Molecular Machines and Devices
Beilstein Nanotechnology Symposium 2014

This symposium brought together the scientific communities working in one-dimensional electronic transport, molecular electronics and synthetic molecular machines. The emphasis was on the physical aspects of these subjects with an equal mix of theory and experiment. Many vivid discussions lead to a comprehensive exchange of ideas.

Ron Naaman from the Weizmann Institute presented intriguing and puzzling results on experiments detecting spin polarization for electrons that traverse chiral molecules. He argued how the helical structure couples the spin and orbital parts of the wave function, even for organic materials, which are usually expected to have a very small spin orbit coupling. This interpretation was heavily debated, and continued at coffee breaks, lunches and at the late-night bar sessions. It was not consistent with conventional descriptions of spin orbit coupling, and thus begs for further experiments and theoretical investigations. In another talk by Wilfred van der Wiel (Twente), giant room temperature magnetoresistance effects in molecular wires were reported. He ascribed these huge effects to spin blockade effects. Using scanning tunneling microscopy and spectroscopy Wulf Wulfhekel (Karlsruhe) showed how single molecules can be used as spintronic devices. The additional spin degree of freedom allows one to realize molecular magnetoresistance sensors and spintronic memristors.

Synthetic molecular motors have been recently designed by several groups. Karl-Heinz Ernst (Zürich) showed how such molecules, which were synthesized by the group of Ben Feringa in Groningen, can be deposited onto metal surfaces and made to rotate with pulses from an STM tip. Alberto Credi (Bologna) takes this even a step further, showing how these concepts can be used to make the molecules perform work and to construct light-driven molecular pumps. Sense Jan van der Molen (Leiden) analyzed the dynamics of Feringa-type molecular motors in detail, which provides an approach for optimizing their efficiency. Herre van der Zant’s group (Delft) designed a molecular motor based on a dipole moment with proper anchoring groups. As a first step in the exploration of this intriguing design, they used the quantum mechanically controlled break junction technique to study the mechanical stability and electronic transparency of the various anchoring groups.

For the case where these synthetic motors require an external periodic excitation for their operation, an entirely different approach was presented in the theoretical contributions by Tchavdar Todorov (Belfast) and Mads Brandbyge (TU Denmark). They predict that a strong non-equilibrium electron current in a wire exerts a force on the atoms of the wire. This force has several components, one of which can be identified as the electron wind force in electromigration. But there appears to be another component that results from the Berry phase of the electrons and acts in the manner of a Lorentz force, driving the atoms around circular trajectories. In addition, by taking the phonon bath of the leads into account, they find that the heat of a symmetric nanowire should have a strong asymmetrical distribution. This calls for experimental confirmation!

Many contributions addressed recent progress in molecular electronics. While the progress in recent years has been impressive, and the level of control and understanding has advanced much further than was expected some years ago, most work is aimed at fundamental studies. Although large scale applications are probably awaiting resolution of the top-contact problem for self-assembled monolayers, Richard McCreery (Alberta) showed a first device that has already reached the market. Stefan Tautz (Jülich) used an STM equipped with a tuning fork sensor to study molecules in an STM junction. This elegant method allows the simultaneous measurement of electronic transport, structural changes and forces. Given the amount of information that is provided by this technique, many scientists have identified this as a fruitful route. As an example, we mention Leonhard Grill (Graz) who removed single molecules and oligomers from a surface while simultaneously measuring the forces and the electron transport through the molecule.

The lively debates and the exciting research questions which were posed now call for a follow-up. This has led to the planning of a Thematic Series of the Beilstein Journal of Nanotechnology on the theme of the symposium.

Jan M. van Ruitenbeek
Mario Ruben
Harold Zandvliet