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

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.

This research proposal is focused on delivering fundamental science that will have high academic impact in the short-term, with strong potential for industrial, economic and societal impact in the longer term (5-10 years). These impacts will require additional multi-disciplinary research beyond the scope of the present 36 month project, in order to take the new molecular components, physical characterisation and theoretical models from the laboratory to real-world applications. The main beneficiaries of this proposal are academics and industries engaged in studying the conversion of heat into electricity with unprecedented efficiency. The fundamental processes associated with these phenomena in organic molecules that are self-assembled in monolayers have not been systematically studied, and they are a new direction for research in thermoelectric systems. There is growing demand for new thermoelectric materials from the information-technologies, aerospace and automotive industries. This research, therefore, will help to retain the UK's internationally competitive position in the field of molecular electronic technologies. This highly interdisciplinary project is designed in a self-consistent way, covering the chemical synthesis of new molecules, their assembly on surfaces, molecular-scale experimental and theoretical physics, and laboratory demonstrations of thermoelectric effects. Our goals will be achieved by novel strategies that involve maximising phonon scattering at the electrode-molecule interface and designing molecules with built-in destructive phonon interference over a range of relevant frequencies. This project will be the first time that room temperature phonon interference (RTPI) and quantum interference (RTQI) have been exploited simultaneously in the same device and the resulting combination of low thermal conductance and high Seebeck coefficient will lead to molecular junctions with extraordinary thermoelectric efficiency. The project, therefore, will lay the foundations for high performance thermoelectric thin-film devices and could lead to a step change in the understanding of thermoelectric processes.
In the latter stages of the project (around month 30) a multidisciplinary workshop will be organised to bring international academic colleagues and current and potential industrial partners together to show-case the current status of the fields of molecular electronics materials, thermoelectrics and energy harvesting. A successful outcome of this project will also be very stimulating to the wider academic community and industrial partners who are engaged in the broad areas of synthesis and assembly of organic materials, mechanisms of charge transport, molecular electronics, sensors and surface science.
Many companies worldwide and in the UK are likely to benefit directly or indirectly from this project, e.g. IBM Zurich Research Laboratories (Switzerland), Rolic Technologies (Switzerland), Siemens (Germany), NSG Pilkington (Lathom), Quantum Base (Lancaster), NPL (Teddington), Toshiba (Cambridge and Japan), and VTT (Finland). The PDRAs employed on the project will benefit from joining the applicants' very active and successful research groups, which stimulate creativity and adventure in an interdisciplinary training environment. The PDRAs will, therefore, be ideally placed to learn new research-related and transferable skills and build strong independent research careers. They will be especially equipped to develop and lead future programmes in organic and molecular electronics and to contribute to the commercial development of technologies which derive from this fundamental science. This aligns closely with EPSRC's remit 'Development of Future Leaders', both for academia and industry. Finally, this project seeks to identify the first steps to a new type of technology that has important societal and economic impacts and the potential to address pressing problems in the real world.