Atomic fingerprinting of heteroatoms using noncontact atomic force microscopy*

Richard P. Feynman said the following during his talk at the APS annual meeting in 1959: “If you have a strange substance and you want to know what it is, you go through a long and complicated process of chemical analysis.… It would be very easy to make an analysis of any complicated chemical substance; all one would have to do would be to look at it and see where the atoms are. The only trouble is that the electron microscope is one hundred times too poor.” Feynman’s idea has been, partially, realized through the use of noncontact atomic force microscope (nc-AFM). Nc-AFM with a functionalized CO tip made visualization of molecules and atoms possible. This atomic resolution dazzles the scientific world and opens new avenues of science. However, the measured nc-AFM images are sometimes hard to interpret. To be specific, using the common CO tip can barely distinguish heteroatoms (such as N, S, etc.) from C atoms.

We employ real-space pseudopotentials constructed within density functional theory code, PARSEC (http://real-space.org/), to simulate nc-AFM images using various functionalized tips at different tip heights. Our simulation work has demonstrated that using nc-AFM with a chemically inert tip (CO tip) followed by a metallic tip (Cu tip) is a promising method to discriminate heteroatoms from C atoms within molecules.