Udo Schwingenschlogl

Abstract:

This presentation gives an overview of ab-initio calculations [1-3] addressing the thermoelectric performance of MXenes. Specific examples include a comparison of Ti₂CO₂, Zr₂CO₂, and Hf₂CO₂ in order to evaluate the role of the metal atom. The lattice thermal conductivity is demonstrated to grow along the series Ti-Zr-Hf in the temperature range 300-700 K, resulting in the highest figure of merit in the case of Ti₂CO₂. Flat conduction bands promote the thermopower in the case of n-doping. Functionalization effects are studied for Sc₂C, which is semiconducting for various functional groups, including O, F, and OH. The lowest lattice thermal conductivity is found for OH functionalization. Therefore, despite a relatively low thermopower, Sc₂C(OH)₂ may be interesting for intermediate-temperature thermoelectric applications due to a high electrical conductivity. We also discuss results on heterostructures built from MXenes and transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding. They contribute significantly to thermal transport and compensate for reduced contributions of the acoustic phonons (strong scattering in heterostructures), such that the thermal conductivities become similar to those of the constituent MXenes.