Atomic Force Microscopy and Spectroscopy - Dynamic Force Spectroscopy and Biomolecular Recognition - page 60

(a)

(b)

1.0 μ m

1.0 μ m

1.4
1.2
1
0.8
0.6
0.4
0.2
0

3
2
1
0
-1
-2

0 1 2 3 4 5 0 1 2 3 4 5

Scan position [ μ m] Scan position [ μ m]

FIGURE 2.5 (a) Atomic force microscopy image obtained in contact mode of a monomolec-
ular L-α-dipalmitoyl-phosphatidycholine (DPPC) film adsorbed on a mica substrate. The
image is color coded, that is, dark areas represent the mica substrate and light areas the DPPC
film. (b) The simultaneously recorded friction image shows lower friction on the film (dark
areas) as on the substrate (light areas). The graphs at the bottom represent single scan lines
obtained at the positions marked by a dark line in the images at the top. Sample and experiment
by L. Chi and J.-E. Schmutz, University of Munster.

for the bending of the cantilever. This procedure, however, needs a calibration of the
cantilever bending.
The situation gets even more disappointing if attractive tip-sample forces are
present as shown in Figure 2.3b. In this case, the attractive forces are so large that
cannot be counterbalanced by the soft cantilever anymore. Mathematically speak-
ing, the gradient of the tip-sample forces is larger than the spring constant of the
cantilever (Burnham & Colton, 1989). Therefore, an instability occurs if

∂ F ts ( z )

k cant < (2.11)

∂ z

As a result, the tip “jumps” toward the sample surface during an approach.
This effect strongly influences static mode AFM measurements in air and vac-
uum where strong long-range attractive forces are present as exemplified by a typi-
cal force-versus-distance curve shown in Figure 2.6. Here, the force acting on the tip
recorded during an approach and retraction movement of the cantilever is depicted.