Scanning probe microscopy
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As the name suggests, the heart of an SPM is a probe that is
scanned over the sample surface to build up some form of image.
The type of image you get depends on the interaction that is
measured by the probe. Images can be produced that reflect many
different properties of the sample. The sample height information
(topography), usually forms one aspect of the image, but images
can also be collected that show other properties, including
mechanical, electrostatic, optical, or magnetic information
about the sample surface.
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Different probes and measurement systems are used for some
of the different properties that can be measured, but one requirement
is that the interaction between the probe and the sample is
localised in some way. The measured signal must be dominated
by some small region of the sample close to the tip, so that
an image of the sample can be formed as the tip is scanned over
the surface. This implies that the interaction must have a strong
distance dependence, so that only the nearest parts of the sample
contribute to the interaction felt by the tip. The range of
the interaction will be one factor in the final resolution of
the instrument. When the interaction has a very strong distance
dependence, such as the electron tunelling current used in STM,
the resolution can be good enough to "see" individual
atoms.
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| Since the measured signal
should be dominated by the small region of probe and sample that
are closest together, the actual probe does not need to be an
isolated point. The probe can be part of some larger structure
that is more convenient to mount and scan. The size of the probe
can be relatively large, perhaps hundreds of microns or more,
but if the interaction has a short enough range then the signal
will be dominated by the very tip region of the probe, so that
resolutions can still be achieved in the range from atomic distances
to microns. |
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| The idea of a probe measuring
a local interaction and building up an image is relatively straightforward,
but the actual implementation of a system with a resolution in
this range is technically challenging. Many factors came together
in the development of scanning probe microscopy, including the
development of piezoelectric materials that made it possible to
reproducibly position and scan components with a sub-nanometre
precision. |
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