| No.91, September 2009 |
The Scanning Tunneling
Microscope (STM)
This article was written by the Newsletter editor Burt Stumpf,
an emeritus professor of physics at Ohio University in Athens,
Ohio. He is a Fellow of the Acoustical Society of America and a
fellow of the Ohio Academy of Science. Burt taught undergraduate
and graduate courses in physics. He also is the author of the
textbook Analytical Acoustics published by Ann
Arbor Science. Burt is an active member of the Appalachian Section
of AAPT.
This microscope makes use of scanning a surface with a beam of
electrons. It depends on the fact that if two electrodes (one is a
fine metal tip and the second is the conducting surface to be
examined) are brought very close together, electrons can go from one
to the other electrode. Nonconducting surfaces need to be coated with
a thin conducting layer for the microscope to work. A voltage
difference is placed across the gap between the metal tip and the
conducting surface as the tip scans across the surface. The tip has
to move up and down with the topography of the surface in order to
keep the electrode current constant. The distance of the tip from the
surface at each point is also kept constant. As the surface goes up
and down so does the tip. A sensor detects this tip motion and
produces a current itself which is then processed and displayed on a
monitor screen. This display shows a "picture" of the surface
topography features. The approximate resolution for the horizontal
direction is 0.2 nanometers and for the vertical 0.01 nanometer. One
purpose of STM is to study different surfaces as to their properties
to conducting. A good reference for the STM is the Penguin Dictionary of Physics.
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