Break junction

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

Measurement principle on macroscale

The measurement process has a fundamental resemblance to a bascule bridge. With the variety of possible designs all MCBJ techniques have two common characteristics: A flexible sample with a separable, nanoscopic conductive bridge in the middle and a mechanism for continuous, slow repeatable bending and relaxing of the sample.

In the experiment, the sample is fixed in a three-point bending device and controlled by a mechanical drive or piezo mechanism bent from the bottom or back relaxed, so that separate or close the contacts in the center. In this case, a variable voltage is applied to the break contact and the measured current through the sample.

An important role in the bending device plays the translation factor (reduction ratio) between the vertical displacement of the thrust screw and horizontal distance change in the electrode, which is relatively high accuracy from the sample parameters ( sample length, thickness and length of the bridge ) may determine. Alternatively, the multiplication factor can be calculated also taking advantage of the exponential dependence between current and maximum distance from the tunnel current. With the smallest bridge lengths thus translation factor values ​​up to 10-7 theoretically achievable. The use of piezo motors also allows for the precise adjustment of the vertical position and velocity, which can lead to very precise adjustment of the electrode spacing in the angstrom range.

Principle of measurement at the nano scale

MCBJ the 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. Upon application of a constant voltage, the sample is slowly evenly bent and the current through the gold electrode was measured. First we see a continuous decrease of the current intensity according to Ohm's law, as the sample diameter gradually reduced during the bending. Forming less than a hundred metal atoms the smallest constriction, quantum effects play an increasingly important role and it is observed a step-like decrease of the conductivity. Finally, the conductivity Quant G0 is observable as a single atom links two electrodes.

Gold is very malleable and ductile, which often results in the case of slow elongation of the formation of atomic wires and visibly in power measurements as a plateau. The presence of molecules plays no role in this section measurement due to the low molecular conductivity.

Are the atomic contacts dissolved observed tunnel current is to achieve a stable configuration as the exponential decay of the current, at which only a few molecules bridging the electrodes. In this case, the current flow through the molecules is stronger than the vacuum tunneling. Further stretching leads to the separation of the molecular contacts, so that in the end only a single molecule of connecting the electrodes. In this case, the current drops stepwise. Since the sulfur-gold bond is stronger than the gold - gold bond, it also comes in this case to the formation of atomic gold wires, which are reflected in conductivity measurements in the plateau.

The continuation of the samples bend the electrodes permanently disconnect and the sample must be relaxed in order to repeat the measurement.

Variations

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

Type of sample

Two types of samples are particularly common: Macro wires and nanostructures ( nanowires ). Macro wires are made ​​of a thin metal wire in the center of a taper is defined. This simple and inexpensive approach is used for demonstration purposes at present rather as means of nanostructuring process samples produced have significantly better properties.

Effort, cost and time consumption in the production of nanowires are indeed significantly larger, but such samples have much higher stability, much smaller translation factor, lower risk of contamination and are also easier to study (especially with REM). We observed significant differences in structure sizes and production methods, but usually Nanostructures can be defined using electron beam lithography, and then coated with metal by chemical or physical etching processed (this step leads to the formation of metal bridge in the mid- sample ). The bridge width may be less than 20nm.

Electrode materials

There are two requirements for the electrodes:

The choice of electrode materials affect the conductivity of the metal - molecule -metal system (MMM ) system and is, among other things because of this very important. Mostly you used gold because of the large ductility, high stability against oxidation and a strong bond to anchor organic groups. Some experiments show, however, that, for example, Silver can provide stable MMM contacts with higher conductivity values. Platinum and other precious metals such as rhodium or palladium are, due to their relatively high catalytic activity not as good as electrode materials. Other base metals can oxidize strong. The formation of an oxide layer on the surface hinders the electrical investigation. An interesting approach to face the rapid development of carbon-based electronics is the conduct of MCBJ experiments with carbon nanotubes.

Bending mechanism

A piezo actuator has several advantages over mechanical engines with respect to their stability, reliability and associated accuracy. Moreover, the application of mechanical drives is affected by large hysteresis and heat production. Therefore, the piezo - actuators are preferably used.

Pros and Cons

In summary, the following advantages for MCBJ:

• The system is relatively stable and the test results are reproducible.
• Many individual measurements can be performed during a test with a sample, the statistical evaluation easier.
• Distances between the electrodes are set accurately. The distance change in a day is usually in the angstrom range even at room temperature.
• Contact electrodes are less sensitive to external vibration. Furthermore, there are the contacts of the same material and have approximately the same shape, which permits the formation of symmetric linkages.
• Clean contact surface (vacuum environment is not required). Contacts can be cleaned by applying a large voltage again.
• In contrast to STM MCBJ are compatible with possible nanoelectronics components.
• Relatively low cost of experimental setup, sample preparation, measuring execution and maintenance.

MCBJ also has some disadvantages:

• Little control over the exact molecular bonding of the electrodes and electrode configuration and molecular arrangement on the atomic scale.
• Investigation of long and (or) poor conducting molecules is difficult, there have been no test results reported for more than 5 nm with molecules.
• Investigation of molecules in solutions is problematic.
• Large dependence of the conductivity of the binding site leads to poor comparability of test results and discrepancies between theory and experiment.
• For the analysis of a large number of measurement results is required to be processed by means of lengthy statistical analysis.
• Both electrodes are usually made of the same material, the choice of electrode materials is limited.
• An elaborate production of lithographic structures is usually required.
• The Abstandskalibration for electrodes is complicated and error prone.
• The measuring method is very slow compared to STM.