Contribution of Chemical Bonding to the Force in Atomic Force Microscopy

Non-contact atomic force microscope (NC-AFM) has become a powerful tool. It can provide the atomic structure and chemical bonding information at the atomic scale. Three kinds of tip-sample interactions are often concerned: including van der Waals interaction, electrostatic interaction, and chemical bonding interaction. In this work, the chemical bonding interaction between the tip and a Pb film is clearly demonstrated by NC-AFM based on a Q-plus force sensor. The tip-sample interaction energy versus the bias voltage was obtained and fitted by a parabolic function to find the effective local contact potential difference, which decreased with increasing tip-sample distance. Such a trend is caused by the wave function overlap. Thus, the decay length of the electron wave function was estimated. Oscillation of the decay length with film thickness was also observed, which can be attributed to the thickness-dependent quantum well states in the Pb islands.