A critical look at surface force measurement using a commercial atomic force microscope in the noncontact mode

The use of commercial atomic force microscopes (AFM) operating in the noncontact mode for surface force measurements is critically reviewed. Approach curves (i.e., vibration amplitude versus tip–surface distance) using standard microfabricated tips are discussed with respect to the basic theory of an equivalent harmonic oscillator. Different artifacts are addressed. In particular, we show theoretically and experimentally that the force exerted by the layer of air confined between the cantilever and the surface is a major contribution to the force on the cantilever. However, by carefully choosing the parameters (essentially the vibration amplitude) for the measurement of the approach curve, and by taking into account the damping within the confined air layer, we succeeded in measuring reliable surface force profiles with commercial AFM in the air and in describing them quantitatively by dispersion force interactions.