Scanning tunneling microscopy (STM) was used to probe electron-tunneling barriers for molecular monolayers physisorbed onto the surface of highly oriented pyrolytic graphite using the tip modulation method. In this way direct comparisons were made between the typical topographical data and the tunneling barrier measurements. Measurements were conducted for non molecular orbital (MO)-mediated tunneling (in the case of monolayers of stearic acid), as well as MO-mediated tunneling (in the case of monolayers of a planar polycyclic aromatic compound, indolo[2,1-b]quinazoline-6,12-dione, aka tryptanthrin). For the non MO-mediated tunneling case, the barrier height map was observed to correlate with the electrostatic potential (ESP) of the adsorbed molecules; indeed, the effects of the ESP distribution were used to interpret previously unreported subtle differences between two different conformers of stearic acid adsorbates. For the MO-mediated tunneling case, the barrier height map was observed to correspond to the spatial properties of the MO directly, with little correlation to the ESP distribution of the molecule, as indicated by excellent agreement of the experimental data with STM barrier height simulations developed by the authors from density functional theory computational results of the molecules themselves.

Sub-molecular tunneling barrier measurements of molecular adlayers at the solution-graphite interface

2019 Spring National Meeting

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