Feedback stabilized force-sensors: a gateway to the direct measurement of interaction potentials

A plot of the force interaction between a tip and a surface as a function of their separation constitutes a force curve which is like a fingerprint identifying a particular physical system, its characteristics dependent on the nature of the tip and sample and the potential between them. Unfortunately, if the tip is mounted on a compliant cantilever for high force sensitivity, then at some point it will snap into contact with the surface as a result of high force gradients or thermal or mechanical noise. Such jumps complicate data interpretation because they leave discontinuities in the force curve. In atomic force microscopy they can be particularly detrimental, as the release in energy may cause permanent blunting of the carefully fabricated tip structure. As every set of images starts with a force curve measurement, in the form of an initial tip-sample approach, instrument instabilities are of widespread concern to probe microscopists as well as those interested in localized point contact measurements.