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

doi:10.1073/pnas.2113967119

Direct visualization of polaron formation in the thermoelectric SnSe

Proc Natl Acad Sci U S A. 2022 Jan 18;119(3):e2113967119. doi: 10.1073/pnas.2113967119.

Authors

Laurent P René de Cotret¹, Martin R Otto¹, Jan-Hendrik Pöhls¹, Zhongzhen Luo², Mercouri G Kanatzidis², Bradley J Siwick^{3

4}

Affiliations

¹ Department of Physics, Center for the Physics of Materials, McGill University, Montréal, QC H3A 2T8, Canada.
² Department of Chemistry, Northwestern University, Evanston, IL 60208.
³ Department of Physics, Center for the Physics of Materials, McGill University, Montréal, QC H3A 2T8, Canada; bradley.siwick@mcgill.ca.
⁴ Department of Chemistry, McGill University, Montréal, QC H3A 0B8, Canada.

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.

Keywords: Thermoelectric; electron–phonon coupling; polarons.