Biomedical Engineering Reference
In-Depth Information
x distance: 1.6 µm
2.3 µm
3.7 nm
0 nm
0
0
2.3 µm
z height: 1.14 nm
6
5
4
3
2
1
100%
77%
59%
Pb
Sn
Si
0.0
1.2 1.6
Maximum attractive total force (nN)
0.4
0.8
2.0
2.4
Fig. 3.9. Example non-contact AFM images. Top: examples of non-contact AFM images in ambient
conditions (air) - individual DNA molecules (left) and 1 nm nanoparticles (right) [123]. Bottom
images: non-contact AFM in UHV conditions for individual atom identification. Left: atomically
resolved NC-AFM image of Si, Sn and Pb atoms on an Si(111) substrate - some atoms may be
differentiated based on apparent size, but identification is not possible. Middle: short-range chemical
force measured over each atom is dependent on the chemical nature of the atoms. Right: the same
image as on the left, with atoms coloured according to the colour scheme in the middle. Adapted
from [8], with permission. (A colour version of this illustration can be found in the plate section.)
identify the attractive force above individual atoms which could be correlated to their
chemical identity [8]. Further examples of the applications of non-contact-AFM to obtain
atomically resolved information are given in Section 7.1.5.
3.1.2.2 Intermittent-contact mode
Although the first experiments in dynamic AFM aimed to carry out non-contact AFM, it
was not long before the advantages of using a dynamic mode that allows the probe to touch
the sample (that is, pass into the repulsive regime) were discovered [97]. For intermittent-
contact AFM, feedback is usually based on amplitude modulation [108] and the tip-
sample interaction passes from the 'zero-force' regime, through the attractive regime, and
into the repulsive regime, as shown in Figure 3.10.
The fact that the tip-sample interaction moves through all three regimes has several
important implications:
(i) There is tip-sample repulsive interaction, i.e. tip and sample touch each other,
leading to the possibility of sample or tip damage, however:
