The interaction between molecules is an important fundamental issue in the research of chemistry[1,2], materials[3,4], biology[5,6], and other related areas. For two conjugate molecules close to each other, there has been a lot of experimental evidence to prove that they can be stacked with each other through the intermolecular interaction between π electron clouds, which provides an efficient path for electron transport between molecules[7,8]. However, there is no experimental evidence showing that the σ-σ stacking interactions between two non-conjugated molecules can offer efficient charge transport through the supramolecular interactions between two monomers. This is mainly due to the generally underestimated weak intermolecular σ-σ stacking interactions compared with π-π stacking interactions. Recent theoretical studies have indicated that σ-σ stacking interactions and π-π stacking interactions may be equally important in stacked aromatic rings[9,10] and some pioneering studies using self-assembled monolayer (SAM) junctions demonstrated that lateral transport could occur via the through-space pathway among the adjacent hydrocarbon molecules within the SAM[11,12]. Single-molecule electrical characterization techniques provide us with a means to investigate charge transport through supramolecular junctions with controllable gap distance between the two electrodes[13,14].
Here, we selected non-conjugated cyclohexanethiol and single-anchored adamantane molecules to fabricate and investigate charge transport through σ-σ stacked supramolecular junctions using the scanning tunneling microscope break junction (STM-BJ) technique. Our measurements demonstrated experimentally that the existence of σ-σ stacking interactions between neighboring non-conjugated cyclohexanethiol molecules is efficient enough to serve as a pathway for charge transport. We found that there are two different conductance states for σ-σ stacked cyclohexanethiol junctions formed during the break junction measurement, and the charge transport capacity of the stacked cyclohexanethiol dimer junction is comparable to that of the π-π stacked benzenethiol junction.
Similar results were also obtained when we investigated single-anchored adamantane molecules, which have a highly symmetric cage structure consisting of four identical cyclohexane rings in the armchair configuration. The current-voltage characteristics demonstrate the existence of stacked molecular junctions with a symmetrical geometry configuration connected between the electrode pair, and the flicker noise analysis suggests that the pathway of charge transport through these σ-σ stacked molecular junctions has an obvious through-space characteristic. The specific configuration of these stacked molecular junctions is proposed, and the charge transport through σ-σ stacking intermolecular interactions is supported by theoretical calculations. Our findings open an avenue for the fabrication of supramolecular junctions using non-conjugated molecules, and this will increase the structural diversity of molecular devices and materials.
An abstract figure of this work. Left: the σ-σ stacked supramolecular junction formed with two cyclohexane rings. Right: the π-π stacked supramolecular junction formed with two benzene rings. The bluish-purple and pink regions between two molecules indicate the σ-σ interactions and π-π interactions, respectively. The red arrows with electrons show the charge transport from one molecule to the other one. The equal-arm balance with two ends balanced depicts that the charge transport capability of σ-σ interactions can be comparable to that of π-π interactions.
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