Download Advanced Instrumental Analysis

Advanced Instrumental Analysis
December 3, 2007
Cassidy McCurdy
Scanning Tunneling Microscopy
Scanning Tunneling Microscopy (STM) is a way to visualize surfaces on a
nanoscale level. By passing a current through a very sharp needle point onto the sample
surface the exact topography of each atom can be determined. STM is based on the
principles of quantum mechanical tunneling, where electrons move from one area to
another area through an area that it logically should not be able to move through. The
electron can only move between two states that are equal, which means that the electron
must enter an electron orbital that is at the same energy level as the electron orbital it left.
This specificity allows the STM to create a more accurate picture of what the surface of
the sample is like.
The setup of a STM is actually very simple, a thin, very sharp probe hovers over
the sample, and a current is passed through the needle. The probe will either hold a
constant current and the height of the probe will change to keep the current constant, or
the current will change to keep the height of the probe constant. This is done in a near
vacuum chamber, but there are many STM setups that can perform well without a
vacuum. The changes in current or height are sent to the data acquisition device, and that
data is processed into a visual image. Most of the images obtained from STM setups are
in black and white, where the shade of gray correlates to the value being read. Images
obtained by STM that are colorized are always colorized after the fact, to make the image
more readable or realistic.
The Scanning Tunneling Microscope was invented in 1981, and its inventors,
Gerd Binnig and Heinrich Rohrer, won the Nobel Peace prize because of it in 1986.
They put together the quantum mechanical equations that allow STM to work, which are
the most complicated part of the process. The total equation involves terms for the probe,
the sample, and the barrier between them that the electrons must travel through. It was
John Bardeen, a man who won the Nobel Peace prize twice, who discovered that if the
quantum terms for the probe and the sample could be discovered the term for the barrier
could be solved for, and without the term for the barrier it would be impossible to
interpret the data from a STM setup.
Since the invention of the scanning tunneling microscope, there have been many
adaptations of the machine that allow it to find out much more about a sample. Most of
the adaptations are fairly minor. If the probe is grounded and a voltage drop is induced
across the sample the STM become a ST Potentiometer, which gives all the information
of a potentiometer but at an atomic level. By replacing the probe with a probe covered
with a thing magnetic substance you turn a STM into a Spin Polarized STM. The
magnetic coating allows the electrons being transferred to be spin oriented, and since they
must tunnel to a equivalent state it is possible to find the magnetic moment of the sample
on a nanoscale range. One of the more extensive adaptations occurs in Photon Scanning
Tunneling Microscopy. The probe in this case is a fiber optic cable that has a sharpened
end. Light is shined through a prism from two directed due to a beam splitter. The prism
acts as the sample holder, and the light that exits the prism passes through the sample and
hits the fiber optic cable. The sample must be extremely thin so the light can pass
through, but the data obtained comes from an evanescent field created by the light
interacting with itself as it leaves the prism. The end result is a picture of the sample that
is very similar to the result of a STM. The difference being that a STM can only observe
metallic or semi-conducting materials, while the photon STM can observe any sample
that can be made thin enough to be translucent.
STM is a powerful method for getting visual representations of sample beyond the
limit that optical lenses have. If a STM setup has the adaptability to change the probe or
to perform potentiometry then a single setup can figure out a lot about a single sample, all
of which can be visually represented.
References
P S Carney, R A Frazin, S I Bozhevolnyi, V S Volkov, A Boltasseva, and J C Schotland,
"A computational lens for the near-field," Accepted by PRL (2004).
“principals of scanning tunneling microscopy”.
http://dpmc.unige.ch/gr_fischer/localprobe.html#potentio
http://www.wikipedia.com/scanning_tunneling_microscopy