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Yashoda's Vision: Microscopy at All Levels
The poet William Blake wanted "to see a world in a grain of sand and heaven in a flower." Yashoda, the foster mother of Lord Krishna, saw in Krishna's wide-open mouth all the galaxies and the universe. These poetic and mystic visions are turning into reality. Ever since the Dutch janitor Anton van Leewenhoek turned a lens on a thimbleful of water and saw the "Little Animals," microscopy has been on the move. The desire to see ever-decreasing sizes was fueled by Henry Clifton Sorby, the Sheffield petrographer, when he used the optical microscope to observe the lamellar structure of pearlite. Since then, the optical microscope has been perfected, and a host of microscopes based on electrons and ions have been developed. Specifically, the goal of atomic resolution has been achieved in three distinct ways. Erwin Müller invented the field-ion microscope in Berlin in the early 1950s. The microscope worked on the point-projection principle and provided in a simple and elegant way magnifications in excess of one million. Individual atoms came to be seen. What was more impressive was that these atoms could be field-evaporated one at a time. When this could be done at liquid-helium temperature to tungsten, the metal with the highest melting point, it was an awesome demonstration. Curiously, even though atomic resolution was achieved, it was soon shown that dislocations can be invisible under some circumstances, and even one component of the atoms in intermetallics could be invisible! A further development took place, whereby the evaporated ion could be chemically identified. The development in electronics and computation ensured that the reconstruction of the structure in three-dimensional atomic detail was possible. The 3D, or tomographic, atom probe has given a virtuoso performance in unraveling many nanoscale phenomena in exquisite detail. When it was realized that electrons have a dual nature, both particle and wave, the optical microscope was imitated by replacing photons with electrons. As the electrons had a much smaller wavelength, the resolution imposed by the Abbe limit was pushed down to lower values. Improvement in instrumentation over the decades has led to microscopes with 1-Å resolution. The images correspond to the projection of atomic columns. It is increasingly realized that the interpretation of the high-resolution images are considerably more complex than was realized in the beginning. This once again proves the adage that there is no problem, however complex, that cannot be made, after a great deal of study, even more complex. By combining scanning with transmission microscopy, A.V. Crewe was able to achieve atomic resolution. This microscope has no lenses, yet by a legerdemain based on the reciprocity principle, it achieves the same contrast as a conventional transmission electron microscope. In a most significant achievement, J.H. Spence and co-workers have been able to image electronic orbitals in cuprite with the startling revelation that copper atoms can bond to each other in a covalent fashion. R.D. Young, a student of Erwin Müller, conceived the idea of a field-emission topographic instrument. Its fruition as the scanning tunneling microscope, by Binning and Rohrer, has brought out a powerful tool for not only looking at surfaces in atomic detail, but also as a tool for nanolithography. Its offshoots, such as the atomic force microscope, make it possible to study mechanics at the atomic scale. Our views of friction, wear, lubrication, and corrosion are being transformed as a result. If the past is prologue to the future, we may anticipate further developments. Powerful microscopes are expensive affairs and are threatening a "microscopic divide" among nations. However, experiments are already under way so that the Internet may be used for remote use of microscopes. Thus, a scientist sitting in Bangalore may be able to operate the atomic resolution microscope (ARM) at the Lawrence Berkeley National Laboratory, bringing together two Silicon Valleys-a world apart-through the magic of silicon. One can also see the increasing use of mathematics in microstructures. Stereology is an amazing subdiscipline. The elegant but surprising conclusion that volume, area, line, and point fraction are all equal to each other is but the beginning. Three-dimensional reconstruction from optical images has already changed our view of the topology of microstructure in steels. Its further use is bound to change our concepts of the microstructures of deformed and transformed materials. Many new materials invoke new geometries. Thus, quasicrystals can be understood as periodic structures in higher dimensions. The structure of fullerenes, Schwarzite, and nanotubes can be understood by appealing to curved space manifestations of flat two-dimensional graphite. There are extraordinary connections between close-packing of spheres, stacking of tetrahedra, and protein folding that are just beginning to be explored. One sees further use of geometry in micros-copy. It is also clear that in probing for atomic detail one may miss the forest for the trees. It is evident that many phenomena are governed by events in the mesoscale. Thus, just as much as multiscale modeling is required, multiscale micros-copy will be inevitable. The journey to see the very small has been long and arduous, but has also been exciting and rewarding. The coming decades will see microscopes functioning as laboratories and workshops so that by this convergence we will design new materials architectures, observe them as they are synthesized, and measure their properties in a unified fashion. S. Ranganathan is a professor at the Indian Institute of Science, Bangalore; Honorary Visiting Professor at Banaras Hindu University; and a visiting professor in the Institute of Materials Research, at Tohoku University in Sendai, Japan. He is an internationally recognized authority in field-ion and electron microscopy, structure of interfaces, and quasicrystals. He is a Fellow of four Science and Engineering Academies of India, and has been elected as an Honorary Member by the Indian Institute of Metals and was named Materials Scientist of the Year by the Materials Research Society of India. Ranganathan received his PhD degree from Cambridge University. Materials Challenges
For The Next Century presents a series
of articles speculating on the role of materials in society in
the coming century and beyond.
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