An STM consists primarily of a needle and a computer. A needle tip is scanned across the surface of the sample under investigation. If a small electric potential difference is maintained between tip and sample, then a current will flow between them once the tip gets very close, even though electricity will not normally flow through air or vacuum. The current only flows when the electron clouds surrounding the atoms of tip and sample overlap, and this occurs only to an appreciable extent when the tip-sample distance is of atomic dimensions. This effect is called the tunneling effect.
There are several other scanning microscopes that resemble the STM. The most widespread is the Atomic Force Microscope, where the tip is placed on a small cantilever. When the distance between tip and sample is within atomic dimensions, forces between tip and sample cause the cantilever to bend. A laser ray reflected from the cantilever can serve to detect the cantilever's movement and thus gain information about the forces at play.
Many similar instruments have been developed. If one places a ferromagnetic substance on the tip, it can be used to probe the sample's magnetic properties. Or by measuring the force as a function of the tip-sample distance, one can quantify the sample's hardness. Such microscopes are commonly grouped under the heading of Scanning Probe Microscopes.
Reproduced from Scientific American, August 1985, p. 53.
By permission of Ian Worpole.