Mechanical Tuning Of Electronic Structures And Thermoelectric Properties Of Bi₂Se₃

The ability of thermoelectric materials to convert heat which is abundant around human living environment, including terrestrial heat, industrial waste heat, vehicle exhaust waste heat, and solar heat, into electricity attracts a lot of attentions from researchers. Thermoelectric material Bi2Se3 with layered structure is equiped with several advantages, including relatively low thermal conductivity, low costs and weak bipolar conduction effect compared with widely used Bi2Te3. Therefore, the researches in Bi2Se3 are of great significances from the perspectives of science and application. In order to achieve a high figure of merit of Bi2Se3, mechanical tuning has been adopted in this thesis. In this thesis, density-functional theory was used to investigate the effects of stress and strain on electronic structures of Bi2Se3, and the Semi-classic Boltzmann transport theory was taken to study the impacts of stress and strain on the thermoelectric properties of Bi2Se3, and the main research work and conclusions are as follows:1. The effects of stress on the electronic structures and thermoelectric properties of Bi2Se3 have been investigated. It is found that the band gap of Bi2Se3 is broadened under stress, and the number of extremum values of band structure near fermi level grows, as well as the proportion of light bands increased. The results of thermoelectric properties calculations indicate that seebeck coefficient of p-type Bi2Se3 increases with increasing stress at lower hole concentration range while decreases at higher one, and the electric conductivity and maximum value of power factor with respect to relaxation time rise with increasing stress. The seebeck coefficient of n-type Bi2Se3 increases with increasing stress at most electric concentration range, and the power factor with respect to relaxation time rises with increasing stress. The thermoelectric properties of both n- and p-type Bi2Se3 have been enhanced by stress with thermoelectric properties of p-type Bi2Se3 higher than that of n-type one, and the carrier concentrations are obtained.2. The effects of strain on electronic structures and thermoelectric properties of Bi2Se3 have been studied. It reveals that the number of extremum values of band structure near fermi level is increased by both compressive and tensile strain, and camel-back band shape appears in band structure at several places. In addition, the proportion of light bands elevates somewhat. The slope and localization of density of states of conduction band near fermi level are increased and strengthened by compressive strain. The maximum value of power factor with respect to relaxation time can be increased by both compressive and tensile strain, while the maximum value of power factor with respect to relaxation time of p-type Bi2Se3 under compressive strain is higher than that of tensile one, suggesting compressive strain is more favorable for enhancing thermoelectric properties of p-type Bi2Se3. Maximum value of power factor with respect to relaxation time of n-type Bi2Se3 is increased by compressive strain while decreased by tensile one. The results of thermoelectric properties of Bi2Se3 mentioned above show that power factor of Bi2Se3 can be tuned by strain, and we have obtained the optimized carrier concentrations for power factor, offering good theoretical guidances to experiments.