Quantum-Interference-Enhanced Thermoelectricity (QUIET).

Quantum interference is a mechanism which can be used to manipulate the electrical properties of a single molecule by exploiting the property that an electron can be considered to be a wave as well as a particle. It turns out that constructive or destructive interference of electrons within individual organic molecules can be engineered precisely by the addition of various atomic groups to the molecule or by carefully selecting the connection of the molecule to external electrodes. Although the dream of manipulating quantum interference in single molecules has been discussed for many years, experimental evidence of room-temperature interference effects in single-molecule junctions was reported only recently. Building on these demonstrations of quantum interference, QuIET aims to deliver the next breakthrough by designing and realising technologically-relevant materials and devices, which exploit quantum interference at room-temperature and above.

Waste heat from information technologies currently results carbon emissions which are comparable to those of the total global aviation industry. QuIET aims to address this global challenge by inventing new materials, which efficiently convert this waste heat into useful electricity. Our target materials are thin films formed from single layers or a few layers of molecules, sandwiched between planar electrodes. Quantum interference will be used to optimise their ability to convert waste heat into electricity and for on-chip cooling. This will be achieved by designing, synthesising and measuring molecules with a high Seebeck coefficient, which determines the voltage generated when a temperature difference is applied to the two sides of a molecule or a thin film. Conversely, if a voltage is applied across a molecule, the closely-related Peltier coefficient determines the magnitude of the cooling effect that can be created.

It turns out that the Seebeck coefficient is proportional to the number of electrons within the molecule and also how the density of electronic states is distributed with energy. Both of these can be manipulated in certain families of organic molecules using quantum interference. A third property important for heat recovery (the first two being the electrical conductance and the Seebeck coefficient) is the thermal conductance, which needs to be low. Within a bulk material it is difficult to engineer simultaneously high electrical conductance and low thermal conductance. However for single molecules or thin molecular films attached to electrodes, the thermal conductance can be engineered by selecting slippery anchor groups, for binding the molecules to the electrodes and by introducing soft internal mechanical degrees of freedom, which further reduce phonon transport.

The technical challenges that this proposal addresses are four-fold. The first is to identify theoretically families of molecules that will have the propensity for large quantum interference effects, and to predict which atomic groups and which anchor groups will optimise their properties. The second is to synthesise these molecules and the third is to measure their properties at the single molecular level to feed back to the theoretic models. The fourth and final challenge is to investigate whether these superior properties persist when the molecules are turned into a vast parallel array of molecules, known as a self-assembled molecular layer. Understanding the hurdles that need to be overcome to realise quantum interference effects at room temperature in macroscopic thin-film arrays of molecules, will help identify the first steps to a new type of technology that has important societal and economic impacts in the real world and addresses pressing problems with on-chip cooling and energy-efficient heat recovery.

Recent experimental demonstrations of quantum interference (QI) in single-molecule devices at room temperature have opened up the possibility that QI could be exploited to design new molecular materials with the ability to convert waste heat into electricity with unprecedented efficiency. To realise this potential in the laboratory, this project aims to demonstrate that room-temperature QI-enhanced thermoelectricity, high electrical conductance and low thermal conductance in single molecules can be transferred without significant degradation in parallel arrays of self-assembled molecular layers (SAMs), thereby setting the groundwork for high-performance thermoelectric thin film devices. The planned work in this program is fundamental in nature, because although all molecules of a given kind are identical, their atomic-scale environments are all different and therefore the effect of intermolecular interactions, defects in the electrodes and imperfections in molecular arrays needs to be built into any design strategy for molecular-scale thermoelectric films. In addition, the control of QI in parallel arrays of molecules sandwiched between parallel planar electrodes has yet to be established, the molecules needed to achieve this control have to be synthesised and the principles needed to achieve the delicate balance between high electrical conductance and low thermal conductance, combined with a high Seebeck coefficient in molecular thin films have to be demonstrated.

Once established, the design rules we identify will enable future economic and societal impacts, which would be realisable in the next 5-10 years in areas such as on-chip cooling and waste heat recovery, particularly from data farms and the environment for powering the internet of things. These long-term impacts will require further multi-disciplinary translational research beyond the scope of the project, which takes the molecular films and devices from our laboratory demonstrations to real-world application. IBM, NPL and Sigma Aldrich will provide advice on maximising the impact of our outputs, and we plan to engage with other industries through 'Energy Lancaster', Imperial's 'Energy Futures Lab' and the Knowledge Transfer Partnership on Thermoelectricity (TEMPEST).

In the nearer term, we plan to develop a set of experimental and theoretical methodologies that will have a more immediate impact in academic and industrial research laboratories. In particular, we will fill a UK capability gap, since there are no UK groups currently measuring thermopower in molecular-scale thin films. Such thermopower measurements, as well as providing insight into the thermoelectric properties of molecular materials, are particularly instructive for probing the mechanism of charge transport. The underpinning QI effects have not been experimentally applied to SAMs and our project will deliver the first experimental measurements of thermopower in such systems. 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 and their environments.

The project will also have direct impact on our early career researchers, as their research training and experience in chemical synthesis, theory and molecular-scale measurements will allow them to contribute to future developments in molecular and organic electronics, molecular thermoelectrics and molecular sensors and to the commercial exploitation of the new disruptive technologies emerging from the science.

Workshops held in months 18 and 36 will contribute to the dissemination of our outputs to end users in academia and industry.