First-principles Study Of The Regulation Of Thermoelectric Performance Of Bi₂Se₃ Nanofilms

Thermoelectric materials provide a safe,reliable,environmentally friendly and all solid state power generation and refrigeration,which plays a more and more important role in alleviating energy crisis,environmental pollution and climatic change.Two-dimensional layered nanostructures have become the frontier and hot spot in the field of thermoelectric materials in recent years due to their enhanced thermoelectric properties.Based on density functional theory and Boltzmann transport theory,the effects of biaxial strain and external electric field on the electronic structure and thermoelectric transport properties of monolayer Bi₂Se₃ were systematically investigated in this thesis.（1）Based on density functional theory and Boltzmann transport theory,the effects of biaxial strain on the band structure and the thermoelectric transport properties of monolayer Bi₂Se₃ were systematically investigated.The band gap of monolayer Bi₂Se₃ increases with increasing compressive strain and decreases with increasing tensile strain.As the strain increases,the valence band converges and becomes flat,the density of states at the edge of the valence band becomes steeper,and the velocity of carriers decreases,so that the Seebeck coefficient of the p-type doped monolayer Bi₂Se₃ increases,and the electrical conductivityσ/τdecreases.Since the strain has relatively little effect on the conduction band edge,this makes the variation of the Seebeck coefficient and conductivityσ/τof the n-type doped monolayer Bi₂Se₃ with strain insignificant.Compared with the maximum power factor S²σ/τwithout strain,the maximum power factors of n-type and p-type doped monolayer Bi₂Se₃with a tensile strain of 6%are increased by 3%and 8%,respectively.This study can provide a theoretical reference for how to use biaxial strain to regulate the electronic structure and thermoelectric properties of two-dimensional materials.（2）Based on density functional theory,the geometry structure,electronic structure and charge transfer of monolayer Bi₂Se₃ under the external electric field were systematically studied.The results show that when the electric field strength is in the range of 0 V/（?）to 0.6V/（?）,the conduction band and valence band near the Fermi level of monolayer Bi₂Se₃ are insensitive to the external electric field,and the band gap hardly changes with the external electric field.This is mainly due to the shielding effect.Under the shielding effect,the structure of monolayer Bi₂Se₃ is less affected by the external electric field,which makes the energy band of monolayer Bi₂Se₃ insensitive to the external electric field.This study can provide a theoretical reference for how to regulate the electronic structure and physical properties of two-dimensional materials using the external electric field.