). While the studies on PE0/PMMA blends involved melt blending or nonaqueous solutions, in the present case, both PEO and HPC are water soluble. In addition, the skeletal chain cross-sections of PEO and HPC differ appreciably.

Aqueous solution of PEO and HPC (1% wt/v) were prepared. Blends of varying composition were made from these solution by mixing for one hour using a magnetic stirrer.
Scanning tunneling microscopy of metal particles on graphite
Authors: Bailey J. M. , Schmidt L. D.
Published: June 2020

Supported metal catalysts are typically 10 to 100Å diameter particles on inert supports such as Al2O3 or graphite. The shape of catalytic particles determines the number and type of active sites on which a reactant can adsorb. The particle morphology can be studied by TEM which gives atomic resolution, but little information about the surface topography, or by SEM which yields topographic information but only at resolutions of >20Å. However, Scanning Tunneling Microscopy (STM), a surface analysis technique developed in 1981, is a very sensitive surface probe with vertical resolution of >0.1Å. In this work, catalytic systems of small platinum and rhodium particles supported on graphite substrates have been studied using STM to determine the surface morphology of the particles.

The supported particle samples were prepared by first evaporating approximately 20Å of metal film on an Highly Oriented Pyrolitic Graphite (HOPG) substrate. A typical platinum sample was then placed in a tube furnace and annealed in hydrogen at 850°C for 4 hours; a typical rhodium sample was annealed in hydrogen at 750°C for 2 hours.
Topographical signaling for cell differentiation in the plant pathogenic fungus, Uromyces
Authors: Hoch H.C.
Published: June 2020

Fungal plant pathogens invade host cells with a variety of specialized infection structures, however, for most fungi the appressorium is developmentally the first and most important structure to be formed in preparation for host colonization. It must be positioned at an appropriate site on the host in a timely way so that subsequent infection can be assured. For fungi which cause rust diseases of plants, positioning the appressorium is the most critical stage because invasion of the host can occur only via the stomata. Uredospores of these fungi (e.g.,Uromyces appendiculatus) germinate and grow, directed by the leaf (bean) surface topography toward stomata where they cease growth and develop appressoria directly over the stomatal openings. Development of the appressorium is accompanied by ameboid-like migration of the cytoplasm into the ballooning hyphal tip, DNA synthesis and nuclear division, synthesis of several “differentiation” proteins, and a rearrangement of the cytoskeleton. An orderly succession of subsequent infection structures (e.g., infection pegs, vesicles) follow in a preprogrammed sequence once the initial developmental process has been started.

My research goals have been to determine what feature(s) of the host plant signals infection structure formation and how the fungus perceives these signals.
Ultrastructural analyses of local circuits in the olfactory system
Authors: Greer C.A. , Kaliszewski C.K. , Cameron H.A.
Published: June 2020

Information processing within the mammalian olfactory bulb, following transduction of odor stimuli by the receptor cells of the epithelium, occurs at two distinct levels. First, in the glomerular layer dendrodendritic synapses from a subpopulation of interneurons onto projection neurons (mitral and tufted cells) modulate the influence of arriving sensory information. The second level of processing occurs within the external plexiform layer where a second population of interneurons, granule cells, form reciprocal dendrodendritic synapses with mitral and tufted cells to modulate the flow of information out of the olfactory bulb. This second population of interneurons has been of interest, in part, due to the recent recognition that subpopulations form microcircuits preferentially with either the mitral or the tufted cell projection neurons, thus supporting the notion of parallel processing pathways. As part of a continuing effort to characterize the properties of these neurons and their dendritic circuits we have been utilizing light microscopy of selectively stained neurons and both conventional and high voltage electron microscopy of selectively stained neurons to study the geometry and synaptology of the olfactory bulb granule cells.

Sprague-Dawley rats, 12 - 60 days postnatal, were perfused with 1% paraformaldehyde and 1% glutaraldehyde followed by immersion in the fixative for 8-12 hrs. After washes in 0.1M phosphate buffer the tissue was passed through a conventional dehydration series, embedded in EPON and routinely stained with uranyl acetate and lead citrate following sectioning. In addition, littermates were processed for Golgi-EM. Tissue examined utilizing conventional EM (60 - 100kV) was cut at approximately 70 nm and mounted on formvar slotted grids. Tissue examined utilizing high voltage EM (800 - 1000 kV) was cut at 1 - 5 um and mounted on mesh grids prior to carbon coating. Serial sections studied with conventional EM were reconstructed and morphometrically assessed utilizing a PC based 3D reconstruction program.
Molecular analysis of the tight junction
Authors: Stevenson Bruce R. , Heintzelman Matthew B. , Anderson James Melvin , Citi Sandra , Braun I. Deborah , Mooseker Mark S.
Published: June 2020

The tight junction (zonula occludens) constitutes a selectively permeable barrier in the paracellular pathway of most epithelia. It is also thought to play a role in the maintenance of the cell surface compositional asymmetry characteristic of epithelial cells. The identification of ZO-1 and cingulin, the first two proteins found to be exclusively associated with the tight junction, permits novel investigations of this important epithelial cell structure at the biochemical level.

ZO-1 is a high molecular weight polypeptide (>200 kD) found at the tight junctions of a variety of epithelia as well as endothelia, and ultrastructural localization studies on isolated liver plasma membranes indicate that this molecule is clustered at the points of membrane contact on the cytoplasmic surface of the junction. Physical analysis demonstrates ZO-1 to be an elongated, monomeric, phosphorylated protein, peripherally associated with the junctional membrane. Cingulin was originally isolated from chicken intestine, and, like ZO-1, is a peripheral membrane component of the junction which exhibits an elongated shape. Antibodies directed against this molecule show two primary bands at 140 kD and 108 kD on immunoblots of several epithelial tissues, although the relationship between these two elements remains undefined. Little information currently exists regarding the relationship of ZO-1 and cingulin to each other or to tight junction structure or function. We report here a comparison of the immunocytochemical properties of these junctional components as well as an examination of the phosphorylation state of ZO-1.
Nuclear magnetic resonance microscopy
Authors: Lauterbur Paul C.
Published: June 2020

Nuclear magnetic resonance imaging can reach microscopic resolution, as was noted many years ago, but the first serious attempt to explore the limits of the possibilities was made by Hedges. Resolution is ultimately limited under most circumstances by the signal-to-noise ratio, which is greater for small radio receiver coils, high magnetic fields and long observation times. The strongest signals in biological applications are obtained from water protons; for the usual magnetic fields used in NMR experiments (2-14 tesla), receiver coils of one to several millimeters in diameter, and observation times of a number of minutes, the volume resolution will be limited to a few hundred or thousand cubic micrometers. The proportions of voxels may be freely chosen within wide limits by varying the details of the imaging procedure. For isotropic resolution, therefore, objects of the order of (10μm) may be distinguished.

Because the spatial coordinates are encoded by magnetic field gradients, the NMR resonance frequency differences, which determine the potential spatial resolution, may be made very large. As noted above, however, the corresponding volumes may become too small to give useful signal-to-noise ratios. In the presence of magnetic field gradients there will also be a loss of signal strength and resolution because molecular diffusion causes the coherence of the NMR signal to decay more rapidly than it otherwise would. This phenomenon is especially important in microscopic imaging.
The role of cell-extracellular matrix interactions in neural crest cell migration
Authors: Bronner-Fraser Marianne
Published: June 2020

The formation of the embryo involves intricate cell movements, cell proliferation, and differentiation. The neural crest has long served as a model for the study of these processes because these cells: 1. migrate extensively along characteristic pathways during embryogenesis. 2. give rise to diverse and numerous derivatives, including pigment cells, adrenal chromaffin cells, and the ganglia of the peripheral nervous system; and 3. are accessible to surgical, immunological, and biochemical manipulations during both initial and certain later stages in their development. We are in the process of identifying factors that influence cell migration and differentiation in the neural crest system.

Neural crest cells follow two primary migratory pathways in the trunk: a dorsolateral route underneath the skin, and a ventral route through the somite. Within the somites, neural crest cells preferentially migrate through the rostral half of each sclerotome but are absent from the caudal sclerotome. The regions through which neural crest cells migrate are lined with extracellular matrix (ECM) molecules. Because of the intimate relationship between neural crest cells and the surrounding matrix, it has been proposed that the ECM plays an important role in the initiation, guidance, and cessation of neural crest cell movement.
Intercellular junctions in thin sections of human senile cataracts
Authors: Costello M. J. , Oliver T. N. , Cobo L. M.
Published: June 2020

The ultrastructural analysis of normal and cataractous lenses is often hindered by incomplete fixation especially of the centers of the lenses. To overcome this limitation, we have achieved good preservation by chemically fixing 50-200 um thick vibratome sections1 of six human senile cataractous lens nuclei obtained after extracapsular surgery. The remaining nuclear core of each lens was deep yellow to amber and contained central opacities. Fresh vibratome sections (15-30 per lens) were examined in the light microscope (FIG. 1) or were prepared for EM. Fixed and embedded whole vibratome sections were usually bisected and examined along a diameter by trimming successive plateaus (FIG. 2 inset) which gave us access to most regions within the lenses.

Whole sections in the light microscope (FIG. 1) consistently showed rings of varying density. Undetectable in uncut lenses, the rings may represent slight discontinuities in composition, refractive index or adherence of layers of fiber cells. These rings roughly correspond to the location of cracks in sections which appear after EM processing (FIG.2 & 3). Fiber cells within these lenses generally displayed complex and irregular shapes, but the cells were well preserved with distinct cell boundaries. We emphasize here the common features of the nuclear cataracts which all contained severe enough opacities to warrant surgical removal but in which the cells appeared to remain intact. Adjacent cells interdigitate with irregular protrusions (FIG. 4). Differential cytoplasmic density between neighboring cells is suggestive of variations in refractive index which might cause excessive light scattering. In this same region in FIG. 4 we also note that dense staining material is present in the extracellular space appearing between the undulating membranes of adjacent cells (FIG. 4, inset). We know of no other report of such deposits in normal or abnormal lenses. The thickness of these deposits may also be sufficient to scatter light. A prominent feature of the lens preparations regardless of location is the presence of undulating membranes which are part of tongue-and-groove junctions (FIG. 5). It is noteworthy that the convex side of the curves is consistently well-preserved and is probably the crystalline array of protein noted in other studies, including in human autopsy lenses. It is not known whether cell communication occurs in the nucleus via gap junctions, but structures with the distinctive morphology of gap junctions are present even within senile cataracts (FIG. 5). FIG. 5 also shows a complex clustering of undulating junctions. The multilamellar membrane cluster in FIG.6, which is similar to structures reported in Emory mouse cataracts, is not commonly seen within the mass of lens tissue but is seen at cut surfaces and within cracks where cells are pulled apart.
Electron Microscopy in the diagnosis of lysosomal storage diseases
Authors: Vogler Carole , Rosenberg Harvey S.
Published: June 2020

Diagnostic procedures for evaluation of patients with lysosomal storage diseases (LSD) seek to identify a deficiency of a responsible lysosomal enzyme or accumulation of a substance that requires the missing enzyme for degradation. Most patients with LSD have progressive neurological degeneration and may have a variety of musculoskeletal and visceral abnormalities. In the LSD, the abnormally diminished lysosomal enzyme results in accumulation of unmetabolized catabolites in distended lysosomes. Because of the subcellular morphology and size of lysosomes, electron microscopy is an ideal tool to study tissue from patients with suspected LSD. In patients with LSD all cells lack the specific lysosomal enzyme but the distribution of storage material is dependent on the extent of catabolism of the substrate in each cell type under normal circumstances. Lysosmal storages diseases affect many cell types and tissues. Storage material though does not accumulate in all tissues and cell types and may be different biochemically and morphologically in different tissues.

Conjunctiva, skin, rectal mucosa and peripheral blood leukocytes may show ultrastructural evidence of lysosomal storage even in the absence of clinical findings and thus any of these tissues can be used for ultrastructural examination in the diagnostic evaluation of patients with suspected LSD. Biopsy of skin and conjunctiva are easily obtained and provide multiple cell types including endothelium, epithelium, fibroblasts and nerves for ultrastructural study. Fibroblasts from skin and conjunctiva can also be utilized for the initiation of tissue cultures for chemical assays. Brain biopsy has been largely replaced by biopsy of more readily obtained tissue and by biochemical assays. Such assays though may give equivical or nondiagnostic results and in some lysosomal storage diseases an enzyme defect has not yet been identified and diagnoses can be made only by ultrastructural examination.
Facilitated migration of cells from a transected peripheral nerve over collagen fibers: in vivo observations
Authors: Wasserman Arthur J. , Rizvi Azam , Zazanis George , Silver Frederick H.
Published: June 2020

In cases of peripheral nerve damage the gap between proximal and distal stumps can be closed by suturing the ends together, using a nerve graft, or by nerve tubulization. Suturing allows regeneration but does not prevent formation of painful neuromas which adhere to adjacent tissues. Autografts are not reported to be as good as tubulization and require a second surgical site with additional risks and complications. Tubulization involves implanting a nerve guide tube that will provide a stable environment for axon proliferation while simultaneously preventing formation of fibrous scar tissue. Supplementing tubes with a collagen gel or collagen plus extracellular matrix factors is reported to increase axon proliferation when compared to controls. But there is no information regarding the use of collagen fibers to guide nerve cell migration through a tube. This communication reports ultrastructural observations on rat sciatic nerve regeneration through a silicone nerve stent containing crosslinked collagen fibers.

Collagen fibers were prepared as described previously. The fibers were threaded through a silicone tube to form a central plug. One cm segments of sciatic nerve were excised from Sprague Dawley rats. A control group of rats received a silicone tube implant without collagen while an experimental group received the silicone tube containing a collagen fiber plug. At 4 and 6 weeks postoperatively, the implants were removed and fixed in 2.5% glutaraldehyde buffered by 0.1 M cacodylate containing 1.5 mM CaCl2and balanced by 0.1 M sucrose. The explants were post-fixed in 1% OSO4, block stained in 1% uranyl acetate, dehydrated and embedded in Epon. Axons were counted on montages prepared at a total magnification of 1700x. Montages were viewed through a dissecting microscope. Thin sections were sampled from the proximal, middle and distal regions of regenerating sciatic plugs.
Effects of sterilization modes on the mechanical strengths of plaster of paris implants
Authors: Cowden K. , Giammara B. , Devine T. , Hanker J.
Published: June 2020

Plaster of Paris (calcium sulfate hemihydrate, CaSO4. ½ H2O) has been used as a biomedical implant material since 1892. One of the primary limiting factors of these implants is their mechanical properties. These materials have low compressive and tensile strengths when compared to normal bone. These are important limiting factors where large biomechanical forces exist. Previous work has suggested that sterilization techniques could affect the implant’s strength. A study of plaster of Paris implant mechanical and physical properties to find optimum sterilization techniques therefore, could lead to a significant increase in their application and promise for future use as hard tissue prosthetic materials.

USG Medical Grade Calcium Sulfate Hemihydrate Types A, A-1 and B, were sterilized by dry heat and by gamma radiation. Types A and B were additionally sterilized with and without the setting agent potassium sulfate (K2SO4). The plaster mixtures were then moistened with a minimum amount of water and formed into disks (.339 in. diameter x .053 in. deep) in polyethylene molds with a microspatula. After drying, the disks were fractured with a Stokes Hardness Tester. The compressive strengths of the disks were obtained directly from the hardness tester. Values for the maximum tensile strengths σowere then calculated:where (P = applied compression, D = disk diameter, and t = disk thickness). Plaster disks (types A and B) that contained no setting agent showed a significant loss in strength with either dry heat or gamma radiation sterilization. Those that contained potassium sulfate (K2SO4) did not show a significant loss in strength with either sterilization technique. In all comparisons (with and without K2SO4and with either dry heat or gamma radiation sterilization) the type B plaster had higher compressive and tensile strengths than that of the type A plaster. The type A-1 plaster however, which is specially modified for accelerated setting, was comparable to that of type B with K2SO4in both compressive and tensile strength (Table 1).