Biomedical Engineering Reference
In-Depth Information
Operating principles of intermittent-contact AFM
In IC-AFM the probe is oscillated with a large amplitude, typically in the range of
1-100 nm [108], and the feedback is usually based on the amplitude signal. In most
cases, the probe is oscillated by an additional piezoelectric element attached to the probe
holder (see Chapter 2), although it is also possible to excite the cantilever vibration by
other methods, e.g. by an external magnet, with a magnetically coated cantilever [124,
125], which may reduce fluid vibration when imaging in liquid. In fact, rather than driving
the probe directly, the most common excitation method for fluid imaging is to excite the
entire fluid cell holder, which causes the liquid to vibrate, acoustically driving the
cantilever [126, 127]. Often, in addition to the amplitude signal, the delay in the phase
of the probe oscillation is recorded. Oscillation amplitude and phase are illustrated in
Figure 3.12.
The amplitude is reduced by the contact with the sample surface, and so an amplitude
set-point is set by the user, and the amplitude is the error signal in IC-AFM. In a similar
way to deflection in contact mode, the amplitude signal in intermittent contact may be
used as an illustration of the shape of the sample. Again, like the deflection signal, the
amplitude signal shows where the feedback system has not yet compensated for changes
in sample height, so for best height data, the amplitude signal should be minimized.
An example image showing the relation between height and amplitude data is shown in
Figure 3.13. Note that like deflection images in contact mode, the
z
scale of amplitude
images in IC-AFM is usually in volts, unless specifically calibrated. It's common practice
to remove this scale for publication as it has no practical use.
In addition to height and amplitude data, the phase-shift may also be saved as an image.
The reason why saving this data is useful is not obvious, and this information was largely
ignored in early intermittent-contact AFM. In fact, the phase of the oscillating cantilever
is strongly affected by the probe tip-sample interactions, so it can be a useful way of
distinguishing materials. As a Non-topographic mode, phase imaging is covered in
Section 3.2.3.2.
Applicability
Intermittent-contact mode is a very widely applied technique, and is currently the most
commonly applied technique for imaging in air. In liquid, IC-AFM mode is also very
Fig. 3.12. Illustration of the effect of intermittent contact on the cantilevers' oscillation. The free
oscillation (solid) is modified when in contact with a surface (dashed) by a reduction in amplitude
and a phase shift.
