Various Operation Modes of AFM

In vacuum, the long-range attractive van der Waals interactions as well as the short-range chemical forces are the predominant interaction between sample and tip (provided that electrostatic Coulomb interactions are carefully compensated or negligible and the tip is non-magnetic). The van der Waals forces are caused by the interaction of fluctuations in the electromagnetic field and are attractive when the tip approaches the surface, the chemical forces originate from Pauli-exclusion and nuclear-repulsion. The attraction increases until tip and sample have approximately the distance of a chemical bond. When distance between tip and surface is further decreased the interaction becomes repulsive (for more details see [83]). 

The different force ranges define the operation modes of the AFM. In contact mode, the repulsive chemical force is predominant. In the dynamic mode, the oscillation amplitude can be wider than the force potential displayed in Fig. 10.20. The interactions thus must be integrated over the oscillation cycle [84, 85[. 

42) Even with a very stiff cantilever, the distance between tip and sample cannot be decreased below a few tenths of a nanometer due to the 

When the dynamic mode is applied in the non-contact regime (typically in vacuum), the attractive forces dominate. When the dynamic mode is applied in the intermittent regime (typically in air or liquids) repulsive forces are the predominant interaction. 

Constant force the deflection of the cantilever is kept constant by the feedback circuit, which moves the scanner up and down in z-direction. In this case, an image of constant force is generated from the movement of the scanner. The advantage of this method is, that with a cantilever deflection held constant, the total force exerted on the sample remains constant during the measurement. This is the preferred mode for most applications. 

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