The Scanning Tunneling
The scanning tunneling
microscope (STM) is a type of electron microscope
that shows three-dimensional images of a sample. In
the STM, the structure of a surface is studied using
a stylus that scans the surface at a fixed distance
An extremely fine
conducting probe is held close to the sample.
Electrons tunnel between the surface and the stylus,
producing an electrical signal. The stylus is
extremely sharp, the tip being formed by one single
atom. It slowly scans across the surface at a
distance of only an atom's diameter. The stylus is
raised and lowered in order to keep the signal
constant and maintain the distance. This enables it
to follow even the smallest details of the surface it
is scanning. Recording the vertical movement of the
stylus makes it possible to study the structure of
the surface atom by atom. A profile of the surface is
created, and from that a computer-generated contour
map of the surface is produced.
The study of
surfaces is an important part of physics, with
particular applications in semiconductor physics and
microelectronics. In chemistry, surface reactions
also play an important part, for example in
catalysis. The STM works best with conducting
materials, but it is also possible to fix organic
molecules on a surface and study their structures.
For example, this technique has been used in the
study of DNA molecules.