3.5 Scanning modes of the AFM
One of the interesting effects of scale is the answer to the question of whether the probe needs to come into contact with the surface of the sample being scanned. The cantilever on which the probe tip is mounted is a very compliant structure. The control system of the AFM ensures that the deflection of the cantilever, and hence the force it exerts on the surface, is maintained within very strict limits. Figure 7 shows how the force between two atoms varies as a function of their separation. Force above the x-axis is attractive; that below is repulsive. The curve crosses the x-axis where the net force is zero, and the distance along the x-axis at this point corresponds to the mean separation of the atoms. If two atoms from different pieces of material get this close to one another, they are in a state that is commonly known as ‘in contact’.
An important ideal of any form of microscopy is to damage the sample as little as possible, and so the AFM tracks the surface, maintaining a separation sufficient to ensure that no atoms are dislodged from their bound positions. Depending on the type of substrate being scanned, the tip of the AFM probe may be maintained at different distances from the surface. In the contact mode it will be at (or very slightly below) the separation marked r0 on the figure, where the powerfully separation-dependent repulsive force begins to be felt. For soft substrates, such as biomolecules, the probe may be operated at a greater separation, never actually touching the surface, but still close enough to feel the longer-range attractive force component of the interaction.