Electron microscope is a device that uses a beam of electrons to magnify a specimen. Electron microscopes can resolve (give clear pictures of) features much smaller than those visible through optical microscopes. Optical microscopes use visible light and lenses to magnify images. Electron microscopes can resolve much smaller images than can optical microscopes. Some electron microscopes can even resolve individual atoms. Electron microscopes serve as a major research tool in biology, chemistry, medicine, metallurgy, physics, and nanoscience, the study of extremely small things. Ernst Ruska and other German scientists built the first electron microscope in 1931.
In both electron and optical microscopes, the wavelength (distance between wave crests) of the beams that magnify the image limits the resolving power. An electron microscope can resolve smaller features because the wavelength of the electrons it uses is much shorter than that of visible light. The shortest wavelength of visible light is about 400 nanometers. One nanometer equals about 1/1,000,000 millimeter (1/25,400,000 inch). The electron beams used in most electron microscopes have wavelengths much less than 1 nanometer. There are three kinds of electron microscopes: (1) transmission, (2) scanning, and (3) scanning transmission.
Transmission electron microscopes
pass a beam of electrons through a specimen 100 or fewer nanometers thick. The specimen scatters or absorbs part of the beam. Electromagnets called magnetic lenses focus electrons that pass through the specimen to a fluorescent screen, photographic plate, or digital camera. Transmission electron microscopes can magnify a specimen up to several million times.
Scanning electron microscopes
focus the electron beam so that it strikes a small spot on the specimen. The beam then scans the specimen in a regular pattern. As electrons strike the specimen’s surface, the surface throws off other electrons, called secondary electrons. A collector counts the secondary electrons point by point as the beam scans the specimen. A computer screen displays the result—a sharp image of surface features. Scanning electron microscopes are usually used to look at surfaces of thick specimens. Scanning electron microscopes can magnify a specimen’s surface by up to several hundred thousand times.
Scanning transmission electron microscopes
pass a beam through a specimen in much the same way that transmission electron microscopes do. Instead of remaining fixed, however, the beam scans the specimen to produce an image. Albert Crewe and other American scientists used one to produce the first images of atoms in 1970.
Cryo-electron microscopy
(cryo-EM) involves using a transmission electron microscope to study a specimen at extremely low temperatures. Biological materials are difficult to study with an electron microscope, because radiation from the intense electron beam destroys biomolecules (molecules from living things). In addition, all electron microscopes are high-vacuum devices. Biomolecules collapse in a vacuum (space that contains little matter). The low temperatures used in cryo-EM are useful for protecting such specimens from the vacuum and from the harmful radiation created by an electron microscope. Cryo-EM allows researchers to portray biological materials in a natural state, at a high resolution.
See also Microscope; Scanning probe microscope.
