AFM: Dynamic Modes
Non-contact mode belongs to a family of AC modes, which refers to the use of an oscillating cantilever. A stiff cantilever is oscillated in the attractive regime, meaning that the tip is quite close to the sample, but not touching it (hence, non-contact). The forces between the tip and sample are quite low, on the order of pN (10-12 N). The detection scheme is based on measuring changes to the resonant frequency or amplitude of the cantilever as the interaction between the tip and sample dampen the oscillation.
Dynamic Force Mode
Also known as... tapping mode, intermittent-contact mode, acoustic AC mode, oscillating mode
A stiff cantilever is oscillated closer to the sample than in noncontact mode. Part of the oscillation extends into the repulsive regime, so the tip intermittently touches or taps the surface. Very stiff cantilevers are typically used, as tips can get stuck in the water contamination layer.
The advantage of tapping the surface is improved lateral resolution on soft samples. Lateral forces such as drag, common in contact mode, are virtually eliminated. For poorly adsorbed specimens on a substrate surface, the advantage is clearly seen.
Schematic of dynamic AFM mode (DFM) operation
In Phase Contrast mode, the phase shift of the oscillating cantilever relative to the driving signal is measured. This phase shift can be correlated with specific material properties that effect the tip/sample interaction. The phase shift can be used to differentiate areas on a sample with such differing properties as friction, adhesion and viscoelasticity. The technique is used simultaneously with DFM mode so topography can be measured as well.
Force modulation refers to a method used to probe properties of materials through sample/tip interactions. The tip (or sample) is oscillated at a high frequency and pushed into the repulsive regime. The slope of the force-distance curve is measured which is correlated to the sample's elasticity. The data can be acquired along with topography which allows comparison of height and material properties.
Force Modulation Mode applies a dynamic force while in contact mode