Mechanically controlled break contacts

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The mechanically controlled break junction ( English mechanically controllable break junction, MCBJ ) is a robust, highly stable method that is especially chemical, electrical and thermal properties for the investigation of nano-contacts has been developed, as well as single molecules, and is widely used.

Measurement principle on a macro scale

The measuring process is basically similar to a bascule bridge. With the variety of possible designs, all MCBJ techniques have two common features: a pliable sample with a separable, nanoscopic conductive bridge in the middle, and a mechanism for continuously, slowly repeatable bending and relaxing of the sample.

When carrying out the experiment, the sample is fixed in a three-point bending device and bent in a controlled manner from below by means of a mechanical drive or piezo mechanism or relaxed again, so that the contacts in the middle separate or close. A variable voltage is applied to the break contact and the current through the sample is measured.

The transmission factor (reduction ratio) between the vertical displacement of the thrust screw and the horizontal change in distance between the electrodes, which can be determined with relatively high accuracy from the sample parameters (sample length, thickness and bridge length), plays an important role in the bending device. Alternatively, the translation factor can also be calculated from the tunnel current using the exponential dependence between current and peak distance. With the smallest bridge lengths, translation factor values ​​of up to 10 −7 can theoretically be achieved. The use of piezo motors also allows precise adjustment of vertical position and speed, which can lead to very precise adjustment of the electrode spacing in the Angstrom range.

Measuring principle on the nanoscale

The MCBJ sample is immersed in a molecular solution, then rinsed and dried so that only the molecules bound to the gold atoms remain on the electrode surface. When a constant voltage is applied, the sample is slowly bent evenly and the current through the golden electrodes is measured. First you can see a continuous decrease in the current strength according to Ohm's law , as the sample diameter gradually decreases as it bends. If fewer than a hundred metal atoms form the smallest constriction, quantum effects play an increasingly important role and a gradual decrease in conductivity is observed. Ultimately, the conductivity quantum G 0 can be observed, since only a single atom connects two electrodes.

Gold is very plastic and ductile , which in the case of slow expansion often leads to the formation of atomic wires and appears as a plateau in current measurements. The presence of molecules is irrelevant on this measurement section due to the low molecular conductivity.

If the atomic contacts are broken, a tunnel current is observed as an exponential decrease in the current strength until a stable configuration is reached in which only a few molecules bridge the electrodes. In this case the current flow through the molecules is stronger than the vacuum tunneling. Further expansion leads to the separation of the molecule contacts, so that in the end only a single molecule connects the electrodes. The current drops in stages. Since the sulfur-gold bond is stronger than the gold-gold bond, atomic gold wires are formed in this case as well, which are reflected in plateaus during conductivity measurements.

As the specimen continues to bend, the electrodes will finally separate and the specimen must be relaxed in order to repeat the measurement.

Variations

There are numerous variations of the MCBJ technique. The most important role is played here, firstly, the choice of the sample type (especially nanostructures and macro wires), secondly, electrode materials (mostly gold ) and thirdly, the bending mechanism (mechanical or piezo drive).

Sample type

Two types of samples are particularly common: macro wires and nanostructures (nanowires). Macro wires are made from a thin metal wire with a taper defined in the center. This simple and inexpensive concept is currently used more for demonstration purposes, since samples manufactured using nanostructuring processes have significantly better properties.

The effort, cost and time consumed in the production of nanowires are considerably greater, but such samples have much higher stability, a much smaller translation factor, a lower risk of contamination and are also easier to examine (especially with SEM ). Significant differences are observed in feature sizes and manufacturing methods, but i. d. As a rule, nanostructures are defined using electron beam lithography , coated with metal and then processed using chemical or physical etching processes (this step leads to the formation of a metal bridge in the middle of the sample). The bridge width can be less than 20 nm.

Electrode materials

There are two requirements for the electrodes:

  1. The contacts must arise when two nanoscopically sharp tips are separated in order to enable the measurement of quantized atomic and molecular conductivity.
  2. Contacts must be repeatedly opened and closed for statistical analysis.

The choice of electrode materials influences the conductivity of the metal-molecule-metal system (MMM system) and is very important for this reason, among other things. Most of the time, gold is used because of its great ductility , high stability against oxidation and a strong bond to organic anchor groups. However, some experiments show that z. B. Silver can provide more stable MMM contacts with higher conductivity values. Platinum and similar noble metals such as rhodium or palladium are not so suitable as electrode materials due to their relatively high catalytic activity. Other base metals can oxidize heavily. The formation of an oxide layer on the surface hinders the electrical examination. An interesting approach in view of the rapid development of carbon-based electronics consists in carrying out MCBJ experiments with carbon nanotubes .

Bending mechanism

A piezo drive has several advantages over mechanical motors in terms of their stability, reliability, and associated accuracy. In addition, the use of mechanical drives is impaired by large hysteresis and heat production. This is why piezo drives are preferred.

Pros and cons

In summary, there are the following advantages for MCBJ:

  • The system is relatively stable and the measurement results are reproducible.
  • Many individual measurements can be carried out with a sample during an experiment, which simplifies statistical evaluation.
  • The distances between the electrodes are precisely adjustable. The change in distance within a day is usually in the Angstrom range, even at room temperature.
  • Contact electrodes are not very sensitive to external vibrations. In addition, the contacts are made of the same material and shape, which allows symmetrical bonds to be formed.
  • Clean contact surface (vacuum environment not required). Contacts can be cleaned again by applying a high voltage.
  • In contrast to STM , MCBJ are compatible with possible nanoelectronic components.
  • Relatively low costs for test setup, sample preparation, measurement implementation and maintenance.

MCBJ also has some disadvantages:

  • Little control over the exact binding of molecules to the electrodes, as well as electrode configuration and molecular arrangement on the atomic scale.
  • Investigation of long and (or) poorly conducting molecules is made more difficult; no measurement results with molecules longer than 5 nm have been reported so far.
  • Studying molecules in solutions is problematic.
  • The great dependence of the conductivity on the binding site leads to poor comparability of test results and discrepancies between theory and experiment.
  • A large number of measurement results are required for the analysis, which are processed by means of lengthy statistical evaluation.
  • Both electrodes are usually made of the same material, the choice of electrode materials is limited.
  • A complex production of lithographic structures is usually necessary.
  • The distance calibration for electrodes is complicated and error-prone.
  • The measurement method is very slow compared to STM.

See also

Molecular Nanotechnology

Individual evidence

  1. ↑ Grüter, L. Mechanically controllable break junction in liquid environment: a tool to measure electronic transport through single molecules , Dissertation, Basel 2005. (PDF file; 6.98 MB)
  2. ↑ Huang, Z .; Xu, B .; Chen, Y .; Ventra, MD; Tao, N. Measurement of Current-Induced Local Heating in a Single Molecule Junction . Nano Letters 6, No. 6, pp. 1240-1244, 2006. doi : 10.1021 / nl0608285
  3. ↑ Zhou, X.-S .; Liang, J.-H .; Chen, Z.-B .; Mao, B.-W. An electrochemical jump-to-contact STM-break junction approach to construct single molecular junctions with different metallic electrodes . Electrochemistry Communications 13, No. 5, pp. 407-410, 2011.
  4. ↑ Karthäuser, S. Control of molecule-based transport for future molecular devices . J. Phys. Condens. Matter 23, No. 013001, pp. 1-16, 2011.
  5. ↑ Kaneko, S .; Nakazumi, T .; Kiguchi, M. Fabrication of a Well-Defined Single Benzene Molecule Junction Using Ag Electrodes . The Journal of Physical Chemistry Letters 1, No. 24, pp. 3520-3523, 2010. doi : 10.1021 / jz101506u
  6. ↑ Marquardt, CW; Grunder, S .; Blaszczyk, A .; Dehm, A .; Hennrich, F .; Lohneysen, H. v .; Mayor, M .; Krupke, R. Electroluminescence from a single nanotube molecule nanotube junction . Nature Nanotechnology 5, No. 12, pp. 863-867, 2010.