Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution-Graphite Interface: Nonorbital-Mediated Tunneling

摘要

Semiquantitative apparent tunneling barrier measurements were collected of stearic acid adsorbed onto highly oriented pyrolytic graphite from 1-phenyloctane. A detailed investigation of the topography revealed an interesting geometric phenomenon, namely, the observation of different conformers within a single monolayer; this phenomenon has not been previously reported, despite the large volume of work on these and similar systems in the literature. Apparent barrier height images showed two separate effects. The apparent barrier height map is dominated by the distribution of surface charge (as tracked by electrostatic potential) in regions where significant polarity exists (i.e., near the carboxyl groups). Conversely, in regions where there is little variation in electrostatic potential (i.e., near the interlaced alkane chains), the manifestation of the topography dominates the apparent barrier height. The electron tunneling involved in imaging these types of monolayers is not mediated by adlayer molecular orbitals since their energies fall outside of the Fermi levels of the substrate and the tip under the tunneling conditions described. These studies provide a basis for further investigations into barrier height tunneling spectroscopy of molecular monolayers on graphite.