Periodic Reporting for period 3 - QuIET (Quantum Interference Enhanced Thermoelectricity)
Reporting period: 2020-02-01 to 2022-07-31
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.
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.
QuIET’s seven partners have combined their complementary expertise in synthesis, modelling, characterization and fabrication, interacting actively and getting a strong feedback from each other. Various strategies to tune QI effects in molecular junctions have been explored by synthesizing prototype molecules that are predicted to present 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.
The thermopower and conductance of different series of molecules connected to gold electrodes have been measured using a scanning tunneling microscope (STM). Our results demonstrate the enhancement of the thermopower using QI via controlled chemical modifications, by mechanical manipulation, and by the presence of unpaired electrons. We have performed a detailed characterisation of the transport properties of selected molecules both at ambient and at low temperatures using a mechanically-controlled break junction (MCBJ) and we have implemented graphene-based three-terminal devices and three-terminal electromigration devices for thermopower measurements. These electromigration devices are fully equipped with heaters and gates that enable the full characterization of a single-molecule junction. The applicability of this method goes beyond the project goals of QuIET. We have developed a new ac method for the simultaneous and continuous measurement of thermopower and conductance during the evolution of the junction, as well as the acquisition of thermopower images. The thermopower of self-assembled monolayers (SAMs) has also been explored, and we have confirmed the transfer of quantum interference in single molecules into self-assembled molecular arrays. Using extremely sensitive micro electro-mechanical systems (MEMS) sensors, we have measured the thermal conductance of single molecules, and a new hot-wire-thermocouple STM probe for the measurement of thermal conductance has being developed and demonstrated.
A new release of the multi-scale simulation tool “Gollum” for molecular-scale electronics, thermoelectrics and phonon transport in realistic environments was delivered and theoretical modelling has been realised for the experimentally observed thermopower, conductance and thermal conductance and for the behaviour of the molecules in the junction.
Prototype devices have been developed to exploit thermoelectric effects in molecular junctions, namely, nanoparticle arrays and vertical molecular junctions. Graphene nanoribbon three-terminal devices have also been developed. We have also collaborated actively with groups outside the consortium providing theoretical support and receiving molecules or theoretical backing. The research needs, the scientific questions and the technological opportunities of thermoelectric devices based on molecular junctions have been also analyzed in detail.
This work has led to 96 scientific papers in high-impact peer reviewed journals and one book. Two patents have been filed covering experimental developments. One conference has been organized.
The thermopower and conductance of different series of molecules connected to gold electrodes have been measured using a scanning tunneling microscope (STM). Our results demonstrate the enhancement of the thermopower using QI via controlled chemical modifications, by mechanical manipulation, and by the presence of unpaired electrons. We have performed a detailed characterisation of the transport properties of selected molecules both at ambient and at low temperatures using a mechanically-controlled break junction (MCBJ) and we have implemented graphene-based three-terminal devices and three-terminal electromigration devices for thermopower measurements. These electromigration devices are fully equipped with heaters and gates that enable the full characterization of a single-molecule junction. The applicability of this method goes beyond the project goals of QuIET. We have developed a new ac method for the simultaneous and continuous measurement of thermopower and conductance during the evolution of the junction, as well as the acquisition of thermopower images. The thermopower of self-assembled monolayers (SAMs) has also been explored, and we have confirmed the transfer of quantum interference in single molecules into self-assembled molecular arrays. Using extremely sensitive micro electro-mechanical systems (MEMS) sensors, we have measured the thermal conductance of single molecules, and a new hot-wire-thermocouple STM probe for the measurement of thermal conductance has being developed and demonstrated.
A new release of the multi-scale simulation tool “Gollum” for molecular-scale electronics, thermoelectrics and phonon transport in realistic environments was delivered and theoretical modelling has been realised for the experimentally observed thermopower, conductance and thermal conductance and for the behaviour of the molecules in the junction.
Prototype devices have been developed to exploit thermoelectric effects in molecular junctions, namely, nanoparticle arrays and vertical molecular junctions. Graphene nanoribbon three-terminal devices have also been developed. We have also collaborated actively with groups outside the consortium providing theoretical support and receiving molecules or theoretical backing. The research needs, the scientific questions and the technological opportunities of thermoelectric devices based on molecular junctions have been also analyzed in detail.
This work has led to 96 scientific papers in high-impact peer reviewed journals and one book. Two patents have been filed covering experimental developments. One conference has been organized.
QuIET has established a baseline of knowledge and skills for a future technology based on SAM-based thermoelectricity, that could lead to low-cost ecologically friendly thermoelectric devices. QuIET has established the principles for efficient thermoelectric devices based on QI and demonstrated QI-based functionality in several scalable platforms at room temperature. Employing QI for thermoelectric energy conversion is an entirely new approach, with extremely high potential. QuIET’s contribution to the understanding of the effect of QI on the thermal and electrical transport will have a strong influence on research in the field of thermoelectric energy conversion. QuIET has opened up completely new research directions, aimed at exploiting QI for a range of applications, beyond 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 gained may also find application in the fields of nano-optics, plasmonics, and phononics.