Impact of air damping in quantitative dynamic AFM-measurements

As oscillating AFM probe approaches the sample, the amplitude and phase shift of the cantilever begin to change at distances of tens of microns away from the surface. Consequently, before the start of semicontact tip-surface interactions, the dynamic characteristics of the cantilever are shifted from their undisturbed values. This change is attributed mainly to the air damping in the gap between the cantilever and the sample. The damping is due to the peculiarities of geometry of sample surface as well as the properties of the probe. The observed behavior is studied by harmonic oscillator model. The results show that damping in the near-surface gap is described by introducing a single additional parameter into the model. The values of this parameter are calculated for various rectangular cantilevers. The numerical simulations and experimental data treatment illustrates a significance of influence of air damping in the gap on the determination of the physical and mechanical constants of the material.