Many efforts have been made to search for materials that maximize the thermoelectric (TE) figure of merit, ZT, but for decades, the improvement has been limited because of the interdependent material parameters that determine ZT. Recently, several breakthroughs have been reported by applying nanotechnology. To further enhance ZT, a clear understanding of electronic and thermal transport is necessary. The objectives of this thesis are: 1) to evaluate the electronic and thermal performance with a Landauer approach using full band electronic bandstructure and a full dispersion description of phonons, 2) to show how the Landauer treatment gives new insights to the understanding of thermoelectrics, and 3) to discuss possibilities for enhancing TE performance. We first present a Landauer approach for computing TE parameters using a full band electronic bandstructure. The full band results are related to the more common effective mass formalism. Next, a full dispersion description of phonons is used to calculate the thermal conductivities of bulk and thin films using a Landauer approach. It is shown that simplified dispersion models for phonons should be used with caution and that the Landauer approach provides a relatively simple (but accurate) technique to treat phonon transport from the ballistic to diffusive regimes. We also address the question of how to engineer the electronic structure to enhance the performance of a thermoelectric material by re-visiting from a Landauer perspective the question of what bandstructure produces the best thermoelectric device performance. Next, we explore the possibilities of increasing ZT through multi-barrier structures, quantum engineered graphene and molecules, high valley degeneracy, or by distorting the density-of-states. Finally, we shift our attention to nanocomposite thermoelectric materials and discuss a new approach to model nanocomposite TE devices. Using polycrystalline graphene as a testbed of our model, we study how grain boundaries affect the electronic performance of large-area polycrystalline graphene and propose the new approach of 'percolation-doping by nanowires' to beat the transparency-conductivity constraints.
Changwook Jeong received his Doctor of Philosophy in August 2012.
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