ANALYSIS OF ADENINE AND THYMINE ADSORBED ON GRAPHITE BY SCANNING TUNNELING AND ATOMIC FORCE MICROSCOPY

Analysis of AFM of adenine and thymine adsorbed onto a hot graphite surface shows that these molecules are adsorbed in patches typically 50 nm wide and 5 nm high, but does not provide any discernable atomic structure. In contrast, STM image scans that contain both parts of the graphite substrate and of the adsorbate would mislead one to believe that the adsorbates consist of just one ordered monolayer of either adenine of thymine (like alkylcyanobiphenyl on graphite [D.P.E. Smith et al., Nature 344 (1990)641]). From these STM images the lattice dimensions, structural periodicities, and the epitaxy of the adsorbed molecules with respect to the basal plane of graphite can be determined. The aromatic regions are strongly detected with near-atomic resolution in both molecules, while the various sidegroups are not well resolved. Thus STM can discriminate between purines and pyrimidines - if they are held in thick patches.