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 distanc e) using standard microfabricated tips are discussed with respect to t he basic theory of an equivalent harmonic oscillator. Different artifa cts are addressed. In particular, we show theoretically and experiment ally that the force exerted by the layer of air confined between the c antilever 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, a nd by taking into account the damping within the con tined air layer, we succeeded in measuring reliable surface force profiles with commerc ial AFM in the air and in describing them quantitatively by dispersion force interactions. (C) 1997 American Institute of Physics. [S0034-67 48(97)05310-0].