Biomedical Engineering Reference
In-Depth Information
0
m
12
m
0
µ
m
12
µ
m
µ
µ
0
µ
m
3.7
µ
m
0
µ
m
1.36 V
12
µ
m
0
µ
m
12
µ
m
-10 V
Fig. 3.13. Intermittent-contact AFM images of human red blood cells. Height (left) and amplitude
(right) images shown.
widely applied, although it is subject to a number of difficulties specific to operation in
liquid, namely that mechanical excitation of the cantilever can lead to excitation of the
fluid and fluid cell as well [128], and a lack of clear understanding of the contrast
mechanisms [108, 129, 130]. The operation of IC-AFM mode in liquid, as well as in air,
is discussed in Section 4.3. Intermittent-contact AFM is not commonly applied in vacuum,
due to restrictions in bandwidth due to increase of Q in vacuum [117]. An extremely wide
range of samples have been studied by intermittent contact-mode AFM, some of these are
illustrated in Chapter 7.
Higher harmonics imaging
A recent development in Intermittent-contact AFM is the use of modes of resonance other
than the fundamental one. This may either be by a passive technique, by measuring the
vibration at these higher modes, can involve excitation at multiple frequencies. Addition
of such capabilities to an AFM is relatively simple, the main requirement being that a
lock-in amplifier capable of monitoring the very high frequencies. Figure 3.14 shows
illustrations of the first four modes of a beam-shaped cantilever. The requirement for a
high-frequency amplifier is because higher modes of real cantilevers are likely to have
extremely high frequencies. Because the modes are anharmonic, the second mode is not
necessarily at double the frequency of the fundamental (i.e.
f
2
6¼
2
f
1
), but may be as high
as six times the fundamental frequency [131]. In any case, having two lock-ins is useful
because it is advantageous to be able to monitor both
f
1
and
f
2
simultaneously.
Fig. 3.14. Illustrations of the first four normal resonance modes of a beam-shaped cantilever.
