Microscopy, Electron (Including TEM and SEM)

Changing the Rules

By the 1920s, it was increasingly apparent that electron beams acted somewhat like beams of light—they cast shadows, they could be reflected, and (in 1927) researchers at Bell Laboratories showed they could be diffracted. That same year, Hans Busch showed that a magnetic coil could focus an electron beam in the same way that a glass lens focuses light. This was the first indication that a new kind of microscope could be built with electrons as the imaging radiation. Since the resolution of a microscope is directly proportional to the wavelength of radiation shining on the sample, and electron wavelengths are more than three orders of magnitude smaller than the wavelength of visible light, such a microscope would have a much higher resolution than traditional light microscopy.

Such a microscope was developed in the early 1930s by Ernst Ruska and Max Knoll at the Technical University of Berlin. The Berlin team soon realized that two broad types of electron optical microscope were possible. In one, the beam would simply pass through a thin sample onto a detector—the transmission electron microscope or (TEM). In the other, a beam of electrons would move back and forth over a sample. This beam would either be reflected off the sample, or would excite the sample and cause it to emit its own “secondary” electrons. The reflected and secondary electrons would enter a detector, be amplified, and turned into a new electron beam that would be scanned back and forth in the same fashion as the original beam.

Unlike the TEM, this scanning electron microscope, or (SEM), would produce images that resembled (though were higher resolution than) ordinary light microscope images. However, the SEM's complex circuitry, and the fact that its resolution would be lower than the TEM's, meant that TEM was developed first. By the end of the 1930s, TEM and SEM had formed long-lasting alliances with different groups of users. TEM's need for very thin samples made it more attractive to metallurgists and others interested in material properties. Life scientists found SEM more attractive because it did not require thin samples and because the images it produced were more comparable to light microscopy. Though there are many, and important, exceptions, the general outlook of [Page 418]TEM toward materials science and SEM toward biology continues to inform use of these instruments in nano-science. Since TEM has higher resolution, the biologists who use it rather than SEM tend to be those looking at the very smallest constituents of life (e.g., viruses or cell organelles) where the line between chemistry and biology is fuzziest.

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