Single-molecule wire

Figure 12.12 (a) Idealised single-molecule transport experiment, (b) Single-molecule transport experiment with optimal structural control based on a (1) epitaxial molecular monolayer with well-characterised adsorbate-substrate bond and (2) a specific, chemically well-defined tip-molecule contact, (c) Mechanically gated single-molecule wire based on experiment in b and STM tip retraction. 

The Liverpool group recently pioneered electron transport studies with a single molecule wire containing the V core unit bound symmetrically between the tip of an STM and a gold substrate [228,237,293,297,298]. This work was extended by the Jiilich/Bern group in a strictly electro chemically controlled environment and combined with in-situ structure studies [231, 236,269,296]. Based on the statistical analysis of a large number of current-distance (z t-Az) and cur rent-time (h-t) traces both groups demonstrated that junction conductance increases upon the potential-induced transition from the stable dication to the radical cation V state. 

Aragones AC, Darwish N, Saletra WJ, P6rez-Garcia L, Sanz F, Puigmarti-Luis J, Amabilino DB, Diez-Perez 1 (2014) Highly conductive single-molecule wires with controlled orientation by coordination of metalloporphyrins. Nano Lett 14 4751 756... 

Monatomic Gold Wires with Single Molecules An Ab Initio Simulation. Physical Review Letters, 89, 186402-1-186402-4. 

Ballmann S, Hieringer W, Seeker D, Zheng Q, Gladysz JA, Gorling A, Weber HB (2010) Molecular wires in single-molecule junctions charge transport and vibrational excitations. ChemPhysChem 11(10) 2256-2260... 

Haiss W, Nichols RJ, van Zalinge H, Higgins SJ, Bethell D, Schiffrin DJ (2004) Measurement of single molecule conductivity using the spontaneous formation of molecular wires. PCCP 6(17) 4330-4337... 

An alternative method to position two electrodes at nanometer distances apart is the mechanically-controlled, break junction (MCBJ) technique. An ultra-thin, notched Au wire on a flexible substrate can be broken reliably by pushing on the Au with a piezoelectric piston, cracking the Au (Fig. 4). This produces a gap between the Au shards whose size can be finely varied to 1 A by a piston or control rod [46, 47]. When UE molecules with thiol groups on both ends are present in a surrounding solution, the gap can be adjusted until the molecules can span it. A dilute solution means the number of spanning molecules will be small, and the least-common-multiple of current flow among many junctions indicates those spanned by a single molecule [47]. 

How does one reliably wire a single molecule to macroscopic leads ... 

As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

Fig. 10 Simple circuit diagrams of the different series and parallel association of molecular wires Mi and M2 discussed in the text. The two molecular wires are (a) bonded in series, (b) connected in parallel on the metallic pads, (c) forming a single molecule with one intramolecular node, and (d) forming a single molecule with two intramolecular nodes...

Figure 20. A representation of the technique used in the mechanically controllable break junction for recording the current through a single molecule, (a) The gold wire was coated with a SAM of the molecular wires (b) and then broken, under solution (c), via extension of the piezo element under the silicon surface (see Figure 19). Evaporation of the volatile components and slow movement of the piezo downward (see Figure 19) permits one molecule to bridge the gap (d) that is shown, in expanded view, in the insert. The insert shows a benzene-1,4-dithiolate molecule between proximal gold electrodes. The thiolate is normally FI-terminated after deposition end groups denoted as X can be either FI or Au, the Au potentially arising from a previous contact/retraction event.

In principle a STM should be adequate to measure the electrical resistance of a single molecule since it suffices to measure I-V curves of the metal (tip)-molecule-metal (substrate) system. However, published results in the literature concerning this subject have to be considered cautiously because of the generally unknown nature of the molecule-metal contacts. An illustrative experiment demonstrates the relevance of the interface (Kushmerick et al, 2002). This experimental work studies charge transport using the cross-wire tunnel junction technique, where two... 

The predictive power of the luminescent PET sensor principle is again apparent here. Further, the benzocrown ether and the amine receptors would selectively bind Na" and H, respectively. A remarkable feature here is that no molecular wiring is needed to allow the human operation of this two-input molecular device. The device self-selects its own ion inputs into the appropriate signal channels by means of the chemoselective receptor modules. Since the output signal is fluorescence, even a single molecule can interface with detectors in the human domain, including the dark-adapted eye. Tanaka s 45 is another example where fluorescence quenching is achieved only when Ba and SCN are present. This was mentioned in Section 6. Similarly, several sensor systems—1,17, and 21—could be employed... 

J. Mater. Chem. 2005,15, 3260 N. Tao, Electrochemical Fabrication of Metallic Quantum Wires, J. Chem. Ed. 2005,82, 720 S. Lindsay, Single-Molecule Electronic Measurements with Metal Electrodes, J. Chem. Ed. 2005, 82, 727 R. A. Wassel and C. B. Gorman, Establishing the Molecular Basis for Molecular Electronics, Angew. Chem. Int. Ed. 2004,43, 5120. 

Ashwell, G. J., Urasinska, B., Wang, C. S., Bryce, M. R., Grace, I., Lambert, C. J., Single-molecule electrical studies on a 7 nm long molecular wire. Chem. Commun. 2006, 4706-4708. 

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