基于扫描探针技术的低维Bi基拓扑绝缘体热电特性研究

As one of the state-of-art room temperature thermoelectric (TE) materials, Bi-based materials have the broad application prospect. It is the key problem to fully improve the thermoelectric figure of merit (ZT). In recent years, most of Bi-based excellent thermoelectric materials, such as Bi2Te3 and Bi2Se3，can be also topological trivial. Topological insulators (TI) are a new state of quantum matter which is characterized by insulating bulk and conducting surface states. This unusual topological surface states with high mobility (high σ) can potentially enhance ZT in a thermoelectric device based on TI surface state. The project aims to explore the inherent relations between their surface states protected by topology and thermoelectric performance of low-dimensional Bi-base topological insulators, where the dimension, thickness and ambipolar effect on their thermoelectric power will be detected. A thermoelectric imaging method based on contact-mode Atom Force Microscopy (AFM) probe techniques will be used to characterize and measure the local thermoelectric power, which can yield high-sensitivity imaging of structural variation and state density inhomogeneity on the submicron-scale. Based on these results，by doping and field effect tuning, we will try to control and manipulate the thermoelectric properties of Bi-based topological insulator by tuning their electronic structure, which will be very important for their applications in new thermoelectric devices.

Bi基材料是室温附近性能最好的一类热电材料，具有广泛的应用前景。而如何最大限度地提高材料的热电优值是当今热电材料研究的主要问题。近年来，研究发现许多Bi基热电材料，如Bi2Te3，Bi2Se3等，具有拓扑绝缘体特性。拓扑绝缘体作为一种全新的量子物质态，其主要特征是体内绝缘而表面导电，而这种特殊的表面态具有较高的迁移率，可能在提高材料热电优值方面得到应用。本项目将利用基于接触式原子力显微镜进行热电扫描功能化改造后的探针技术，从维度、层厚和双极效应三个方面，对低维Bi基拓扑绝缘体的热电特性进行在微纳米尺度上的研究，探索拓扑绝缘体表面态对材料热电输运特性的影响。在此基础上，结合掺杂和场效应调控，尝试通过调控Bi基拓扑绝缘体电子态结构来调控其热电性能，为设计和构建基于拓扑绝缘体材料的新型热电器件提供指导和依据。

本研究侧重于Bi-基拓扑绝缘体薄膜的热/电特性研究。首先，采用分子束外延的方法在SrTiO3、Au 薄膜和graphite等多衬底上制备出了厚度可控、成分可调的(Bi1-xSbx)2Te3拓扑绝缘体性薄膜，并用扫描隧道显微镜及其谱学技术对(Bi1-xSbx)2Te3拓扑绝缘体薄膜的电子态性质进行了系统的表征。其中，基于低温扫描隧道显微镜谱学扫描技术的表面态准粒子干涉测量发现，在(Bi1-xSbx)2Te3拓扑绝缘体表面除了本征的狄拉克表面态外，还首次在导带区域发现了一套具有类似手性自旋特性的新的表面态。结合第一性原理计算结果，这套新出现的自旋劈裂的表面态可以被归结为是由于Sb对Bi的取代引起的。这种电子态中出现的新的输运通道可能对Bi-基拓扑绝缘体的热学方面性能也产生影响。其次，为了研究Bi-基拓扑绝缘体材料中的热电性能，我们发展了基于扫描原子力显微镜平台的微纳米尺度热电测量方法，并利用新近报道的具有优异热电性能的SnSe单晶纳米薄片对该测试平台的基本性能进行了测试。最后，利用该新测量平台，对在Au薄膜衬底上制备的Bi2Se3薄膜进行了热电性能的初步表征，测量得到的Seebeck系数为负值，这与Bi2Se3薄膜通常表现为n掺杂型的结果是相一致的。上述结果为进一步研究Bi-基拓扑绝缘体的热电特性提供了很好的基础和平台。