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The future of high-resolution electron microscopy
Ultramicroscopy18 (1985)463-468 North-Holland, Amsterdam
463
THE FUTURE OF HIGH-RESOLUTION ELECTRON MICROSCOPY J.M. COWLEY Department of Physics, Arizona State University, Tempe, Arizona 85287, USA Received 10 July 1985
In summary of the final discussion session of the conference on high-resolution electron microscopy, the prospects for the use of high-resolution imaging and related techniques are reported to be exciting for the analysis of crystal structures, crystal defects and crystal surfaces, but considerable difficulties in instrumentation may prevent rapid progress in some directions. In other directions the limitations imposed by the nature of the interactions of electrons with matter have already been approached.
1. Introduction The final session of the ASU Centennial Conference on High-Resolution Electron Microscopy was organized as an open discussion on the topic of this report. As is usual in these cases, it was not left to the whim of the audience to determine the topics to be discussed. A panel of experts were selected to cover the main areas of possible subject matter and they were asked, at short notice, to present short summaries to introduce the topics. It was, of course, an exercise in optimism to expect any expert to cover the future of his field in a few minutes or to expect adequate discussion of the topics raised by the assembly of noted authorities in the audience. We ran out of time very rapidly, with many loose ends still dangling. The chosen experts and their given topics were Bruce Hyde: Large unit cell materials. John Spence: Small unit cell materials. John Cowley: Making use of higher resolution. Christian Colliex: Using analytical EM signals. Archie Howie: Surfaces and small particles. Owen Saxton: Image simulation and image processing. This is not a report of what these people said or of the, often hvely, interchanges with the audience. No verbatim records were kept of the proceedings and no attempt will be made to reproduce a
complete or chronologically correct summary of the session. This report is merely a biased, personal recollection of part of the subject matter. It is based in part on notes left by the panel of experts; but while the author is heavily indebted to the experts for their contributions, they can in no way be held responsible for his interpretations of their statements or opinions. References to the hterature will not be included in this review. The relevant papers are all in these proceedings. It is evident to all that high-resolution electron microscopy (HREM) is in a hvely state of development, with new and exciting possibilities opening up for important progress in many areas. The flood of orders being received by the manufacturers for the new, medium-energy, ultra-high-resolution microscopes is remarkable, particularly in view of their high costs in a period of some financial stringency. Clearly people are expecting a great deal from the instruments and believe that the next stage of HREM will be worth the money.
2. S t m c t ~ e analysis of crystals For the large-unit-cell materials of solid state chemistry and mineralogy the first impact of HREM came with the realization that the cation distributions could be seen clearly, or reasonably
0304-3991/85/$03.30 © Elsevier Science Pubhshers B.V. (North-Holland Physics Pubhshing Division)
464
J.M. Cowley/ Future of high-resolution electron microscopy
inferred, for an important range of materials, mostly oxides, with the microscope resolutions of 3-4 ,~, then available. With improving resolution the cation sites are more sharply defined. The structural data for perfect or imperfect regions can be derived with more precision. The quantitative measurement of intensities and comparison with calculations can be the basis for significant advances. After all, if the "resolution" of an X-ray diffraction experiment were defined in the same way as for an electron microscopy experiment the resolution limit for Cu K a radiation would be 0.77 A, but by using accurate intensity measurements and a suitable set of assumptions regarding the sample and the measuring technique (periodicity, kinematical scattering) the atom positions are found with a precision approaching 10-3 A. A great virtue of the electron microscope technique is usually considered to be that the assumptions of periodicity are unnecessary and the deviations from periodicity, which are often more important than the periodic average structure, can be seen with the same resolution. However, the broader principle remains that, given sufficient accuracy of intensity measurement and sufficient knowledge of parameters defining the instrumental and specimen conditions, any desired trade-off can be made between pre-knowledge of specimen conditions (extent of the periodicity, the form of the defects) and the precision of structure determination. The measurement of intensities will increasingly replace the taking of pictures. As is already the case for X-ray diffraction, the manipulation of masses of data with a large computer is rapidly becoming a major part of any structural investigation. The requirements for accurate values for the instrumental parameters defining the defocus, aberrations and alignment of the microscope are already evident and have prompted the moves towards digitized feedback systems to automatically adjust the electron optics of the microscope. Progress in this direction is inevitable because at the moment very few operators have the experience, insight and understanding of the principles involved to make all the adjustments needed to obtain the best possible high-resolution performance from any of the new instruments. Either the
display systems for the microscopes must be drastically changed, with image intensifier systems, automated digital read-out of astigmatism, misalignment, defocus etc. to guide the operator, or else the whole operation must be automated to eliminate most of the human intervention. The latter, pushbutton-black box, approach is attractive in principle but difficult in practice because for electron microscopy, more than for other techniques, continuous operator intervention may be required to modify or redirect experiments in progress. Also it may be dangerous in that the illusion is readily created that less intelligence is required to understand the limitations of the black box than to perform the functions personally. All this implies an ever-increasingrole for the computer in electron microscopy. The straightforward calculation of images for comparison with pictures presents no fundamental problems even with improved resolution, but for the extension to in-line acquisition and analysis of images and CBED patterns and for interactive systems for complete structure analysis procedures, the development of computer capabilities does not seem to have been rapid enough. The fast hard-wired frame stores and array processors are a boon but they lack the flexibility and accessibility of the host computer. Some way to combine the virtues of both would be highly desirable. The requirements for accurate values of the specimen parameters of thickness and orientation pose an important challenge. At the moment the tendency is to rely on the experience and intuition of the operator or else to leave these as free parameters in the matching with computed image intensities. The obvious answer appears to be the combination of CBED with HREM imaging from the same specimen area and the matching of both with computer calculations, but there is a need for improved engineering to eliminate the uncertainties introduced by stray fields which can affect the relative alignments when the switch is made from one instrumental mode to the other (except in the case of STEM instruments). The second generation of structural problems can now be approached more readily. The materials involved are the oxides and sulfides with more close-packed action arrays and also alloys and
J.M. Cowley / Future of high - resolution electron microscopy
semiconductors. Previously ohly the few high-resolution, high-voltage microscopes have approached the resolution needed to show unambiguously the atom separation of 1.5 to 2.5 A in even the most favorable projections of these materials. For the small unit cell materials, the metals and semiconductors, the crystal structure itself is rarely in question. The structures of the defects present overwhelming problems in all but a few very favorable special cases where linear or planar defects are aligned with the incident beam. Even there uncertainties arise because of radiation damage effects and unknown defect terminations at the top and bottom surfaces. It is understandable that emphasis should swing towards the more obvious and approachable defects, the free surfaces of crystals almost parallel to the beam. The initial results of such studies are important in themselves but also provide valuable guides to the solution of the more general problems.
3. The requirements for better resolution. Undoubtedly further improvements in the resolution of the instruments will be made in future years, whether by decreasing, or correcting for, the aberrations of the objective lens or by increasing the accelerating voltage, or both. It may be as well to note, at this stage, that an number of other factors are involved if full use is to be made of the nominal resolution improvement. Decrease of A, the least resolvable distance (as defined for Scherzer conditions), will give better contrast for isolated, unsupported, atoms but it will not, in general, make it easier to interpret images of thin crystals. The projection approximation, which says that the image represents a generally non-linear representation of the projection of the object structure, will be worse for a given thickness for decreased A at the same voltage but may improve slightly if the voltage, V, is increased. The prospect for validity of the weak phase object approximation (WPOA) which implies linear imaging theory (and use of simple enveloPe functions) is, if anything, worse, because the approximation becomes worse as A decreases and the improvements with V becomes less as V is in-
465
creased. The projected charge density approximation, which gives a linear relationship of the image intensity to the projected charge density, will always improve as A is decreased, but that applies only for a resolution of about 2A or more. For thicker specimens, except perhaps for crystals in axial orientations, problems will soon arise from the limited depth of focus, which is proportional to A2. Thus for 1 A resolution at 200 keV, the depth of focus will be less than 20 A. This implies that the high voltage and lens current stabilities must be greatly improved (by a factor of 10 or more for this case). Also the fine focus steps on the objective lens current must be much smaller than at present. Other instrumental adjustments will likewise have to be made with much greater precision. The precision of alignment and stigmation will have to be improved by factors proportional roughly to Zi-1, as will be the requirements for stability of the instrument with respect to both mechanical vibrations and stray fields. Finally, one must take into consideration that the radiation damage to the specimen will increase, roughly as A-2, or worse, because to see or record images at better resolution it is necessary to use higher magnifications and preferably, the recording should be done faster, at TV or higher rates. It may be possible to devise detection systems of greater efficiency and speed. The ultimate is a system in which each individual electron is recorded, but that does not take us far beyond present-day technology. We are rapidly aproaching the fundamental limitation on collecting informarion by use of electron scattering. The impression is that by spending much greater amounts of money on much more complicated instruments we may be able to see more detail but on smaller and less typical specimens of fewer types of materials. No doubt this will be done. But most scientists will be happy to trade off any improvement in resolution, beyond that required to "see atoms", for the immediate benefits of being able to use their microscopes to solve practical problems of solid state science. According to the nature of the problem, this may be done by coupling the HREM imaging with EELS or XEDS analysis or microdiffraction a n d / o r making provi-
466
J.M. Cowley/ Future of high-resolution electron microscopy
sion for controlled specimen environments and in situ specimen treatment.
4. High-resolution analytical microscopy For each new detector fitted to an electron microscope, a new subdiscipline is born. The spectroscopies which may be added measure excited state properties of crystals, which are often difficult to relate to the ground state observed in an HREM image but each provides information of importance for some aspect of the study of the chemical or physical properties. Given an image showing, to some approximation, the distributions of atoms, the questions of most immediate importance for many areas of solid state science are: what atoms are present and in what quantifies? The analytical methods of EELS and XEDS have been developed until signals identifying the nature of the atoms can be obtained from clumps of only a few, or a few tens of atoms, but for quantitative analysis much greater sample volumes are needed. The limitations are fundamental, depending on the small cross-sections for the inelastic scattering processes. Some improvement should become possible with improved detection devices: parallel detection systems for EELS and large-angle detectors, surrounding the specimen, for XEDS. By using larger and larger volumes of material, one can get increasingly accurate data from the weaker or lower contrast signals: information on valence states from near-edge fine structure (ELNES) and on the local environments of atoms from EXELFS. For semiconductors and the like, there are the means to study band structures and even localized energy states in band gaps. For defects in semiconductors the electronic states can be explored with cathodoluminescence, which provides high spatial resolution, millivolt energy resolution and the sensitivity needed to see the low concentrations of impurities otherwise invisible in HREM, ELS and XEDS, The limitation on the information from the analytical technique is commonly the radiation damage done to the specimen: the breaking of bonds, the knock-on damage, the beam-induced
diffusion and beam-induced desorption. The concept is growing, but not yet well developed, that the combination of signals available may provide an excellent means for the study of these radiation-induced processes at an atomic level.
5. Environmental control It becomes increasingly obvious when one observes very .thin films and small particles in a HREM instrument that the usual layer of contamination or the amorphous debris left by the specimen preparation technique may be more than just a nuisance, superimposing a random phasecontrast background on the image. It may well be an active contributor to the state of the specimen, interacting in an ill-defined and poorly predictable way with the specimen material under the intense irradiation by the incident beam. Even if chemically inert, it will form a barrier layer, obscuring the imaging or diffraction from surface structural features and preventing the study of meaningful surface reactions. Hence there is currently an increasing emphasis on the need for adequate control of the specimen surfaces during both preparation and observation. This necessarily involves creating ultra-high-vacuum conditions in the specimen chamber, and preferably in the whole microscope, and this inevitably requires major, expensive redesign and reconstruction of major portions of the instrument. If this can be done, however, retaining the HREM capabilities, major new fields of application of electron microscopy will be opened for exploration. High-resolution studies of surface structure with high-contrast imaging of atom-high steps has been shown to be possible by both transmission and reflection. With proper control of surface preparation and cleanliness, and controlled specimen temperature and environment, all the interferences from wide-area surface science on the processes of surface reconstructions, surface reactions and crystal growth can be explored directly in terms of atom positions and atom movements. Reactions of thin films and small particles can be followed under known and controlled chemical and physical conditions, with TV-rate recording where necessary.
J.M. Cowley/ Future of high-resolution electron microscopy
It is desirable to establish the cleanliness of surfaces for such experiments and to identify the chemical composition of the surface layers by use of the well established techniques of LEED, AES, XPS and so on, in addition to the high-energyelectron methods of EELS, XEDS and microdiffraction, and this should be done in situ or, at least, in the same ultra-high-vacuum system. Only in this way can the observations be related to the wealth of results in the surface science literature.
467
The initial moves in this direction have provided encouragement for a major development. The future will show how far it may be possible to go towards these ends.
Acknowledgments The author is indebted to Ray Carpenter, John Spence and Graeme Wood for helpful comments.
463
THE FUTURE OF HIGH-RESOLUTION ELECTRON MICROSCOPY J.M. COWLEY Department of Physics, Arizona State University, Tempe, Arizona 85287, USA Received 10 July 1985
In summary of the final discussion session of the conference on high-resolution electron microscopy, the prospects for the use of high-resolution imaging and related techniques are reported to be exciting for the analysis of crystal structures, crystal defects and crystal surfaces, but considerable difficulties in instrumentation may prevent rapid progress in some directions. In other directions the limitations imposed by the nature of the interactions of electrons with matter have already been approached.
1. Introduction The final session of the ASU Centennial Conference on High-Resolution Electron Microscopy was organized as an open discussion on the topic of this report. As is usual in these cases, it was not left to the whim of the audience to determine the topics to be discussed. A panel of experts were selected to cover the main areas of possible subject matter and they were asked, at short notice, to present short summaries to introduce the topics. It was, of course, an exercise in optimism to expect any expert to cover the future of his field in a few minutes or to expect adequate discussion of the topics raised by the assembly of noted authorities in the audience. We ran out of time very rapidly, with many loose ends still dangling. The chosen experts and their given topics were Bruce Hyde: Large unit cell materials. John Spence: Small unit cell materials. John Cowley: Making use of higher resolution. Christian Colliex: Using analytical EM signals. Archie Howie: Surfaces and small particles. Owen Saxton: Image simulation and image processing. This is not a report of what these people said or of the, often hvely, interchanges with the audience. No verbatim records were kept of the proceedings and no attempt will be made to reproduce a
complete or chronologically correct summary of the session. This report is merely a biased, personal recollection of part of the subject matter. It is based in part on notes left by the panel of experts; but while the author is heavily indebted to the experts for their contributions, they can in no way be held responsible for his interpretations of their statements or opinions. References to the hterature will not be included in this review. The relevant papers are all in these proceedings. It is evident to all that high-resolution electron microscopy (HREM) is in a hvely state of development, with new and exciting possibilities opening up for important progress in many areas. The flood of orders being received by the manufacturers for the new, medium-energy, ultra-high-resolution microscopes is remarkable, particularly in view of their high costs in a period of some financial stringency. Clearly people are expecting a great deal from the instruments and believe that the next stage of HREM will be worth the money.
2. S t m c t ~ e analysis of crystals For the large-unit-cell materials of solid state chemistry and mineralogy the first impact of HREM came with the realization that the cation distributions could be seen clearly, or reasonably
0304-3991/85/$03.30 © Elsevier Science Pubhshers B.V. (North-Holland Physics Pubhshing Division)
464
J.M. Cowley/ Future of high-resolution electron microscopy
inferred, for an important range of materials, mostly oxides, with the microscope resolutions of 3-4 ,~, then available. With improving resolution the cation sites are more sharply defined. The structural data for perfect or imperfect regions can be derived with more precision. The quantitative measurement of intensities and comparison with calculations can be the basis for significant advances. After all, if the "resolution" of an X-ray diffraction experiment were defined in the same way as for an electron microscopy experiment the resolution limit for Cu K a radiation would be 0.77 A, but by using accurate intensity measurements and a suitable set of assumptions regarding the sample and the measuring technique (periodicity, kinematical scattering) the atom positions are found with a precision approaching 10-3 A. A great virtue of the electron microscope technique is usually considered to be that the assumptions of periodicity are unnecessary and the deviations from periodicity, which are often more important than the periodic average structure, can be seen with the same resolution. However, the broader principle remains that, given sufficient accuracy of intensity measurement and sufficient knowledge of parameters defining the instrumental and specimen conditions, any desired trade-off can be made between pre-knowledge of specimen conditions (extent of the periodicity, the form of the defects) and the precision of structure determination. The measurement of intensities will increasingly replace the taking of pictures. As is already the case for X-ray diffraction, the manipulation of masses of data with a large computer is rapidly becoming a major part of any structural investigation. The requirements for accurate values for the instrumental parameters defining the defocus, aberrations and alignment of the microscope are already evident and have prompted the moves towards digitized feedback systems to automatically adjust the electron optics of the microscope. Progress in this direction is inevitable because at the moment very few operators have the experience, insight and understanding of the principles involved to make all the adjustments needed to obtain the best possible high-resolution performance from any of the new instruments. Either the
display systems for the microscopes must be drastically changed, with image intensifier systems, automated digital read-out of astigmatism, misalignment, defocus etc. to guide the operator, or else the whole operation must be automated to eliminate most of the human intervention. The latter, pushbutton-black box, approach is attractive in principle but difficult in practice because for electron microscopy, more than for other techniques, continuous operator intervention may be required to modify or redirect experiments in progress. Also it may be dangerous in that the illusion is readily created that less intelligence is required to understand the limitations of the black box than to perform the functions personally. All this implies an ever-increasingrole for the computer in electron microscopy. The straightforward calculation of images for comparison with pictures presents no fundamental problems even with improved resolution, but for the extension to in-line acquisition and analysis of images and CBED patterns and for interactive systems for complete structure analysis procedures, the development of computer capabilities does not seem to have been rapid enough. The fast hard-wired frame stores and array processors are a boon but they lack the flexibility and accessibility of the host computer. Some way to combine the virtues of both would be highly desirable. The requirements for accurate values of the specimen parameters of thickness and orientation pose an important challenge. At the moment the tendency is to rely on the experience and intuition of the operator or else to leave these as free parameters in the matching with computed image intensities. The obvious answer appears to be the combination of CBED with HREM imaging from the same specimen area and the matching of both with computer calculations, but there is a need for improved engineering to eliminate the uncertainties introduced by stray fields which can affect the relative alignments when the switch is made from one instrumental mode to the other (except in the case of STEM instruments). The second generation of structural problems can now be approached more readily. The materials involved are the oxides and sulfides with more close-packed action arrays and also alloys and
J.M. Cowley / Future of high - resolution electron microscopy
semiconductors. Previously ohly the few high-resolution, high-voltage microscopes have approached the resolution needed to show unambiguously the atom separation of 1.5 to 2.5 A in even the most favorable projections of these materials. For the small unit cell materials, the metals and semiconductors, the crystal structure itself is rarely in question. The structures of the defects present overwhelming problems in all but a few very favorable special cases where linear or planar defects are aligned with the incident beam. Even there uncertainties arise because of radiation damage effects and unknown defect terminations at the top and bottom surfaces. It is understandable that emphasis should swing towards the more obvious and approachable defects, the free surfaces of crystals almost parallel to the beam. The initial results of such studies are important in themselves but also provide valuable guides to the solution of the more general problems.
3. The requirements for better resolution. Undoubtedly further improvements in the resolution of the instruments will be made in future years, whether by decreasing, or correcting for, the aberrations of the objective lens or by increasing the accelerating voltage, or both. It may be as well to note, at this stage, that an number of other factors are involved if full use is to be made of the nominal resolution improvement. Decrease of A, the least resolvable distance (as defined for Scherzer conditions), will give better contrast for isolated, unsupported, atoms but it will not, in general, make it easier to interpret images of thin crystals. The projection approximation, which says that the image represents a generally non-linear representation of the projection of the object structure, will be worse for a given thickness for decreased A at the same voltage but may improve slightly if the voltage, V, is increased. The prospect for validity of the weak phase object approximation (WPOA) which implies linear imaging theory (and use of simple enveloPe functions) is, if anything, worse, because the approximation becomes worse as A decreases and the improvements with V becomes less as V is in-
465
creased. The projected charge density approximation, which gives a linear relationship of the image intensity to the projected charge density, will always improve as A is decreased, but that applies only for a resolution of about 2A or more. For thicker specimens, except perhaps for crystals in axial orientations, problems will soon arise from the limited depth of focus, which is proportional to A2. Thus for 1 A resolution at 200 keV, the depth of focus will be less than 20 A. This implies that the high voltage and lens current stabilities must be greatly improved (by a factor of 10 or more for this case). Also the fine focus steps on the objective lens current must be much smaller than at present. Other instrumental adjustments will likewise have to be made with much greater precision. The precision of alignment and stigmation will have to be improved by factors proportional roughly to Zi-1, as will be the requirements for stability of the instrument with respect to both mechanical vibrations and stray fields. Finally, one must take into consideration that the radiation damage to the specimen will increase, roughly as A-2, or worse, because to see or record images at better resolution it is necessary to use higher magnifications and preferably, the recording should be done faster, at TV or higher rates. It may be possible to devise detection systems of greater efficiency and speed. The ultimate is a system in which each individual electron is recorded, but that does not take us far beyond present-day technology. We are rapidly aproaching the fundamental limitation on collecting informarion by use of electron scattering. The impression is that by spending much greater amounts of money on much more complicated instruments we may be able to see more detail but on smaller and less typical specimens of fewer types of materials. No doubt this will be done. But most scientists will be happy to trade off any improvement in resolution, beyond that required to "see atoms", for the immediate benefits of being able to use their microscopes to solve practical problems of solid state science. According to the nature of the problem, this may be done by coupling the HREM imaging with EELS or XEDS analysis or microdiffraction a n d / o r making provi-
466
J.M. Cowley/ Future of high-resolution electron microscopy
sion for controlled specimen environments and in situ specimen treatment.
4. High-resolution analytical microscopy For each new detector fitted to an electron microscope, a new subdiscipline is born. The spectroscopies which may be added measure excited state properties of crystals, which are often difficult to relate to the ground state observed in an HREM image but each provides information of importance for some aspect of the study of the chemical or physical properties. Given an image showing, to some approximation, the distributions of atoms, the questions of most immediate importance for many areas of solid state science are: what atoms are present and in what quantifies? The analytical methods of EELS and XEDS have been developed until signals identifying the nature of the atoms can be obtained from clumps of only a few, or a few tens of atoms, but for quantitative analysis much greater sample volumes are needed. The limitations are fundamental, depending on the small cross-sections for the inelastic scattering processes. Some improvement should become possible with improved detection devices: parallel detection systems for EELS and large-angle detectors, surrounding the specimen, for XEDS. By using larger and larger volumes of material, one can get increasingly accurate data from the weaker or lower contrast signals: information on valence states from near-edge fine structure (ELNES) and on the local environments of atoms from EXELFS. For semiconductors and the like, there are the means to study band structures and even localized energy states in band gaps. For defects in semiconductors the electronic states can be explored with cathodoluminescence, which provides high spatial resolution, millivolt energy resolution and the sensitivity needed to see the low concentrations of impurities otherwise invisible in HREM, ELS and XEDS, The limitation on the information from the analytical technique is commonly the radiation damage done to the specimen: the breaking of bonds, the knock-on damage, the beam-induced
diffusion and beam-induced desorption. The concept is growing, but not yet well developed, that the combination of signals available may provide an excellent means for the study of these radiation-induced processes at an atomic level.
5. Environmental control It becomes increasingly obvious when one observes very .thin films and small particles in a HREM instrument that the usual layer of contamination or the amorphous debris left by the specimen preparation technique may be more than just a nuisance, superimposing a random phasecontrast background on the image. It may well be an active contributor to the state of the specimen, interacting in an ill-defined and poorly predictable way with the specimen material under the intense irradiation by the incident beam. Even if chemically inert, it will form a barrier layer, obscuring the imaging or diffraction from surface structural features and preventing the study of meaningful surface reactions. Hence there is currently an increasing emphasis on the need for adequate control of the specimen surfaces during both preparation and observation. This necessarily involves creating ultra-high-vacuum conditions in the specimen chamber, and preferably in the whole microscope, and this inevitably requires major, expensive redesign and reconstruction of major portions of the instrument. If this can be done, however, retaining the HREM capabilities, major new fields of application of electron microscopy will be opened for exploration. High-resolution studies of surface structure with high-contrast imaging of atom-high steps has been shown to be possible by both transmission and reflection. With proper control of surface preparation and cleanliness, and controlled specimen temperature and environment, all the interferences from wide-area surface science on the processes of surface reconstructions, surface reactions and crystal growth can be explored directly in terms of atom positions and atom movements. Reactions of thin films and small particles can be followed under known and controlled chemical and physical conditions, with TV-rate recording where necessary.
J.M. Cowley/ Future of high-resolution electron microscopy
It is desirable to establish the cleanliness of surfaces for such experiments and to identify the chemical composition of the surface layers by use of the well established techniques of LEED, AES, XPS and so on, in addition to the high-energyelectron methods of EELS, XEDS and microdiffraction, and this should be done in situ or, at least, in the same ultra-high-vacuum system. Only in this way can the observations be related to the wealth of results in the surface science literature.
467
The initial moves in this direction have provided encouragement for a major development. The future will show how far it may be possible to go towards these ends.
Acknowledgments The author is indebted to Ray Carpenter, John Spence and Graeme Wood for helpful comments.