David C. Milan

Electrochemical Single-Molecule Transistors with Optimized Gate Coupling

Electrochemical gating at the single molecule level of viologen molecular bridges in ionic liquids is examined. Contrary to previous data recorded in aqueous electrolytes, a clear and sharp peak in the single molecule conductance versus electrochemical potential data is obtained in ionic liquids. These data are rationalized in terms of a two-step electrochemical model for charge transport across the redox bridge. In this model the gate coupling in the ionic liquid is found to be fully effective with a modeled gate coupling parameter, ξ, of unity. This compares to a much lower gate coupling parameter of 0.2 for the equivalent aqueous gating system. This study shows that ionic liquids are far more effective media for gating the conductance of single molecules than either solid-state three-terminal platforms created using nanolithography, or aqueous media.

Single-Molecule Conductance of Viologen–Cucurbit[8]uril Host–Guest Complexes

The local molecular environment is a critical factor which should be taken into account when measuring single-molecule electrical properties in condensed media or in the design of future molecular electronic or single molecule sensing devices. Supramolecular interactions can be used to control the local environment in molecular assemblies and have been used to create microenvironments, for instance, for chemical reactions. Here, we use supramolecular interactions to create microenvironments which influence the electrical conductance of single molecule wires. Cucurbit[8]uril (CB[8]) with a large hydrophobic cavity was used to host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular complex. Significant increases in the viologen wire single molecule conductances are observed when it is threaded into CB[8] due to large changes of the molecular microenvironment. The results were interpreted within the framework of a Marcus-type model for electron transfer as arising from a reduction in outer-sphere reorganization energy when the viologen is confined within the hydrophobic CB[8] cavity.

Single-Molecule Conductance Studies of Organometallic Complexes Bearing 3-Thienyl Contacting Groups

Abstract The compounds and complexes 1,4‐C6H4(C≡C‐cyclo‐3‐C4H3S)2 (2), trans‐[Pt(C≡C‐cyclo‐3‐C4H3S)2(PEt3)2] (3), trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2(dppe)2] (4; dppe=1,2‐bis(diphenylphosphino)ethane) and trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2{P(OEt)3}4] (5) featuring the 3‐thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3–5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break‐junction (STM‐BJ) and STM‐I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM‐I(s) methods, and a higher feature by the STM‐BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi‐parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non‐resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2–5.

Low variability of single-molecule conductance assisted by bulky metal–molecule contacts

A detailed study of the trimethylsilylethynyl moiety, –CCSiMe3 (TMSE), as an anchoring group in metal|molecule|metal junctions, using a combination of experiment and density functional theory is presented. It is shown that the TMSE anchoring group provides improved control over the molecule–substrate arrangement within metal|molecule|metal junctions, with the steric bulk of the methyl groups limiting the number of highly transmissive binding sites at the electrode surface, resulting in a single sharp peak in the conductance histograms recorded by both the in situ break junction and I(s) STM techniques. As a consequence of the low accessibility of the TMSE group to surface binding configurations of measurable conductance, only about 10% of gold break junction formation cycles result in the clear formation of molecular junctions in the experimental histograms. The DFT-computed transmission characteristics of junctions formed from the TMSE-contacted oligo(phenylene)ethynylene (OPE)-based molecules described here are dominated by tunneling effects through the highest-occupied molecular orbitals (HOMOs). This gives rise to similar conductance characteristics in these TMSE-contacted systems as found in low conductance-type junctions based on comparably structured OPE-derivatives with amine-contacts that also conduct through HOMO-based channels.

Comparing the distribution of the electronic gap of an organic molecule with its photoluminescence spectrum

The electronic gap structure of the organic molecule N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, also known as TPD, has been studied by means of a Scanning Tunneling Microscope (STM) and by Photoluminescence (PL) analysis. Hundreds of current-voltage characteristics measured at different spots of the sample show the typical behavior of a semiconductor. The analysis of the curves allows to construct a gap distribution histogram which reassembles the PL spectrum of this compound. This analysis demonstrates that STM can give relevant information, not only related to the expected value of a semiconductor gap but also on its distribution which affects its physical properties such as its PL.

Unconventional Single-Molecule Conductance Behavior for a New Heterocyclic Anchoring Group: Pyrazolyl

Electrical conductance across a molecular junction is strongly determined by the anchoring group of the molecule. Here we highlight the unusual behavior of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene that exhibits unconventional junction current versus junction-stretching distance curves, which are peak-shaped and feature two conducting states of 2.3 × 10-4 G0 and 3.4 × 10-4 G0. A combination of theory and experiments is used to understand the conductance of single-molecule junctions featuring this new anchoring group, i.e., pyrazolyl. These results demonstrate that the pyrazolyl moiety changes its protonation state and contact binding during junction evolution and that it also binds in either end-on or facial geometries with gold contacts. The pyrazolyl moiety holds general interest as a contacting group, because this linkage leads to a strong double anchoring of the molecule to the gold electrode, resulting in enhanced conductance values.

Influence of surface coverage on the formation of 4,4′-bipyridinium (viologen) single molecular junctions

Single-molecule conductance experiments using the STM-based I(s) method and samples of N,N′-di(4-(trimethylsilylethynyl)benzyl)-4,4′-bipyridinium bis(tetrafluoroborate) ([1](BF4)2) prepared on gold substrates with low-surface coverage of [1](BF4)2 (Γ = 1.25 × 10−11 mol cm−2) give rise to molecular junctions with two distinct conductance values. From the associated break-off distances and comparison experiments with related compounds, the higher conductance junctions are attributed to molecular contacts between the molecule and the electrodes via the N,N′-dibenzyl-4,4′-bipyridinium (viologen) moiety and one trimethylsilylethynyl (TMSE) group (G = (5.4 ± 0.95) × 10−5 G0, break-off distance (1.56 ± 0.09) nm). The second, lower conductance junction (G = (0.84 ± 0.09) × 10−5 G0) is consistent with an extended molecular conformation between the substrate and tip contacted through the two TMSE groups giving rise to a break-off distance (1.95 ± 0.12) nm that compares well with the Si⋯Si distance (2.0 nm) in the extended molecule. Langmuir monolayers of [1](BF4)2 formed at the air–water interface can be transferred onto a gold-on-glass substrate by the Langmuir–Blodgett (LB) technique to give well-ordered, compact films with surface coverage Γ = 2.0 × 10−10 mol cm−2. Single-molecule conductance experiments using the STM-based I(s) method reveal only the higher conductance junctions (G = (5.4 ± 0.95) × 10−5 G0, break-off distance (1.56 ± 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films.