Electron Microscopes

The first electron microscope was built only a few years after the discovery that high-speed electrons had wavelength many times smaller than the wavelength of light. Because of the smaller wavelengths, electron microscope are capable of much higher resolution than optical microscopes. The Beam of electrons can be focused by suitably shaped electric of magnetic fields. For example, the magnetic field of a solenoid acts like a converging lend for electrons. In transmission electron microscope (TEM), an energetic beam of electrons focused by a magnetic condenser lens illuminates the specimen being examined and passes trough to a magnetic objective lens, which forms an intermediate image. A magnetic projector lens then magnifies a portion of the intermediate image to form the final image on a fluorescent screen, a photographic plate, or a semiconductor detector connected to a computer, just as in the case of an optical microscope, the overall magnification is the product of the magnifications of the objective and the projector lenses. With the proper choice of magnetic kens currents, the overall magnification can be as high as 200,000X. The critical limitation is the resolution. Modern instruments are capable of resolving objects smaller than 0,5 nm.

Scanning electron microscopes (SEM), which came into wide usage in the 1970s and 1980s, operate on a different principle from the TEM. In the SEM, a finely focused electron beam screen across the surface of the specimen being examined. As the beam scans across the specimen, the incident electron (primaries) knock out other electrons (secondaries) that come from the area on the specimen were the primary beam is focused. The secondary electrons are collected at a positive electrode. The intensity of the secondary current changes as the primary beam sweeps across the specimen, since more secondaries are generated when the beam strikes a sharply curved edge or sloping surface than when it strikes a flat surface. The intensity information is used to generate a televisionlike image on a cathode ray thbe, which gives an impression of three-dimensional surface relief. The size of the beam (typically less than 10 nm ) limits the resolution of the instrument. In both types of electron microscopes, the extremely small de Broglie wavelengths of high-speed electrons permit imaging with high resolution.