It has hardly anything to do with optical microscopy.
There are no lenses, there is no requirement for a light source
to illuminate the sample, there is no eyepiece to look through;
the microscope itself does not even look like a typical optical
microscope. Developed in the mid-eighties, atomic force microscopy
is nowadays part of a large family of scanning probe microscope
techniques. Their origin lies in scanning tunneling microscopy,
which won its inventors the Nobel prize in 1985. |
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Cynics say an AFM is a hybrid between a lawn mower and an old-fashioned record player.
This is alright to gain an initial insight into the technique, but
the details are of course more complicated. The AFM's needle is
much sharper and the tracking force is a million times lower than
that of a record player. The imaging technique consists of a mechanical
device, which is able to measure very small forces when atoms or
molecules come close together, so it was named atomic force microscopy.
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Cantilevers are at the center of the atomic force microscope.
The principal part of the device called the cantilever is a plate
spring, which is fixed at one end. At the other end it supports
a pointed tip. |
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about cantilevers... |
The tip can be moved across a sample surface line by line, just like a lawn mower in the garden.
The pointed tip is brought into contact with the sample and moved
across the surface. The cantilever deflects as it travels across
the landscape (topography). The cantilever deflection is usually
detected by a laser beam, which is focused on the flat top of the
cantilever. The movements of the reflected light are "seen"
by monitoring the current in different parts of a photodiode. This
information about the tip movement provides three-dimensional images
of the sample. |
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