3.5.1 Contact mode
Contact mode produces images with the highest resolution. This is because when the probe tip is as close as it can be to the surface, the influence of atoms other than the one directly under the probe tip is relatively small. This is a simple geometrical effect – if the tip were withdrawn a large distance from the surface, a large number of atoms would be at a very similar distance from the tip, and therefore would have a similar contribution to the overall force. In contact mode, the repulsive force between tip and sample is sufficient to deflect the cantilever. How much the cantilever is allowed to deflect will determine the force it exerts on the sample, in proportion to its flexural stiffness. Cantilevers of low stiffness are desirable here, because it is easy to ensure that excessive force is not applied to the surface being scanned, since a large deflection results in a small change in the applied force.