【作者】 乌云达来；

【导师】 赵国军；

【作者基本信息】 内蒙古大学 ， 物理学， 2025， 博士

【摘要】 近年来,二维ⅥA族层状材料在热电领域的应用潜力引起了广泛关注。特别是单层过渡金属二硫属化物（TMDs）,因其卓越的载流子迁移率和固有的低晶格热导率,已成为热电研究的核心材料之一。尽管该材料具备优异的性能,但其仍存在进一步优化的可能性。研究指出,通过垂直堆叠范德华（vdW）异质结构,可以显著提升热电性能,且该制备技术已达到成熟阶段。在现有的计算方法基础上,对范德华异质结构的热电输运特性进行了系统性的探究。具体而言,使用了形变势理论来计算载流子迁移率。鉴于在室温条件下,电子-声子散射是载流子输运过程的主要影响因素,结合了第一性原理计算和玻尔兹曼输运理论,对热电输运特性以及电-声子散射的调控机制进行了全面分析。然而,这种计算方法仅适用于声学支散射和低阶散射过程。为了突破这一局限,进一步采用了密度泛函理论和Wannier插值法,以计算所有声子的散射,并详细分析了各向异性效应对声子散射的影响。此外,为了更全面地理解高阶声子散射机制,我们的研究中还运用了Laval-Born-James（LBJ）理论和Zacharias Giustino（ZG）特殊位移法,深入研究了高阶布里渊区域的声子散射情况,并计算了高温环境下的声子散射特性。通过这一系列的综合研究和计算,不仅揭示了二维ⅥA族层状材料在热电领域的巨大潜力,还为未来进一步提升其性能提供了基础的理论依据。具体研究内容如下:（1）热载流子的非平衡弛豫动力学是新型功能材料设计中的一个关键科学问题。电子-声子散射作为热化过程中的主要机制,在决定载流子输运性质方面起着决定性作用。采用密度泛函理论（DFT）结合Wannier方法,研究了二维材料Janus 1T-SnSSe与1T-SnS₂在费米面附近的电子-声子散射率。通过对比分析两种材料的散射率差异,得出了不同结构对热载流子弛豫性质的影响机制。结果表明,在300 K下,Janus 1T-SnSSe的电子-声子散射率明显低于1T-SnS₂。声子模式分辨率分析表明,这种差异主要源于与1T-SnS₂相比,Janus 1T-SnSSe中声学支对散射过程的贡献明显较弱。对载流子弛豫动力学的进一步分析表明,Janus1T-SnSSe中的热载流子表现出更长的弛豫时间,表明非平衡载流子寿命更长（～238 fs）,这有利于减轻能量耗散。进一步分析表明,Janus 1T-SnSSe中热载流子的平均自由径明显优于1T-SnS₂,在热载流子提取效率方面具有显著优势。Janus 1T-SnSSe不仅在热载流子输运性能方面的显著优势,而且为基于其高效光电器件的设计提供了理论指导。（2）基于LBJ理论框架,对2H-MoTe₂双层结构中的多声子非弹性散射过程进行了研究,并通过进一步分析,得到了该过程在总声子散射贡献中的比例。系统的理论计算和分析得出了以下关键发现:第一,多声子非弹性散射的强度与距布里渊区中心的距离成正比。值得注意的是,当波矢量|Q|超过8（?）^-1时,多声子相互作用的贡献显著增加,超过总散射强度的60%,超过了单声子相互作用。此外,所有声子非弹性散射都表现出显著的各向异性特征,沿Γ-M和Γ-K方向具有明显的强度差异。此外,虽然Te₁Te₂集体运动模式对所有声子非弹性散射强度的贡献相对较小,但它对布拉格散射强度有显著影响。这种独特的现象归因于2H-MoTe₂双层结构的中心对称性引起的相位抵消效应。为了确保结果的可靠性,采用ZG理论来验证全声子非弹性散射强度,结果与LBJ理论预测的结果非常接近。本研究的理论进展不仅加深了对过渡金属二硫化物中声子非弹性散射机制的理解,而且为精确控制二维材料的非平衡声子输运特性提供了新的理论基础。（3）采用第一性原理方法,SiC/MoSe₂/SiC范德华异质结的电子输运特性以及热电输运性能进行了全面的探讨。理论计算和输运特性分析证实,异质结表现出典型的I型能带排列,并表现出强烈的载流子输运各向异性。在300-700 K的宽温度范围内,n型导电沟道的电子贡献明显超过p型空穴传输的电子贡献。这种独特的载流子传输特性使该系统能够实现2.40 mW·m^-1·K^-2的峰值功率因数,与传统的单层二维热电材料相比,实现了1-2个数量级的突破性改进。微观机理分析表明,异质结中声学支和低频光学分支之间的强耦合导致声子散射显著增强,将x轴晶格热导率降低到1.51 W·m^-1·K^-1的超低值,与y轴相比降低了20%。这种晶格热输运各向异性是由非谐波效应对声子输运特性的调制引起的。由于其优异的电子输运性能和超低晶格热导率,异质结在700 K下实现了4.86的热电优值（ZT）,与类似材料相比表现出优异的性能。这项工作不仅从能带工程和声子工程的角度阐明了二维异质结构中的热电增强机制,而且为开发具有定向热管理能力的新型热电材料提供了理论基础。热载流子弛豫过程涉及多种散射机制,电-声子散射和多声子非弹性散射均重要,但对能量弛豫贡献不同。例如,Janus 1T-SnSSe发射低频声学支可延长带隙边缘弛豫时间,延缓热载流子热化;然而,对称性破坏使布里渊区域中心Γ点发射的高频ZO光学支抑制弛豫时间延长,加速热载流子热化。在双层2H-MoTe₂材料体系中,多声子散射在高级布里渊区域或高温环境下的非弹性散射及热输运过程中起主导作用。300 K时,多声子散射对热传输特性影响达18%,间接揭示其在该材料中可延长热载流子弛豫时间、减缓热化过程的可能。热载流子弛豫过程影响热电材料输运性质,SiC/MoSe₂/SiC异质结中热载流子弛豫时间随声子频率增加呈指数型衰减,原因是高频区声子非谐效应增强使散射概率上升,导致晶格热导率下降。综上所述,不同材料的声子特性决定了热载流子的弛豫过程。更多还原

【Abstract】 In recent years,two-dimensional（2D）Group-ⅥA layered materials have attracted significant attention for their potential thermoelectric applications.Particularly,monolayer transition metal dichalcogenides（TMDs）have emerged as one of the core materials in thermoelectric research due to their exceptional carrier mobility and intrinsically low lattice thermal conductivity.Although these materials exhibit outstanding performance,there remains room for further optimization.Studies have demonstrated that vertically stacked van der Waals（vdW）heterostructures can remarkably enhance thermoelectric performance,and this fabrication technique has now reached a mature stage.Building upon existing computational methods,our research systematically investigates the thermoelectric transport properties of vdW heterostructures.Specifically,we employ deformation potential theory to accurately calculate carrier mobility.Considering that electron-phonon scattering dominates the carrier transport process at room temperature,we combine first-principles calculations with Boltzmann transport theory to comprehensively analyze thermoelectric transport characteristics and the regulation mechanisms of electron-phonon scattering.However,such computational approaches are only applicable to acoustic phonon scattering and low-order scattering processes.To overcome this limitation,we further implement density functional theory（DFT）coupled with Wannier interpolation to calculate all phonon scattering processes,with detailed analysis of anisotropic effects on phonon scattering.Moreover,to gain deeper insights into high-order phonon scattering mechanisms,we apply the Laval-Born-James（LBJ）theory and Zacharias Giustino（ZG）special displacement method to investigate phonon scattering in high-order Brillouin zones and characterize phonon scattering behavior at elevated temperatures.Through this comprehensive computational study,our research not only reveals the tremendous potential of 2D Group-ⅥA layered materials in thermoelectric applications but also provides fundamental theoretical guidance for further performance enhancement.The specific research contents are as follows:（1）The non-equilibrium relaxation dynamics of hot carriers represent a critical scientific issue in the design of new functional materials.Electron-phonon scattering,as the predominant mechanism in the thermalization process,plays a decisive role in determining the transport characteristics of charge carriers.Using density functional theory（DFT）combined with the Wannier method,we investigated the electron-phonon scattering rates near the Fermi surface in two-dimensional Janus1T-SnSSe and 1T-SnS₂materials.Through comparative analysis of the scattering rate differences between the two materials,we elucidated the influence mechanisms of structural differences on hot carrier relaxation properties.The results indicate that at300 K,the electron-phonon scattering rate for Janus 1T-SnSSe is significantly lower than that for SnS₂.Phonon mode resolution analysis reveals that this difference primarily arises from the substantially weaker contribution of acoustic phonon branches to the scattering process in Janus 1T-SnSSe compared to 1T-SnS₂.Further analysis of carrier relaxation dynamics demonstrates that hot carriers in Janus1T-SnSSe exhibit longer relaxation times,indicating a longer non-equilibrium carrier lifetime（～238 fs）,which is advantageous for mitigating energy dissipation.Further analysis shows that the mean free path of hot charge carriers in Janus 1T-SnSSe is significantly better than that in 1T-SnS₂,indicating a significant advantage in hot charge carrier extraction efficiency.Janus 1T-SnSSe not only has significant advantages in hot carrier transport performance,but also provides theoretical guidance for the design of high-efficiency optoelectronic devices based on it.（2）Within the framework of the LBJ theory,we studied the multiphonon inelastic scattering processes in bilayer 2H-MoTe₂.Further analysis determined the proportion contributed by this process to the total phonon scattering.Systematic theoretical calculations and analyses led to the following key findings:First,the intensity of multiphonon inelastic scattering is directly proportional to the distance from the center of the Brillouin zone.Notably,when the wave vector|Q|exceeds 8（?）^-1,the contribution of multiphonon interactions increases significantly,surpassing60%of the total scattering intensity,exceeding that of one-phonon interactions.Additionally,all-phonon non-elastic scattering exhibits significant anisotropic characteristics,with pronounced intensity differences along theΓ-M andΓ-K directions.Furthermore,while the contribution of the Te₁Te₂collective motion mode to all-phonon inelastic scattering intensity is relatively modest,it significantly influences the Bragg scattering intensity.This unique phenomenon is attributed to the phase cancellation effect resulting from the central symmetry of the 2H-MoTe₂bilayer structure.To ensure the reliability of the results,ZG theory was employed to validate the all-phonon inelastic scattering intensity,with results closely matching those predicted by LBJ theory.The theoretical advances presented in this study not only deepen the understanding of phonon inelastic scattering mechanisms in transition metal disulfides but also provide a new theoretical foundation for accurately controlling the non-equilibrium phonon transport characteristics of two-dimensional materials.（3）We comprehensively investigated the electronic transport characteristics and thermoelectric transport properties of SiC/MoSe₂/SiC van der Waals heterostructures using first principles methods.Theoretical calculations and transport characteristic analyses confirm that the heterojunction exhibits typical I-type band alignment and demonstrates strong carrier transport anisotropy.Over a broad temperature range of300-700 K,the electron contribution from the n-type conduction channel significantly exceeds that from the p-type hole transport.This unique carrier transport characteristic allows the system to achieve a peak power factor of 2.40 mW·m^-1·K^-2,marking a breakthrough improvement of 1-2 orders of magnitude compared to traditional single-layer two-dimensional thermoelectric materials.Microscopic mechanism analysis reveals that the strong coupling between acoustic phonons and low-frequency optical branches in the heterojunction leads to substantial phonon scattering enhancement,reducing the x-axis lattice thermal conductivity to an ultra-low value of 1.51 W·m^-1·K^-1,a 20%decrease compared to the y-axis.This lattice thermal transport anisotropy arises from the modulation of phonon transport properties via non-harmonic effects.Due to its excellent electronic transport properties and ultra-low lattice thermal conductivity,the heterojunction achieves a thermoelectric figure of merit（ZT）of 4.86 at 700 K,demonstrating superior performance compared to similar materials.This work not only elucidates the thermoelectric enhancement mechanism in two-dimensional heterostructures from the perspectives of band engineering and phonon engineering but also provides a theoretical foundation for the development of novel thermoelectric materials with directional thermal management capabilities.The relaxation process of hot carriers involves multiple scattering mechanisms,with both electron-phonon scattering and multiphonon inelastic scattering playing important yet distinct roles in energy relaxation.For instance,in Janus 1T-SnSSe,the emission of low-frequency acoustic phonons prolongs relaxation time near the band edge,delaying hot carrier thermalization.However,symmetry breaking induces high-frequency ZO optical phonon emission at theΓpoint of the Brillouin zone,which suppresses the extension of relaxation time and accelerates hot carrier thermalization.In the bilayer 2H-MoTe₂system,multiphonon scattering dominates in high-order Brillouin zones or under high-temperature conditions during inelastic scattering and heat transport processes.At 300 K,multiphonon scattering contributes up to 18%to thermal transport characteristics,indirectly demonstrating its potential to extend hot carrier relaxation time and slow thermalization in this material.The hot carrier relaxation process significantly influences the transport properties of thermoelectric materials.In SiC/MoSe₂/SiC heterostructures,the relaxation time of hot carriers exhibits an exponential decay with increasing phonon frequency,attributed to enhanced phonon anharmonic effects in the high-frequency region that sharply increase scattering probability,leading to reduced lattice thermal conductivity.In conclusion,the phonon characteristics of different materials determine the relaxation dynamics of hot carriers.更多还原