Impact of Junction Formation Method and Surface Roughness on Single Molecule Conductance

In recent years, several experimental studies have shown that different values of single molecule conductance
can be observed for the same type of molecule. Although this observation has been tentatively attributed
either to differing molecular conformations or to differing contact geometries, the reason for the different
conductance groups remains still unclear. To elucidate this issue, a comparison of four different experimental
methods to measure single molecule conductance is presented here for the case of alkanedithiols between
gold electrodes, which is considered to be a model system. Three different fundamental conductance groups
exhibiting low, medium, and high conductance, respectively, were observed for each molecule. The comparison
of measurements performed on surface areas with different step densities reveals that the medium (high)
conductance group can be attributed to the adsorption of one (two) contacting S atoms at step sites, whereas
the low conductance group can be attributed to molecules adsorbed between flat surface regions. This finding
is corroborated by a gap separation analysis for the different conduction groups, by matrix isolation
measurements, and by a comparison of the results presented here with conductance measurements performed
on self-assembled monolayers. The results presented here help to resolve apparent discrepancies in single
molecule conductance measurements and are of general significance for molecular electronics and
electrochemistry, since they show how molecular conductance is influenced by the contact morphology and,
thus, by the atomic structure of the substrate surface.