Modes of operation
Figure 19.4 shows the Lennard-Jones potential for two interacting atoms. An AFM experiment can be recorded in both attractive and repulsive regimes of the Lennard-Jones potential. There are three basic modes of AFM imaging. Another mode, called force spectroscopy is not used for imaging but for characterizing physico-chemical properties of the specimen as discussed later in this section.
Figure 19.4 Lennard-Jones potential and the regions of attraction (orange) and repulsion (green).
Contact mode AFM: In contact mode AFM, the tip is brought in close contact with the specimen (in the repulsive regime) and scanned over the surface. As the tip is in contact with the sample throughout the scan, the frictional forces are very high. This mode of operation therefore may not be suitable for soft samples including biological samples.
Non-contact mode AFM: In non-contact mode AFM, a cantilever with very high spring constant is oscillated very close to the sample (in the attractive regime). The quantities that are measured are changes in the oscillation amplitude and the phase. The forces between the tip and the sample are very small, of the order of piconewtons. This mode is therefore well-suited for very soft samples but resolution is compromised.
Intermittent mode or tapping mode AFM: A stiff cantilever is oscillated so close to the specimen that a small part of oscillation lies in the repulsive regime of the Lennard-Jones potential. The tip therefore intermittently touches the sample while scanning. This mode of imaging allows imaging with very high resolution and has become the method of choice for scanning the soft biological samples.