Atomic Force Microscopy and Spectroscopy - Dynamic Force Spectroscopy and Biomolecular Recognition - page 61

Snap-in

Approach
Retraction

0

F adh Snap-out

Piezo movement

FIGURE 2.6 A schematic of a typical force-versus-distance-curve obtained in static mode.
The cantilever approaches toward the sample surface. Due to strong attractive forces it “jumps”
(snap-in) toward the sample surface at a specific position. During retraction, the tip is strongly
attracted by the surface and the snap-out point is considerably behind the snap-in point. This
results in a hysteresis between approach and retraction.

Upon approach of the cantilever toward the sample, the attractive forces acting on
the tip bend the cantilever toward the sample surface. At a specific point close to
the sample surface, these forces can no longer be sustained by the cantilever spring
and the tip “jumps” toward the sample surface. Now tip and sample are in direct
mechanical contact. A further approach toward the sample surface pushes the tip
into the sample. Since the spring constant of the cantilever is usually much softer
than the elasticity of the sample, the bending of the cantilever increases almost
linearly.
If the cantilever is now retracted from the surface, the tip stays in contact with the
sample at first because it is strongly attracted by the sample due to adhesive forces.
The force F adh is necessary to retract the tip from the surface. The corresponding
snap-out position is always at a larger distance from the surface than the snap-in,
which results in a hysteresis between approach and retraction of the cantilever. This
phenomenon of mechanical instability is often referred to as the jump-to-contact .
Unfortunately, this sudden jump can lead to undesired changes of the tip and/or
sample.
The same type of measurement can be used to measure molecular binding forces
or the elongation of chain-like molecules (for a review see, e.g., Janshoff et al.,
2002 Hinterdorfer & Dufrene, 2006). As shown in Figure 2.7a, chain-like molecules
attached to a surface might eventually pick up the tip and the resulting bending
of the cantilever is detected during the retraction of the cantilever from the sur-
face. Applications of this technique can be found in the other chapters of this
book.