AFM has three differing modes of operation. These are contact mode, tapping mode and non-contact mode.
In contact mode the tip contacts the surface through the adsorbed fluid layer on the sample surface. The detector monitors the changing cantilever deflection and the force is calculated using Hooke’s law:
F = − k x (F = force, k = spring constant, x = cantilever deflection)
The feedback circuit adjusts the probe height to try and maintain a constant force and deflection on the cantilever. This is known as the deflection setpoint.
In tapping mode the cantilever oscillates at or slightly below its resonant frequency. The amplitude of oscillation typically ranges from 20 nm to 100 nm. The tip lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing.
Because the forces on the tip change as the tip-surface separation changes, the resonant frequency of the cantilever is dependent on this separation.
The oscillation is also damped when the tip is closer to the surface. Hence changes in the oscillation amplitude can be used to measure the distance between the tip and the surface. The feedback circuit adjusts the probe height to try and maintain a constant amplitude of oscillation i.e. the amplitude setpoint.
In non-contact mode the cantilever oscillates near the surface of the sample, but does not contact it. The oscillation is at slightly above the resonant frequency. Van der Waals and other long-range forces decrease the resonant frequency just above the surface. This decrease in resonant frequency causes the amplitude of oscillation to decrease.
In ambient conditions the adsorbed fluid layer is often significantly thicker than the region where van der Waals forces are significant. So the probe is either out of range of the van der Waals forces it attempts to measure, or becomes trapped in the fluid layer. Therefore non-contact mode AFM works best under ultra-high vacuum conditions.
Comparison of modes
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