Figure 1 compares the movement of the probe tip relative to the sample surface for images being acquired between in contact AFM and in non-contact AFM. Contact AFM uses the physical contact between the probe tip and the sample surface, whereas non-contact AFM does not require this contact with the sample. In Non-Contact mode, the force between the tip and the sample is very weak so that there is no unexpected change in the sample during the measurement. Therefore, Non-Contact AFM is very useful when a biological sample or other very soft sample is being measured; the tip will also have an extended lifetime because it is not abraded during the scanning process. On the other hand, the force between the tip and the sample in the non-contact regime is very low, and it is not possible to measure the deflection of the cantilever directly. So, Non-Contact AFM detects the changes in the phase or the vibration amplitude of the cantilever that are induced by the attractive force between the probe tip and the sample while the cantilever is mechanically oscillated near its resonant frequency.  

K_eff= k₀ - F¡ (1)

When the attractive force is applied, k_eff becomes smaller than ko since the force gradient is positive. Accordingly, the stronger the interaction between the surface and the tip (in other words, the closer the tip is brought to the surface), the smaller the effective spring constant becomes. This alternating current method (AC detection) makes more sensitive responds to the force gradient as opposed to the force itself. Thus, it is also applied in such techniques as MFM (Magnetic Force Microscopy) and Dynamic Force Mode AFM.

A bimorph is used to mechanically vibrate the cantilever. When the bimorphs drive frequency reaches the vicinity of the cantilevers natural/intrinsic vibration frequency (f₀), resonance will take place, and the vibration that is transferred to the cantilever becomes very large. This intrinsic frequency can be detected by measuring and recording the amplitude of the cantilever vibration while scanning the drive frequency of the voltage being applied to the bimorph. Figure 2 displays the relationship between the cantilevers amplitude and the vibration frequency. From this output, we can determine the cantilevers intrinsic frequency.

On the other hand, the spring constant affects the resonant frequency (f₀) of the cantilever, and the relation between the spring constant (k₀) in free space and the resonant frequency (f₀) is as in Equation (2).  

 f₀ = (k₀ / m)^1/2  (2)  

As in Equation (1), since keff becomes smaller than k₀ due to the attractive force, feff too becomes smaller than fo as shown in Figure 3 (a). If you vibrate the cantilever at the frequency f₁(a little larger than fo) where a steep slope is observed in the graph representing free space frequency vs. amplitude, the amplitude change at f₁ becomes very large even with a small change of intrinsic frequency caused by atomic attractions. Therefore, the amplitude change measured in f₁ reflects the distance change between the probe tip and the surface atoms. 

NC-AFM on biological samples

NC-AFM vs. Tapping mode