AFM imaging: Interactions between the probe and sample
At very close tip-sample distances (a few angstroms), a strong repulsive force appears between the tip and the sample surface due to the overlap of atomic orbitals. The repulsive force increases as the distance between the tip and sample surface decreases.
As the probe tip scans the surface at this close distance, a feedback system to the piezoelectric scanner raises and lowers the sample to keep a constant repulsive force between the tip and the sample surface. A plot of this upward and downward motion (z), as a function of the tip x - y position on the sample surface, provides a high-resolution image of the surface topography. This mode of operation is called “contact mode”. When the repulsive force is in place, the tip and sample are considered to be in 'contact'. High resolution is possible because very small changes in separation (z) (see Figure 2), lead to large changes in force which is the parameter being monitored. However, for some samples such as biological specimens or soft material such as some polymers, 'contact' of the surface with the tip can damage the sample.
To overcome this problem, an alternative set-up is that the tip vibrates rapidly up and down and only 'taps' or comes very close to the sample surface while at the bottom of its oscillation. This mode, referred to as “tapping mode”, “semicontact mode”, or “dynamic mode”, is the most common mode of operation as it prevents sample damage. However, now changes in separation (z) lead to smaller changes in force (see Figure 2), and hence the resolution of images from tapping mode is not as high as images from contact mode.
"Non-contact" mode AFM uses longer range forces which are attractive. These attractive forces occur at tip-sample surface distances of > 10 nm.