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Looking At The World Differently Electron microscopy is the standard method for visualizing individual nanoscale structures. A complete visualization of the nano world, however, will require characterization by multiple, complementary beams of radiation. |
When we wish to tell someone that we understand, we often say, "I see," a phrase confirming that the ability to visualize something is an essential part of understanding. In the emerging field of nanoscale science and technology, the visualization of nanoscale structures will be essential to understanding them. Among imaging methods where the specimen is irradiated by an incident radiation, the instruments that best provide this ability are electron microscopes. Whereas an optical microscope is limited to the micrometer scale, an electron microscope can resolve features as small as a fraction of a nanometer. In addition, unlike photons, electrons interact strongly with matter, producing signals that provide data about a sample's physical and chemical properties. Although powerful, electron microscopy gives an incomplete view of the nano world, just as looking at a flower with light of only one wavelength—for example, with red laser light—would give an imperfect idea of the flower's true colors. We can now look beyond electron microscopy imaging by simultaneously illuminating our specimen with multiple radiations. Chosen from the "rainbow" of electromagnetic and particle beams available, giving us in effect two or more "eyes," each of these radiations is capable of discerning different kinds of information from the same region of our sample. So-called "dual-beam" microscopes, which have both electron and ion sources, are commercially available and are becoming common tools for advanced materials analysis. When ions strike the sample surface, they sputter, or erode, the surface at roughly a nanometer per second. Thus, an ion beam can be used as a scalpel to cut into the specimen, revealing previously unknown internal details. The ions also generate secondary particle emissions, both electrons and ions, from a very shallow region of the surface. The secondary electrons carry information about the surface topography and crystal structure, while the secondary ions can be collected and identified to analyze the chemical composition of the top few atomic layers of the surface. By using the incoming ion beam also to erode the surface, scientists can study the variation in a sample's composition not only in the place of the surface but also as a function of depth. Such a dual-beam microscope will be available at ORNL's new Center for Nanophase Materials Sciences. Photon beams with wavelengths that span the electromagnetic spectrum provide a wide range of specimen excitations. Consider the additional information obtained when a fine beam of X rays is projected onto a specimen. Like electron beams, X-ray beams can stimulate emissions of X rays from the specimen, potentially enabling the determination of the specimen's elemental composition and chemical formula. However, in contrast with electron beams, X-ray beams generate negligible background signal. Consequently, "X-ray fluorescence" can detect the presence of a few parts per million of an element, albeit at more limited spatial resolution, while conventional electron beam fluorescence is typically limited to several percent concentration. Lower-energy photons—those from light in the visible or infrared spectrum—can also be employed as an additional beam for "Raman spectroscopy" studies of a variety of nanostructures, including nanotubes, polymers, and semiconducting thin films. In this technique the wavelength of the incoming light is changed by interaction with a sample. A measurement of the differences provides a unique fingerprint that can identify different polymer types, show whether carbon nanotubes are acting more like semiconductors than metallic conductors, or measure the local mechanical strain in thin-film layers. In summary, the secrets of the nano world will be fully illuminated by imaging and analysis with a variety of complementary beams of particles or radiation. We anticipate in the near future the development of poly-beam instruments that will take advantage of the additional information provided by X-ray photons, light beams, and possibly other, more exotic radiation sources.—David C. Joy, ORNL-University of Tennessee Distinguished Scientist, and Ian M. Anderson, Metals and Ceramics Division, ORNL
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