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STM Techniques
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The Scanning Tunneling Microscope (STM) is a non-optical microscope that scans an
electrical probe over a surface to be imaged to detect a weak electric current flowing
between the tip and the surface. The STM (not to be confused with the scanning electron
microscope) was invented in 1981 by Gerd Binnig and Heinrich Rohrer of IBM's Zurich
Lab in Switzerland. |
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Although initially greeted with some skepticism by materials
scientists, the invention garnered the two a Nobel Prize in Physics (1986). The
STM allows scientists to visualize regions of high electron density and hence infer
the position of individual atoms and molecules on the surface of a lattice. Previous
methods required arduous study of diffraction patterns and required interpretation
to obtain spatial lattice structures. The STM is capable of higher resolution than
its somewhat newer cousin, the atomic force microscope (AFM).
Both the STM and the
AFM fall under the class of scanning probe microscopes. The STM can obtain images
of conductive surfaces at an atomic scale 2 × 10−10 m or 0.2 nanometer, and
also can be used to manipulate individual atoms, trigger chemical reactions, or
reversibly produce ions by removing or adding individual electrons from atoms or
molecules. The acronym STM can mean either scanning tunneling microscope or scanning
tunneling microscopy. This microscope has an extremely sharp stylus that scans the
surface. The stylus is so sharp that its tip consists only of one atom.
Strictly,
as the tunnelling current is such a short ranged phenomenon (which is what gives
STM its impressive resolution), tunnelling normally only occurs through the furthest
extremity of the stylus - which might itself appear to be rather blunt on a larger
scale. The STM is a non-optical microscopy technique which employs principles of
quantum mechanics.
A sharp probe (the tip), whose end is as sharp as a single atom,
moves over the surface of the material under study, and a voltage is applied between
the probe and the sample surface. Depending on the voltage applied, electrons will
tunnel through the potential barrier between the surface and probe, resulting in
a weak electric current. The direction of the tunneling depends on the polarity
of the electric field. The magnitude of this current is exponentially dependent
on the distance between probe and the surface.
For tunneling to occur, the substance
being scanned must be conductive (or semiconductive). Insulators cannot be scanned
by STM, as the electron has no available energy state to tunnel into or out of due
to the band gap structure in insulators. In one scanning mode, a servo loop (feedback
loop) keeps the tunneling current constant by adjusting the distance between the
tip and the surface (constant current mode).
This adjustment (and adjustments in
any spatial direction) is accomplished by placing an electric field across a piezoelectric
element, which deforms relative to the voltage of the electric field. By scanning
the tip over the surface and measuring the height (which is directly related to
the voltage applied to the piezo element), one can thus model the surface structure
of the material under study. STMs can reach sufficiently detailed resolution to
resolve single atoms. The STM tip will come within a nanometers of the sample surface.
If the tip makes contact with the surface, the tip "crashes" into the surface and
must be replaced. |
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