Molecular dynamic simulation of tip-polymer interaction in tapping-mode atomic force microscopy

We present a molecular dynamic study of the interaction between an amorphous silica tip (SiO2) and an amorphous poly-(methyl- methacrylate) substrate under conditions relevant for tapping-mode atomic force microscopy. To capture the actual dynamics of the tip, we use the dynamic contact simulation method [Kim, J. Appl. Phys. 112, 094325 (2012)]. We obtain force-displacement relationships both for neat polymer substrates and a sample with a sub-surface nanotube and extract the local stiffness and energy dissipation per cycle. The simulations capture non-trivial aspects of the interaction that originate from the viscoelastic nature of the polymer including an increase in repulsive interaction force during approach with tip velocity and an increase in adhesion during retraction with decreasing tip velocity. Scans of local stiffness and dissipation over the samples reveal intrinsic variability in the amorphous polymer but also the effect of local surface topography on the extracted properties as well as the ability of the method to detect a sub-surface nanotube. This insight and quantitative data should be valuable to interpret the results of atomic force microscopy studies.