Biomedical Engineering Reference
In-Depth Information
Fig. 3.2. Simplified force-distance curve showing contact (repulsive region) scanning regime.
A deflection-distance curve, which is the raw data from which a force-distance curve is measured,
has a similar shape. Right: illustration of probe bending in each regime.
determine tip-sample interactions, and is the basis for several non-topographic AFM
modes, such as force spectroscopy and nanoindentation. More information about the use
of such capabilities of force-distance curves, and how they are converted from deflection
to force is given in Section 3.2.1, and Section 4.5, respectively.
Considering the approach curve shown in Figure 3.2, when the tip is far from the
sample surface, the cantilever is considered to have zero deflection; as the tip appr-
oaches the surface, it normally feels first an attractive force, and a 'snap-in' occurs, as
the tip becomes unstable and jumps into contact with the surface. As the instrument
continues to push the cantilever towards the surface, the interaction moves into the
'repulsive' regime, i.e. the tip is now applying a force to the sample, and the sample
applies an opposite force to tip. In this regime, a combination of cantilever bending and
sample compression will be occurring according to the relative compliances of the
sample surface and AFM probe. If the direction of movement is reversed, the interaction
passes again into the attractive regime, and the tip stays on the surface until instability
occurs once more, and the tip snaps off the surface. It is within the repulsive regime that
contact-mode imaging usually occurs (for example, at the point labelled 'set-point' in
Figure 3.2). In other words, in contact-mode AFM, the tip of the probe is always
touching the sample. This has the following important implications for contact-mode
AFM:
1. As a result of the repulsive force between the tip and the sample, the sample may be
damaged or otherwise changed by the scanning process.
2. Conversely, the tip could also be damaged or changed by the scanning process.
3. As the tip and sample are constantly in contact with each other as the tip moves along
the sample, in addition to the normal force they apply to each other, lateral forces are
experienced by both probe and sample.
4. The contact between the tip and the sample means that the nature of the sample
surface may affect the results obtained. This means that the technique can be
sensitive to the nature of the sample.
