期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：3
DOI：10.1073/pnas.2113967119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
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.
