There are three types of electron microscopes: transmission electron microscopes (TEM), scanning electron microscopes (SEM) and scanning-tunneling electron microscopes (STM).
TEMs transmit electron beams through a thin section or slice of a specimen to create an image. TEMs are particularly useful for studying the insides of cells.
With SEMs, the specimen is usually coated with an ultra-thin layer of gold atoms. The electron beam scans over the surface of the specimen, exciting electrons on the surface. When these surface electrons are emitted (as secondary electrons), they are collected by special devices that create an image out of them.
SEMs are especially useful for studying the surfaces and structures of cells. With their great depth of field, SEMs produce 3-D images.
STMs can display things as infinitesimal as the individual atoms on an object’s surface. They scan specimen surfaces in the same way as SEMs, but they use an electrically charged tip that is placed within nanometers of the surface of the specimen. Electrons “jump” between the tip and the specimen surface in what’s called the tunneling current, hence the name of this kind of microscope.
As the tip is moved back and forth across the specimen, the current varies according to whether the tip is right over an atom or over the space or trough between atoms. A computer creates an image based on these differences in current.