Electrostatic and contact forces in force microscopy

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Abstract
We have been measuring the electrostatic and contact forces between a tip and a graphite surface in a force microscope, which uses a polarizing optical interferometer. For large distances where the electrostatic force predominates, the data are analyzed in terms of a model which introduces the elongated shape of an actual tip. We find that the macroscopic tip has to be taken into account when analyzing the experimental data. The model allows us to deduce the effective radius of the tip which is operative in the electrostatic interaction. We also analyze the contact problem. The attractive part is consistent with van der Waals (vdW) forces. The repulsive regime shows an anomalously small level deflection which is attributed to the deformation of the sample surface. The adhesion of the tip sample is also measured.

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