Tapping Mode AFM operates by scanning a tip attached to the end of an oscillating cantilever across the sample surface. The cantilever is oscillated at or near its resonance frequency with an amplitude ranging from 20nm to 100nm. The frequency of oscillation can be at or on either side of the resonant frequency of the cantilever. The tip lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of each oscillation. Changes in the vibration frequency of the cantilever vibrating near its resonance frequency are measured. The feedback loop maintains a constant oscillation amplitude, and therefore, a constant tip-sample interaction is maintained during imaging.
Like contact mode, tapping mode can be operated in ambient and liquid environments. Tapping mode has a number of advantages compared with contact mode, including greatly reduced lateral and normal forces being applied to the sample surface. Therefore, tapping mode is preferred to image samples with structures that are weakly bound to the surface or soft samples. Figure 5 is an example of a tapping mode image in air of carbon nanotubes on a silicon surface.
Tip choice for tapping mode in air
The tip taps the surface and must not adhere or stick to the surface due to capillary forces. The cantilevers are therefore designed to have spring constants, on average, between, 2 N/m and 50 N/m. Resonant frequencies for tapping mode in air cantilevers are usually between 50 to 400 kHz. Figure 6a is an optical image of a typical tapping mode probe while Figure 6b is an SEM image of a tapping mode probe showing the tip in more detail. The probes are usually made of silicon and have tip diameters of approximately 20 or less.