Biomedical Engineering Reference
In-Depth Information
is used as the feedback signal. However, if we compare the left- and right-hand sides of
the split photodetector, we obtain the lateral deflection signal. When measuring this
signal, the technique is sometimes called lateral force microscopy, or LFM. The reason
why measuring this can be useful is that this signal contains information about
the mechanical interaction of the probe tip with the sample surface. The lateral twisting
of the cantilever is a measure of the friction encountered by the tip as it scans over the
sample. Thus, this signal is sensitive to the nature (shape and frictional properties) of the
surface. For this reason, LFM is sometimes also called friction force microscopy (FFM),
and the lateral signal is sometimes referred to as the friction signal, although the signal
obtained laterally contains more information than just the friction felt by the tip. It is
important to bear in mind that the lateral bending is coupled with vertical bending of the
tip, and contains information about the shape of the sample, as well as its material,
because friction depends on the slope the tip is travelling along [77, 188]. However,
using this technique it
is
possible to get quantitative information about variation in
sample properties. Some examples of this are shown in Section 7.1.4. A discussion on
calibration of lateral signals is included in Section 4.2.
As mentioned previously, it is not normally necessary to measure AFM height signals in
more than one fast scanning direction. The situation in the case of the lateral deflection
data is somewhat different. The lateral deflection signal will normally always be different
in the two directions, as the cantilever will twist by a certain amount assuming there is
some measurable lateral component to the tip-sample force (i.e. friction). Therefore, even
on perfectly flat, homogeneous samples, the two images will be different from each other
in the magnitude and possibly sign of the signal. In general, changes of slope will affect
forwards and backwards scans oppositely, and changes in friction due to material contrast
will give greater or smaller difference between the forward and reverse scans. This is
shown schematically in Figure 3.18.
From Figure 3.18 it is possible to see that changes in slope and changes in material
contrast have different effects upon the lateral deflection signal. If the user subtracts the
left-to-right and right-to-left signals from each other, in the case of the slope change, the
result will be a signal with almost no contrast. However, in the case of the material friction
change, the resulting signal will be sensitive to the sample friction. Larger friction will
give a greater difference between the forward and reverse scans, while lower friction will
give a smaller difference. Thus, collecting both forward and reverse direction scans and
subtracting them in LFM can give useful information [160, 189].
3.2.3.2 Phase imaging
'Phase imaging' in AFM refers to recording the phase shift signal in intermittent-contact
AFM. In 1995 for the first time, the phase signal was described as being sensitive to
variations in composition, adhesion, friction, viscoelasticity as well as other factors [190].
Then in 1996 Garcia and Tamayo suggested that the phase signal in soft materials is
sensitive to viscoelastic properties and adhesion forces, with little participation by elastic
properties [191]. It has been a common assumption ever since that phase contrast will
show adhesion or viscoelastic properties [192, 193]. In fact, as shown in the examples of
phase contrast in Figure 3.19, phase contrast from material properties is seen in a wide
variety of samples, but also reflects topometric differences (differences in slope). This is
because the phase is really a measure of the energy dissipation involved in the contact
