An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a higher resolving power than light microscopes and can reveal the structure of smaller objects.
Electron microscopes are used to investigate the ultra structure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis.
When the electron beam interacts with the specimen, it loses energy by a variety of mechanisms. The lost energy is converted into alternative forms such as heat, emission of low energy secondary electrons and high-energy backscattered electrons, light emission (cathodoluminescence) or X-ray emissions, all of which provide signals carrying information about the properties of the specimen surface, such as its topography and composition. The image displayed by an SEM maps the varying intensity of any of these signals into the image in a position corresponding to the position of the beam on the specimen when the signal was generated.
Electron microscopes are expensive to build and maintain, but the capital and running costs of confocal light microscope systems now overlaps those of basic electron microscopes. The samples largely have to be viewed in a vacuum, as the molecules that make up air would scatter the electrons.
Scanning electron microscopes operating in conventional high-vacuum mode usually image conductive specimens; therefore non-conductive materials require conductive coating (gold/palladium alloy, carbon, osmium, etc.). The low-voltage mode of microscopes makes the observation of non-conductive specimens without coating possible.
Non-conductive materials can also be imaged by a variable pressure (or environmental) scanning electron microscope. Small, stable specimens such as carbon nanotubes, diatom frustules and small mineral crystals (asbestos fibers, for example) require no special treatment before being examined in the electron microscope. Samples of hydrated materials, including almost all biological specimens have to be prepared in various ways to stabilize them, reduce their thickness ultrathin sectioning) and increase their electron optical contrast (staining).