热电材料中几种典型电子能带结构效应的实验研究

Band structure engineering approach is crucial for optimizing the performances of thermoelectric materials. Recently, several typical novel electronic band structures, such as the localized resonant electronic energy states, the convergence of electronic bands, as well as the multiple electronic bands, are discovered to be very effective for enhancing electrical properties of thermoelectric materials. So far, the interpretation of electronic band structure mainly relies on the complementary efforts of the electrical transport measurements and the band structure calculations. However, the direct and precise measurements on the electronic band structures of thermoelectric materials have not been reported yet. . In this project, Mg2Si1-xSnx, SnTe and PbTe1-xSex are selected as the research objects. The molecular beam epitaxy technique is used for growing high quality and single-crystal-like films of these materials. The techniques of scanning tunneling microscope and angle-resolved photoemission spectroscopy are utilized for directly and precisely characterizing the typical electronic band features in these materials, such as bands converging with composition and temperature, as well as the localized resonant energy states due to elemental doping. Through the direct measurements of electronic band structures, this project aims to verify the correctness and precision of electronic band structures obtained from theoretical calculations, and to clarify the influence of chemical composition and temperature on the electronic band structures. Further, the direct relation between electrical transport and electronic band structures could be established, which will be used to improve thermoelectric properties of these materials.

电子能带结构优化是提高热电材料性能的重要途径。近几年国际上发现了几种典型电子能带结构新效应可用来显著提高材料的电性能，包括局域电子共振态效应、电子能带结构收敛效应、多电子能带输运效应等。迄今为止，热电材料中电子能带结构的解析主要依赖电输运测量结合电子能带结构的理论计算。然而，电子能带结构的直接精确表征在热电材料研究中还未见报道。. 本项目以Mg2Si1-xSnx、SnTe和PbTe1-xSex材料为研究对象，拟采用分子束外延技术生长高质量的单晶薄膜，通过扫描隧道显微镜和角分辨光电子能谱技术直接精确表征这几种材料中存在的典型电子能带结构效应：电子能带随组分和温度收敛的效应以及掺杂引入局域共振态效应。通过电子能带结构的直接测量，验证电子能带结构理论计算结果的正确性和准确性，揭示组分、温度等对电子能带结构的影响规律，建立电子能带结构与电性能之间的直接关系，为优化热电性能提供指导。

电子能带结构工程是优化热电材料电输运性能的重要途径之一。然而，热电材料中电子能带结构的研究目前主要通过电输运结合理论计算来推测，缺乏直接实验表征的有力支撑。本项目提出采用MBE制备单晶薄膜并通过STM和ARPES进行电子能带结构直接精确表征的可行思路，研究了几种典型热电材料的电子能带结构特征及其调控机制。本项目所取得的重要研究成果如下。. （1）为实现电子能带结构的直接精确表征，通过MBE工艺设计制备出高质量的SnTe、Bi2Te3、Mg3Sb2和MoTe2单晶薄膜，通过调节衬底温度和元素束流比实现了对成分和点缺陷结构的精确调控。.（2）阐明了Bi2Te3的导带和价带具有六重简并特征，发现了本征点缺陷显著影响导带电子结构的新现象，揭示了Bi2面缺陷的调控规律及其无序/有序插层调控电子能带结构的新规律，在反位缺陷TeBi为主要点缺陷结构的Bi2Te3单晶薄膜中获得了超高的热电功率因子可达5.05mWm-1K-2。.（3）阐明了SnTe中轻价带为四重简并以及重价带为十二重简并的特性，发现了点缺陷结构结构能显著调节费米能级和空穴浓度的新规律，证实了随温度升高引起轻价带的能量位置不变，而重价带的能量位置上升的价带收敛现象。.（4）阐明Mg3Sb2的价带结构特征以及晶格应力调控有望实现多个价带收敛的新规律，发现了Mg3Sb2中温度驱动的Lifshitz电子相变（费米能级随温度升高显著朝导带偏移），揭示了该Lifshitz电子相变是n型Mg3Sb2基化合物获得宽温域优异电输运性能的一个主要原因。.（5）阐明了1T’-MoTe2中自旋轨道耦合是调控其电子能带结构的重要途径，通过形成Bi2Te3/1T’-MoTe2异质结实现了1T’-MoTe2中拓扑带隙的打开，提供了一种新的量子自旋霍尔物相。.本项目圆满完成项目任务书中所列几种典型电子能带结构的实验研究及其调控规律的研究内容，已发表6篇SCI论文，全面完成了本项目的研究目标。本项目的探索为后续进一步利用电子能带工程优化热电材料的电输运性能提供了重要实验支撑。