Tunneling Microscopy (STM)
is a non-optical,
very high-resolution microscopy
technique that is used for obtaining images of conductive surfaces at
atomic scale level (~2 angstroms, i.e., 0.2 nanometer). Just like
the AFM, it is a type
of scanning probe microscopy, employing an atomically sharp probe tip
that is scanned over the sample surface in order to accomplish its
imaging function. The equipment used for STM is called a
which also goes by the acronym of 'STM'.
Aside from atomic level imaging, STM can also be used to alter the
sample by manipulating individual
atoms, initiating chemical reactions, and creating ions.
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on the basis of a phenomenon known as
'quantum mechanical tunneling.'
This phenomenon is characterized by the fact that a very small current
will flow between a sharp metal probe tip and the surface of an
electrically conductive material if: 1) a voltage is applied between the
tip and the conductive material; and 2) the tip is positioned just a few
nanometers away from the sample surface, i.e., no contact is made
between the tip and the sample.
The current produced by tunneling is
'tunneling current', which is on the order of one nanoamp for an
applied voltage of 1 V.
The amount of tunneling current produced is exponentially dependent on
the distance between the tip and the sample surface. This sensitivity of
the tunneling current to the tip's distance from the sample is utilized
by the STM in its operation.
i.e., the distance between the tip and the sample surface is kept
constant in order to keep the tunneling current constant. Since
the topography of the sample surface changes, the ST microscope must
move the probe tip according to how the sample topography varies in
order to keep the tip-to-sample distance constant.
The tip is mounted on a
which controls the position of the tip in three dimensions relative to
the sample. The piezo element that moves the tip towards or away from
the sample surface is controlled by a
circuit that monitors the tunneling current in order to determine
whether the tip is too close or too far from the surface. This
feedback circuit supplies the electrode of the piezo element with a
control voltage that moves the tip in the right direction to keep the
distance of the tip from the sample constant.
As the tip is
line by line
over a small area of the sample surface in the x-y plane, topographic
data based on the z-axis position of the tip (which,
in turn, is based on the tunneling current)
is collected by the computer of the STM. The image of the
topography of the sample may then be reconstructed from the collected
data. Under the right conditions, high-quality STM's can produce images
with sufficient resolution to show individual atoms. STM images
are commonly presented in greyscale, with protrusions shown in white and
depressions in black.
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STM is one of
the most important tools for surface physics and chemistry studies.
With the ability to show the structure of the uppermost layer of atoms
or molecules, STM can be used to reveal surface defects, display the
morphology of various depositions, or measure the
of a wafer in the angstrom-range. STM may also be used in the study of
conduction or charge transport mechanisms.
STM can also
be used to
accurately, by pushing or dragging them with the tip at low
temperatures. Electrons emitted by the tip can also be used to alter the
sample. The ability of STM to serve as a tool for 'rearranging' atoms
has made it an important tool in
STM does not
need a vacuum in order to operate, although it is usually operated in an
ultrahigh vacuum environment to avoid contamination or oxidation of
sample surfaces when high-resolution imaging of metals or semiconductors
is required. Surface oxidation reduces the conductivity of the sample's
surface and affects the tunneling current, resulting in imaging
operates on the flow of the tunneling current, it can not be used on
non-conductive samples. It may be possible to coat a
non-conductive sample with a conductive layer such as gold to make it
observable under an STM, but this coating step can mask hide certain
features or degrade imaging resolution.
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