Biomedical Engineering Reference
In-Depth Information
is true is that in ambient conditions, a capillary layer of water will form between the tip
and the surface. One effect of this is to 'pull' the AFM tip onto the surface, often
applying an even stronger force than the force applied (via the set-point) by the operator.
Thus it is easy to unwittingly apply a very large force (
nN) to the sample in contact-
mode AFM in ambient conditions. In water, these forces do not exist, so it is easier to
image with a very gentle force. For this reason, and due to some complications of
imaging in dynamic modes in liquids (see the next section), imaging in liquid is a strong
point of contact mode. As mentioned previously, contact mode also works well in high-
speed AFM, and some high-speed AFM set-ups use this mode exclusively [36].
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3.1.2 Oscillating modes
In the first paper on AFM, Binnig and Quate acknowledged the potential benefits of
oscillating the cantilever in an AFM, and compared the results of using an oscillating
probe with those from contact mode. At the time contact mode gave far better results,
probably due to the nature of the probe used [19]. Although the use of oscillating modes
were revisited shortly afterwards [105], it was several years before oscillating probe
modes became popular, and for quite a while nearly all AFM was carried out in contact
mode. The primary motivation for using oscillating mode in an AFM is to take advantage
of the signal-to-noise benefits associated with modulated signals. Thus, an AFM that has
oscillating modes can measure images with a small probe-sample force.
There are now a large number of dynamic modes of operation, and even more names for
those modes. However, all of these modes are variations on a theme. The cantilever is
oscillated, usually with an additional piezoelectric element, and typically at its resonant
frequency. When the oscillating probe approaches the sample surface, the oscillation
changes due to the interaction between the probe and the force field from the sample.
The effect is a damping of the cantilever oscillation, which leads to a reduction in the
frequency and amplitude of the oscillation. The oscillation is monitored by the force
transducer (i.e. by the optical lever in most AFMs), and the scanner adjusts the
z
height via
the feedback loop to maintain the probe at a fixed distance from the sample, just as in
contact-mode AFM. The only real differences between the various oscillating modes
available are in the size (amplitude) of the oscillation applied to the probe, and the method
used to detect the change in oscillation. The general principle of oscillating AFM modes is
shown in Figure 3.5.
Irrespective of the many different terms used to describe the techniques, there are
actually only a few kinds of conditions used in oscillating imaging modes. The user can
decide to set either a small or a large applied oscillation amplitude, and sometimes can
decide how to detect the change in probe oscillation. Some instruments may only have
one detection scheme implemented. The instrumental set-up schematic is shown in
Figure 3.5. An oscillating signal is generated, and applied to the cantilever mechanically,
such that the probe is oscillated close to its resonant frequency. The oscillation of the
probe is monitored as it is brought close to the sample surface. The detected change in
the oscillation (whether detected via amplitude, phase or frequency), is used in a
feedback loop to maintain the probe-sample interaction constant. The choice of small
or large amplitude has a considerable practical effect, as is illustrated in Figure 3.6.
Using a small oscillation amplitude (Denoted by the arrow A), it is possible to maintain
