Operation - JPK instruments nanowizard afm Handbook

2.4 Operation

Contact mode
In contact mode, the tip never leaves the surface, so this mode can be used for
very high resolution imaging, such as atomic resolution of inorganic crystals or the
images of protein crystals showing the subunits of the proteins. The maximum
vertical force is also controlled, so the compression of the sample can be limited.
The lateral forces as the tip moves over the surface can be a problem in some
situations, but can actually be an advantage in other situations. The lateral
deflection can give information about the friction force between the tip and the
sample, and can show up areas that may have the same height, but different
chemical properties.
In contact mode, the setpoint value is the deflection of the cantilever, so a lower
value of the setpoint gives a lower imaging force.
Intermittent contact mode
In intermittent contact mode, the tip is not in contact with the surface for most of the
oscillation cycle. The lateral forces can therefore be much lower, and this mode
can be used for imaging samples such as molecules that are not firmly stuck down
on the surface, without moving them around.
The cantilever is usually driven close to a resonance of the system, to give a
reasonable amplitude for the oscillation and also to provide phase information. The
phase of the cantilever oscillation can give information about the sample properties,
such as stiffness and mechanical information or adhesion. The resonant frequency
of the cantilever depends on its mass and spring constant; stiffer cantilevers have
higher resonant frequencies.
In intermittent contact mode, the setpoint value is the amplitude of the oscillation,
so a higher setpoint value means less damping by the sample and hence a lower
imaging forces.
Cantilevers and spring constants
Different imaging modes tend to use cantilevers with different properties. In contact
mode, the deflection of the cantilever is controlled as the tip is scanned over the
surface. A softer cantilever means that a lower force can be used to give the same
deflection. Often lower forces give better imaging, so the softest cantilevers are
generally used for contact mode imaging. Many cantilevers are available with
spring constants (k) below 0.5 N/m.
Stiffer cantilevers are usually used for intermittent contact mode, particularly in air.
These generally have a resonant frequency of 200 – 400 kHz, and spring constants
of more than 10 N/m. These stiffer cantilevers give more stable imaging in air,
since the cantilever is able to break free of the capillary forces when the tip touches
the sample. As there can be very low average deflection values during careful
imaging, the stiffer cantilevers do not necessarily damage the surface.
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8 JPK Instruments NanoWizard Handbook Version 2.2