Anal. Chern. 1982, 54, 83 R-86 R (281) Ratzlaff, K. L. Anal. Chem. 1980, 52, 1415-1420. (282) Stieg, S.; Nieman, T. A. Anal. Chem. 1980, 52, 798-800. (283) Bonnell, I. R.; Defreeso, J. D. Anal. Chem. 1980, 52,139-142. (284) Stewart, J. E. Anal. Chem. 1981, 53,1125-1128. (285) Owens, G. D.; Margerum, D. W. Anal. Chem. 1980, 52, 91 A-106 A. (286) Owens, G. D.; Taylor, R. W.; Ridley, T. Y.; Margerum, D. W. Anal. Chem. 1980, 52,130-138. (287) Kaye, W.; Barber, D.; Mlarasco, R. Anal. Chem. 1980, 52,437 A-442
A. (288) Nichols, C. S.; Dernas, J. N.; Cromartle, Anal. Chem. 1980, 52, 205-207. (289) Bush. C. A.; Palapaiti, S.; Daben, A. Anal. Chem. 1961, 53, 1140-1 142. (290) Marino, D. F.; Ingle, J. D., Jr. Anal. Chem. 1981, 53,845-850. (291) Stieg, S.; Nleman, T. A. Anal. Chem. 1980, 52,800-804. (292) Durham, 8.; Geren, C. R. Chem., Biomed. and Envlron. Insf. 1981, 7 7 , 77-84. (293) Efstathiou, C.E.; Papastathapoulas, D. S.; Hadjlioannou, T. P. Chem., Biomed. Environ. Inst. 1981, 7 7 , 49-56. (294) Vijan, P. N.; Wood, G. R. Anal. Chem. 1981, 53, 1443-1450. (295) Jacintho, A. 0.; Zagatto, E. A. G.; Bergamin, H.; Krug. F. T.; Reis, B. F.; Burns, R. C.; Kowalski, B. R. Anal. Chlm. Acta 1981, 730,243-249. (296) Fukamachi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 719, 389-393.
(297) McCarthy, J. P.; Jackson, M. E.; Ridgway, T. A.; Caruso, J. A. Anal. Chem. 1981, 53,1512-1518. (298) Na, H. C.; Niemczyk, T. M. Chem., Blomed. Environ. Inst. 1981, 7 7 , 305-312. (299) Kato, K.; Sagitani, Y. Chem., Blomed. Environ. Inst. 1981, 7 7 , 85-104. (300) Frazer, J. W. Anal. Chem. 1980, 52, 1205 A-I219 A. (301) Frazer, J. W.; Brand, H. R. Anal. Chem. 1980, 52, 1730-1738. (302) Holtzman, J. L. Anal. Chem. 1980, 52,989-991. (303) Fasano. B. M.; Nogar, N. S. Chem., Biomed. Environ. Inst. 1981, 7 7 , 331-339. (304) Bertrand, R.; Dubois, J.-E.; Toullec, J. Anal. Chem. 1981, 53, 219-223. (305) Ellerton, R. R. W.; Mayne, E. W. Anal. Chem. 1980, 52, 773-774. (306) Kelter, P. 6.; Carr, J. 0. Anal. Chem. 1980, 52, 1552. (307) Kelter, P. 6.; Carr, J. 0. Anal. Chem. 1979, 57, 1828-1833. (308) Maeder, M.; Garupp, H. Anal. Chim. Acta 1980, 122, 303-315. (309) Megehl, P. L.; Johnson, D. C. Anal. Chlm. Acta 1981, 124. 303-314. (310) Baitenspergei, U.; Eggll, R. Anal. Chlm. Acta 1981, 723, 107-115. (311) Chesney, D. J.; Anderson, J. L.;Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981, 124, 315-321. (312) Rubinson, K. A.; Glibert, T. W.; Mark, H. B., Jr. Anal. Chem. 1980, 52, 1599-1551.
Electron Micxoscopy John M. Cowley Department of Physics, Arizona State University, Tempe, Arizona 85287
In the 4 years since the previous review of the nonbiological aspects of electron microscopy, there have been important consolidations of the technical resources. Techniques previously mentioned as thle exploratory research of a few laboratories have become aclcepted, standardized, quantified, and widely applied. The necessary equipment is available commercially and comprehensive books and reviews provide adequate introductions for new users. There has been a lively growth in both the quantity and quality of electron microscopy applied to the materiahi sciences in industrial and academic environments. At the same time possibilities for important new advances are seen, arising in part from the ready availability of improved instrumentation, including minicomputers and field emission guns, and partly from the more complete understanding of the theoretical basis of electron diffraction and imaging.
INSTltUMENTATION Ultra-High-Resolution Microscopes. Since it has become clear that the high-resolutionmicroscopy of crystals can provide information of immediate and fundamental importance for many areas of solid-state science, there has been renewed emphasis on thie production of instruments for this purpose. The resolutiion has been improved and the requirement has been met that the best resolution should be attained using a goniometer stage which is necessary for aligning crystals into pireferred axial orientations. There are two competitive approaches to ultra high resolution. In Japan there are several high-resolution high-voltage (0.5-1.5 MV) electron microscopes which have shown crystal structure images which are directly interpretable for resolutions of about 2 A, i.e., the arrangement of atoms in the unit cell and, in favorable cases, in crystal defects can be inferred because the intensity distribution in the image is a direct, if nonlinear, representation of a projection of the distribution of electrostatic potential which has maxima at atom position. One such instrument has been made in England ( I ) . There are, as yet, none in American although one is on order from Japan for the Lawrence-Berkeley laboratory (2). On the other hand the more conventional 100-keV microscopes have been considerably improved and, especially when the voltage has been increased to 200 kV, directly interpretable images can be achieved with resolutions approaching 2.5 A. For these microscopes thLe detail visible in the images may be 0003-2700/82/0354-83R$06.00/0
much finer than this, down to the 1 A level, but such detail is not directly interpretable since the relationship of the intensity distribution to the specimen structure is greatly perturbed by lens aberrations. In principle it should be possible to interpret such fine detail by comparison with images computed from models of the structure or, in very favorable cases, by use of holographic or image processing techniques. In practice there are a few cases where these methods have been used with some degree of success, but the requirements for their successful application are normally so exacting that their wider use is inhibited. On the other hand the high-voltage, high-resolution microscopes do not usually show such fine detail since instabilities in the high voltage and other electrical supplies are more difficult to control and limit the image detail, whether directly interpretable or not, to the 2-A level. It remains to be seen whether the next major advances in high-resolution imaging will come as a result of engineering improvements to provide better stabilities for high-voltage machines or as a result of advances in the recording and processing of the data from the lower voltage machines allowing the better resolutions to be achieved on a routine basis by the more indirect methods. Scanning Transmission Electron Microscopy (STEM) Instruments. The original STEM instruments of Crewe and associates (3)were designed and were mostly used for single atom imaging and for biological applications. The STEM instruments now produced commercially by VG Microscopes Ltd, England, are increasingly applied in the materials science areas. They are built, and often subsequently adapted further by the users, to allow for special modes which are particularly suitable for crystalline inorganic specimens. In spite of the notable successes in the imaging of isolated heavy atoms ( 4 ) ,STEM instruments are not usually competitive with the conventional TEM in producing high-quality, high-resolution micrographs, especially in the common bright-field modes. For imaging, their special virtue lies in the possibility of using special detector configurations to select any part of the diffraction pattern to form the image (5). However more rapid development is taking place in the analytical applications. For any position of the incident electron beam on the specimen the illuminated area gives a detectable microdiffraction pattern, an X-ray signal with the characteristic emission lines from the elements present, and 0 1982 American
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an energy loss spectrum showing the characteristic energy loss peaks of the elements present. Since the diameter of the incident beam on the specimen, formed by imaging the high-brightness source in a cold field emission electron gun, is about 5 A, the possibility exists of doing chemical and crystallographic analysis of very small volumes of material. A comprehensive survey of the present status of STEM is contained in the published proceedings of the conference on STEM in Solid State Science held in Arizona in 1981 (6). Analytical Electron Microscopes (TEM-STEM). Increasingly instrument manufacturersare attempting to provide all the capabilities of the STEM for microanalysis and microdiffraction, together with the normal TEM high-resolution imaging capabilities, in a single AEM instrument. Inevitably some compromise is necessary. It is rarely possible with a combination instrument to achieve the performance of an instrument specialized for particular modes of operation. Currently image resolutions in the 3-4 8, range are anticipated from instruments which can give microdiffraction from regions of 20 A diameter and microanalysis by energy dispersive X-ray spectroscopy or by electron energy loss spectroscopy from regions of diameter 200-300 A. When special field-emission guns are fitted to such instruments, these latter figures are improved by at least a factor of 2 (7). Computerization. The use of minicomnuters is having increasing impact on many aspects of election microscopy, as is evident from the proceedings of the symposium held at the recent meeting of the Electron Microscopy Society of America (8). The scanning systems are particularly well suited to computerized operations since the observed data is produced as time-varying intensity signals or pulse trains from one or several parallel detectors. The computer may be made to control the position of a beam being scanned over the specimen, the energy loss settin of an energy loss spectrum, or the energies of X-rays detectecfin an energy dispersed X-ray detector. The intensity count in digital form is then correlated with the control signals. Various forms of on-line processing of the intensity data are possible or the data may be stored for subsequent off-line processing by a separate computer system. Among the more ambitious computer operated systems attached to STEM instruments are those capable of collecting up to four images simultaneously in digital form (9) so that correlations between images can be used on-line or off-line to provide special types of information. For example, comparisons of images obtained with electrons which have lost various selected amounts of energy can be used in principle (and, to a limited extent so far, in practice) to provide unambiguous matminas of the distribution of various elements in STEMimages (io). The on-line analysis of high-resolution TEM images has been recognized as desirable for a number of vears since Fourier transform analysis is the most powerful i n d convenient way to assess the instrumental parameters of defocus and lens aberrations which strongly influence the image intensity distributions. Until recently the only feasible way to carry out such analysis was by optical diffractometry of recorded photographic images. The availability of TV-rate digital frame storage devices has now made it feasible to record images in digital form and perform the Fourier transform by use of an array processor within a few seconds (11). Then the instrumental parameters can be measured and adjusted before the images are taken rather than afterward. Extensions of this system to allow image processing for contrast enhancement, detection of periodic or nonperiodic components of images, and other purposes will no doubt follow. Further possible applications of the digital frame storage system include the rapid quantitative recording of the microdiffraction patterns and shadow images (holograms) produced in a STEM instrument (12). Facilities. Until a few years ago, electron microscopes were cheap enough in relation to general funding levels that they could be bought for the exclusive use of research groups or individuals. The only exceptions were the high-voltage microscopes for which the cost was so high and the areas of application so relatively specialized that their existence was usually justified by making them into facilities available for use by a broad section of the scientific community. Recently, however, the rapid increase in sophistication and cost of even the 100- and ZOO-keV instruments has become 84R
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so great in relationship to levels of available funding that it is necessary to consider facility-type operations in which the more advanced instrumentation is concentrated in selected laboratories where a group of specialists can provide expert guidance and assistance with user projects. Among the recent installations available for nonbiological work is the Facility for High Resolution Electron Microscopy at Arizona State University established under the NSF Regional Instrumentation Facilities Program (13). Also at the Lawrence Berkeley Laboratory an ultra-high-resolution high-voltage instrument will form the nucleus of a facility operation supported by the Department of Energy (2).
IMAGING OF CRYSTAL STRUCTURES The direct imaging of crystal structures, showing both the ordered periodic arrangements and, in many cases, the distributions and detailed form of the crystal defects with near atomic resolution, has in recent years become a widely used and important technique for many areas of solid-state science. An excellent account of the state of the art and of the range of applications is provided by the proceedings of the 1979 Nobel Symposium on Direct Imaging of Atoms in Crystals and Molecules (14). The applications to the study of minerals and complicated oxide systems have been particularly productive. The technique still has important limitations. The resolution is still not sufficient to allow the clear distinction of atoms which have separations of 1.5-2 A, which is the range of interatomic distances occurring most frequently in the projections of structures. It is often difficult to obtain three-dimensionalinformation rather than merely projections of structures. The usable thickness of sample is usually limited to about 50 A for 100-keV electrons with only a small increase a t higher voltages if the best possible resolution is achieved (15). When the resolution limit is approached it is rarely possible to make intuitive interpretations of the image contrast with confidence. Interpretations must be confirmed by comparison with computed images. Fortunately computer programs for this purpose are now readily available and a complete image simulation package is available commercially. When the instrumental and specimen parameters are very well controlled, it is possible to use computer simulation for the interpretation of image detail which is on a finer scale than the accepted “interpretable resolution”. Olsen and Spence (16)have been able to distinguish between models for defects in silicon, involving atom rows separated by 3 or less, on the basis of images taken with a microscope of a nominal resolution of about 4 A. Bourret et al. (17)have been able to derive information on dislocation cores in germanium by comparison with calculated images and Bursill et al. (18) have been able to interpret fine detail of the structure of the small planar defects in diamonds. An excellent introduction to the principles of imaging with electrons is provided by the book of Spence (19). Imaging processing techniques have been studied for many years as a possible means for converting fine image detail into interpretable information concerning the specimen. Extensive treatments and discussions of the possibilities are given in the books of Saxton (20), Misell (21),and Hawkes (22). For the most part the proposed techniques have relied on the weak phase object approximation which is valid only for very thin specimens containing relatively light atoms. Some apparent success has been achieved using thin films of amorphous carbon as test objects (23)although the nature of the specimen in this case precluded the possibility that the processing should provide more information about its structure. More obvious improvements of the images are seen to result from the fully digital processing of images of chlorinated phthalocyanine crystals by Kirkland and Siege1 (24). In this latter case it seemed unlikely that the linear, first-order scattering theory of the weak phase object approximation could be satisfactory for a crystal containing rows of chlorine and copper atoms in the beam direction. When the nonlinear terms associated with large phase changes were included, the computations became lengthy but the image improvement was notable (25).
a
ANALYTICAL ELECTRON MICROSCOPY Microanalysis. The techniques of microanalysis, using either the characteristic X-rays generated in the sample or the characteristic energy losses of the electrons transmitted
ELECTRON MICROSCOPY Jdm M. carln,la OSMn Rofs~orolFir+ k s at MZOM state unhwslty. HS was ban in Australla and W e d hk B.Sc. degree al Ihe unvsrslty of Adelaide in 1942 and hk MSc. and D.Sc. degrees at me same university in 1945 and 1957. HS atlendsd MIT from 1947 to 1949 and ob mined hk FhD. degree mere. From 1945 Io 1962 he wwked in Ihe chemical phvaica ~iM101IhecMnmMlwealmsciemwkand Industrial Research Organization in Meibourne. W n became Rofessor of Physks at me University 01 Melbourne in 1963. In 1970 he moved to this country lo take hk Present writan. He has been a Director 01 ihe E k i o n Micros~pySociety of Amerlca and a member of the Executive COmmmee of the International Unbn of Crystallography. He was elected Fellow of Royal Society of London in 1979.
through the sample have become increasingly important tools for materials science. The emphasis in recent years has been on establishing the techniques for routine, reliable analysis, making them more quantitative, and defining the necessary experimental conditions with more precision. A comprehensive account of the bases for these techniques is provided by the publication of the book based on a symposium of the Electron Microscopy Society of American, the Introduction to Analytical Electron Microscopy (26). In electron energy loss spectroscopy (EELS) increasing attention has been given to the detailed form of the inner shell energy loss. While the position and magnitude of the peaks provide information on the nature and relative concentrations of the elements present, the fine structure near the sharp leading edges can be interpreted in terms of the atom bonding (23.The long high-energy tail in this peak shows oscillations which may be interpreted in the same way as the structure of the edges for X-ray absorption in the technique of EXAFS. On the basis of the ELS spectra, the technique of EXELFS gives the same type of information as EXAFS concerning the confiiations of neighboring atoms about any particular type of atom in the structure (28). The EXELFS methods tend to suffer from difficulties of low signal strengths and high background levels, but they do have important advantages over EXAFS in that they are most conveniently applied to elements of low atomic number and the spectra can be obtained from extremely small volumes of soecimen material. ’ T(tdate, the emphasi has heen in exploring the poasibilitiea for this technique I t has been shown that the numbers of nearest neighbor atoms of a given atom type can be determined and asymmetries in bonding can be demonstrated for strongly anisotropic materials such as graphite (29). The number of applications where new information about structures has been derived is as yet very small. X-ray Emission and Channeling. An interesting and useful technique for determining atom positions in crystals arises from the observation of the interaction of diffraction effects with the emission of characteristic X-rays. When a crystal is set to give strong diffraction from a prominent set of lattice planes, a standing wave field is set up in the crystal with a high intensity of electrons at a position relative to the lattice planes which varies with the deviation from the exact Bragg angle for diffraction. This gives rise to the well-known channeling phenomena (30). If a particular type of atoms has relatively high concentrations a t the position where the electron intensity is greatest, the probability of emission of the characteristic X-rays from that type of atom will be enhanced. By varying the crystal orientation and observing the X-ray emission, it is possible to determine the occupancy of particular lattice sites. The method has been applied successfully by Taft0 to the determination of whether particular types of metal atoms occupy the octahedral or tetrahedral sites in spinel structures. The sites have been determined for elements present in concentrations of only 1or 2%. It is possible that the effects of channeling on ELS spectra may be used in the same way (31). Microdiffraction and CBED. The advent of field emission guns and of STEM instruments or attachments has allowed the practical exploitation of methods for obtaining
diffraction patterns from very small specimen areas. There are two main linea of development of these methods with the emphasii b e i respectively on observation of diffraction from extremely small areas, down to 5 A or less in diameter, and in observation of high-resolution and wide angle diffraction patterns from larger areas of diameter 100 A ot more. Microdiffraction patterns from regions of about 5 A diameter are necessarily obtained with relatively high angles of incident beam convergence so that the diffraction spots tend t o overlap and produce interference effects (32). If the unit cell dimensions of a thin crystal specimen are greater than 5 A, it is possible to observe the variation of diffraction intensities as the incident beam is moved about within the unit cell, illuminating different groupings of atoms (33). For immediate practical applications, it is usually preferable to decrease the incident beam convergence so that the diffraction spots are relatively small and well separated. The beam diameter at the specimen is then increased to IC-20 A, but this is sufficiently small for many investigations of microcrystalline or near amorphous specimens, crystal defects, or very small particles. The immediate advantage over normal larger area diffraction is that instead of the diffuse haloes or vague ring patterns given by the larger regions, one can record single-crystal patterns from individual microcrystal regions and thereby study new phases, epitaxial relationships, and defect configurations not otherwise detectable (34). Since it is possible with special equipment to observe these microdiffraction patterns with a TV camera and record them on videotape, applications to transient phenomena and radiation sensitive materials are possible (33). The alternative approach to microdiffraction using a convergent incident beam emphasizes the analysis of the intensity distribution in the diffraction patterns resulting largely from dynamical diffraction processes. The technique is then generally referred to as convergent beam electron diffraction (CBED). In principle specimen areas as small as 5 A or less in diameter can be used but practical considerations make it convenient to use regions of diameter of 100 A or more. The symmetry elements present in the CBED pattern intensities reflect the symmetry of the crystal region being studied. Because many-beam dynamical diffraction effects are involved, the information on crystal symmetry is much more complete than that derived from X-ray diffraction or other kinematical diffraction methods (35). The presence or absence of a center of symmetry can be deduced and even the handedness of the structure can be derived. Application of CBED to an increasing number of crystals has resulted in unambiguous space-group determinations which have frequently been a t variance with prior deductions from X-ray diffraction data (36). In other contexts, the CBED patterns which include the wide angle scattering effects associated with the so-called upper layer lines, show valuable evidence of the symmetry and periodicities of the crystal unit cell in three dimensions, avoiding the normal limitations of two-dimensional projections (37). Since these methods can be applied to very small regions for which the extended investigations required for collection of three-dimensional data is scarcely feasible, they have applications for investigations of small solid-state precipitates and inclusions (38). They also can be very effective in detecting changes of symmetry or lattice orientation associated with crystal defects.
SURFACE STRUCTURE The customary tethniquea for the study of surfaee structure, composition and energetics, including LEED, AES. SIMS. etc. have been developed recently to an extraodinary extent, being sensitive in some cases to the presence and properties of a small fraction of a monolayer. However, they have the common deficiency of poor spatial resolution which is usually no better than 1pm. The use of high-voltage electrons in TEM and STEM instruments is increasingly providing an alternative approach with high spatial resolution as its main advantage but with, as yet, relatively poor capabilitiee for surface compositional and energetics analysis. For special thin film samples viewed in transmission, it is possible to see single heavy atoms on light atom substrates and to make inferences regarding their motions in relation to the surface structure using either bright-field TEM (39) or dark-field STEM (4). Atom-high surface steps have long ANALYTICAL
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been made visible with near atomic lateral resolution (40)and detailed studies ofthe atom distributions on surfaces are being made by comparison with computed images of model surfaces (41).
Reflection electron microscopy (REM) with fast electrons scattered at grazing incidence from solid surfaces was explored in the 1950s but abandoned when scanning electron microscopy using secondary was s' a more convenient and flexible technique with comparable r~Olution. The revival of REM in the 197% using highresolution TEM instruments. stressed the use of diffracted electrons from crystal surfaces (42), providing a means for studies of crystal defects and structural variations in conjunction with the diffraction pattern evidence, in close analogy with the dark-field modes of transmission microscopy of crystals. A major limitation of the REM and most of the TEM methods for studying surfaces is that the relatively poor vacuum of the commercially available instruments (now claimed to be about lo-' torr) makes it difficult to obtain clean, uncontaminated surfaces. Important advances have been possible with special TEM instruments (43) and the commercial STEM machines (44,45) with vacuums better than lo4 torr. In particular, Yagi, Honjo, et al. of Tokyo Institute of Technology have incorporated surface treatment capabilities in a high vacuum TEM instrument and have produced spectacular and important pictures of silicon surfaces (46). Monatomic steps on the surface are clearly visible. Surface superlattices such as the well-known 7 X 7 structure can be observed as they grow or are transformed on the surface at temperature (47). Dislocations intersecting the surface are visible and their interactions with surface structures are evident. With an attainable resolution of better than 20 A, the technique can obviously rovide an abundance of new information not accessible Ey any other method. The advantages of the STEM instrument for REM work include the possibility of combining the high-resolution imaging of the surfaces with microdiffraction from any chosen surface feature, which may be of diameter as small as the resolution limit of about 10 A (45). Also the possibility exists for adding surface microanalysis by detection of either the characteristic X-ray emission or by ELS on the surface diffracted electrons. Use of ELS in the low energy-loss range shows promise of providing data on the energy states of the surface electrons (48). LITERATURE CITED
(1) Cosslett, V. E.; Camps, R. A,; Saxton, W. 0.; Smlth, D. J.; Nixon, W. C.; Ahmed, H.; Catto, C. J. D.; Cleaver, J. R. A,; Smlth, K. C. A,; Tlmbs, A. E.; Turner, P. W.; Ross, P. M. Nature (London)1979, 287, 49. (2) Gronsky, R. I n "38th Annual Proceedings of the Electron Mlcroscopy Society of America"; Bailey, G. W., Ed.; Claitors Publ. Divls.: Baton Rouge, LA, 1980; p 2. (3) Crewe, A. V.; Wall, J. J . Mol. B i d . 1970, 4 8 , 375. (4) Crewe. A. V. Chem. Scr. 1978, 14, 17. (5) Cowley, J. M.; Spence. J. C. H. Ultramlcroscopy 1979, 3, 433. (6) Zeitler, E., Ed, Ultramicroscopy 1981, 7 (no. 1). (7) Carpenter, R. W. Phys. Today 1981, 3 4 , 34. (8) Bailey, G. W., Ed. "39th Annual Proceedings of the Electron Microscopy Society of America"; Claitor's Publ. Divis.: Baton Rouge, LA, 1981; 246-273 and 298-309.
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