Multifrequency electrostatic force microscopy in the repulsive regime

Atomic force microscopy on polymer surfaces is commonly carried out in the repulsive amplitude modulation mode ( tapping mode). In order to achieve an additional measurement of electrostatic quantities under ambient conditions a higher flexural eigenmode is used for electrical characterization of the surface during imaging. The higher eigenmode is resonantly excited by a small ac-bias voltage between tip and sample. Thus, topography and electric potential are measured simultaneously by this multifrequency electrostatic force microscopy method. However, the strong mechanical non-linearity of the repulsive tip-sample contact may induce a complex dynamics. To minimize possible cross-talk between topographic and electrostatic information, the oscillation amplitude of the eigenmode used for electrostatic characterization has to be kept much smaller than the amplitude of the eigenmode used for topographic imaging. Additionally, image data such as topography, mechanical phase and electrostatic information needs to be verified and possible interferences between the information channels have to be identified. The capabilities of this resonant multifrequency operation principle are demonstrated by characterizing charge patterns on a polymer electret. The resonant multifrequency technique is an extension to the conventional amplitude modulation mode and thus can be easily implemented in most atomic force microscopes.