Issue 25, 2019

Enhanced thermoelectric performance of GeTe through in situ microdomain and Ge-vacancy control

Abstract

A highly reproducible sample preparation method for pure GeTe in a rhombohedral structure without converting to the cubic structure up to ∼500 °C is reported to show control of the Ge-vacancy level and the corresponding herringbone-structured microdomains. The thermoelectric figure-of-merit (ZT) for GeTe powder could be raised from ∼0.8 to 1.37 at high temperature (HT) near ∼500 °C by tuning the Ge-vacancy level through the applied reversible in situ route, which made it highly controllable and reproducible. The enhanced ZT of GeTe was found to be strongly correlated with both its significantly increased Seebeck coefficient (∼161 μV K−1 at 500 °C) and reduced thermal conductivity (∼2.62 W m−1 K−1 at 500 °C) for a sample with nearly vacancy-free thicker herringbone-structured microdomains in the suppressed rhombohedral-to-cubic structure phase transformation. The microdomain and crystal structures were identified with HR-TEM (high-resolution transmission electron microscopy) and powder X-ray diffraction (XRD), while electron probe micro-analysis (EPMA) was used to confirm the stoichiometry changes of Ge : Te. Theoretical calculations for GeTe with various Ge-vacancy levels suggested that the Fermi level shifts toward the valence band as a function of increasing the Ge-vacancy level, which is consistent with the increased hole-type carrier concentration (n) and effective mass (m*) deduced from the Hall measurements. The uniquely prepared sample of a near-vacancy-free GeTe in a rhombohedral structure at high temperature favoured an enhanced Seebeck coefficient in view of the converging L- and Σ-bands of the heavy effective mass at the Fermi level, while the high density domain boundaries for the domain of low carrier density were shown to reduce the total thermal conductivity effectively.

Graphical abstract: Enhanced thermoelectric performance of GeTe through in situ microdomain and Ge-vacancy control

Supplementary files

Article information

Article type
Paper
Submitted
02 Apr 2019
Accepted
16 May 2019
First published
17 May 2019

J. Mater. Chem. A, 2019,7, 15181-15189

Enhanced thermoelectric performance of GeTe through in situ microdomain and Ge-vacancy control

K. S. Bayikadi, R. Sankar, C. T. Wu, C. Xia, Y. Chen, L. Chen, K. Chen and F. Chou, J. Mater. Chem. A, 2019, 7, 15181 DOI: 10.1039/C9TA03503F

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