Imaging modes - practicalities
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Contact mode
In conctact 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 between the tip and the sample,
and can show 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.
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Intermittent contact mode
In intermittent conctact 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.
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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.
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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
damage the surface. |
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For intermittent contact mode in liquid, the capillary force
is not a problem, and softer cantilevers are often used. "Contact
mode" cantilevers are often used for intermittent contact
mode in liquid conditions. The resonant frequencies are much
lower, and the damping of the liquid around the cantilever has
a strong effect on the resonance.
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more
about cantilevers... |
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The spring constant of a cantilever can be estimated from its
geometry and the properties of the material it is made from.
The spring constant depends very strongly on the thickness of
the cantilever, however, and this can be difficult to measure
accurately. If a calibrated reference cantilever is available,
then the cantilevers can be pushed against one another to compare
the deflection of one cantilever by the other, and hence measure
the spring constant of the unknown cantilever.
For soft cantilevers, another option is to measure the thermal
noise and calculate the spring constant. This is an attractive
option, since the cantilever is not damaged by the measurement,
and no extra equipment is required.
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