Van der Waals and capacitive forces in atomic force microscopies

In this article we show that in the atomic force microscopy experiments performed on a metallic surface, there is always a long range electrostatic force in addition to the van der Waals forces. This capacitive force is due to the contact potential between the tip and the surface and exists even without external applied potential. We have calculated this capacitive force for a real geometry of the tip-sample system and compared it to the van der Waals force calculated for the same geometry. We conclude that the electrostatic force is always dominant for a tip-surface distance larger than half of the tip radius of curvature. (C) 1999 American Institute of Physics. [S0021-8979(99)04421-7].