On determining the contact point in force measurements by atomic force microscopy methods

Abstract

Atomic force microscopy (AFM) is a widely used technique for obtaining information about the three-dimensional structure and physical and mechanical properties of surfaces. In nanoindentation methods, an AFM probe is brought to the sample from a distance and then interacts with the surface. Accurate determination of the point of contact of the probe tip with the surface in these measurements is necessary for reliable calculation of both the non-contact interaction constants and the structural and mechanical properties of the material. The tip-surface contact is preceded by a rapid jump of the probe to the surface due to attractive forces in experiments conducted in ambient conditions. If the stiffness of the probe beam or the sample does not allow the probe to deform the surface, then the end of the probe jump (minimum deflection of the beam as the probe approaches the material) is usually taken as the start of the contact. If the surface is soft enough, contact may occur earlier than the end of the rapid jump, i.e., the tip will begin to penetrate into the surface before the jump is complete. In this work it is shown that the contact of the tip with a deformable polymer surface occurs before the completion of the jump of the tip to the surface. It is reasonable to take the point corresponding to the maximum change in the beam deflection as the beginning of the contact between the probe and the material. The significant speed of the approach of the tip to the surface under the action of attractive forces leads to a loss of experimental data in the section of the curve near the transition from non-contact to contact interaction. Insufficient rate of data capture and high approach velocity can cause significant points of the force curve to be missed, i.e., lead to errors in determining the beginning of the contact both for soft and hard surfaces.