A variation of scanning probe microscopy that relies on the quantum mechanical phenomenon of electrons ‘tunnelling’ through barriers by virtue of their wavelike properties. It detects the very small current created by tunnelling electrons when a fine-tipped metal conductor is scanned over the surface of a sample. The amount of current flowing depends on the distance between the metal probe and the surface. If the apparatus is operated in constant-current mode, feedback from a current sensor is used to alter the position of the probe relative to the surface to maintain a constant current. The deflections of the probe (or the sample) then correspond to the surface topography. The data are processed by a computer and a grey-scale map of the surface is produced. Although most suitable for imaging substances that are electrical conductors, STM is now being used for studying organic systems and biological materials at the nanometre scale, by first applying the nonconductive material as a thin film to a conductive substrate, such as graphite.
Subjects: Biological Sciences.