FRICTION EFFECTS ON FORCE MEASUREMENTS WITH AN ATOMIC-FORCE MICROSCOPE

The deflection of an atomic force microscope (AFM) cantilever by a sam ple that is approached or withdrawn can be used to produce a Plot of f orce versus sample Position. These force curves are becoming important for investigating intermolecular forces and surface properties. Howev er, many components of the force curves are not yet understood. Here w e show that friction as the tip slides on the surface contributes to t he hysteresis in the contact part of the force curve. This sliding ari ses when the cantilever is mounted at an angle (about 15-degrees in ou r microscope) to the surface, which forces the tip to move forward aft er contact is made during the approach. While the cantilever bends upw ard, it is bowed forward by the friction. This creates an offset in th e contact line which is important to interpreting the curves, the part icular when force curves are converted into curves of force versus sep aration distance, since the contact line is then taken as zero separat ion. In addition, the sudden onset of the friction at contact can caus e a discontinuity in the force curve which sometimes appears as a shor t range attractive interaction. The friction effect on the contact lin e is uncoupled from the hysteresis in the noncontact part of the curve , which arises from the viscosity of the medium in which the measureme nt is made.