Mechanical break junction

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...

By a "mechanical break junction" (MBJ) technique [134], nanometerwide gaps were fabricated between Au electrodes, bithiols were inserted into the gap, and their resistance was measured [135]. The maximum measured conductance (45 nS at 1-V bias) at the point of maximum slope of the first large current through the molecule was due to a single molecule in the gap [135] if two molecules were there, then the conductance increased by a factor of 2. 

Here we report on the second approach. To conduct an experiment as the one sketched in Figure 12.12a would be the dream of every experimentalist in the field. Indeed, many mechanical break junction experiments have been reported which come quite close to this ideal [93-95]. However, a major unsolved problem of all these experiments is that there are currently no robust methods to image and determine the precise adsorption site and conformation of the molecule in the break junction [96]. But at the same time it is known that the connection between molecule and electrode greatly affects the current-voltage characteristics, sometimes even more than the properties of the molecule itself [97]. This poses a serious problem for the interpretation of single molecule transport data. 

He, H.X., C.Z. Li, and N.J. Tao. 2001. Gonductance of polymer nanowires fabricated by a combined electrodeposition and mechanical break junction method. Appl Phys Lett 7S (6) 811-813. 

The work by Kruger et al. was initiated by an investigation into the influence of mechanical force on the thiolate - gold interaction which was relevant to mechanical break junctions. In this review we focus on the aspects of this work which relate to mechanochemistry, and not to the mechanical strength of gold nanowires. Their research applied first principles molecular dynamics simulations to investigate the abstraction of an ethylthiolate molecule from an Au(211) substrate, thus investigating the response of the substrate - molecule interaction to an external force applied to the surface normal (Fig. 7). 

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)... 

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