Atomic Force Microscope: Imaging Modes

Tapping Mode:

• In general, most samples develop a liquid meniscus layer. Hence, the probe tip is kept close enough to sample for detecting short-range forces, but away from the tip sticking to surface presents (see Figure 19.05).

Figure 19.05: Schematic of tapping mode imaging.

• In this mode, the cantilever is driven to oscillate up and down at near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder. However, the amplitude of this oscillation is greater than 10 nm, typically 100 to 200 nm. The interaction of forces acting on the cantilever when the tip comes close to the surface, Van der Waals forces, dipole-dipole interactions, electrostatic forces, etc. cause the amplitude of this oscillation to decrease when the tip moves closer to sample surface. An electronic servo uses the piezoelectric actuator to control the height of the cantilever above the sample surface. The servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample.

• A tapping AFM image is produced by imaging the force of the intermittent contacts of the tip with the sample surface. This method lessens the damage done to the surface and the tip compared to contact mode.