Influence of experimental conditions on apparent AFM tip-surface contact in air

Accurate determination of the onset of the contact between the atomic force microscope (AFM) tip and the surface in force measurements is necessary both for calculating the constants of non-contact interactions and for determining the structural and mechanical properties of the materials. In an air environment the contact is preceded by a rapid jump of the tip to the surface due to attractive forces. If a surface is not deformed by a probe of a given stiffness, the end of the jump (minimum of the deflection of the cantilever) is taken as the onset of the contact. In this work, it is shown that the tip contacts with the deformable surface before the completion of the jump. It is reasonable to use the point of the extreme deflection velocity as the onset of the contact. A dynamic mass-spring model of the interaction of the tip with the viscoelastic material has shown that the high deflection velocity and the low rate of data acquisition (bandwidth) are the reasons for significant errors in determining the contact from the experimental force curve. In the most cases, the observed contact point is lower than the actual one (under certain conditions, the opposite situation is also possible) and the measurement error is at least 50 % (depending on the experimental settings, the properties of the probe and the material).