To explain the way STMs work in more detail, a voltage is applied between the sharp probe tip and the surface
of the object while the tip is brought to within a nanometers or so. At this distance, the tunnel effect causes
a weak current (the tunnel current) to flow between the object surface and the probe tip, even though they are
not in contact.
What is important here is that the strength of the tunnel current changes by one order of magnitude with changes
of only a fraction of a nanometer in distance between the tip and the object surface. This means that a tunnel
current may arise between the atom at the tip of the probe and the nearest atom to it on the object surface,
but not the atom next to it. As such, the probes of STMs can be quite thick in nano-scale terms, just as long
as they have tips of only one atom.
STMs are in real life controlled by a feedback loop to ensure that the tunnel current remains constant. This
is done by adjusting the height of the probe while moving the probe horizontally in tiny increments over the
surface. The resolution of STMs can be an amazing 0.005 nm, enabling observation of individual atoms and the
way they are arranged on the surface of the object.
