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.