In the context of biological specimens, it is in principle desirable to quantitatively map, rather than just point analyze, the distribution of physiologically important elements, and to do so at subcellular resolution. Presently, this can be accomplished by electron energy loss spectrum-imaging (EELSI) in both the scanning transmission electron microscope (STEM) and the energy-filtering transmission electron microscope (EFTEM). Until recently, this approach has been of limited value for mapping the particularly important element Ca, mainly because intracellular total Ca concentrations are normally quite low (<5 mmol/kg dry weight) and because the background in the vicinity of the Ca L23 edge is complex and requires precise background modeling to extract the very weak Ca signals. As a result, the Ca signal is usually not high enough to reach detection threshold during a practical EELSI acquisition time.

Electron energy-loss spectrometry and energy-filtered imaging allow the possibility of detecting, quantifying and mapping of boron. Boron spatial distribution in biological tissue is of particular interest for boron neutron capture therapy (BNCT) for cancer. We have studied the limits of boron quantification and mapping using electron energy-loss spectroscopy and energy-filtered imaging.

To evaluate the concentration limits for boron mapping and quantification three types of specimens were used. First, a uniform boron layer of well known thickness deposited onto of 50 nm-thick carbon film was used to determine the limits for boron quantification. Second, samples for boron mapping with non-uniform boron distribution were prepared by electron-beam evaporation of boron onto a shadow-masked 50 nm-thick carbon film. Third, tobacco mosaic virus (TMV) in a water solution of boronophenylalanine fructrose (BPA-F) was deposited onto a 2 nm—thick carbon film.

Imaging of sample regions with a thickness significantly larger than the extinction length and strong thickness variations introduces two major problems for transmission electron microscopy (TEM) : (i) inelastic scattering increases the energy width of the transmitted electrons and therefore the resolution decreases (ii) the contrast differences caused by thickness variations can be higher than the dynamic range of the detector system.

Both problems can be solved by using energy filtering techniques. The advantage here is that for energy filtered imaging the resolution limit is not determined by the sample thickness but by the width of the energy selection aperture. Fig. 1 shows three envelope functions of the temporal coherence calculated for different values of the energy width. The functions were plotted for an acceleration voltage of 200 kV and a high voltage stability of 2 ppm.

EELS with a TEM is an established spectroscopic technique that is used to provide both qualitative and quantitative chemical information. Analysis of EELS spectral shapes provides information on the coordination, oxidation state, and spin states of atoms in minerals. Despite the availability of parallel EELS detectors for almost fifteen years, geological applications are relatively rare, mainly because of the high level of expertise required by the operator in both acquiring and analyzing the data. For example, core-loss edges can be difficult to see on the background and elemental quantification is not yet routine.

The goal of this review is to discuss the uses and interpretations of the EELS spectra of a range of minerals. We will focus on the interpretation of the electron-loss near-edge structures (ELNES) of the core-loss edges and concentrate primarily on: a) determination of the oxidation state of transition elements and other metals, and b) identification of local anion and cation coordinations. In addition, we will explore the potential of ELNES as a bonding probe.

The resolution in Electron Energy Loss Spectroscopy (EELS) in (S)TEM is presently limited by the instabilities of the high voltage supply (typically 0.2 eV), by the resolution of the spectrometer (typically 0.7 eV), and by the energy spread of the electron source (typically 0.7 eV for a Schottky field emitter). An increase of this resolution to better than 0.1 eV would enable chemical analysis on a nanometer scale. We are presently following hardware as well as software approaches in order to reach this energy resolution in a 200 kV (S)TEM.

The hardware side of our project focuses on the three limiting factors mentioned in the previous paragraph. The instabilities of the high voltage supply are mostly noise and drift, which can be reduced significantly by isolating the high voltage tank, thermally as well as acoustically. In addition, Gatan is developing an imaging filter for this project, which has sub-0.1 eV resolution.

Electron energy-loss spectroscopy imaging is now becoming an established and indispensable tool in high-resolution materials characterization. Whether the imaging approach is based on post column filters attached to a conventional transmission electron microscope (TEM) or on parallel spectrometers on a scanning (field emission) TEM, energy filtered (EF) microscopy is having a significant impact in materials science (e.g. Ref. 1). Elemental mapping, however, is not the only imaging tool accessible with EF microscopy. EELS images and spectra are often only part of the data that is obtained when samples are characterized in an analytical TEM. Complementary information is also obtained from EDS spectra which are often analyzed independently from the EELS data and from EELS near edge structures. When the EELS fine structure is altered due to changes in chemical state of the probed atoms, additional information is accessible and can potentially be used in novel methods of image acquisition.

Chemical segregation is fundamental to a host of geologically interesting phenomena. These include exsolution, weathering, dynamic recrystallization, and interface formation. Investigation of these mineralogical processes centers on the relationship between crystal structure and chemistry. While there is an abundance of electron microprobe information available at the micron scale, Energy- Filtered Imaging (EFI) offers the first opportunity to investigate the chemistry of these diverse processes at the nearly atomistic level. In particular, we have been using Energy-Filtered Imaging (EFI) in the TEM to acquire quantitative compositional information at the scale of individual unit cells (<1 nm spatial resolution) in exsolved pyroxenes. Along with the two-orders-of-magnitude improvement in spatial resolution, EFI has greater quantum efficiency and the possibility of lighter element detection than X-ray spectroscopic methods.

The complex, subsolidus phase relations of many minerals are routinely exploited by petrologists investigating the thermal and tectonic evolution of the earth, moon and meteorites.

Particulate, mixed-valence transition metal oxides are frequently used for battery, catalytic and magnetic applications. For example, the Li ion exchange battery exploits charge transfer of mixed Mn+3, Mn+4 materials. Charge localization and phase separation, especially at particle surfaces, are critical issues for determining the materials’ useful properties, be it catalytic activity or saturation magnetization. The ability to image the charge localization and correlate this with crystallographic information would be extremely useful in the study of this class of materials. Using energy-filtered transmission electron microscopy (EFTEM), valence maps of Mn and Co with a ∼ 2 nm scale have been obtained for bulk samples. In principal this technique can de directly extended to the case of particulate samples, however there are some additional experimental challenges, such as thickness and edge effects, that must be addressed. We demonstrate here the feasibility of valence mapping of particulate samples, and discuss the factors that limit quantitative data extraction from the maps.

The ability to obtain elemental maps in a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) is extremely useful in the analysis of materials, and semiconductor devices such as ULSI's and GMR heads. Hitachi has developed a new type of elemental mapping system, consisting of a STEM (Hitachi, HD-2000) equipped with a two-window electron energy filter. In-situ calculation of the energy-filtered signal makes it possible to observe real time elemental mapping images with nanometer resolution.

Figure 1 shows a schematic of the elemental mapping system. In the STEM, electrons are generated from a cold field emission gun and accelerated to a potential of 200 kV. The electrons arc focused by the objective lens into a small probe (<1 nm), which is then rastered over the specimen using scanning coils. Transmitted electrons are collected by an energy filter, which is located beneath the specimen., and consists of quadrupole lenses, a magnetic prism spectrometer and two kinds of electron beam energy detectors.

The increasing availability of fast, multiprocessor high performance computing facilities has revolutionized iterative electronic structure calculations of solids, particularly for those systems possessing little or no symmetry. This combined with the semi-commercial dissemination of well-supported software packages for the calculation of both occupied and unoccupied electronic states at high levels of approximation has opened up the possibilities for more routine modeling and understanding of electron energy loss near-edge structure (ELNES) data obtained in the TEM/STEM, and hence the elucidation of chemical bonding in solids at high spatial resolution.

The two modeling approaches presented and compared here involve ab-initio band structure calculations based on the full linear augmented plane wave (FLAPW - WIEN97) approach, and full multiple scattering (MS) clusterbased methods. We have performed a systematic study of transition metal carbides and nitrides and compared the modeling results with high resolution EELS data.

Thin Cu films were grown on single crystalline α-Al2O3 by molecular beam epitaxy at a substrate temperature of T = 800°C. The nominally 100 nm thin film consists of islands, which have diameters of 0.5-1 μm. High-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) were applied to obtain information about the atomic and electronic structure of the interface. HRTEM studies were performed on the Jeol JEM ARM 1250 operated at 1250 kV (point resolution of 0.12 nm). Analytical studies were conducted on a dedicated scanning TEM (VG HB 501) operated at 100 kV and equipped with a parallel EELS (Gatan 666).

HRTEM of the Cu/ Al2O3-interface shows an atomically abrupt interface and reveals an epitaxial orientation relationship (1120)s[0001]s || (111)Cu <211>Cu (S denotes sapphire) (FIG.l). Close-packed planes and directions in both crystals are parallel to each other resulting in misfits of 2% and 7% in <211>Cu- and <110>Cu-directions, respectively. The interface is not coherent, but in both directions no misfit dislocations are detectable.

It has been demonstrated that electron energy loss spectrometry (EELS) can be used to probe the electronic structure of materials on the near-atomic scale. The electron energy loss near edge structure (ELNES) observed after the onset of a core edge reflects a weighted local density of final states to which core electrons are excited by fast incident electrons. Lately ‘atomic resolution EELS’ and ‘column-by-column spectroscopy’ have become familiar themes amongst the EELS community. The next generation of STEMs, equipped with spherical aberration (Cs) correctors and electron beam monochromators, will have sufficient spatial and energy resolution, along with the superior signal to noise required, to detect small changes in the ELNES from atomic column to atomic column.

Core loss ELNES provides information about unoccupied states, but the structure observed in spectra is sensitive to changes in the underlying occupied states, and thus to the bonding in the material.

Most materials used in technology are polycrystalline. Therefore, the properties of the internal interfaces often control the properties of the materials (interface-controlled materials). In single phase polycrystalline materials, grain boundaries are the internal interfaces (homophase boundaries) of interest, whereas in composites interfaces between different components (heterophase boundaries) play a crucial role. It is of great importance to investigate the structure, composition and bonding of the different interfaces. The atomistic structure of specific interfaces is obtained by high-resolution transmission electron microscopy studies, whereas analytical electron microscopy (AEM) investigations produce information on the composition of interfaces. AEM studies include energy-dispersive X-ray spectroscopy (EDX) as well as electron energy-loss spectroscopy (EELS). From the energy-loss near-edge structure (ELNES) of ionization edges measured by EELS, information on the local electronic structure at interfaces can be obtained. Information on bonding and its modifications by segregated atoms can be extracted from the ELNES studies.

In recent years, GaN and its alloys have been the subject of an intense global research effort to develop its optoelectronic properties in the blue-green region of the spectrum. Of particular interest has been the fact that despite a high density of threading dislocations, on the order of 108 to 1010 per cm2, thin film devices retain their ability to emit light. The origin of this behavior remains unclear, and it has even been suggested that reducing the number of defects by employing different growth techniques does not necessarily increase the crystal's lasing abilities. As research is now aiming to grow GaN on silicon (silicon has a large lattice-mismatch to GaN) in order to develop an inexpensive LED technology, dislocations are likely to remain a major issue. It is therefore essential that we develop a fundamental understanding of the electronic structure of these defects, in order to determine their effect on the properties.

Perovskite-type oxides with high electronic and ionic conductivity are very promising materials for use as dense ceramic membranes for oxygen separation. For the successful implementation of practical ceramic membranes, a full understanding of the parameters controlling the degree of non-stoichiometry, i.e. the defect chemistry is essential. A combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) can be used to directly image crystal and defect structures and the effect of the structures on the local electronic properties (i.e. oxygen coordination and cation valence). Here the defect chemistry in SrCoO3-δ before and after a reduction treatment at high temperatures is investigated in the JEOL 201 OF STEM. This material is known to exist in a wide a variety of phases with different crystal structures, compositions and valence states of cobalt, and can be highly oxygen deficient.

There has been a rejuvenation of interest in the field of metallic glasses in the last decade, ever since the discovery of alloy compositions which allow bulk glass formation. By definition, bulk metallic glasses (BMGs) are characterized by a critical cooling rate < 10 K/s or a minimum dimension > 1mm. We have chosen to examine the structure of the Pd-Ni-P system in order to explain, from an atomic structural point of view, its precipitously high glass-forming ability in this system with respect to those of the binary alloys. With three constituent elements the study of the Pd-Ni-P system remains tractable in comparison to other BMGs that often contain five or more elements. This system has a critical cooling rate lower than 1 K/s and is one of the simplest prototypes of a BMG. However, binary alloys of transition metals (e.g. Pd, Ni, Cu) with metalloids (P, Si, etc.) will not form BMGs even though they can be forced to retain a glassy structure with rapid quenching from the liquid phase.

Up to 47 mass% of gadolinium can be dissolved in a baseline borosilicate glass (mol%) 20 B2O3, 5 A12O3, 60 SiO2, and 20 Na2O. Understanding the dissolution of Gd in borosilicate melts is important in the optimization of glass formulations. However, probing amorphous materials with spectroscopic techniques is challenging research when little knowledge of the local chemical environment of the glass network is available. Electron energy loss fine structure (ELFS) spectroscopy is one available technique that has several advantages for probing the local structure of amorphous materials. This technique gives local atomic structure information equally well for both amorphous and crystalline materials. There is high sensitivity for low Z elements like boron, oxygen, aluminum, and silicon whereas x-ray absorption fine structure (XAFS) technique faces experimental difficulties. Hence, results from ELFS can be complimentary to the results from XAFS. For medium to high Z elements, one can use the L- or M-edges of the elemental spectrum.

For the SESAM/SATEM project is for testing and as a first development step a new fully corrected imaging energy filter of the Ω type built. It is incorporated in a 200keV EFTEM equipped with an LaB6 cathode and a objective lens with a spherical and chromatic aberration coefficient of Cs = Cc = 1.2mm. This EFTEM will be used this year by the SESAM project group at the University Tiibungen, Germany and the MPI fur Metallforschung, Stuttgart. At the same time this kind of filter will be also mounted in the final SATEM.

The energy filter is similar to the design of the so-called FHI-filter suggested by Rose and Lanio [1,2] and built at the Fritz-Haber-Institut of the MPG by Krahl [3]. The total length of the proposed filter is half of the FHI-filter and nearly the same as the length of the LEO 912Ω filter.

It has been already pointed out that the medium voltage microscopes of 300kV to 400kV have some advantages in the analytical capabilities of EDS and EELS as compared to those of 200kV). The P/B ratios and the spatial resolution for the analysis will be improved with the increase of the accelerating voltages as well as lattice resolution. In order to improve the spatial resolution of inelastic filtered images, we have recently developed a new 300 kV omega-filter electron microscope with a field emission gun. In the paper, some characteristic features of the new microscope and its application results are described.

The new microscope have a 300kV field emission gun, an omega-filter, EDS, digital STEM, a slow-scan CCD, an imaging plate and TV camera. Some characteristic features of the new microscope are summarized in Table 1. Based on a calculation of probe diameter as a function of probe current at 300kV in a Shottkey type gun with a brightness of 7xl08A/cm2sr and Cs of 0.6mm, a minimum probe size (FWHM) is estimated to be about 0.2nm (Fig. 1).