Zirconium carbide belongs to the group of refractory compounds having NaCl structure with high chemical inertness and melting point, characteristics that have made these materials to play a prominent role as hard coatings and these properties are closely related to their electronic structure. The electronic configuration of ZrC is: (Xe) 5d2 6s2, one s electron being able to be promoted to a d orbital forming four equivalent sd3 hybrid orbitals, favoring covalent bonds maintaining, however, a percentage in ionic character of about 30%.

Some attention has been paid in the last two decades in studying the optical properties of transition metal carbides and nitrides by electron energy loss spectroscopy (EELS), being TiC and TiN the most extensively reported.

In this work the optical constants of commercial powder ZrC were obtained using a Gatan Parallel Detection Electron spectrometer (model 766) attached to the CM-200 transmission electron microscope (TEM).

EFTEM has become a very useful tool in characterizing material properties, because it allows studying local chemical and electronic properties of a specimen with nanometer level spatial resolution. There are two spectroscopic imaging methods have been established to obtain both the spatial and the spectra information, I (x, y, ΔE), which can be quantitatively evaluated. One technique, as known as spectrum-image method, is based on scanning the electron beam across the sample to acquire the EELS spectrum with two-dimensional information. Conversely, the other method retrieves the spectra in a specific position from a series of energy loss images, as known as imaging-spectrum method. The spectrum-image method can be achieved with better energy resolution, however, a long acquisition time. Compared to spectrum-image method, which acquires a series of spectrum with scanning probe, the advantage of imaging-spectrum method is the capability of simultaneously recording abundant information in a series of maps.

Energy filtered (EF) imaging and energy loss fine structure analysis have now become routine tools to characterize very complex microstructures at high spatial resolution. When combined with energy dispersive x-ray spectroscopy microanalysis in a field emission gun transmission electron microscope (TEM) and with samples prepared by a large array of techniques such as FIB or microtoming, these analytical techniques can tackle real-life and more fundamental problems in materials science. This paper presents some examples of this work that highlight the complementarity of information obtained either from EF imaging, near-edge structures and EDS microanalysis. These examples are taken from work in the development of battery materials for automotive applications and in the study of ultrathin magnetic layers. Other examples of high spatial resolution energy loss fine structure at interfaces of high-K dielectric-Si in interface are shown in elsewhere in these proceedings.

It is not uncommon for an electrical failure in a microelectronic device to be traced back to an individual resistive contact, such as a W contact lined by TiN to the underlying metal silicide [1]. Identifying the root cause of the defective cell then requires the capability of extracting interfacial chemistry and microstructure with near atomic resolution, which is achieved by recording EELS and EDX data either along 1-d lines or from 2-d arrays. EELS data are characterized by intrinsically low signal-to-background ratios and plural inelastic-scattering effects, which presents a challenge to the reliability of the methods used to extract quantitative information from a spectrum image. Commercially available software packages by Emispec and Gatan currently provide routines for power-law or polynomial background fits and allow the net counts under peaks or edges to be determined from each pixel. in both cases each spectrum in the series is processed in the same manner.

Energy filtered transmission electron microscopy (EFTEM) has been developed in recent years to be a highly flexible tool for spatially resolved microanalysis. The basic technique has been extended to allow fully quantitative mapping of single elements, or a large range of elements using an image-spectroscopy approach . All these techniques are still limited by the nature of the TEM as a structure projector; any elemental map is only a 2D projection of what is in reality a 3D structure. A fully 3D approach to EFTEM analysis would offer additional insights into many materials problems and make possible meaningful analysis of certain systems for the first time. Three-dimensional electron microscopy (electron tomography) offers one path for the reconstruction of 3D structures from 2D projections. Although this has been well developed for solving the structure of biological macromolecules and viruses its use in the physical sciences is quite new.

Recent developments in detector technology [1] for EELS and Energy Filtered TEM has made possible to obtain large number of spectra and energy filtered images during very short exposure times. This in turn opens the exciting possibility of studying time dependent processes in the electron microscope, during exposure to the electron beam as well as the study of different radiation sensitive samples which are being degraded during lengthily data recording. This kind of data recording generates a large amount of data and manual data analysis should be avoided in order to be able to fully benefit from the improved sensitivity and increased speed of these new detectors. Thus a fast, real-time data analysis system is highly desirable.

A system for real-time data analysis (spectra classification) of data generated from such a detector has been simulated in a program based on the object oriented C++ framework ROOT [2][3].

As techniques for electron energy-loss spectroscopy (EELS) reach a higher degree of optimization, detection limits for analyzing biological structures are approaching those predicted by theory. in favorable specimens, single atom detection is predicted for elemental maps acquired by means of the scanning transmission electron microscope (STEM) equipped with a field emission source, paralleldetection EELS and a spectrum-imaging system. to obtain such results, the electron detector should have a detective quantum efficiency close to unity and a well behaved point-spread function; such design features are now available with a cooled charge-couple device (CCD) array. The energy-filtering transmission electron microscope (EFTEM) provides a complementary approach to mapping elements occurring at higher concentrations but distributed over larger regions of the specimen. Use of an optimized CCD detector in the EFTEM now enables accurate quantitation in addition to high analytical sensitivity, albeit not at the single atom level.

There has been considerable interest in recent years in simulating the complete electron energy loss (EELS) spectrum to help the analyst make suitable judgments on experimental parameters such as collection angles and acquisition times. The spectrum is characterised by edges that arise from the excitation of inner shell electrons to the first available empty state. The edge threshold is approximately given by the inner shell binding energy. Each edge sits on the background of the tails of all the edges from inner shell excitations of lower energy (see Figl). The background from plasmons decays very quickly with energy and even the broadest plasmon makes a negligible contribution to the intensity for losses above about 150eV. The visibility of an edge (and also the detectability of the corresponding element) can be related to the intensity of the edge compared to the background although it is more common to use the jump ratio, which is the ratio of the intensity in the region after the edge E+B to the intensity of the background in the edge region, B.

Electron energy loss spectroscopy (EELS) probes electronic excitations of a solid on the atomic scale. The widespread availability of first-principles calculations has lead to an explosion of theoretical calculations of EELS spectra. Agreement between theory and experiment is generally reported to be good at the typical energy resolutions in commercial microscopes of 0.7-1.3 eV. However a brief survey of the X-ray absorption literature suggests that the anticipated introduction of monochromators, along with improvements in energy stability, and spectrometer resolution will unmask many more effects that cannot be predicted as precisely as they can be measured.

The shape and binding energy of a core excitation is determined by both the ground state electronic structure (initial state effects) and the reponse to the excited electron-hole (final state effects) (Fig. 1). Errors in the initial state, such as the systematic errors in band gaps (and hence band offsets) are inherent in the local density approximation eigenvalues used to simulate EELS spectra.

Electron energy-loss spectroscopy (EELS) is now a standard tool to investigate the local chemistry and bonding of defects in solids. to first order, the energy thresholds of the ionization edges in EELS spectra are determined by the identity of the element [1], while small shifts are induced by different bonding coordination and charge states [2]. The shapes of ionization edges in EELS spectra reflect the local bonding environments. We present first-principles calculations that incorporate electron-hole interactions and are in excellent agreement with experimental data obtained with X-ray absorption spectroscopy (XAS) and EELS. The superior energy resolution in XAS spectra and the new calculations make a compelling case that core-hole effects dominate core-excitation edges in all of the materials investigated: Si, SiO2, MgO, SrTiO3 and SiC. These materials differ widely in their dielectric constants leading to the conclusion that core-hole effects dominate all core-electron excitation spectra in semiconductors and insulators.

The promise of advanced technological applications in optical and electronic devices has led to a significant recent research effort in the structure-property relationships of defects in GaN. in particular, the major scientific issues arise from the high density of threading dislocations induced during thin film growth by film-substrate lattice mismatch. There is still debate as to the exact effect of these dislocations on the overall properties; they may or may not be electrically active and are thought to decrease the lifetime of devices. As such, a fundamental understanding of the electronic properties of these defects will facilitate the development of new and improved devices.

The analysis of the electronic structure of dislocations in GaN is performed here by a combination of atomic resolution electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) and multiple scattering (MS) simulations. Experimentally, a Z-contrast image of the dislocation core is obtained first1 and used to position the probe for EELS.

Electron loss near edge structure (ELNES) reflects transitions of inner shell electrons to site- and symmetry-projected unoccupied electronic states, which in a solid lie above the Fermi level and may be calculated using advanced electronic structure calculations. The direction of momentum transfer, q, imparted during the inelastic scattering process is dependent on the angle over which the scattered electrons are collected. It is usual to resolve the momentum transfer, into components parallel and perpendicular to the incident beam direction (q∥and q⊥). in a specimen region which is crystallographically anisotropic, the exact direction of the momentum transfer and hence energy states available to the excited electron will depend on the specimen orientation with respect to the incident electron beam. Practically, this can present problems where quantitative comparisons and/or modelling of electronic structure between different specimen regions at different orientations, or within a polycrystalline material, or even between different samples are required.

Introduction: Atomically resolved chemical analysis by means of electron energy (core-) loss spectroscopy has been postulated since the technical availability of highly focused sub-nm STEM beams. We study the feasibility of this approach, as compared to the alternative of spatial difference spectra, where interfacial resolution is achieved by subtracting boxes at and apart from the interface. A nanobeam EELS signal is generated by the dynamical propagation of a conical wave through the specimen. This wave acts as 3D-excitation envelope for inelastic scattering and is needed to link the ab-initio calculated atom- and angular-momentum-projected density-of-state (DOS) functions to measurable EELS signals.

Projected DOS: For the rhombohedral twin grain boundary in sapphire (FIG 1) as model system, DOS for p states in oxygen have been calculated 4 as described in , for the most stable twin structure obtained by ab-initio calculations.

In the last 6-7 years there has been considerable experimental and theoretical interest in GaN and its alloys for use in light emitting applications. GaN is a direct gap semiconductor with a bandgap at ∼ 3.4 eV. When alloyed with in and/or Al the bandgap can be tuned to any wavelength ranging from UV to red. to further the development of GaN based devices, reliable methods are required to measure and predict the optical and electronic properties. EELS is one of the few techniques that can study these properties with high spatial resolution (< 1 nm) if a small electron probe is used, such as that in a scanning transmission electron microscope (STEM). High spatial resolution is necessary as, to improve efficiency, devices are normally based on quantum well structures.

Gd3+ doped Ce oxides are very promising candidates as electrolytes for solid oxide fuel cells operating at ∼ 500 °C. For their successful commercial implementation, a full understanding of the defect chemistry in the bulk and at grain boundaries is essential. in particular, the contribution of the grain boundaries to the total ionic conductivity through such effects as the segregation of impurities, dopants and vacancies is of crucial importance. Here the effect of the atomic structure on the local electronic properties, i.e. oxygen coordination and cation valence at grain boundaries of the fluorite structured Gd0.2Ce0.8O2-x ceramic electrolyte is investigated by a combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the JEOL 201 OF STEM (operating at 200keV, and aligned for a probe size ∼ 0.2 nm).

Preliminary O K- and Ce M45-edges were acquired from points in the grains (A and B) and grain boundary shown in Figure 1.

LiCoO2 is currently the preferred cathode material in secondary Li-ion batteries. Stoichiometric LiCoO2 has rhombohedral symmetry and belongs to the R3m space group. This layered structure has Li and Co ions ordered along (111) planes to form alternating cation layers. The Co3+ ion, located in octahedral sites, forms a strong bond with neighboring oxygen atoms to produce O-Co-O sheets. Li layers are sandwiched between these sheets of CoO2.

It has traditionally been accepted that in lithium metal oxides (LiMO2) the valence of the transition metal compensates for the charge on the intercalated Li. The theoretical boundaries for Li concentration in an electrode of LixCoO2 are 0≤x≤l. Using a simple ionic charge transfer model, the fully-lithiated material has ions with charges of Li+, Co3+, and O2-. in the CoO2 end member the ions have charges of Co4+ and O2- This simple model incorrectly leads to the conclusion that the oxygen valence is virtually unaffected by Li intercation.

Introduction

Fluid transport systems of organisms [1] in general and the blood vascular system in particular are considered to be optimally designed. From Murray's laws [2] it is concluded that an arterial bifurcation where the diameter of the parent vessel (d0) relates to the larger (d1) and the smaller daughter vessel (d2) according to d03 = d13 + d23 is optimal. Interestingly, existing data predominantly refer to arterial branchings of the fully developed circulatory system of mammals. to the best of our knowledge there are no data available on arterial bifurcations and venous mergings in an initially growing but then regressing tubular system of blood vessels as it is found in the gill filter apparatus of the anuran tadpole where the highly complex three-dimensional vascular network totally disappears at the end of metamorphosis.

Microwave-assisted processing of biological samples began with Mayers. Subsequently, many investigators reported using microwave ovens for various aspects of biological sample processing for light microscopy (LM) and/or transmission electron microscopy (TEM.) The use of ice-encased fixation and water-immersion resin polymerization marked the beginning of rapid, reproducible microwave-assisted processing techniques.

Mcrowave protocols are now often used for LM and TEM processing. Examples include paraffin embedding for LM, in situ hybridization, decalcification, immunological staining plus clinical studies and research projects for TEM.

Recently my lab has begun to microwave process biological samples for scanning electron microscopy (SEM.). We fix, dehydrate and dry with hexamethyldisilazane in less than 1 ½ hours. The only part of the process not utilizing microwave assist is the final 15 minute drying step in a conventional oven.

The history of microwave-assisted processing has been dominated by the idea that microwave heating was an integral part of the equation. The separation of a microwave component from the heating effects of the radiation during sample processing has been experimentally difficult. Combined with this difficulty has been the closed cavity design of microwave ovens. This design is typical of laboratory and household ovens and results in the formation of “hot” and “cold” spots within the chamber. These spots produce regions in close proximity to each other which have widely varying heating effects on samples.

A second factor to consider with microwave heating is the effect wattage output has on rate and extent of microwave induced heating. Peak wattage outputs of all laboratory and most household microwave ovens are in excess of 650W. As a result the vast majority of all microwave-assisted protocols are based on heating parameters associated with high wattage processing.

The investigation of connective tissue ultrastructure has historically relied on chemical fixation to stabilize micro architecture. These fixatives are not effective in retaining many matrix molecules, including proteoglycans, and allow precipitation of many other components in subsequent processing steps. The result is a preponderance of open space between matrix components. Cells shrink considerably and a precipitation of molecules in highly concentrated regions leads to artifacts including the lamina lucida and lamina densa of basement membranes. Cryo-fixation at high pressure followed by freeze substitution seeks to immobilize tissue components within vitreous (non-crystalline) ice, which is later substituted with acetone at temperatures below the recrystalization temperature of water. Formation of large ice crystals during any part of the preparation protocol significantly distorts structure and can be recognized in the compound microscope. Recognition of smaller freezing artifacts, including microcrystalline ice, is imperative for interpretation of resulting images.