Contact mode/ constant force AFM operates by scanning the tip across the sample surface while monitoring the change in cantilever deflection with a split photodiode detector. A feedback loop maintains a constant deflection between the cantilever and the sample by vertically (z) moving the scanner at each (x,y) data point. The vertical movement is used to build an image. By maintaining a constant cantilever deflection, the force between the tip and the sample remains constant. Spring constants usually range from 0.01 to 1.0 N/m, resulting in forces ranging from nN to μN in an ambient atmosphere. Operation can take place in ambient and liquid environments.
Contact mode/ constant height AFM operates by keeping the height (z) of the scanner constant, and using the cantilever deflection, as monitored by the split photodiode detector, to directly generate the topographical image. Contact mode/ constant height mode is most useful for atomic-scale images of atomically flat surfaces.
Contact mode is useful for obtaining 3D topographical information on nanostructures and surfaces. A disadvantage of this mode is that, because the tip is in constant 'contact' with the sample surface, large lateral forces can be exerted on the sample as the tip is raster scanned over the surface resulting in damaged samples and deformed images.
Tip choice for contact mode in air
Since the tip is in essence in contact with the surface, the spring constant of the cantilever is not as stiff as for tapping mode operation in air. Spring constants tend to be <1 N/m, and the cantilever geometry is generally V-shaped to minimize cantilever twisting and buckling. Resonant frequencies for the cantilevers of contact mode probes are usually between 10 to 80 kHz. Figure 4a is an optical image of a typical contact mode probe while Figure 1b is an SEM image of a typical contact mode probe showing the tip in more detail. The probes are usually made of silicon nitride and have tip diameters of approximately 20 to 40 nm.
Figure 4. (a) Optical image of tapping mode cantilever. (b) SEM image showing the tip is more detail