Molecular Junctions based on Aromatic Coupling

S. Wu, M.-T. González, R. Huber, S. Grunder, M. Mayor, Ch. Schönenberger & Michel Calame
University of Basel, Klingelbergstrasse. 82, CH-4056 Basel
Adapted from S. Wu et al., Nature Nanotech. 3, 569 (2008).

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligophenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic π-π coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

To determine the electronic properties of devices based on single molecules [1, 2, 3], a single or a few molecules need to be wired between at least two electrodes, a source and a drain electrode. We use a gold wire with a constriction in its center that is continuously stretched, resulting in a narrowing of its diameter at the constriction (Fig.1a & b). This process is carried on down to the atomic scale and ends with the breaking open of the gold bridge, giving access to two atomic contacts [4]. This technique, termed mechanically controllable break junction (MCBJ), can be used to form molecular junctions in a liquid environment [5, 6]. Traditionally, molecules are synthesized with two terminal anchor groups (typically -SH) at both ends which allow the immobilization of the molecule between the two atomic contacts. The electrical conductance G is measured while opening the junction. When the Au bridge is stretched, G(z) decreases, showing conductance plateaus for G values above the quantum conductance unit G₀ ≡ 2e²/h (Fig.1c). There is a so-called “last plateau” at G ≈ G₀. This last plateau corresponds to a single atom Au bridge. If the junction is elongated further, it breaks open. The down-jump in conductance typically stops at a value of G ≈ 10^-3 G₀, when electron tunneling between the electrodes sets in. Electron tunneling with a constant tunneling barrier height results in a linear dependence of log(G) versus z as observed in the measurements. To form molecular junctions, the breaking process is performed in presence of a solution containing molecules bearing two anchor groups. Because a Au-S bond is stronger than a Au-Au bond [7, 8], Au atoms are pulled and migrate to the ends of the Au electrodes forming elongated tips when the electrodes are further separated apart. This process continues until the force which has been built up exceeds the limit given by the Au-Au bond. Then, the molecular junction breaks open. In our study we use conjugated oligo-phenylene ethynylene (OPE) molecules as a model system. Conjugated molecules are interesting candidates for electron transport due to the delocalization of electrons throughout the molecular backbone [9, 10]. Such a structure results in a lower HOMO-LUMO gap (~ 3 eV) as compared to that of saturated molecules (~ 7 eV), leading to a higher charge transport efficiency through the molecule. Measuring the electrical conductance G during the breaking process in presence of molecules, we can anticipate that G will stay approximately constant during stretching when a molecular junction forms until the junction breaks open (Fig.1d). A statistical approach is used, in which sequential open-close cycles are performed to repeatedly form molecular junctions. Conductance histograms are then built as shown in Fig. 1c & d and Fig. 2. A peak in the histogram, e.g. at G ≈ 10^-4 G₀ in Fig. 1d, represents the signature of the molecular junction formation. For molecules with a single anchor group, one would a priori presume that no stable metal-molecule-metal junction can form since the molecules cannot attach on both sides of the junction. The data in Fig. 2 compare conductance histograms obtained for OPE molecules bearing two (top) or only one (bottom) anchor group. While the peak in the top histogram is expected due to the presence of the two anchor groups, that of the second histogram comes more as a surprise. It clearly shows that stable molecular bridges can form, even if a single anchor group is present in the molecule. We think that the connection between the electrodes is made possible by π-π stacking interaction between a pair of adjacent molecules [11, 12, 13]. If one molecule is anchored via its thiol linker group on e.g. the left electrode, another one bound to the right electrode can complete the mechanical assembly of the junction via π-π coupling through the phenyl rings. This interpretation is supported by the shift of G_peak to lower values by more than one order of magnitude. A reduced average conductance value can indeed be expected because a molecular bridge formed by a stacked pair of molecules will be longer (~ 29.1 Å) than a single dithiol molecule anchored between Au electrodes (~ 20.7 Å). The distance that electrons have to tunnel between the Au electrodes is therefore slightly larger for the stacked bridge. From the study summarized above, we can infer that intermolecular π-π stacking interaction between monothiol molecules composed of alternating phenylene and ethynylene units is strong enough to induce the formation of molecular junctions. This is a significant finding for molecular electronics. Intermolecular aromatic stacking plays a determinant role in stabilizing nanoobjects. The importance of π-π overlap has long been recognized in thin-film organic electronics, molecular mechanics, and especially in biomolecular and supramolecular chemistry. We show here that π-π stacking can also be used as the dominant guiding force for the formation of molecular bridges in few molecules electronic junctions. These experimental findings provide a strong ground for the design of future electro-mechanical and sensing devices operating at the single molecule level.