Break junction

A break junction is formed by breaking a thin metallic wire to produce a narrow gap between two conductors. Bridging this gap by a single or a few molecules creates a metal-molecule-metal junction, as illustrated in Fig. 10.8. The metallic wire can be broken by mechanical deformation (mechanically controlled break junctions, MCBs) or by electromigration. 

The first molecular measurement using MCBs was reported in 1997 by Reed and coworkers [37], who measured the conductance of a single benzene-1,4-dithiol molecule (83a). This work was followed in 1999 by measurements on oligothiophene 105 by Bourgoin and coworkers [38]. 

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Atomic force microscopy of palladium nanowires showing the (a) open break junction before exposure to hydrogen and (b) closed break junction after exposure to hydrogen. 

Fig. 1 Sketches of break junction-type test beds for molecular transport. On the far left is a tunneling electron microscopy (TEM) image of the actual metallic structure in (mechanical) break junctions from the nanoelectronics group at University of Basel. The sketches in the middle (Reprinted by permission from Macmillan Publishers Ltd Nature Nanotechnology 4, 230-234 (2009), copyright 2009) and right (reproduced from Molecular Devices, A.M. Moore, D.L. Allara, and P.S. Weiss, in NNIN Nanotechnology Open Textbook (2007) with permission from the authors) show possible geometries for molecules between two gold electrodes, and (on the upper right) a molecule that has only one end attached across the junction...

There are almost 100 papers that discuss benzene dithiol s conductance. As the point about geometric distributions became well understood, it was realized that statistical analysis was extremely useful. Accordingly, electrochemical break junction techniques, both in their original form of crashed electrodes being separated to form the gap or in the newer electrochemistry form, in which a gap is created and then electrochemically modified, have proliferated. The important thing is that statistical measurements can be made [24, 90], with hundreds or thousands of data points. Not surprisingly, distributions are observed (as the earlier computations had suggested). 

Chen IWP, Fu M-D, Tseng W-H, Chen C-h, Chou C-M, Luh T-Y (2007) The effect of molecular conformation on single molecule conductance measurements of 7i-conjugated oligoaryls by STM break junction. Chem Commun 29 3074—3076... 

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]. 

A break junction can also be created by passing a current (0.5-1.0 V) through an Au nanowire (<20 nm diameter) defined by electron-beam lithography and shadow evaporation. Such electromigrated break junctions (EMBJs) have yielded reproducible 1-3 nm gaps between electrodes [48-50]. 

Another type of break junction, the in situ or STM break junction (STM-BJ), involves pushing an STM tip through a SAM to make contact with the underlying electrode (Fig. 5). The probe tip is then quickly withdrawn, stretching a bridge of metal atoms until the bridge breaks. As the gap widens, SAM molecules can fill it, giving distinctive current flows, until the gap becomes too wide for molecules to... 

Fig. 4 Design of the Mechanically-Controlled Break Junction (MCBJ) technique...

Zhou X-S, Wei Y-M, Liu L, Chen Z-B, Tang J, Mao B-W (2008) Extending the capability of STM break junction for conductance measurement of atomic-size nanowires an electrochemical strategy. J Am Chem Soc 130 13228-13230... 

He J, Sankey O, Lee M, Tao N, Li X, Lindsay S (2006) Measuring single molecule conductance with break junctions. Faraday Discuss 131 145-154... 

Keywords Break junction Charge transport Electrolyte gate Metal nanocluster Molecular junction Scanning tunneling microscopy Scanning tunneling spectroscopy... 

In the following we will focus on three molecular electronics test beds as developed and employed for applications at electrified solid/liquid interfaces (1) STM and STS, (2) assemblies based on horizontal nanogap electrodes, and (3) mechanically-controlled break junction experiments. For a more detailed description of the methods we refer to several excellent reviews published recently [16-22]. We will also address specific aspects of electrolyte gating and of data analysis. 

Fig. 2 (a) Schematic representation of a mechanically controlled break junction (MCBJ). The inset shows the SEM image of a nanofabricated gold bridge [40]. (b) Principle of an STM-based break junction experiment (STM-BJ)... 

Scanning Tunneling Microscopy Based Break Junction... 

The fabrication and characterization of atomic metal contacts have been based mainly on electro-deposition/dissolution [182] and break junction techniques (see review [134] and literatures cited therein). In particular, gold nanocontacts have been studied in great detail, due to the chemical inertness of the material, the malleability and ductility of gold. The processes of formation, evolution, and breaking of gold atomic contacts leads to step-like features in the current-distance curves [188, 189]. The abrupt changes in the current (conductance) response were... 

Rahimi M, Hegg M (2009) Probing charge transport in single-molecule break junctions using inelastic tunneling. Phys Rev B 79 081404(R)... 

Rahimi M, Troisi A (2009) Probing local electric field and conformational switching in single-molecule break junctions. Phys Rev B 79 113413-113414... 

Figure 19. A schematic of the mechanically controllable break junction showing the bending beam formed from a silicon wafer, the counter supports, the notched gold wire which is glued to the surface, the pizeo element for control...

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