Self-assembled molecular monolayers with ultra-low thermal conductance for energy harvesting (QSAMs)

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In single molecules, vibrations due to heat (phonons) and electrons both behave quantum-mechanically like waves and so they can exhibit interference, which can be used to manipulate them. It turns out that constructive or destructive interference of phonons and 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 manipulation of room-temperature quantum interference (RTQI) of electrons in single molecules has been realised recently and is a topic of intense competition between research groups in the UK and abroad, simultaneous control of room-temperature phonon interference (RTPI) has not yet been achieved. This project, called QSAMs, aims to deliver the next breakthrough by designing and realising technologically-relevant materials and devices, which exploit both RTPI and RTQI to yield the next generation of thermoelectric materials.Electricity for information technologies currently results in carbon emissions that are comparable to those of the total global aviation industry. QSAMs aims to address the global challenge of reducing these emissions significantly by inventing new materials that efficiently convert the waste heat produced by data centres (or example) into useful electricity. Our target materials are thin films formed from single layers or a few layers of molecules, sandwiched between flat electrodes. Interference will be used to optimise their ability to convert waste heat into electricity and for on-chip cooling. This will be achieved by modifying the vibrational properties of molecules with a high RTQI-driven 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.A crucial property important for heat recovery (in addition to 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 synthesising Christmas-tree-like molecules (connected to the electrodes at top and bottom), in which the trunk of the molecule is connected to branches coming out of the sides, which oscillate in such a way as to cancel out the phonon waves flowing along the trunk. Phonon transport can be further reduced by selecting slippery anchor groups for binding the molecules to the electrodes, in order to scatter phonons at the contacts between the molecules and electrodes. The technical challenges that this proposal addresses are three-fold. The first is to identify theoretically families of molecules that will have the propensity for large RTQI and RTPI 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 and optimise their properties in a vast parallel array of molecules, known as a self-assembled monolayer. Understanding the hurdles that need to be overcome to realise simultaneously RTPI and RTQI in such macroscopic ultra-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 of on-chip cooling and energy-efficient heat recovery.