Date of Award

12-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Committee Member

Dr. Jian He, Committee Chair

Committee Member

Dr. Murray Daw

Committee Member

Dr. Apparao Rao

Committee Member

Dr. Sumanta Tewari

Abstract

The recent surge of interest in tin selenide (SnSe) is due to the reported record-high thermoelectric figure of merit at elevated temperatures. While the researchers are exerting tremendous efforts to further improve the thermoelectric performance of SnSe via doping and nanostructuring, it is getting more and more apparent that SnSe is fascinating in many aspects of fundamental physics. SnSe is, in many aspects, an outlier of the current materials selection rules for high thermoelectric performance. Hence, answering why thermoelectric performance is high in SnSe should come before addressing how to further improve its thermoelectric performance. To this end, there are three primary questions worth immediate attention, one related to the thermal nature of SnSe: (i) why the lattice thermal conductivity is low in such a simple-structured light element containing binary compound; and the other two related to the electrical nature of SnSe: what is the electronic ground state of this material. The second question can be addressed by answering to sub-questions: (ii) what is the origin of the resistivity anomaly around 50 K; and (iii) what is the nature of the metal-insulator transition driven by Sn deficiency.

In this thesis, we intend to address the three questions by means of the temperature dependent resistivity, Seebeck coefficient, Hall coefficient, specific heat, thermal conductivity, magnetic susceptibility and X-ray diffraction measurements in conjunction with density functional theory calculations. These results presented a grand picture how SnSe evolves structurally, electrically, and thermally from a low temperature metallic state to a high temperature semiconducting state toward a promising thermoelectric performance. In particular, we found that (i) high quality pristine SnSe single crystals exhibit a metallic ground state (that is, a small but robust Fermi surface with multiple pockets) with a coexisting band gap; (ii) off-stoichiometry and doping could destabilize the metallic state; (iii) the low-lying optical phonon modes and strong anharmonicity contribute to the low lattice thermal conductivity. We obtained a thermoelectric figure of merit ZT ∼ 1.0, ∼ 0.8 and ∼ 0.25 at 850 K along the b, c and a directions in high quality single crystalline SnSe. We have also discussed the formation of the Fermi surface in relation to Sn vacancies and the disorder induced metal-to-insulator transition in light of Anderson localization.

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