Single-molecule junctions

Lortscher E, Ciszek JW, Tour J, Riel H (2006) Reversible and controllable switching of a single-molecule junction. Small 2(8-9) 973-977... 

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... 

Venkataraman L, Klare JE, Nuckolls C, Hybertsen MS, Steigerwald ML (2006) Dependence of single-molecule junction conductance on molecular conformation. Nature 442 904-908... 

Park YS, Widawsky JR, Kamenetska M, Steigerwald ML, Hybertsen MS, Nuckolls C, Venkataraman L (2009) Frustrated rotations in single-molecule junctions. J Am Chem Soc 131 10820-10821... 

Single-Molecule Junctions of Thiol-Terminated Biphenyls. 154... 

Single Molecule Junctions with Nitrile-Terminated Biphenyls. 156... 

Electrochemically Gate-Controlled Conductance of 4,4 - Bipyridine Single-Molecule Junctions. 158... 

The following fundamental aspects represent some of the key challenges in attempts to understand charge transport in (single-) molecule junctions in more detail ... 

Figure 19b displays the 2D histogram of the experimentally obtained conductance of N4 plotted vs distance [63]. The distance scale z is normalized with respect to z = 0 at G = 0.7 G0, to a common point. The chosen procedure is justified, because of the steep decay of the tunneling current after breaking of the last atomic contact. The histogram counts the occurrence of [log(G/Go), z ] pairs in a 2D field. Figure 19b exhibits the features of gold quantum contacts at G > Go, and a second cloud-like pattern in [10 5 10 4 G0, 0 0.5 nm]. We attribute the latter to the formation of single-molecule junctions of only one type. The center of the cloud is located at G = 3.5 4.5 x 10 5 Go, close to the peak position in the ID histogram (Fig. 19a). The extension of the cloud along the distance scale is around 0.5 nm, close to the typical length of the plateaus (the inset of Fig. 19a).

Attachment sites for a molecule within a single molecule junction... 

The take-home message here is that conductivity measurements in single-molecule junctions are difficult to analyze without the support of quantum mechanical calculations that include the metal electrodes. This is very much the domain of specialists, and the simple rules discussed for analyzing elastic tunneling spectra in other junction types generally do not apply for metal-single-molecule-metal junctions. 

Paulsson M, Krag C, Frederiksen T, Brandbyge M (2009) Conductance of alkanedithiol single-molecule junctions a molecular dynamics study. Nano Lett 9 117-121... 

Fig. 2 (a) Schematic diagram of single molecule junction conductance measurement, (b) Structures of a subset of the biphenyl series studied, shown in order of increasing twist angle (6) or decreasing conjugation, (c) Biphenyl Junction conductance as a function of molecular twist angle. (Reprinted with permission from [9])... 

Enormous progress has been achieved in the experimental realization of such nano-devices, we only mention the development of controllable single-molecule junctions [8]-[22] and scanning tunneling microscopy based techniques [23]— [44]. With their help, a plethora of interesting phenomena like rectification [18], negative differential conductance [9,35], Coulomb blockade [10,11,15,16,21, 23], Kondo effect [11,12], vibrational effects [10,13,14,16,21,25,31-33,35,36], and nanoscale memory effects [34,39,40,42,44], among others, have been demonstrated. 

Electrolyte Gating in (Single) Molecule Junctions Containing Redox-Active Viologen Derivatives... 

An electrochemically assisted jump-to-contact process has also been applied in the development of a modified STM-BJ approach by fhe present author s group [70]. This technique has the advantage that it allows the construction of chemically well-defined, atomic-size contacts, and is possible with both chemically active and/or soft metals. The metal contacts created in this way have been confirmed as having a well-defined structure, suitable for studying the mechanical properties of the nanocontacts [62, 71]. As in the conventional STM-BJ approach, the pair of metal electrodes created after breaking the contact provides the electrodes to construct metal-molecule-metal molecular junctions. These improvements have extended the capability of conventional STM-BJ to create a variety of metal nanocontacts and single-molecule junctions beyond the Au-molecule-Au junctions. 

Liu, Z., Ding, S.Y., Chen, Z.B. etal. (2011) Revealing the molecular structure of single-molecule junctions in different conductance states by fishing-mode tip-enhanced Raman spectroscopy. Nature Communications, 2, 305. 

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