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Electrons shoot down molecular wires

Electronic communication at the nanoscale: this diagram shows molecular wires of different lengths which act as bridges between two molecules. Zinc porphyrin is used as the electron donor (left); fullerene is used as the electron acceptor (right). (Image: FAU)

FAU chemists work on better solar cells and faster mini computers

FAU chemists have made an important step towards the vision of nanoscale electronic components. They have shown that electrons can be transported much faster over rigid molecular wires than over flexible ones. Together with partners in Japan, the researchers are working on more effective, lower-cost solar cells and tiny, high-performance electronic components. Their findings have recently been published in the renowned online journal Nature Chemistry.

We have only just got used to the fact that high-performance computer chips are no longer visible to the naked eye, yet researchers are already penetrating even smaller dimensions with their work on nanoelectronics. The aim of the FAU researchers’ activities is to use individual molecules as electronic components in order to connect, save and process data in a logical way. For these molecules to interact with one another, they have to be connected at the nanolevel by what are known as molecular wires.

‘We are in the process of gaining a basic understanding of how electrons are transported through molecular wires,’ says Prof. Dr. Dirk M. Guldi from the Chair of Physical Chemistry I at FAU. ‘Conducting electric current through nanoscale wires has proved to be an effective tool which can be used to characterise the transport properties of wires of various lengths.’

Which structure is best?

In conventional methods, the two ends of the nanoscale wire are connected to two electrodes, allowing the conductivity to be measured. The working group led by Dirk Guldi is using a different approach: covalent bonding. In this method, carbon-based molecules act as the electron donor at one end of the molecular wire and as the electron acceptor at the other. When the donor is stimulated by light, for example, it releases an electron which is transported through the nanowire to the acceptor. The duration of the electron transfer over wires of various lengths is measured using ultrafast laser spectroscopy, which has a temporal resolution of just a few femtoseconds. ‘These measurements provide us with information about which molecular structures are particularly suitable for transferring charge,’ explains Christina Schubert from Prof. Guldi’s working group.

One of the main problems in electron transfer is the molecular movements in the nanowire, the rotation around single bonds, for instance. However, in order to transfer electrons over comparatively long distances of up to 36.4 Å, wires need to be both rigid and flat. The Erlangen chemists are working on this with research partners from the University of Tokyo. The Japanese working group led by Prof. Dr. Eiichi Nakamura specialises in developing new synthesis reactions and supplies the researchers in Erlangen with rigid carbon wires which can be used to study electron transport at the nanoscale.

Significantly stronger effects than expected

During their experiments with the rigid carbon bridges, the FAU researchers have discovered for the first time that the electronic communication between the electron donor and the electron receptor is much stronger than in comparable, non-rigid carbon wires – three to four times stronger in fact.

They also found out that the electron-vibration coupling which makes inelastic electron tunnelling possible had a significant effect on the charge transfer. The inelastic tunnel creates a new path for the electron which leads to a much quicker charge recombination. The team of chemists led by Dirk Guldi have now discovered that electron-vibration coupling is around twice as strong in rigid carbon bridges as it is in flexible wires. ‘We have been able to prove inelastic electron tunnelling at room temperature for the first time,’ says Christina Schubert, ‘Now we’re working on constructing tailor-made nanowires with new functions for molecular components.’

Solar cells of the future: low-cost, environmentally friendly and flexible

The research groups’ visionary goal is to develop integrated components and electronic circuits at the molecular level which can be used for data processing, and to make the use of solar energy many times more efficient than it is today. Their research focusses on on carbon nanostructures – fullerene, carbon nanotubes and graphene – which are studied in solutions, in transparent films and on electrode surfaces. Researchers at FAU’s Department of Chemistry have already started integrating these carbon components into solar cells to make them more efficient. ‘In the future, organic solar cells could replace the silicon solar cells used today,’ says Dirk Guldi. ‘They cost much less to produce, the materials used are environmentally friendly, and the modules themselves do not have to be rigid but can be installed on roofs, façades and even windows in the form of flexible, semi-transparent foils.’

Further information:

Prof. Dr. Dirk M. Guldi
Phone: +49 9131 8527340
dirk.guldi@fau.de

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