Scanning tunneling microscopy of adenine layers self-assembled on the graphite surface: Molecular packing and crystallite morphology

Scanning tunneling microscopy (STM) has been performed in air on layers of the nucleic acid base, adenine, adsorbed to graphite surfaces following evaporation of saturated aqueous solutions. Our results indicate that the structure of the monolayer is indistinguishable from that formed in ultra high vacuum by molecular beam epitaxy. We have observed multiple adjacent domains, lattice mismatches, multilayer formation and lateral diffusion phenomena exhibited by growth and dissolution processes. We can interpret these observations in terms of a model of the epitaxially registered adsorbate structure stabilized by intermolecular hydrogen bonds. Our examination of the adsorbate as a function of tunneling conditions and scan direction also demonstrates the effect of these parameters on the observed STM image contrast and the monolayer stability.