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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - QuIET (Quantum Interference Enhanced Thermoelectricity)

Teaser

Summary

Generation of electricity from heat via the Seebeck effect is silent, environmentally friendly and requires no moving parts. Waste heat from automobile exhausts and industrial manufacturing processes could be used to generate electricity economically, provided materials with a high thermoelectric efficiency (characterised by a high dimensionless thermoelectric figure of merit ZT) could be identified. Conversely, Peltier cooling using such materials would have applications ranging from on-chip cooling of CMOS-based devices to home refrigerators.

Currently-available thermoelectric materials are semiconductors, typically based in Bi, Pb, Te or Se, which are toxic, expensive and have limited efficiency, which constrains them to niche applications. Wide scale applications of thermoelectricity require new low-cost materials with much higher thermoelectric efficiency.

The ability to manipulate quantum interference (QI) in molecules creates the potential for high-performance and sustainable materials with unprecedented thermoelectric efficiencies. To deliver the next breakthrough, QuIET will exploit QI at the single-molecule level and then translate this enhanced functionality to technologically-relevant thin-film materials and devices above room temperature. We will establish the feasibility of using QI in molecular junctions at room temperature to enhance their Seebeck coefficient, and combine this with mechanical tuning of molecules and electrode-molecule couplings to minimise their phonon thermal conductance. This will be achieved by combining theoretical predictions of their electronic and vibrational properties with our ability to design and synthesize new molecules and to measure their Seebeck coefficient and thermal conductance. Furthermore, we will establish strategies to exploit this QI functionality in many-molecule thin films providing a basis for the design of practical devices and new materials to solve the optimization challenge of combining high efficiency with low cost and toxicity.

The objectives of QuIET are to:

1. Develop single-molecule junctions with high thermopower by taking advantage of QI.
2. Enhance thermoelectric efficiency of single-molecule junctions by minimising the phonon contribution to thermal conductance.
3. Create self-assembled monolayers (SAMs) that preserve the high thermopower and low thermal conductance obtained in single-molecule junctions, and further enhance their thermoelectric properties via intermolecular interactions.
4. Identify implementation routes of scalable thermoelectric devices based on molecular junctions.

To deliver these objectives, QuIET will bring together the multidisciplinary expertise of leading scientists in molecular synthesis, experimental quantum transport in nanosystems, and modelling and theoretical calculations in the nanoscale.

Work performed

During this first year, QuIETâ€™s seven partners have combined their complementary expertise in synthesis, modelling, characterization and fabrication, interacting actively and getting a strong feedback from each other. We have synthesised molecules which enable the variation of electronic pathways leading to Quantum Interference (QI) effects. These molecules incorporate anchor groups for binding to gold or graphene electrodes and for forming single-molecule junctions, or cross-linking side-groups to form stable self-assembled monolayers. We have measured the thermopower and conductance of different series of molecules connected to gold electrodes using a scanning tunneling microscope (STM), studied the stability of molecular junctions on graphene-based nanogaps, and implemented graphene-based devices for thermopower measurements. Detailed characterisation of transport properties was performed using a mechanically-controlled break junction (MCBJ). Extremely sensitive micro electro-mechanical systems (MEMS) sensors have been used to measure the thermal conductance of single molecules. The thermopower of self-assembled monolayers (SAMs) and the thermal conductance in thin films were measured and a new hot-wire STM setup for the measurement of thermal conductance in SAMs was built and tested. We have delivered a new release of the multi-scale simulation tool â€œGollumâ€ for molecular-scale electronics, thermoelectrics and phonon transport in realistic environments. Theoretical modelling has been realised for the experimentally observed thermopower, conductance and thermal conductance and for the behaviour of the molecules in the junction. We have also collaborated actively with groups outside the consortium providing theoretical support and receiving molecules or theoretical backing.

We have obtained the following results: a) we have demonstrated the enhancement of the Seebeck coefficient of molecular junctions using QI via controlled chemical modifications of the electron pathway and the energy alignment of the molecular levels; b) we have also demonstrated the control of QI by mechanical manipulation; c) a new graphene-based testbed that will allow the investigation of QI control via an electrostatic gate has been developed and molecules with anchor groups specifically adapted to this testbed have been synthesized; d) the measurement of thermal conductance at the single-molecule level has been achieved; e) we have also obtained promising values of the Seebeck coefficient of self-assembled monolayers and developed new tools for measuring thermal conductance in thin films of molecules; f) additionally, powerful theoretical tools have been developed to take into account phonons, interaction between molecules, environmental effects on thermal and electrical conductance and capable of modelling full devices.

Final results

QuIET will establish a solid baseline of knowledge and skills for a future technology based on SAM-based thermoelectricity, opening the opportunity of low-cost ecologically friendly thermoelectric devices. QuIET aims at establishing the principles that will make possible (or show to be possible) efficient thermoelectric devices based on QI. We shall demonstrate experimentally the principles of QI-based functionality in several scalable platforms at room temperature. The knowledge gathered during the timeframe of the project will enable the development of practical efficient thermoelectric devices. Employing QI for thermoelectric energy conversion is an entirely new approach, with extremely high potential. New findings and understanding of the effect of QI on the thermal and electrical transport will have a strong influence on current research in the field of thermoelectric energy conversion. QuIET is a high-risk project, but if successful will have a transformative impact on science and technology. It will open up completely-new research directions, aimed at exploiting QI for a range of applications, beyond our applications of QI-enhanced thermoelectric energy conversion. Understanding and controlling the various types of QI mechanisms will have significant impact on the entire field of nanoscience. Insights will be gained which may also find application in the fields of nano-optics, plasmonics, and phononics.â€ƒ