Another Way Of Thinking About Imaging Modes - JPK instruments nanowizard afm Handbook

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.
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.
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.
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.

2.3 Another way of thinking about imaging modes

The imaging modes can also be thought of in terms of the forces between the tip
and surface. Generally, when two objects are brought together, the long range
forces are attractive, and the force becomes repulsive when the objects are close
together. The longer-range attractive forces are usually van der Waals forces and
capillary forces, and then the repulsive interaction takes over at short ranges, when
the objects are in "contact" and the electron orbitals begin to overlap. The situation
may be a lot more complex, however, when electrostatics and other interactions
from soft samples in liquid are taken into account.
Broadly, though, a general curve can be drawn of the tip-sample force against
distance, and the different operating modes can be matched with different parts of
the curve. An example is shown, which demonstrates the main features. The
curve is a general approximation, however, and different samples will have very
different curves in practice. Negative force (below the axis) is attractive in this
diagram, and positive force (above the axis) is repulsive. As the tip and sample
approach from a long distance, the attractive force increases to some minimum in
the curve. Approaching beyond this minimum reaches a relatively sharp upwards
part of the curve into the repulsive regime.
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6 JPK Instruments NanoWizard Handbook Version 2.2