【作者】 秦丹；

【导师】 吕京涛；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2019， 博士

【摘要】 热电材料可以实现热能与电能的相互转换,且不需要传动部件和无废物排放。因此,随着能源短缺与环境污染问题日益严重,对热电材料的开发与研究具有重要的现实意义。但较低的热电转换效率,严重制约着热电材料广泛深入的应用发展。因而,材料的热电转换效率成为近些年热电领域研究的焦点问题之一,其大小由热电优值ZT来衡量:ZT=S²σT/（κ_e+κ_L）。其中塞贝克系数S、电导率σ、电子热导率κ_e由材料的电子性质决定,而晶格热导率κ_L则由材料的声子性质决定。电输运系数之间不是相互独立的,存在较强的耦合关系,不能单独调控。通常,通过能带工程和载流子浓度优化可以提高功率因子S²σ;降低材料维度不仅有助于解耦电输运系数的依赖关系,还可以降低声子热导率,从而提高热电优值。基于该理论,我们选取几种低维材料,利用第一性原理计算结合玻尔兹曼输运理论研究其热电性能。首先,我们研究了能带工程对单层ZrSe₂热电性能的影响。通过对单层ZrSe₂施加双轴拉伸应力,调控其能带结构,我们发现,当应变为7.5%时,价带简并程度最高,功率因子达到最大。同时,应变导致晶格形变,声学模式变软,声子热导率降低。综合上述两个因素,该应变下获得最优的热电性能。在合适的载流子浓度,对于p型和n型掺杂,室温下热电优值分别为3.84和4.58,这远远优于平衡状态下的热电优值。我们的研究表明,通过施加应力进行能带调控,是提高材料热电性能的一种有效途径。其次,我们研究了双层ZrSe₂与HfSe₂的热电输运性能。基于前人对块体及单层ZrSe₂与HfSe₂的理论与实验研究,我们构建了双层ZrSe₂与HfSe₂模型。通过计算发现,这两种材料是具有简并导带的间接带隙半导体,并且态密度呈现楼梯状的2D特征,这些因素为该材料提供了较高的功率因子。声子计算结果表明,较低的声学声子频率以及光学与声学模式的强耦合,降低了声子热导率。结合电子与声子输运的计算结果,我们发现室温下n型掺杂双层ZrSe₂与HfSe₂最佳热电优值分别为1.84和3.83。该结果远远优于其块体的热电性能。我们的研究表明,二维过渡金属硫化物可以通过调整其原子层数来提高其热电性能。然后,我们系统研究了单层PdSe₂的热电输运性能。实验上人们已经实现了对二维PdSe₂的微机械剥离,并发现该材料的载流子迁移率比较大。我们通过理论计算得到了单层PdSe₂的稳定结构、机械性质以及热电输运性能。我们发现,单层PdSe₂是间接带隙为1.38 eV的半导体,空穴的最大迁移率为1929 cm²V^-1s^-1。面内的有效弹性模量相当小,说明这种结构柔性较大,在施加应力的情况下不易损坏。而且,该材料的晶格热导率比较小。结合较高的塞贝克系数与低的晶格热导率,室温时最佳掺杂浓度下的热电优值为1.1。我们的研究结果表明像PdSe₂这类在面内呈现五边形的材料,其二维结构具有特殊的机械性能与良好的热电性能,这可为未来对这类材料进一步研究提供参考与启示。最后,我们计算了酞菁分子吸附在不同种金属表面的稳定性,解释相应的实验现象。以此为基础,选取稳定构型,我们构建分子结器件,研究了其热电输运性能。我们考虑了SnPc分子在Au（111）和Cu（111）表面的吸附与扩散。计算结果表明,SnPc分子在Cu（111）面上的结合能比在Au（111）面上的要大1 eV,这说明SnPc分子更容易吸附在Cu（111）上。SnPc分子吸附在Au（111）上的各个构型之间的扩散能垒比在Cu（111）上的要小。我们的理论计算结果验证了相关实验现象,也为进一步的实验提供理论支持与参考。同时,我们选取上述稳定构型,利用Au（111）或Cu（111）为电极构建分子器件,并研究了在零偏压下的热电输运性能。虽然该器件的ZT值不是很理想,但是可以采用有效的方法,例如掺杂、应力、电场或者磁场来调控器件的输运性质,提高其性能。更多还原

【Abstract】 Thermoelectric materials can directly convert heat into electricity without any moving parts or waste emission.Therefore,the research on thermelectrics has important practical significance under the background of increasingly serious environmental pollution and energy shortage.However,the thermoelectric conversion efficiency is low,which seriously restricts the extensive and in-depth application of thermoelectric materials.The performance of thermoelectric materials is primarily assessed by the dimensionless figure-of-merit,ZT=S²σT/（κ_e+κ_L）,where the Seebeck coefficient S,the electrical conductivityσ,and the electronic thermal conductivityκ_e,are determined by the electronnic property of materials,while the lattice thermal conductivityκ_Lis determined by the phonon performance.The electrical transport coefficients are not independent.In fact,they are strongly coupled with each other and can not be controlled alone.In general,the power factor S²σcan be improved by band engineering and carrier concentration optimization.Reducing material dimension not only helps to decouple the dependence of the electrical transport coefficient,but also reduces the phonon thermal conductivity.Thereby,the above approaches are beneficial for improving the figure of merit.Based on this theory,we investigate the thermoelectric properties of several low-dimensional materials by the first-principles calculations combined with Boltzmann transport theory.Firstly,we investigate the thermoelectic performance of the monolayer ZrSe₂ through band engineering.By applying biaxial tensile stress to the monolayer ZrSe₂ and adjusting its energy band structure,we obtain the maximized power factor due to the highest valence band degeneracy when the applied strain is 7.5%.At the same time,the phonon thermal conductivity decreases since the acoustic mode becomes soft under the strain.Combining the above two factors,the optimal thermoelectric performance is obtained under the strain.At the appropriate carrier concentration,for p-type and n-type doping,the figure of merit at room temperature is 3.84 and 4.58,respectively,which is far superior to those in equilibrium.Our research shows that energy band engineering by applying stress is an effective approach to improve the thermoelectric properties of materials.Secondly,we study the thermoelectric transport properties of the bilayer ZrSe₂ and HfSe₂.Based on the previous theoretical and experimental researches on bulk and monolayer ZrSe₂ and HfSe₂,we construct bilayer ZrSe₂ and HfSe₂ structures.It is found that the two materials are indirect band gap semiconductors with degenerate conduction bands,and the state density exhibits stair-like two-dimensional features,which provide a higher power factor for the material.The phonon calculation results show that the lower acoustic phonon frequency and the strong coupling of the optical and acoustic modes reduce the phonon thermal conductivity.Combining the calculation results,we find that the optimal ZT of ntype doped bilayer ZrSe₂ and HfSe₂ is 1.84 and 3.83 at room temperature,respectively.These results are far superior to the thermoelectric properties of their bulk conterparts.Our research shows that the thermoelectric properties of two-dimensional transition metal chalcogenides can be improved by adjusting their atomic layer numbers.Then,we systematically investigate the thermoelectric transport performance of monolayer PdSe₂.The two-dimensional PdSe₂ has been exfoliated from bulk crystals experimentally,which has been found to have relatively large carrier mobility.Based on the theoretical calculations,we obtain the stable structure,mechanical properties and thermoelectric transport properties of monolayer PdSe₂.The results show that the monolayer PdSe₂ is a semiconductor with an indirect band gap of 1.38 eV,and the maximum mobility of holes is 1929 cm²V^-1s^-1.The effective elastic modulus in plane is rather small,indicating that the structure is relatively flexible and not easily damaged under the application of stress.Moreover,the lattice thermal conductivity of this material is relatively small.Combined with the high Seebeck coefficient and low lattice thermal conductivity,the figure of merit at the optimum doping concentration at room temperature is 1.1.Our results show that materials such as PdSe₂ which exhibit pentagon in-plane structure have special mechanical properties and good thermoelectric properties.Our studies can provide reference for further research on such materials in the future.Finally,we calculate the stability of phthalocyanine molecules adsorbed on different metal surfaces and explain the corresponding experimental phenomena.Based on this,we select the stable configurations to construct molecular junction devices to study their thermoelectric transport properties.We consider the adsorption and diffusion of SnPc molecules on the Au（111）and Cu（111）surfaces.The calculation results show that the binding energy of SnPc molecule on Cu（111）surface is 1 eV larger than that on Au（111）surface,which indicates that SnPc molecule is more easily adsorbed on Cu（111）.The diffusion energy barrier between the various configurations of SnPc molecules adsorbed on Au（111）is smaller than that on Cu（111）.Our theoretical calculations verify the relevant experimental phenomena and provide theoretical support and reference for further experiments.At the same time,we build the molecular device from the above stable configuration,using Au（111）or Cu（111）as the electrodes,and investigate the thermoelectric transport performance under zero bias.Although the ZT value of the device is not high enough,effective methods such as doping,stress,electric field or magnetic field can be used to regulate the transport properties of the devices and improve their performance.更多还原