Cambridge Encyclopedia » Cambridge Encyclopedia Vol. 23

electron microscope - History, Types, Sample Preparation, Disadvantages

image specimen beam resolution

A microscope using a beam of electrons instead of light, and magnetic or electrostatic fields as lenses. If considered as a wave system, the electron beam has a much higher frequency than visible light, and so provides a much higher resolution. In the transmission electron microscope, the direct passage of the beam through the specimen produces an image on a fluorescent screen. The specimen must be very thin, but the resolution is high: c.0·2–0·5 nm. In the scanning electron microscope, the specimen is scanned by the beam, which produces secondary electron emission. The consequent current produced can be amplified and the signal fed to a cathode ray screen to give the image. The specimen can be thicker, and an image of some depth produced, but resolution is limited to c.10–20 nm. The scanning tunnelling microscope, invented by Gerd Binnig and Heinrich Rohrer in 1982, has a resolution of a few Ångströms and can image down to atomic scales. In this technique, electrons migrate between the sample surface and the microscope tip via the process of quantum tunnelling. Tunnelling current is very sensitive to tip–surface distance; an image is constructed using the control signal passed to the tip positioning system which maintains a constant tip–sample distance. Binnig and Rohrer were awarded the 1986 Nobel Prize for Physics.

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