The Scanning Tunneling
- Preparation of Specimen
An STM specimen needs a substrate that is
extremely flat, down to the atomic level. If the specimen is
uneven, the STM probe will have difficulties in scanning very
steep pits or ridges. Other defects, such as single atoms sitting
on an otherwise flat substrate, will also be a problem since
the atoms themselves may function as unwanted STM probes, destroying
the desired image.
One of the most commonly used STM substrates is a special
form of graphite (highly oriented pyrolytic graphite or HOPG).
It is a naturally layered material that is easy to prepare and
relatively inert. A fresh surface can be obtained as easily as
pressing a piece of adhesive tape to the surface and peeling
away the top layer. The resulting surface will have large flat
areas useful for scanning.
Other popular materials that provide large, atomically flat surfaces include
mica, quartz, and silicon. These materials are insulators, so to be used for
STM a thin layer of noble metal (mainly gold or platinum) is deposited on the
surface. Annealing (heating and then slowly cooling) the metal layer helps
to smooth the surface and produce large flat areas.
A thin gold film can also be grown while deposited
on a flat mica surface and then stripped away. The surface of
the gold that was in contact with the mica will be very flat
over large areas.
Once a flat surface has been achieved, it is important
to keep it free of contamination. A typical dust particle consists
of millions of atoms, and could easily destroy the STM probe. Other
hazards include chemical reactions between the specimen and the surrounding
air that result in impurities on the substrate. These impurities
could eventually make the STM probe crash. To prevent these problems,
many STMs operate in high vacuum. Other techniques include periodic
heating of the sample in a neutral atmosphere to remove impurities
from the surface.