Electrostatic forces between sharp tips and metallic and dielectric samples

A detailed analysis of electrostatic interactions between a dc-biased tip a nd a metallic or insulating sample is presented. By using a simple method t o calculate capacitances and forces, tip shape effects on the force versus tip-sample distance curves are dicussed in detail. For metallic samples the force law, except for a constant background, only depends on the tip radiu s of curvature. In contrast, for dielectric samples the forces depend on th e overall geometry of the tip. Interestingly, we found that the contact (ad hesion) force does not depend on the tip size and is bound by a simple expr ession which only depends on the applied bias and the sample dielectric con stant. (C) 2001 American Institute of Physics.