Fourier transformed atomic force microscopy: tapping mode atomic force microscopy beyond the Hookian approximation

The periodic impact force induced by the tip–sample contact in tapping mode atomic force microscopy (TM-AFM) gives rise to anharmonic oscillations of the sensing cantilever. These anharmonic signals can be understood with a model which goes beyond the common Hookian approximation: the cantilever is described as a multiple degree of freedom system. A theoretical analysis of the anharmonic signals in the light of the extended model shows that these signals contain information on the elastic properties of the specimen surface. In Fourier transformed operation mode of TM-AFM the anharmonic oscillations are analyzed in the frequency domain. This allows for the reconstruction of characteristics of the tip–sample force, like contact time and maximum contact force.