Regulation Of Electronic Structure And Thermoelectic Performance Of Bi₂X₃ （X=Te,Se） And Sb₂Te₂Se Nanofilms

Thermoelectric materials can be used in waste heat power generation and solid-state refrigeration,and their large-scale application can effectively alleviate the current energy crisis and environmental pollution problems.Two-dimensional layered nanofilms have emerged as a frontier and hotspot in the field of thermoelectricity due to their excellent thermoelectric properties.In this thesis,based on density functional theory and Boltzmann transport theory,we systematically investigate the effects of biaxial strain on the electronic structure and thermoelectric properties of monolayer Bi₂Te₃,the effects of an external electric field on the electronic structure of Bi₂Se₃nanofilms with different thicknesses,as well as the effects of strain and external electric field on the electronic structure of monolayer Sb₂Te₂Se.（1）Based on density functional theory and Boltzmann transport theory,the effects of biaxial strain on the electronic structures and thermoelectric transport properties of monolayer Bi₂Te₃was systematically studied.The results show that the biaxial tensile strain increases the band degeneracy and increases the state density near the Fermi level,thereby increases the Seebeck coefficient of monolayer Bi₂Te₃.The effect of strain on the thermoelectric transport coefficients is more pronounced in p-type monolayer Bi₂Te₃compared to n-type monolayer.Tensile strain enhances the power factor of monolayer Bi₂Te₃and simultaneously reduces its thermal conductivity,thereby improves the thermoelectric figure of merit of monolayer Bi₂Te₃.（2）Based on density functional theory,we systematically investigate the effects of external electric field on the electronic structure of Bi₂Se₃nanofilms with different thicknesses.The results show that the effect of external electric field on the electronic structure of Bi₂Se₃is dependent on the layer thickness.For monolayer Bi₂Se₃under the external electric field,there is no significant change in the band structure.However,the band gaps of bilayer and trilayer Bi₂Se₃decrease with increasing external electric field due to the emergence of the Stark effect.Rashba splitting is observed at the conduction band bottom of bilayer and trilayer Bi₂Se₃,and the Rashba coefficient increases with increasing external electric field.For the same external electric field,the larger the layer thickness of Bi₂Se₃nanofilm,the larger the Rashba coefficient.（3）Based on density functional theory,we investigate the effects of biaxial strain and external electric field on the electronic structure of monolayer Sb₂Te₂Se.The results show that tensile strain makes the conduction band steeper and the valence band flatter,and decreases the band gap.The compressive strain has the opposite effect.Tensile strain enhances the density of states of valence band,which improves the thermoelectric performance of monolayer Sb₂Te₂Se.When the external electric field varies from 0 to 0.3V/（?）,the conduction band and valence band of monolayer Sb₂Te₂Se do not change significantly,and the band gap remains almost unaffected,which is mainly due to the shielding effect.