In Non-Contact Mode AFM the cantilever oscillates at a frequency slightly above its resonance frequency (amplitude <10nm) in order to obtain an AC signal from the cantilever. The cantilever's resonant frequency decreases by the van der Waals force and by other long range forces extending above the surface (from 1nm to 10nm above the adsorbed fluid layer). During scanning the tip oscillates above the adsorbed fluid layer on the surface rather than touching it. This mode is ideal for studying soft or elastic samples because the total force between the tip and sample is very low (about 10-12N). The cantilever is oscillated near its resonant frequency (about 100 to 400 kHz) with an amplitude of a few tens to hundreds of angstroms. As the tip comes near the sample surface, the system detects variations in the resonant frequency or vibration amplitude.
Relationship between the resonant frequency of the cantilever and variations in sample topography:
- The resonant frequency of a cantilever varies as the square root of its spring constant.
- The spring constant of the cantilever varies with the force gradient experienced by the cantilever.
- The force gradient (derivative of the force versus distance curve) changes with tip-to-sample separation.
- Therefore, changes in resonant frequency of the cantilever can be used to measure changes in the force gradient, which reflect changes in the tip-to-sample spacing, or sample topography.
The force in the non-contact regime is low, making it more difficult to measure than that of contact mode. In addition, cantilevers must be stiffer so they are not pulled into contact with the sample surface. For these reasons a more sensitive AC detection scheme is needed.
With the constant aid of feedback, the system moves the scanner up and down at each (x,y) data point in order to keep the resonant frequency and amplitude constant. The average tip-to-sample distance is therefore kept constant as well. It is this motion of the scanner that is used to generate the data sheet and stored to form the topographic image of the sample surface.
Non-contact AFM is preferable for measuring soft samples and will not suffer from tip or sample degradation effects that sometimes result from contact AFM. If monolayers of condensed water are present on the surface of the sample it is likely a non-contact scan will be distorted because it will not penetrate the water in order to scan the sample surface, but will scan the surface of the water instead.