| Overview | Atomic Force Microscopy | Scanning Tunneling Microscopy | NSOM
Scanning Tunneling Microscopy
The development of the family of
scanning probe microscopes starts with the original invention
of the STM in 1981. Gerd Binnig and Heinrich Rohrer developed
the first working STM while working at IBM Zurich Research
Laboratories in Switzerland. This instrument would later
win Binnig and Rohrer the Nobel prize in physics in 1986.
The STM works by scanning
a very sharp metal wire tip over a surface. By bringing
the tip very close to the surface, and by applying an electrical
voltage to the tip or sample, we can image the surface at
an extremely small scale – down to resolving individual
The STM is based on several
principles. One is the quantum mechanical effect of tunneling.
It is this effect that allows us to “see” the
surface. Another principle is the piezoelectric effect.
It is this effect that allows us to precisely scan the tip
with angstrom-level control. Lastly, a feedback loop is
required, which monitors the tunneling current and coordinates
the current and the positioning of the tip.
Tunneling is a quantum mechanical
effect. A tunneling current occurs when electrons move through
a barrier that they classically shouldn't be able to move
though. In classical terms, if you don't have enough energy
to move "over" a barrier, you won't. However,
in the quantum mechanical world, electrons have wavelike
properties. These waves don’t end abruptly at a wall
or barrier, but taper off quite quickly. If the barrier
is thin enough, the probability function may extend into
the next region, though the barrier! Because of the small
probability of an electron being on the other side of the
barrier, given enough electrons, some will indeed move through
and appear on the other side. When an electron moves though
the barrier in this fashion, it is called tunneling.
tells us that electrons have both wave and particle
like properties. Tunneling is an effect of the
The top image shows us that when an electron
(the wave) hits a barrier, the wave doesn't
abruptly end, but tapers off very quickly
- exponentially. For a thick barrier, the
wave doesn't get past.
The bottom image shows the senario if the
barrier is quite thin (about a nanometer).
Part of the wave does get through, and therefore
some electrons may appear on the other side
of the barrier..
Because of the sharp decay
of the probability function through the barrier, the number
of electrons that will actually do this is very dependent
upon the thickness of the barrier. The actual current through
the barrier drops off exponentially with the barrier thickness.
To extend this description
to the STM: The starting point of the electron is either
the tip or sample (depending on the setup of the instrument).
The barrier is the gap (air, vacuum, liquid), and the second
region is the “other side” – tip or sample,
again, depending on the experimental setup. By monitoring
the current through the gap, we have very good control of
the tip-sample distance.
The piezoelectric effect
was discovered by Pierre Curie in 1880. The effect is created
by squeezing the sides of certain crystals, such as quartz
or barium titanate. The result is the creation of opposite
charges on the sides. The effect can be reversed as well;
by applying a voltage across a piezoelectric crystal, it
will elongate or compress.
These materials are used
to scan the tip in an STM, and most other scanning probe
techniques. A typical piezoelectric material used in STMs
is PZT (Lead Zirconium Titanate).
Electronics and the Feedback Loop
Obviously, you need electronics
to measure the current, scan the tip, and translate this
information into a form that we can use. A feedback loop
constantly monitors the tunneling current and makes adjustments
to the tip to maintain a constant tunneling current. These
adjustments are recorded by the computer and presented as
an image in the STM software. Such an setup is called a
“constant current” image. In addition, for very
flat surfaces, the feedback loop can be turned off and only
the current is displayed. This is a “constant height”
is cabable of acquiring remarkable images on the most extreme
scale, easily resolving atomic structure in the right environments.
For some very interesting work with STM, see the IBM
The Quantum Corral
This STM image shows the direct of standing-wave
patterns in the local density of states of the Cu(111)
surface. These spatial oscillations are quantum-mechanical
interference patterns caused by scattering of the
two-dimensional electron gas off the Fe adatoms and
Courtesy of International Business
Machines Corporation. Unauthorized use not permitted