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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.
Haiss et al. (2009) studied this question.