Supramolecular Nanorings for Exploring Quantum Interference

Young's famous double-slit experiment of 1803 demonstrated that light behaves as a wave. The light emerging from the slits has a characteristic intensity pattern originating from constructive and destructive interference. Later it was found that when single particles (photons or even molecules) pass through a double slit they produce similar interference patterns; this experiment became the key piece of evidence for wave-particle duality.

A Mach-Zender interferometer is similar to Young's double-slit setup, except that light is split into two routes using mirrors. When the light is recombined, constructive or destructive interference occurs, depending on the difference in the phase of the light from the two routes. Subtle differences in the path-length, or refractive index, can easily be detected, because they determine the phase difference, and thus they control the interference.

This project aims to synthesise and test a "molecular Mach-Zender interferometer" consisting of a molecule with two charge-transport paths; interference between the two transmission channels controls whether the whole system is conductive (in phase) or non-conductive (out of phase). Thus these molecules are expected to be sensitive to magnetic or electric fields which can change the relative phases of the two channels. Furthermore quantum interference effects tend to produce sharp changes in transmission with electron energy, which can result in strong thermoelectric effects. This project is concerned with exploring fundamental principles, but in the long term, this research has the potential to generate commercially disruptive technologies, such as thermoelectric devices for scavenging thermal energy, and transistors with reduced power requirements, abrupt switching and small footprints.

This project if a thoroughly integrated collaboration of three research groups focusing on (1) Oxford: design and synthesis of molecular structures, (2) Liverpool: testing of single molecule conductance and thermopower, and (3) Lancaster: theory and computational simulation, to guide the interpretation of the experimental data, and the design of new molecular structures.

At present there exists a no-man's land between the 15-nm length scale accessible to top-down technologies, such as electron-beam lithography, and bottom-up technologies such as chemical synthesis. The molecules investigated in this project are 3 nm across, but can be increased in size up to around 10 nm. This project is therefore a significant step towards bridging this crucial technology-scale gap, at the limit of Moore's law.

Recent experimental demonstrations of quantum interference (QI) in molecular devices have given great impetus to the scientific community and this is likely to develop into an important and thriving area of molecular electronics research. This project aims to show that QI can be achieved, understood and controlled in supramolecular nanorings, thereby realising molecular-scale analogues of Mach-Zehnder interferometers. Our combination of synthesis, measurement and theory will enable us to establish design rules for achieving room-temperature control of QI in molecular devices. These design rules will lay the ground for potential economic and societal impacts, which could result from future technological implementation in areas highlighted below. Given the relatively early stage of such research into QI in complex single-molecule systems, and into how QI impacts on their electrical and thermoelectric properties, we expect research activities in this area to escalate in the next 5 years. The beginnings of this escalation are evidenced by the recent outputs listed in 'Pathways to Impact'. These include recent results from IBM Zurich, who have made a substantial investment in the development of future molecular-scale thermoelectric technologies.

Our programme will extend the boundaries of single-molecule electronics and QI research, by developing molecular electronic components with a higher level of functionality and complexity than those studied previously. We will also fill a UK capability gap, since there are no UK groups currently measuring thermopower at the molecular scale. Such thermopower measurements, as well as giving insight into the thermoelectric properties of molecular materials, are particularly instructive for probing the mechanism of charge transport. They have not been experimentally applied to single-molecule nanorings and our project will deliver the first experimental measurements of thermopower in such systems. We will also achieve impact by making a new single-molecule experimental technique available to the research community capable of thermopower measurements on a wide variety of contact materials which will be adapted from Liverpool's I(t) technique. This method will offer the advantage of recording large statistical datasets for both thermopower and I-V characteristics at fixed tip-sample gap separations, and may eventually offer more rapid screening of thermopower characteristics of single molecule junctions. In addition, we will develop and disseminate theoretical methods beyond the present state-of-the-art for modelling the thermoelectric and electrical properties of complex molecular junctions, including the ring systems described in this programme.

It is widely recognised that if QI phenomena in molecular interferometers could be fully harnessed, they would lead to delicate sensors for chemical analytes. Furthermore molecular systems have the potential to provide materials with unprecedented thermoelectric efficiencies provided their QI properties can be tuned. Our programme aims to achieve fundamental understanding of the complex interplay between QI and thermoelectric properties. By carrying out fundamental research into QI phenomena in molecular devices, the UK will be in an excellent position to provide a knowledge base for future technological exploitation in such areas.

The project will also directly benefit the early career researchers appointed within the project, as their research training and experience (in e.g. chemical synthesis, theory and single molecule measurements) will provide them with the skills background needed to contribute to future development of technological and commercial opportunities in areas such as molecular and organic electronics, molecular thermoelectric, molecular sensors and related areas.