MONOLAYER GUANINE AND ADENINE ON GRAPHITE IN NACL SOLUTION - A COMPARATIVE STM AND AFM STUDY

Nucleation, growth, and molecular packing structures of monolayer guanine and adenine on graphite have been studied in NaCl aqueous solution (containing guanine and adenine) using both STM and AFM. Guanine and adenine condense into monolayer films on graphite spontaneously. The films dissolve at low substrate potentials, starting either from step edges or by developing pits in the films, and they grow back Bt high potentials. Both the nuclei in the growth and the pits in the dissolution processes tend to be line shaped, which suggests anisotropic line tensions for the monolayer islands. High resolution AFM images show that guanine molecules pack into a rectangular lattice, on the basis of which a hydrogen bonded network model is proposed. In sharp contrast, high resolution STM images reveal more complex features including a superperiodic structure superimposed on the ordered guanine lattice. The superperiodic structure can be attributed to an electronic modulation of the underlying graphite lattice on the guanine lattice. Both AFM and STM show that adenine packs into a centered rectangular lattice, from which a hydrogen bonded network model is also deduced. The molecular packing structures of monolayer guanine and adenine measured by by AFM are independent of the substrate potential over a broad potential range, while the electronic states probed by STM are sensitive to the potential. The STM images also depend upon tip-substrate bias and tunneling current.