Using a newly developed atomic force microscopy (AFM) simulator ACCESS (AFM simulation code for calculating and evaluating surface structures), effects of the atomic arrangement at the tip apex and tip-sample distance on AFM image resolution were examined. A tip which has an atom protruding at its apex and is scanning in the repulsive force range is found to be necessary for obtaining atomically resolved AFM images. The second atomic layer of the tip determines the force characteristics of the system, as well as the AFM image phase shift. Since in actual AFM systems these two effects are convoluted, it is apparent that scanning under the same applied force does not necessarily mean the same tip-sample distance or the same image resolution, unless one is sure that the atomic arrangement at the proximity of the tip apex is the same. It is also found that surface point defects mirror the atomic arrangements of the tip apex in the AFM images, both in attractive and repulsive force ranges, indicating their possible use in tip apex evaluation at the atomic level.

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 (fcc) (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.

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Simulation works on the effects of tip apex geometries on atomic force microscopy (AFM) images were examined. Tips and samples employed in those simulations were mostly made of a single component. Short-range interatomic potentials such as Lennard-Jones and Morse were used. With these potentials, it was found that a single atom tip (a tip with an atom protruding at its apex) is necessary for obtaining true atomic resolution. In many cases flat tip geometries (tips with multiple atoms at their apexes) produce various images that do not correspond to the surface atom arrangements, which may lead to various faulty AFM image interpretations.