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

Abstract

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.

Soft versus hard junction formation for α-terthiophene molecular wires and their charge transfer complexes

“…The observation of more than one conductance value for a single dithiol molecule (with trace selection) agrees with previous work on alkanedithiols 1,12,14,19 and also on conjugated dithiols. 20 In this instance, the maximum in the unselected data (ca.…”

“…Using this preamplifier, we could resolve two conductance values, one (2.56 × 10 −6 G 0 ) below the minimum observable in the earlier experiments and the other (8.91 × 10 −6 G 0 ) correlating with the lower of the two values observed with the 100 nA preamplifier. Table I summarises the data; the factors by which the three different conductance groups seen for uncomplexed 1 differ (from lowest to highest, 3.5 and 5.5) are similar but not identical to the factors typically observed for alkanedithiols (4.4 and 4.0 for 1,8-octanedithiol, for example 14 ). Turning to the factor by which each conductance value is boosted by TCNE complexation, these are (from lowest conductance value to highest), respectively, 72, 70, and 46 (using the means of STM-BJ and I(s) values where both are available).…”

“…14 This, together with the fact that higher-conductance junctions break down at shorter retraction distances, is consistent with the different conductance values corresponding to different possible contact geometries (both at step edges or at least at sites where the Au contact atom has higher coordination; one at a step edge and the other flat, or both adsorbed at a flat site), with thiolate adsorbed to a Au step edge (or similar higher-coordination Au environment) giving a higher conductance. 1,14 The most commonly observed conductance value, when using the STM-BJ method or the I(s) method with relatively high set point current, i.e., relatively short initial tip-sample distance, was the intermediate value of 5.2 × 10 −5 G 0 . In all of the latter work, trace selection was used to construct the histograms; only those traces with clear plateaus were included in the analyses.…”

“…This is consistent with the observation of Haiss et al that this conductance group is normally the most prevalent. 14 We have recently described a substantial conductance increase for junctions involving molecule 1 ( Figure 2) and related molecules upon charge transfer complex formation with the electron-deficient tetracyanoethylene (TCNE). For consistency between the two groups collaborating in this study, we used the STM-BJ method (without any data selection), although the resulting conductance histograms were extremely broad as is often the case with thiol contact groups.…”

Research Update: Molecular electronics: The single-molecule switch and transistor

“…A remark is in place here. The conductance, found for a single octanethiol molecule (∼7-8 nS), is about two orders of magnitude larger than the value reported by Haiss et al 29 It is difficult to determine the exact cause of this difference, but we emphasize on the large number of experiments, involving different octanethiol molecules and STM tips, resulting in a remarkable reproducibility of the data. 19 It might be that the contact geometries in both experiments are different, or that the effective tunneling pathway is much shorter in our case.…”