AG Scheer
Mesoscopic Systems

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Molecular Electronics Group

With the ongoing miniaturization of microelectronics, functional elements in electronic circuits may soon consist of only a couple of electrons or molecules. We therefore address the question how the physical laws which hold for macroscopic solids become modified when one deals with very small structures. It turns out that down-scaling of electronic properties from the macroworld to the atomic or molecular level does not work at all. For example, the famous Ohm's law does not hold anymore, because the resistance does not scale with the length of a ''quantum wire''.

Once having realized this fundamental issue, one immediately conceives this as a chance to develop new concepts. Instead of continuously scaling down, as in industrial chip designs, one considers building up electronic circuits with tailored properties ''bottom up''. The first questions which have to be answered now are: ''Which atoms or molecules have to be combined in which way to achieve the desired properties?'' and: ''Which physical and chemical properties determine the electrical conductance of atomic-size or molecular-size circuits?''.

Part of the research in our group is dedicated to answer the last question, in particular for single-atom contacts. However, for the design of devices with complex functionality, single atoms appear to be too simple, but particular molecules with built-in functionality are under debate. The field of ''Molecular Electronics'' has been opened by the seminal proposition of A. Aviram and M. Ratner in 1974 to build a diode from a single molecule. Since then, it took almost 20 years before the first molecular diode was experimentally realized. By now, molecular electronics is a broad field of research world-wide. An overview of the field until 2005 can be found in the book ''Introducing Molecular Electronics'' by Cuniberti, Fagas, Richter [Introducing Molecular Electronics, Lecture Notes in Physics, Springer, 2005].


Probing Molecular Transport by Multiple Andreev Reflections

Figure 1: Two measured Pb single atom contacts with fits, both exhibiting sets of three transmission channels but with different transmission probability: [0.99,0.38,0.38] and [0.52,0.29,0.20]
Figure 2: Artistic view of the established contact of a molecule bridging a gold nano-bridge with induced superconductivity.
David Weber (pdf)

Funding by SFB 767 Project C2

The properties of molecular transport can be analyzed by measuring the conductance or its derivative, giving first insights to molecular levels and coupling to the electrodes.[1] Furthermore, excitations of the molecule can be found by inspecting the inelastic electron tunneling spectrum (IETS) via the d²I/dV² signal.[2]
However, the possible composition of several conduction channels of electronic transport in a metal-molecule-metal junction has not been explored in much detail yet. With the help of shot noise measurements, it has been shown that Pt-H2-Pt junctions with a conductance of 1 G0 accommodate indeed a single perfectly transmitted channel.[3]
The aim of this project is to analyze the transport properties of a molecular junction with respect to the number and transmission of channels. For this purpose a superconducting MCBJ design is combined with highly conductive molecules like 1,4-benzendithiol. If a contact through the molecule is established, an I-V spectrum will reveal the composition of channels via multiple Andreev reflections (MAR).[4] This can only be done at very low temperatures well below the critical temperature TC. For this purpose, a 1K stick was equipped with break junction mechanics and high frequency filters to reach high energy resolution for MAR spectra in Pb-MCBJ.
First results obtained for single atom Pb contacts can be seen in Figure 1, where two different contacts have been analyzed via MAR spectroscopy to distinguish their channel composition. They contacts both show a similar conductance, but the transmission channel composition clearly differs.

[1] L. Zotti, Th. Kirchner, J.-C. Cuevas, F. Pauly, Th. Huhn, E. Scheer, A. Erbe, Small 6, 14, 1529–1535 (2010)
[2] J. C. Cuevas, E. Scheer, World Scientific, Molecular Electronics: An Introduction to Theory and Experiment (2010)
[3] R. H. M. Smit, Y. Noat, C. Untiedt, N. D. Lang, M. C. van Hemert, J. M. van Ruitenbeek, Nature 419, 906-909 (2002)
[4] E. Scheer, P. Joyez, D. Esteve, C. Urbina, M. H. Devoret, Phys. Rev. Lett. 18, 3535 (1997)
[5] J. C. Cuevas, A. Martin-Rodero, A. Levy Yeyati, Phys. Rev. B 54, 7366 (1996)

Elastic and inelastic shot noise measurement on single molecular junctions.

(a) Experimental (black) IET spectrum for the junction with conductance of 0.23 G0 and theoretical (red) one for a configuration with the conductance of 0.26 G0. (b) Shot noise as a function of the bias voltage applied across the Au-BDT-Au junction. The zero bias conductance for this junction is 0.23 G0. The red line is the fit to the total current noise, yielding τ1 = 0.23
Amin Karimi (pdf)

Funding by SFB 767

The study of shot noise for junctions formed by single molecules offers interesting new information that cannot be easily obtained by other means. At low bias it allows determining the transmission probability and the number of current carrying conductance channels [1,2].

We developed a new and versatile measurement system enabling measurements of the noise in a rather broad range of conductance values without the necessity of double wiring. The first results on single-molecule Au-1,4-benzenedithiol (BDT)-Au provide evidence that the current is carried by a single conduction channel throughout the whole conductance range from 0.24 G0 to 0.01 G0. Figure 1 (a) shows the experimental (black) IET spectrum for the junction with conductance of 0.23 G0 and theoretical (red) one for a configuration with the conductance of 0.26 G0. Figure 1 (b) shows the shot noise as a function of the bias voltage applied for the same junction with the zero bias conductance of 0.23 G0 [2]. The red line is the fit to the total current noise and yield one channel with the transmission probability of τ1 = 0.23. In the next step, we will investigate the effects of phonon scattering on the electronic current noise through nano junctions. Equivalent measurements have recently been reported to be able to reveal inelastic transport contributions to the current through gold atomic contacts [3].

[1] D. Djukic and J. M. van Ruitenbeek, Nano Lett. 6, 789-793 (2006).

[2] A. Karimi, S. G. Bahoosh, M. Herz, R. Hayakawa, F. Pauly and E. Scheer, Nano Lett., 2016, 16 (3), 1803–1807

[3] Manohar Kumar, Rémi Avriller and Jan M. van Ruitenbeek, Phys. Rev. Lett. 108, 146602 (2012).

Electronic Transport through Organic Molecules in Solution

Experimental sketch
Experimental setup, including liquid cell and electromagnetic shielding
K. Luka-Guth in cooperation with T. Huhn
Funding by SFB 767

With this setup it is possible to investigate organic molecules in their natural chemical environment. This is very important because in solution the molecules have more degrees of freedom and thus they are more flexible in the contact. In order to contact the molecules, the MCBJ-technique is used. During the measurement, the contact electrodes are permanently opened and closed while a constant voltage of 100 mV is applied and the resistance is measured. After every opening and closing cycle, the molecule can change its arrangement in the contact. Thereby, it is feasible to collect as much configurations of the molecule as possible. Additionally, current-voltage characteristics are recorded at different electrode gaps.

Molecule sketch
Sketch of the fluorescent molecule trapped between gold electrodes

Two different projects have been carried out with this setup. In the first one, a specially designed fluorescent molecule containing a chromophore, which gives the molecule a lilac color, has been analyzed. The current-voltage characteristics of this molecule show a hysteresis and makes it therefore very promising as a possible molecular storage element. In the second project, the influence of end and side groups on the electronic transport through organic molecules has been investigated.

For information about this project, please refer to the Öffnet externen Link im aktuellen FensterSFB 767 project page.





Charge Transport via Single-Molecules at Low-Temperature

A. Karimi, D. Weber, S. Hambsch

Funding by SFB 767 Project C2

Single-molecules are a very promising candidate for elements of an active electronic device. Until now, the study of charge transport via single-molecules has been successively progressed in nanoscale single-molecule devices. However, there is still not much knowledge about the charge conduction depending on the molecular intrinsic properties. The charge transport through a single-molecule can be modulated by light or microwave irradiations as well as, mechanical forces. To reveal the charge transport mechanism, the inelastic electron tunneling spectroscopy (IETS) technique is used for solid-state single-molecules in mechanically controllable break junctions (MCBJs) at low-temperature (< 4.2 K) as seen in Fig 1.

Additionally, we take advantage of versatile external stimuli like temperature dependence, current-voltage characteristics and magnetic field dependence. The latter is shown in Fig 2 where we compared the signal of a pure gold contact with a reference molecule (Benzenedithiol) and a magnetically active molecule with a radical group (TEMPO-OPE). The radical molecule shows a large change in conductance with applied magnetic field. This effect could be used to address and control specific molecules later on.

Fig 1: IETS performed on different electronic configurations of the same molecule.
Fig 2: Dependence of the electric conductivity of single-molecule junction depending on the magnetic field.