Nanophononic metamaterials: engineering phonons to convert between heat and electricity

Thermoelectrics are materials which convert temperature gradients into electricity, or vice versa. These materials can harvest waste heat from industrial processes, turning it into electricity, or use current to provide efficient cooling. These require a material with high electrical conductivity and low thermal conductivity. For metals these are contradictory, since both current and heat are carried by electrons; however, in semiconductors the heat is carried by phonons. Previous semiconductor research has focused on introducing defects to scatter heat-carrying phonons, but electrons often scatter from these too, lowering the electrical conductivity.

We propose a novel approach, using nanoscale engineering to tailor a material’s phonons. By patterning the material’s surface, specific phonon modes may be created which hybridise with heat-carrying phonons in the bulk material, reducing the thermal conductivity 100-fold. The bulk material is completely unchanged, so electrical properties are unaffected. This new class of nanostructures is known as nanophononic metamaterials (NPMs), and have the potential to have higher thermoelectric figures of merit than any previously known material.

Fabricating these nanostructures is challenging, and it is vital to focus on specific NPMs which can deliver the phononic and electronic properties required for high-efficiency thermoelectrics. We propose to use state-of-the-art quantum mechanical simulations (DFT) to predict and model the electronic and phononic properties of NPMs and, coupled with larger semi-empirical models, to design realistic nanoscale device NPMs with high thermoelectric efficiency. These designs will form the basis for new collaborations with world-leading experimental groups, in order to fabricate the NPMs and verify their performance.

This work will be carried out in collaboration with Dr Mahmoud Hussein (University of Colorado Boulder, USA).