Break junction, mechanically controllable

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]. 

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

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)... 

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...

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.

FIGURE 3.4. (a) Schematic of a benzene-1,4-dithiolate SAM between proximal gold electrodes formed in an mechanically controllable break junction, (b) Typical I(V) characteristics, which illustrate a gap of 0.7 V, and the first derivative G(V), which shows a steplike structure. [Adapted from Ref.35 Reed et al., science 278, 252-254 (1997).]... 

Scanning probe measurements and mechanically controlled break junctions... 

In summary of this section, it can be said that experimental manipulations and conductance measurements on single molecules are still a big scientific challenge, and a lot of the progress that has been recently made has been achieved for particular substrate/molecule systems and can not be easily transferred to other surface materials or types of molecules. The STM is certainly the most versatile instrument for manipulations and measurements on the nanoscale but it is not very suitable for an integration into nano-electronic devices. New techniques such as mechanically controlled break-junctions will have to be further developed for this purpose in the future. 

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. 

Fig. 17.11 The current (right axis) and conductance (left axis) of a molecular junction plotted against the applied voltage. Each plot shows several sweeps of the potential. The different plots correspond to different junctions prepared by the mechanically controlled break junction technique using gold contacts with the molecule shown, (from H. B. Weber, J. Reichert, F. Weigend, R. Ochs, D. Beckmann, M. Mayor, R. Ahlrichs, and H.v. Lohneysen, Chem. Phys. 281, 113 (2002).)...

As may be expected, experimental reality is not as neat as the results of theoretical toy models. Figure 17.11 shows such results obtained using a mechanically controlled break junction technique with gold contacts and the molecule shown. 

Infrared absorption spectroscopy Isophthalic acid Low energy electron diffraction Lowest imoccupied molecular orbital Mechanically controlled break-junction Mercury-sulfate electrode Potential of zero charge q = 0 Quasireference electrode Real hydrogen electrode Reference electrode Alkanedithiols HS(CH2)nSH Self-assembled monolayer(s)... 

Charge transport in Au alkanedithiol Au junctions was measured with a variety of techniques. These include mechanically controlled break junctions (MCBJ [213], STM [205,208,270-275] and conducting AFM break... 

C. Zhou, C. J. Muller, M. A. Reed etal.. Mesoscopic phenomena studied with mechanically controllable break junctions at room temperature in Molecular Electronics (Eds. J. Jortner, M. Ratner), Blackwell Science, Oxford, 1997, pp. 191-213. 

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