Study On The Thermoelectric Transport Properties Of Several Layered Semiconductors

Thermoelectric materials,which can convert waste heat into electric power without involving any emissions,have attracted more and more attention.The efficiency of thermoelectric conversion is governed by the figure of merit ZT=S²σT/κ.However,there are many restrictions on improving the thermoelectric figure of merit due to the couple of transport coefficients.Layered semiconductors can be made into atomic layer thickness,and their two-dimensional quantum confinement effects may have important effects on thermoelectric performance.Based on the first-principles calculations and Boltzmann transport theory,in this thesis,we systematically study the thermoelectric transport properties of several layered semiconductors and their monolayers and heterostructures.The main results are as follows:We firstly investigate the electron and phonon transport properties of bulk layered Bi₂O₂X（X=Se,Te）.Our results indicate that higher power factors are found in the p-type doping.At 300 K,both Bi₂O₂Se and Bi₂O₂Te exhibit ultralow lattice thermal conductivities of 1.14 W/m K and 0.58 W/m K,respectively.The low-frequency optical phonon branches with higher group velocity and longer lifetime also make a main contribution to the lattice thermal conductivity.Due to the weak electrostatic interactions between the layers,the lattice thermal conductivity of Bi₂O₂X shows obvious anisotropy between in-plane and cross-plane directions.The optimal p-type figure of merits can reach 0.53 and 0.62 for Bi₂O₂Se and Bi₂O₂Te,respectively,indicating that the p-type doping is more conducive to improve the thermoelectric performance of Bi₂O₂X.In addition,the spin-orbit coupling significantly reduces the energy difference between the top energy valleys of the valence bands for Bi₂O₂X,thereby reducing the p-type thermoelectric performance,while the n-type thermoelectric properties change little.These results are helpful for the understanding and optimization of thermoelectric performance of layered Bi₂O₂X.Next,we investigate the thermoelectric transport properties of 2D ternary Ti NX（X=F,Cl,Br）monolayers.The large p-type power factor and the low lattice thermal conductivity along the y direction give rise to better thermoelectric performance along the y direction than the x direction,and the highest ZT values at 500 K reach 1.00,0.89 and 1.17 along the y direction in p-type doping for Ti NF,Ti NCl,and Ti NBr monolayer,respectively.These results indicate that Ti NX monolayers are promising 2D anisotropic thermoelectric materials.Based on the recent experimental synthesis of few-layers of Zr S₃,we present a comparative study on the thermoelectric transport properties of bulk and monolayer Zr S₃.For the p-type,the multi-valley degeneracy of valence band leads to the large Seebeck coefficient and the high power factor.For the n-type,the obvious dispersion of the conduction band bottom along the（38）-Y direction results in higher electron mobility,and thus higher electric conductivity and power factor.In addition,the dimensionality reduction in the crystal structure enhances the phonon scattering and decreases the phonon group velocity,and thus reduces the phonon thermal conductivity in Zr S₃monolayer compared to bulk.The optimal n-type thermoelectric figure of merit at 800 K for monolayer Zr S₃can reach 2.44 along the y direction,while it is 1.75 for the n-type doping of bulk Zr S₃.These results indicate that Zr S₃especially in the monolayer form is a promising anisotropic thermoelectric material.Finally,based on experimentally synthesized monolayer WSe₂,monolayer Sn S₂and bilayer WSe₂/Sn S₂heterostructure,we calculate and compare their thermoelectric properties.The results show that the power factor of WSe₂/Sn S₂heterostructure is obviously higher than that of monolayer WSe₂due to the decreased band gap.In term of phonon transport,due to the coupling of low-frequency optical branches with acoustic branches and the weak van der Waals forces between layers,the lattice thermal conductivity of the WSe₂/Sn S₂heterostructure at room temperature is 22.69 W/m K,which is lower than that of the monolayer WSe₂.Therefore,the thermoelectric performance of monolayer WSe₂could be enhanced by this heterostructure,and the figure of merit of the n-type WSe₂/Sn S₂bilayer heterostructure at 800 K can reach 1.16.