NANOTECHNOLOGY—TECHNOLOGY ON A MOLECULAR SCALE - Chemistry: Notable Research and Discoveries - page 51

Electrons tunnel between the gap formed by the STM probe tip and
the surface of the material being scanned. The amount of current
formed by the tunneling electrons depends on the size of the gap,
and as the probe glides above the surface, it creates an atomic
scale map.
probe over the surface to be scanned. As illustrated in
the figure above, the tip of the probe tapers to a single
atom. (These tips are easily made by etching or tearing a
filament of metal.) The machine holds the probe tip above
the surface at a height of about one atomic diameter. A
small voltage, usually about a volt, is applied to the tip, and
a current consisting of electrons begins to flow. A large volt-
age would produce a current by ripping electrons from the
surface, but such a violent event would also ruin the mate-
rial; a single volt cannot normally pull electrons across the
barrier, but it encourages some electrons to “tunnel” to the
other side. This current is extremely dependent on distance.
The magnitude of current can be used to indicate distance,
or the probe tip can be moved up or down to maintain a
steady current, in which case the distance to the surface
is measured by the probe movement. In either case, the
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