Simulation of atomic force microscopy image variations along the surface normal: Presence of possible resolution limit in the attractive force range

Variations of atomic force microscopy (AFM) images as tip-sample distance is varied were examined using a newly developed AFM simulation code ACCESS (AFM simulation Code for Calculating and Evaluating Surface structures) with a Morse-type pairwise potential. A model system consisting of a single atom tip (an “ideal” AFM tip) and a face-centered cubic (fee) (100) surface with or without point defect showed perfect atomic resolution when scanning was performed in the repulsive force range. In the attractive force range, image contrast inversion was observed twice with the increase of tip-sample distance. Simulation on the point defect surface indicated that this inversion is due to a collective force from more than one atom being imaged as one bright spot. Exact correspondence between the sample surface registry and the simulated image in this force range appears to be coincidental.