Theoretical Exploration Of New Two-dimensional Semiconducting Thermoelectric Materials

Thermoelectric conversion technology,which is a kind of green energy technology and can convert waste heat into electricity,avoids excessive consumption of traditional energy.It provides a new way to produce renewable energy and plays an important role in our medium-and long-term energy strategies.At present,the low conversion efficiency is the bottlenecks restricting the development of this technology.Therefore,exploring thermoelectric materials with high efficiency has become an urgent problem.The conversion efficiency of thermoelectric materials is evaluated by the dimensionless figure of merit ZT,which can be expressed as ZT=S²sk^-1T.Hence,increasing the power factor S²sand reducing the thermal conductivitykare the two different basic ideas to optimize ZT value of thermoelectric materials.Dresselhaus proposed that the power factor can be enhanced through the use of quantum confinement effects in the low-dimensional system.The basic mechanism behind is the increase of the density of states near the Fermi level.Reducing the dimension of materials can also have size effect on the heat transport,which increases the degree of freedom to regulate the electrical and heat transport.Therefore,the layered two-dimensional materials with specific structure and electronic properties have become the key point of this thesis.Here,with the focus of optimizing the electrical and thermal properties of two-dimensional materials,we theoretically analyzed the physical mechanism of thermoelectric transport based on the first-principles calculation,Boltzmann electron transport and Boltzmann-Peierls phonon transport equation.This research work may provide a new idea for understanding the design theory of thermoelectric materials and developing new two-dimensional layered semiconductor thermoelectric materials with higher power factor and lower thermal conductivity.The details are as follows:1.Demonstrating two-dimensional layered materials Tl₂O with lower lattice thermal conductivity.The electric transport properties of single-,bi-and tri-layer Tl₂O were analyzed by using first principles calculations and Boltzmann transport theory.The thermal transport property of single-layer Tl₂O was explored through first principles calculations and Boltzmann-Peierls transport equation.Results showed that the band structure changes significantly with the increase of number of layers,resulting in the dramatically increase of Seebeck coefficient.The calculations suggested that the tri-layer Tl₂O exhibits the highest power factor among all the different layers.In Tl₂O monolayer,monovalent Tl contains lone pair electrons and has a heavier mass,which make Tl₂O has stronger anharmonicity and shorter phonon lifetime and thus reduces the thermal conductivity.At room temperature,the thermal conductivity is 0.65Wm^-1K^-1 along the zigzag direction.Low lattice thermal conductivity combined with superior electrical transport properties improve the thermoelectric performance of this materials.This theoretical results show that layered material Tl₂O has great potential in the field of thermoelectricity.2.Polarized covalent bond is an important reason of decreasing lattice thermal conductivity in single-layer IV-V potential thermoelectric compounds.Through the calculation of the electronic structure of IV-Sb?SiSb,GeSb and SnSb?compounds,we found that the bulk structures exhibit the characteristic of metallic conduction,while the monolayers show semiconducting property.For IV-Sb monolayer materials,the strong spin-orbital coupling causes valence band splitting.In the case of p-type doping,parabolic band structures are unfavorable to thermoelectric transport.The calculated band structure verified that the obvious anisotropy of bands'dispersion near the minimum of the conduction band,especially for SiSb.For the n-type of SiSb,the Seebeck coefficient and electronic conductivity are improved by the larger density of states effective mass and lower conductivity effective mass.This decoupled the inverse relation between conductivity and Seebeck.From the thermal conductivity calculations,it shows that the thermal conductivities of monolayer SiSb,GeSb and SnSb increases successively.Strong polarized covalent bond of SiSb is one of the main reasons for its low lattice thermal conductivity.Combining both outstanding electronic properties and low thermal thermal conductivity,SiSb has been confirmed a promising n-type thermoelectric material.This work has been expected to provide new idea for the research and design of thermoelectric materials.3.Demonstrating ternary layered materials GeAsSe and SnSbTe with ultra-high power factor.The carrier relaxation time of GeAsSe and SnSbTe monolayer were investigated by the first principles calculations combined with deformation potential theory.The results indicated that the unique crystal structure of layered GeAsSe and SnSbTe makes the electrical transport property exhibit anisotropic character,especially for p-type doping.The weak coupling between holes and acoustic phonons makes the holes have long relaxation time,leading to the high conductivity in p-type doping.The anisotropy of hole relaxation time results in the anisotropy of conductivity.The conductivity of zigzag?y?direction is much higher than that of x direction.Thus,the higher power factor appears along the zigzag direction.At room temperature,the power factors of GeAsSe and SnSbTe in the zigzag direction are 0.76 Wm^-1K^-2 and 1.16 Wm^-1K^-2,respectively.For SnSbTe,the weak bond leads to lower sound velocity and layer grüneisen parameter,strong anharmonic effect and thus decreasing the lattice thermal conductivity.Because of high power factor along zigzag direction,the p-type doped monolayer GeAsSe and SnSbTe are found to be have much high peak ZT values of 4.4 and 6.5 along this direction at room temperature.This suggests that both materials can be used as promising thermoelectric materials.4.Demonstrating two-dimensional layered materials Ge₄Se₃Te with lower lattice thermal conductivity.The electrical structure of three-dimensional and two-dimensional Ge₄Se₃Te were studied and the results indicated that the weak interlayer coupling reduced the carrier relaxation time,and then in-plane conductivity.Meanwhile,we found that reducing the dimension of Ge₄Se₃Te can increase the power factor.Besides,for monolayer Ge₄Se₃Te,the unique structure with the Se-Ge-Ge-Se?Te?stacking contains Ge-Ge bonds.In the case of monolayer Ge₄Se₃Te,much weaker interatomic bonds give rise to low group velocity and strong optical-acoustic phonon coupling.All these features can be ascribed to the strong anharmonicity of the metal contacts between layers,which result in lower thermal conductivity.At 300K,the thermal conductivity of monolayer Ge₄Se₃Te is1.6Wm^-1K^-1.These theoretical results imply that the synthesized Ge₄Se₃Te has excellent thermoelectric properties.5.Demonstrating the lattice thermal conductivity of multilayer transition metal dichalcogenides can be significantly reduced by constructing heterogeneous structures.The thermal transport properties of WTe₂ monolayer,WTe₂ bilayer,bilayer heterogeneous material WTe₂-MoTe₂ and trilayer heterogeneous material WTe₂-MoTe₂-WTe₂ was analyzed.It was found that the phonon scattering channels of multilayer transition metal dichalcogenides is much more than that of monolayer,especially for multilayer heterogeneous materials,which can be attributed to the presence of van der Waals interactions.In the case of multilayer heterogeneous materials,the acoustic phonon hybridizes with the low frequency optical phonon,which enhance the anharmonic scattering rate and thus a lower phonon lifetime.More three-phonon scattering channels combined with short phonon lifetimes result in trilayer heterogeneous materials exhibiting lower lattice thermal conductivity.At 300K,the lattice thermal conductivity of monolayer WTe₂is 28Wm^-1K^-1,and the lattice thermal conductivity of trilayer heterogeneous materials WTe₂-MoTe₂-WTe₂ is reduced to 3Wm^-1K^-1.The results provide a new idea for the design of transition metal dichalcogenides-based thermoelectric devices.