Direct visualization of polaron formation in the thermoelectric SnSe

Laurent P. René de Cotret, Martin R. Otto, Jan Hendrik Pöhls, Zhongzhen Luo, Mercouri G. Kanatzidis, Bradley J. Siwick*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Article numbere2113967119
JournalProceedings of the National Academy of Sciences of the United States of America
Volume119
Issue number3
DOIs
StatePublished - Jan 18 2022

Keywords

  • Electron-phonon coupling
  • Polarons
  • Thermoelectric

ASJC Scopus subject areas

  • General

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