Special Report
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Chemistry viewed through the eyes of
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Hiah-resolution Microscopy oxygen atoms, In this electron micrograph of a thin crystal of Individual metal Ions are arranged In an almost hexagonal array, each separated by about 3.5 A and surrounded by six or seven a low-temperature form of niobium pentoxlde (Nb2Os)
This Special Report was prepared jointly by Michael Beer, Johns Hopkins University, Baltimore; Ray W. Carpenter and LeRoy Eyrlng, Arizona State University, Tempe; Charles E. Lyman, Du Pont, Wilmington; and John M. Thomas, University of Cambridge, U.K
Improvements during the past few years in electron mi croscopes, both in their resolution and the amount of data that can be extracted from the resulting images, have greatly extended their usefulness in chemical research. It now has become technically feasible, for example, to build transmission electron microscopes—in which the electron beam passes through the specimen—with elec tron lenses good enough to yield reliable images with a resolution close to atomic level limits. In fact, resolution of atoms in aggregate appears to be just around the cor ner. Images of single isolated heavy atoms even now are possible, and the chemical content of spaces a few cubic angstroms in size can be analyzed. Even elusive surfaces can be brought into sharp relief. Meanwhile, advances in scanning instruments—which record signals that are either backscattered electrons or emitted electrons, x-rays, or photons—have enhanced chemists' ability to detect extremely small quantities of materials, obtain three-dimensional information about the structure and defects of crystals, map the 40
C&ENAug. 17, 1981
topographical detail of surfaces, examine the structure of catalysts, or probe the molecular architecture of bio logical materials. These capabilities have been even further enhanced with the development of improved scanning transmission electron microscopes, which combine the transmission of electrons through the specimen with scanning and analytical systems. These developments are of considerable chemical significance. No matter in what area chemists work, they encounter material in the solid state since most elements and even a greater fraction of compounds are solid under normal conditions. Surfaces at high resolution
Resolution of topographical detail on surfaces has been a goal of microscopy for centuries. But the limited depth of field of optical microscopes has prevented all but the lowest magnification. This problem was circumvented in the early 1950's when Charles W. Oatley and his stu dents at the University of Cambridge developed the surface scanning electron microscope. In this instrument, afinelyfocused electron beam scans the surface of the specimen, producing a signal from primary backscattered electrons or secondary electrons emitted from the surface. This signal is used to modulate the intensity of a cathode ray tube that is scanned to form an identical raster (scanning) pattern. The small angles that the electrons make with the optical axis of the in-
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Surface features less than 100 Â in size can be seen in this low-loss micrograph from a scanning electron microscope of the surface of a single crystal of gold 200 À thick. Fine lines are slip lines due to relaxation after vapor deposition
strument are responsible for the large depth offieldin the scanning electron microscope, which can be several hundred times that for optical microscopy. With conventional heated tungsten electron sources, the resolution of a scanning electron microscope is about 100 to 200 Â. Newer electron sources, such as heated lanthanum hexaboride or cold field emission, push the resolution of surface detail down to 20 to 50 A. This resolution level is limited mainly by beam-specimen interactions occurring as electrons penetrate beneath the specimen surface and by the cleanliness of the surface, since most of these devices do not operate at ultra-high vacuum ( c:
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Electron energyloss spectrometer Photomultiplier tube An analytical electron microscope of the transmission/ scanning transmission type, shown here schematically, differs from other types of electron microscopes by having an energy dispersive x-ray spectrometer above the speci men, an electron energy-loss spectrometer below the specimen, detectors for backscattered primary electrons and emitted secondary electrons above the specimen, and deflection coils both above and below the specimen. The upper electron probe deflection coils are used to form scanning images and diffraction patterns either of the transmission type or of specimen surfaces. Most stateof-the-art analytical electron microscopes use a thermionic electron emitter in the electron gun, but more advanced instruments use field emission emitters because they produce a higher electron current density in the small in cident probes needed for microanalysis and microdiffraction studies or for high-resolution scanning transmission electron microscopy
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C&ENAug. 17, 1981
lyzing the complex structure of assemblies, provided appropriate procedures are found for selectively labeling constituents of interest. A compelling need also exists in many areas of biology for procedures for locating chemical elements or molec ular species in various regions of a cell. Characteristic x-rays are emitted when a beam of electrons strongly excites atoms, so that the atoms can be identified in an electron microscope. Alternatively, the energy loss of the beam electron can be measured in a spectrometer so that, again, elements or compounds can be recognized. Even small, diffusible ions can be localized. Tissue can be frozen rapidly to arrest diffusion and kept frozen during microscopy, for instance. Using this technique, the abundance of calcium has been shown to drop in the terminal cisternae of the sarcoplasmic reticulum on ex citation, underlining the key role of calcium ions in triggering muscle contraction. Also, nucleic acid-con taining structures can be recognized through the char acteristic energy loss of phosphorus. Analytical electron microscopy
Chemists who study solids are acutely aware of the complicated relationships between microstructure and properties of solids. High-resolution analytical electron microscopy is the most recent and sophisticated method yet developed for such microstructural analysis. The key to the method is analysis of both the elemental compo sition of the solid and its structure under high-resolution conditions. In this context, high resolution has two equally im portant benefits. First, the structure of the solid, as well as such structural defects as dislocations or internal boundaries, are examined through high-resolution images or by high-spatial-resolution microdiffraction. Second, direct experimental measurements are obtained of the local elemental composition, especially of small regions in heterogeneous solids (such asfine-grainpolycrystalline materials or undergoing the early stages of a precipitation reaction). High-resolution analytical electron micro scopes can do this and more because their lenses and their electron sources focus tiny (down to 5 Â diameter), intense electron probes on specimens and they produce, as well, direct high-resolution transmission images of the specimen's structure. Although analytical electron microscopy basically is simple and straightforward, some of its details are complicated. When a high-energy electron beam (~ 100 kV) is incident on a specimen, a number of signals are emitted that can be detected and used for structural and elemental analysis. These signals result from elastically and inelastically scattered transmitted electrons, characteristic emitted x-rays, backscattered electrons, and secondary emitted electrons. All these signals can be collected simultaneously in a single instrument from a small area of the specimen to permit a high-resolution analysis of its structure and composition. Elastically scattered electrons are collected electronically or on photographic film to form conventional transmission images and microdiffraction patterns of the specimen structure. Because inelastically scattered electrons in the transmitted beams have lost energy to the specimen, the energy distribution in the transmitted beams (the electron energy loss spectrum) directly reflects the elemental composition and bonding in the specimen. The emitted x-ray spectrum is also characteristic of the elemental composition. These two spectra are complementary. Images of the specimen surfaces can
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C&ENAug. 17, 1981
Special Report be formed from the backscattered or secondary emission electrons, or from transmitted elastically or inelastically scattered electrons using specialized experimental methods. An analytical electron microscope usually is one of two types, either a transmission electron microscope with a scanning attachment which can form fixed beam or scanning images or a scanning transmission electron microscope which forms scanning images exclusively. Instrument components unique to an analytical electron microscope are an energy dispersive x-ray spectrometer above the specimen, an electron energy-loss spectrometer below the specimen, backscattered and secondary elec tron detectors, and pre- and post-specimen deflection coils. There also are important differences between the ob jective lenses and vacuum systems used for analytical electron microscopes and conventional microscopes. An analytical electron microscope's objective lens is one of its most critical components. It must perform two func tions: form a high-resolution image and focus a small electron probe on the specimen. In addition, the lens geometry must allow the x-ray spectrometer to be placed close to the specimen. X-ray spectrometers used with these microscopes generally have solid-state detectors with an energy res olution of about 150 eV. The electron spectrometers are usually magnetic prisms with scintillator/photomultiplier detectors. An electron energy-loss spectrum system usually will operate at an energy resolution of about 10 eV for microanalysis, but may be operated at higher resolution (about 1 eV) if fine structure in the loss spec trum, due to solid-state effects, is the primary interest. Data for both types of spectrometry are usually col lected by a multichannel analyzer, but may be stored directly in a minicomputer interfaced to the analytical electron microscope. Computer interfacing for data col lection and instrument control has many advantages and is likely to increase in the future. The electron probe deflection coils above the specimen
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The atomic structure of the surface of a platinum/rhodium alloy catalyst is shown in this field ion micrograph, with a prominent crystallographic (111) pole appearing just to the left of center. We concentric rings correspond to single atom layers, bright spots to single atoms
form scanning images and diffraction patterns either of the high-resolution transmission type or of specimen surfaces. The most advanced analytical electron micro scopes use field-emission electron sources because these produce the highest electron current density in the small incident probes used for microanalysis, microdiffraction, and high-resolution scanning transmission imaging. Crewe and his colleagues at the University of Chicago showed in the late 1960's that field-emission sources are necessary to achieve high resolution in the scanning φ transmission imaging mode. Others suggest that fieldf emission sources are necessary for the best spatial reso A Ε lution in microdiffraction and microanalysis as well. δ Vacuum systems for analytical electron microscopes ο are changing from oil diffusion/mechanical rotary pumps > to dry systems using ion pumps and turbomolecular or C liquid nitrogen sorption pumps for two reasons. First, D 8 high-intensity electron sources require high vacuums for stability and long life; and second, to avoid contamina tion, buildup on specimens must be prevented during 1 high-intensity electron irradiation because it degrades Φ image quality and microanalysis and microdiffraction microscope usually 1 performance. A conventional electron χ operates in a vacuum of about 10~ 5 torr. State-of-the-art !c analytical electron microscopes operate at about 10~9 torr < in the electron gun chamber and about 10" 6 torr in the specimen chamber. Advanced instruments operate at least an order of magnitude below these levels. (Ο
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Analytical performance levels
Although an analytical electron microscope is a com Palladium catalyst particles as small as 10 A In diameter can plicated device, the instruments are well engineered and be distinguished from their charcoal support as dark spots onproduce structural analysis data at a very high rate. Be the bright-field scanning transmission electron micrograph oncause these instruments are designed to acquire many the left and are even more prominent as light spots under different signals from the irradiated specimen, it is rea sonable to ask what level of performance can be achieved dark-field illumination, as shown on the right Aug. 17, 1981 C&EN
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Special Report
