The
scanning tunneling microscope (STM) is now a well-developed tool for
surface analysis. The first reports appeared in 1981 and 5 years
later, Gerd Binnig and Heinrich Rohrer from IBM Zurich were awarded
the Nobel Prize in Physics for its creation. The concept is rather
simple: a sharp, conductive tip is brought to within a few
Ångstroms of the surface of a conductor. A small bias voltage
between the tip and the sample induces a small current to flow even
though the two conductors are separated (though every so slightly).
The current that flows arises from the overlap of the electron
orbitals at the end of the tip and on the surface, and because of
this, the electrons can tunnel across the vacuum barrier between the
t
wo.
When the overlap is large, the current is large and vice versa.
Because of the exponential dependence of the radial 
component
of the wavefunction of electrons on atoms, the current drops
exponentially as the tip withdraws from the surface. This extreme
sensitivity to separation is exploited to make a microscopical
instrument by subsequently rastering the tip across the surface, and
using the current as a feedback signal. The tip-surface separation is
controlled to be constant by keeping the tunneling current at a
constant value. The voltage necessary to keep the tip at a constant
separation is used to produce a computer image of the surface. Under
the best of conditions, the vertical noise floor can approach 500 fm,
about 200 times smaller than an atom. The lateral resolution depends
upon the shape of the tip but it can often achieve 1 or 2
Ångstroms. Here is an image formed on graphite. Each bright
spot correlates to the position of an atom. The hexagonal structure
of the graphite surface is clearly revealed.
We have built our own STM scan head for operation in a UHV chamber but we use the electronics and software associated with a SPM which we purchased from Burleigh Instruments.
In a modification which will bring together elements of the STM and the SNOM, we are modifying our STM so that it can use a conductively coated optical fibre probe tip. This tip will still be conductive and able to provide an atomic resolution STM image of a surface. However, in addition, the tip will be able to pick up any fluorescence induced by the tunneling electron current (electroluminescence) and direct it into a spectrometer. In this manner, we will be able to correlate specific spectral features with particular topological features.
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