Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TherSpinMol (Exploring Thermoelectric and Spintronic properties of Molecular Devices)

Teaser

The development of wearable and diffuse electronics has prompted a global need for converting heat into electrical power. This could be heat from reactors, the sun, or even the human body. Thermoelectric materials are ideal candidates, since they contain no moving parts, can...

Summary

The development of wearable and diffuse electronics has prompted a global need for converting heat into electrical power. This could be heat from reactors, the sun, or even the human body. Thermoelectric materials are ideal candidates, since they contain no moving parts, can be fabricated as thin films and are scalable. However, they currently see only niche application, owing to their very low efficiency. Bulk materials offer little room for progress: the ingredients to reach high efficiencies are mutually contradictory in the bulk. In contrast, theory clearly indicates that molecular nano-materials do not suffer from such constraints and may provide a solution.

TherSpinMol aimed at developing the first molecular thermoelectric devices and discovering new strategies to improve their efficiency. Furthermore, the project worked towards establishing the experimental foundations for molecular spin-caloritronics.

The project has fundamental and applicative significance, aiming at exploring both the physics background of thermoelectric power generation on the single-molecular level and novel ways to create pure spin-currents and their perspective applicability for the reduction of energy consumption in logic elements and energy harvesting.

Work performed

Several device platforms for contacting single molecules have been screened, where graphene was tested most extensively. This led to the discovery of mechanically tunable quantum interference effects in graphene bi-layers (published in Nature Nanotechnology) and mechanically tunable graphene quantum dots (publication submitted). The role of thermal transport through the graphene leads to graphene quantum dots led to a better understanding (and complete modelling) of the thermoelectric behaviour of ‘real’ energy harvesters based on single quantum dots. Furthermore, in a detailed study of the thermoelectric properties of the graphene contacts we discovered that very sensitive, all-graphene thermocouples can be fabricated by varying only the width of a graphene stripe (published in Nano Letters and second publication in preparation).
What is more, a new and highly scalable method to contact single molecules was developed in collaboration with KTH Royal Institute of Technology (published in Nature Communications).

For thermoelectric characterisations of single molecules an ultra-efficient on-chip microheater was developed which can thermally bias single molecules using very low heating powers (published in Applied Physics Letters). This makes the newly developed junctions compatible with very low cryogenic temperatures necessary for detailed thermoelectric characterisations. Using gold as electrode material a high yield of molecular junction formation was achieved. The detailed studies of the thermoelectric properties of single molecules at cryogenic temperatures gave insight into the role of electron-phonon coupling (publication in preparation) or strong correlations like the Kondo regime (publication in preparation) on the thermoelectric efficiency of the device.

Final results

I pioneered measurements of molecular thermoelectricity with full gate control. To this end I developed a device architecture and a robust measurement protocol that allows measuring the thermoelectric properties of single molecules at cryogenic temperatures, over a wide energy range. This allowed me to gain deep understanding of e.g. the role of metallic contacts when performing a thermoelectic measurement or how vibrational degrees of freedom in a single molecule affect their thermoelectric efficiency. This deep insight in the processes which give rise to thermoelectric power generation on the single-molecule level might be extremely valuable for future energy harvesting applications, which puts the successful project TherSpinMol in phase with the objective of the ‘Europe 2020’ strategy for developing secure, clean and efficient energy.
Furthermore, I developed a new strategy to fabricate all-graphene thermocouples. These thermocouples can act as ultra-sensitive local thermoemeters which achieve a temperature resolution down to a few tens of µK and have a foot-print on the order of only 1µm. This would make it possible to fabricate about 200 temperature sensors around the circumference of a human hair, or about 100 sensors below the area of a human cell and could pave the way for controlled local temperature management in biological systems to e.g. fight cancer cells.

Website & more info

More info: https://twitter.com/therspinmol.