Direct visualization of polaron formation in the thermoelectric SnSe.
Direct visualization of polaron formation in the thermoelectric SnSe.
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热电SnSe中极化子形成的直接可视化。
DOI：
10.1073/pnas.2113967119
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发表时间：
2022-01-18
影响因子：
11.1
通讯作者：
Siwick BJ
中科院分区：
文献类型：
--
作者：
René de Cotret LP;Otto MR;Pöhls JH;Luo Z;Kanatzidis MG;Siwick BJ
热电可以利用废热发电，如果其效率得到提高，可以为可持续能源生产做出重要贡献。然而，设计高效的热电材料需要对电传输和热传输之间的基本相互依赖性有深入的了解，为此需要改进我们对载流子如何与晶格振动（声子）耦合的理解。使用先进的超快电子技术，我们探究了 SnSe（性能最高的热电材料之一）中电子-声子耦合的基本性质，以揭示其热电效率的根源。我们发现 SnSe 中的载流子与晶格波强烈相互作用，形成称为极化子的移动晶格畸变，这有助于解释 SnSe 的一些最不寻常的特性。 SnSe 是一种层状材料，目前保持着体热电效率的记录。这种高效率的主要决定因素被认为是声子系统内强非谐波耦合导致的异常低的热导率。在这里，我们通过直接观察材料中极化子的形成表明，SnSe 中载流子系统的性质也是由与声子的强耦合决定的。我们采用超快电子衍射和漫散射来跟踪声子在动量和时间上对带隙上自由载流子光电掺杂的响应，观察驱动载流子局域化的双峰和各向异性晶格畸变。相对较大 (18.7 Å) 的准一维 (1D) 极化子在 300 飞秒时间尺度上形成，较小 (4.2 Å) 3D 极化子的形成时间要长一个数量级 (4 ps)。这种差异似乎是 SnSe 中严重各向异性电子声子耦合的结果，仅对区域中心极性光学声子具有强弗罗利希耦合。这些结果表明，在最佳掺杂水平下，SnSe 中的极化子密度很高。强电子-声子耦合对于这种基准材料的热电性能至关重要，并且对于更广泛的高性能热电材料来说至关重要。
Thermoelectrics can generate electrical power from waste heat and could make an important contribution to sustainable energy production if their efficiency is improved. Engineering efficient thermoelectrics, however, requires a sophisticated understanding of the fundamental interdependencies between electrical and thermal transport, for which improvements in our understanding of how charge carriers are coupled to lattice vibrations (phonons) is needed. Using advanced ultrafast electron-based techniques, we probe the fundamental nature of electron–phonon coupling in SnSe, one of the highest-performing thermoelectric materials, in order to unravel the origins of its thermoelectric efficiency. We show that the charge carriers in SnSe interact strongly with lattice waves to form moving lattice distortions called polarons, which helps to explain some of SnSe’s most unusual properties. SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers across the bandgap, observing the bimodal and anisotropic lattice distortions that drive carrier localization. Relatively large (18.7 Å), quasi-one-dimensional (1D) polarons are formed on the 300-fs timescale with smaller (4.2 Å) 3D polarons taking an order of magnitude longer (4 ps) to form. This difference appears to be a consequence of the profoundly anisotropic electron–phonon coupling in SnSe, with strong Fröhlich coupling only to zone-center polar optical phonons. These results demonstrate a high density of polarons in SnSe at optimal doping levels. Strong electron-phonon coupling is critical to the thermoelectric performance of this benchmark material and, potentially, high performance thermoelectrics more generally.
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