纳米体系晶界量子热输运性质的理论研究

Multi-grain-boundaries are inevitable in integrated devices based on low-dimensional nano materials. Investigation of their thermal transport properties are not only important to fundamental physics, but also promises good applications in both thermoelectrics and low-dimensional quantum devices. Different to electrons, the number of phonon modes contributing to transport is tremendous, which makes detailed analysis difficult. Firstly, we shall develop the mode-matching method in phonon systems by combining the electronic mode-matching method, unfolding algorithm, and nonequilibrium Green’s function method, so as to obtain mode-dependent transmission coefficient. Secondly, we shall develop and optimize real-space first-principles calculations of phonon structures based on a self-developed real-space density functional code to accelerate the computation of phonon structures. Based on these developments, we shall theoretically investigate quantum thermal transport properties in low-dimensional nano systems: reveal the roles of the injection angle, phonon branch, wave vector, group velocities, polarization vectors, the angle and symmetries of grain boundaries during the transport of phonons through a grain boundary. We shall find out quantum filtering mechanism in some particular grain boundaries, and also analyze the impact of local vacancies, defects at grain boundaries. We aim to investigate quantum thermal transport effects at nano grain boundaries, reveal the intrinsic mechanisms, and propose methods to manipulate thermal transport properties by using grain boundaries.

以石墨烯为代表的低维纳米体系的集成器件，不可避免的带来材料中多晶界问题，其热输运性质研究，不仅具有理论意义，还在热电器件与低维量子器件的优化设计上具有重要应用价值。与电子不同的是，对输运性质有贡献的声子模式为数众多，使得深入分析十分困难。为此，我们将首先发展声子分模算法，将电子中的模式匹配算法与反折叠算法和非平衡格林函数方法相结合，得到声子分模透射谱；其次在电子结构实空间计算自主软件的基础上，发展优化算法，提高大规模体系声子结构的计算速度。以此出发，我们将对低维纳米体系的晶界声子输运性质进行系统研究，阐明入射角度、声子模式、波矢、波速、极化矢量、晶界角度和晶界对称性等对声子透射谱的内在影响，探究不同的晶界类型对声子输运的过滤透射机制，分析晶界构型与晶界处局域空位、替代缺陷等对晶界热输运的影响。我们将着重探讨晶界量子热输运效应，揭示其内在机理，并提出利用晶界微观调控体系热输运性质的方法。

晶界在低维纳米体系中呈现出多种特性，对低维体系集成器件的性能有显著影响。其热输运性质研究，不仅具有理论意义，还在热电器件与低维量子器件的优化设计上具有重要应用价值。本课题针对含晶界的低维纳米体系，对其声子量子输运性质进行了系统研究，揭示新的量子效应及其内在机理，为微观调控体系热输运性质提供理论指导。课题执行中取得的主要学术进展包括：1）发展了声子分模算法，将电子中的模式匹配算法与反折叠算法和非平衡格林函数方法相结合，得到声子分模透射谱，揭示二维石墨烯558晶界体系中晶界的谷过滤效应，发现了类法诺共振现象，可用于局域调控声子热导；2)探究了含晶界的一维石墨烯体系的量子热输运性质，阐明了声子波矢匹配机制及其对声子透射性质的重要影响；3）研究了二维多晶过渡族硫属化合物二硫化钼（MoS2）的声子热输运性质，考虑了不同的晶界类型对声子输运的过滤透射机制，得到了晶界对声子局域化与群速度的影响作用，指出必须将晶界的声子透射谱细节考虑在内以精确描述多晶材料的声子输运。4)探索了利用热开关得到量子流增益的方法，总结展望了低维材料体系准粒子量子输运的热控制与量子控制。我们的研究为低维材料体系精准量子热输运控制提供了新的途径。