W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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250 9. APPLICATION OF ATOMIC FORCE MICROSCOPy<br />
to either the probe or the sample substrate <strong>and</strong> observing the amplitude<br />
<strong>and</strong> phase response of the cantilever when the probe is either in contact<br />
with the surface of the material or is in close proximity to a surface<br />
with an intervening fluid layer. Although many forms of the modulation<br />
technique have been employed to characterise predominantly elastic<br />
materials, comparatively few AFM studies use superimposed modulation<br />
to study confined liquids.<br />
Various terms are used to describe dynamic modes of operation<br />
although many of the reported modes are essentially similar in design.<br />
Tapping mode (or intermittent contact) imaging is perhaps the most familiar<br />
dynamic AFM technique. In tapping mode the cantilever is oscillated<br />
at or near its resonance frequency such that when the tip interacts with the<br />
sample surface, the amplitude <strong>and</strong> phase response will change. In response<br />
to this deviation, a feedback loop adjusts the height of cantilever above the<br />
surface to achieve a constant vibration amplitude. Phase contrast images<br />
obtained from tapping mode studies are routinely discussed in terms of<br />
the perceived damping effects induced by the elasticity or compliance of<br />
the sample surface. Most commonly, qualitative interpretation of the phase<br />
data may be used to discern areas of varying stiffness. FMM is essentially<br />
a dynamic mode of operation in which the tip <strong>and</strong> substrate interact under<br />
conditions of a constant (average) force such that the amplitude of the<br />
cantilever oscillation varies about a nominal set point. Modulated force<br />
experiments are often performed at a single point upon the surface, <strong>and</strong><br />
for the case of intervening liquids, the amplitude <strong>and</strong> phase are often monitored<br />
as a function of probe to surface distance.<br />
Considering predominantly elastic surfaces, FMM is often used to<br />
generate surface images, which are generally referred to as either ‘elasticity<br />
maps’ or ‘viscoelasticity maps’, depending upon the interpretation<br />
of the force data. The fundamental approach employs single-point force<br />
modulation [59] in which an ‘AC modulation’ is superimposed upon<br />
either the probe or the sample during a ‘DC’ (i.e. force–distance) experiment.<br />
If the modulation frequency is sufficiently high, the technique may<br />
be employed while scanning a surface <strong>and</strong> is a complementary approach<br />
to point-wise force mapping [28].<br />
The basic method <strong>and</strong> interpretation is considered by Maivald et al.<br />
(1991) [52] who exploit variations in local surface elasticity to provide a<br />
basis for image contrast in inhomogeneous materials. The probe is scanned<br />
across the surface with the feedback loop maintaining contact with a<br />
(nominal) constant cantilever deflection <strong>and</strong> therefore a constant applied<br />
force. If the sample is caused to oscillate a small distance �z 1 in the direction<br />
normal to the surface, the undulating motion of the surface produces<br />
a corresponding superimposed deflection of the cantilever �z 2. For an<br />
infinitely hard sample, the deflection of the cantilever will be such that<br />
�z 1 � �z 2, whereas in the case of softer samples the sample surface will