Imaging modes - introduction
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The detection system measures the cantilever response as the
tip is moved over the surface by the scanning system. In most
AFM systems there is also a feedback loop, which adjusts the
position of the cantilever above the surface as it is scanned,
to take account of the changes in surface height. A value of
the cantilever deflection, for example, is selected and then
the feedback system adjusts the height of the cantilever base
to keep this deflection constant as the tip moves over the surface.
This is known as contact mode imaging.
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There are other ways of operating the system, however, using
dynamic modes where the cantilever vibrates, and this oscillation
of the cantilever is measured rather than the static deflection
of the tip. There are different ways to excite the oscillations
- the cantilever substrate can be shaken directly, or a magnetic
field can be used to drive the cantilever itself if it is coated
with a ferromagnetic layer. In aqueous conditions, the most
common technique is to drive the cantilever acoustically through
the liquid. In all these cases, however, the measurement of
the cantilever oscillation and control systems are the same,
and the cantilever is usually driven close to resonance.
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| In these dynamic modes,
a setpoint amplitude is chosen, and the height adjusted to match
this amplitude through the feedback system. In addition to the
height and error signal information from this constant amplitude
mode, the phase between the drive signal and the cantilever can
also be measured. There are several different dynamic modes, depending
on how much of the oscillation cycle the tip actually makes contact
with the surface. |
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| Intermittent contact mode,
is widely used, and can give a combination of the benefits of
the other modes. The cantilever oscillates and the tip makes repulsive
contact with the surface of the sample at the lowest point of
the oscillation. The lateral forces can be much lower than contact
mode, since the proportion of the time where the tip and sample
are in contact is quite low. There may be a higher normal force
between the tip and sample when they are in contact, however. |
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| In non-contact mode the
cantilever oscillates close to the sample surface, but without
making contact with the surface. This mode is not so widely used,
since the attractive force means that there is a possibility of
the tip jumping into contact with the surface. The capillary force
makes this particularly difficult to control in ambient conditions.
Very stiff cantilevers are needed so that the attraction does
not overcome the spring constant of the cantilever, but the lack
of contact with the sample means that this mode should cause the
least disruption. |
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| Another mode is possible,
where the tip does not leave the surface at all during the oscillation
cycle. This is something like a dynamic form of contact mode,
and is usually called force modulation mode. |
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