College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China.
Institute of Mineral Engineering, Division of Materials Science and Engineering, Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen 52064, Germany.
Nanoscale. 2017 Mar 23;9(12):4295-4309. doi: 10.1039/c7nr01271c.
Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon-phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon-phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga-N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for applications in energy conversion such as thermoelectrics. Our study offers fundamental understanding of phonon transport in ML GaN within the framework of BTE and further electronic structure, which will enrich the studies of nanoscale phonon transport in 2D materials and shed light on further studies.
二维（2D）材料以石墨烯为代表，长期以来一直受到广泛研究。最近，实验成功制备出具有蜂窝结构的单层氮化镓（ML GaN），因其在纳电子学和光电子学中的应用前景广阔，引起了巨大的研究兴趣。考虑到所有这些应用都不可避免地涉及热传输，因此需要对 2D GaN 的声子输运性质进行系统研究。在本文中，我们通过基于第一性原理计算求解玻尔兹曼输运方程（BTE），对 ML GaN 的声子输运性质进行了全面研究，并与体 GaN、2D 石墨烯、硅烯和具有类似蜂窝结构的 ML BN 进行了详细比较。考虑到 ML GaN 的平面结构与石墨烯相似，令人惊讶的是发现 ML GaN 的热导率（κ）（14.93 W mK）比石墨烯低两个数量级，甚至比具有褶皱结构的硅烯还要低。我们通过研究声子支的贡献、模式级声子群速度和寿命的比较、声子-声子散射的详细过程和通道以及势能阱中的声子非谐性，进行了系统的分析。我们发现，与石墨烯和 ML BN 不同，由于 Ga 和 N 原子的原子半径和质量差异较大，2D GaN 中的声子-声子散射选择规则由于对称性降低而略有破坏。从电子结构中可以进一步深入了解。在 ML GaN 中，Ga-d 轨道介导的特殊 sp 轨道杂化导致强烈极化的 Ga-N 键、局域电荷密度及其不均匀分布，引起大的声子非谐性，从而导致 ML GaN 的固有低热导率。本工作中揭示的轨道驱动的 ML GaN 低热导率将使 2D GaN 有望应用于热能转换等领域，如热电学。我们的研究在 BTE 以及进一步的电子结构框架内为 ML GaN 的声子输运提供了基本的理解，这将丰富二维材料中纳米尺度声子输运的研究，并为进一步的研究提供启示。
