Scanning Probe Microscopy - JPK instruments nanowizard afm Handbook

1.3 Scanning Probe Microscopy

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.
Different probes and measurement systems are often used for the different
properties, but one requirement is that the interaction between the probe and the
sample is localized in some way. This is so that the measured signal is dominated
by some small region of the sample closest 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 tunneling
current used in STM, the resolution can be good enough to "see" individual atoms.
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.
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-nanometer precision.
The following diagram shows some of the different forms of scanning probe
microscopy that have been developed. The techniques are usually named after
the interaction that they measure. The list is not complete, as there are many
different forms of scanning probe microscopy, and new techniques are still being
developed. The information in this handbook is mainly concerned with Atomic
Force Microscopy.
®
2 JPK Instruments NanoWizard Handbook Version 2.2