一维图案化氧化锌纳米棒阵列中相干声学声子的超快动力学研究

Thermoelectric materials have attracted much attention due to their capability to convert thermal energy directly into electricity and vice versa. However, the applications of the thermoelectric materials are limited by their relatively low figure of merit ZT, thus finding the way to effectively reduce the lattice thermal conductivity is of great importance to improve their thermoelectric performances. In recent years, nanostructured materials have been proposed and developed to increase the value of ZT through doping and the size effect of nanostructures. Of the nanomaterials, one-dimensional ZnO is an ideal thermoelectric material due to its high Seebeck coefficient and high electrical conductivity. Using femtosecond pump-probe spectroscopy, the ultrafast dynamics of coherent acoustic phonons (CAPs) can be systematically investigated, which could provide useful information for enhancing the thermoelectric properties of the ZnO nanorods. In this project, one-dimensional patterned ZnO nanorod array will be fabricated and its thermoelectric properties will be systematically investigated. Using femtosecond pump-probe spectroscopy, GHz-THz CAPs in ZnO nanorods will be excited and detected, and the effects of the nanorod size and doping concentration on the propagation of the CAPs will be thoroughly analyzed. Through fabricating the ZnO thermoelectric devices based on the patterned ZnO nanorod array, the relationship between the propagating CAPs and the performance of the thermoelectric devices will be examined. The performance of the ZnO thermoelectric devices will also be explored under different temperatures. At last, the service behavior of the ZnO thermoelectric devices will be studied. This project provides a new route for studying the thermoelectric properties of materials with the assistance of femtosecond pump-probe spectroscopy.

热电材料因其可以实现固体状态下热能与电能之间的相互转换而受到广泛关注。针对目前热电材料效率较低的问题，有效地降低材料的晶格热导率是提高热电优值ZT的重要途径之一。近年来，低维纳米材料由于可通过改变尺寸和杂质掺杂两个方面来提高材料的热电性能而成为了一类极具潜力的热电材料。其中一维氧化锌（ZnO）具有较高的塞贝克系数和电导率，是一种较为理想的热电材料。利用飞秒泵浦-探测技术，对一维ZnO纳米棒中相干声学声子的超快动力学进行研究可为其在热电领域的应用提供可靠的理论基础。本项目围绕一维ZnO纳米棒阵列热电性能的优化，利用飞秒泵浦-探测技术激发并探测ZnO纳米棒中的GHz-THz频率的相干声学声子，研究ZnO纳米棒尺寸和元素掺杂浓度对其演化动力学的影响规律，阐明声子输运与热电性能之间的内在关联。构筑基于一维ZnO纳米棒阵列的热电器件，探索其最佳工作温度范围，确定器件安全服役行为。

热电材料因其可实现固体状态下热能与电能之间的相互转化而收到广泛关注。为了实现最佳的热电性能，材料应具备高的塞贝克系数S、高的电导率σ和低的热导率κ，即理想的热电材料应具有“声子玻璃-电子晶体”的特性。其中，材料的热导率κ包括自由载流子对热导率的贡献κe及晶格振动对热导率的贡献κlatt两个部分，后者又被称之为晶格热导率。根据维德曼-夫兰兹定理可知，热导率κe与电导率σ成正比，即在材料电导率增加的同时，自由载流子对热导率的贡献也将增加，从而限制了热电优值的进一步提高。因此，有效地降低材料的晶格热导率κlatt就成为了提升热电材料ZT值的重要途径之一。.项目面向ZnO半导体材料在热电领域中的应用，拟通过掺杂杂质原子来大幅度降低材料的热导率。首先，基于模板法和水热法，实现了一维图案化ZnO纳米棒阵列的限域生长与可控制备，并制备得到了Li或Al掺杂的ZnO薄膜材料。其次，完成了基于ASOPS系统的双红光、双蓝光飞秒泵浦-探测光学平台的搭建。在此基础上，系统研究了相干声学声子在Li或Al掺杂ZnO薄膜中的输运特性，揭示了掺杂原子对相干声学声子的散射规律，并建立了其与热电性能的内在关联。最后，通过第一性原理计算，研究了Li或Al掺杂后晶格畸变对ZnO材料电子和声子的散射规律，以及在不同温度下，掺杂种类和浓度对晶格热导率的影响规律。结果表明，Li掺杂在提升材料导电性的同时，可有效降低ZnO半导体材料的晶格热导率，是一类理想的掺杂元素。