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Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals

Nature volume 508, pages 373–377 (2014)Cite this article

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Abstract

The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment1,2, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m−1 K−1 at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.

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Figure 1: SnSe crystal structure Pnma and ZT values.
Figure 2: Thermoelectric properties as a function of temperature for SnSe crystals.
Figure 3: High-temperature in situ TEM observations.
Figure 4: Theoretically calculated phonon and Grüneisen dispersions, and measured lattice thermal conductivity.

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Acknowledgements

This work was supported in part by Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, and Office of Basic Energy Sciences under award number DE-SC0001054 (L.-D.Z., S.-H.L., Y.Z., H.S., G.T., C.U., C.W., V.P.D. and M.G.K.). TEM work was performed in the (EPIC, NIFTI, Keck-II) facility of the NUANCE Center at Northwestern University. The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, the Keck Foundation, the State of Illinois and Northwestern University.

Author information

Authors and Affiliations

  1. Department of Chemistry, Northwestern University, Evanston, 60208, Illinois, USA

    Li-Dong Zhao, Gangjian Tan & Mercouri G. Kanatzidis

  2. Department of Materials Science and Engineering, Northwestern University, Evanston, 60208, Illinois, USA

    Shih-Han Lo, Yongsheng Zhang, C. Wolverton & Vinayak P. Dravid

  3. Department of Physics, University of Michigan, Ann Arbor, 48109, Michigan, USA

    Hui Sun & Ctirad Uher

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  1. Li-Dong Zhao
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Contributions

L.-D.Z. synthesized the samples, designed and carried out thermoelectric experiments, and wrote the paper. S.-H.L. and V.P.D. performed the TEM characterizations. Y.Z. carried out the calculations. H.S. and C.U. carried out the Hall measurements. G.T. helped with sample synthesis. L.-D.Z., S.-H.L., Y.Z., H.S., G.T., C.U., C.W., V.P.D. and M.G.K. conceived the experiments, analysed the results and co-edited the manuscript. S.-H.L. and Y.Z. contributed equally.

Corresponding author

Correspondence to Mercouri G. Kanatzidis.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 XRD measurement of SnSe on the cleavage plane, and the simulated diffraction pattern.

The (400) reflection plane indicates that the SnSe crystal is cleaved at the plane that is perpendicular to the a axis.

Source data

Extended Data Figure 2 EBSD analysis on a 1 mm2 surface.

a, The all-Euler map showing a large area of sample surface with homogeneous orientation. b, The inverse pole figures of samples cut along the a–b, c–a and b–c planes, respectively. The view down the c-axis (left figure) indicates a slight deviation (∼11°) from it. Scale bar, 200 μm.

Extended Data Figure 3 Hall transport properties of crystalline SnSe.

a, Inverse RH (RH, Hall coefficient), and b, RH/ρ (ρ, electrical resistivity) of SnSe crystals along different axial directions. Inverse RH gives an indication of the carrier concentration; RH/ρ is related to the carrier mobility.

Source data

Extended Data Figure 4 Electronic band structures of low-temperature (Pnma) and high-temperature (Cmcm) phases of SnSe.

a, SnSe at low temperature (Low-T) with the Pnma space group. b, SnSe at high temperature (High-T) with the Cmcm space group. The dashed lines indicate the position of the Fermi level (EF). Inset figures are the first Brillouin zones (BZ) of SnSe with high-symmetry points (red points) that we considered in our band structure calculations. Both phases are indirect-bandgap (Eg) compounds. For the low-temperature phase, the indirect bandgap is along ΓY in its BZ; for the high-temperature phase, the indirect bandgap is from a point within the ΓP direction to the Z point in its BZ.

Extended Data Figure 5 Bandgap of SnSe at room temperature.

Optical absorption spectrum (black trace) and energy bandgap (x-axis intercept) indicate a bandgap of 0.86 eV (red) of SnSe at room temperature. See Methods for details of α and S.

Source data

Extended Data Figure 6 Thermoelectric properties as a function of temperature for crystalline SnSe along different directions.

a, Thermal diffusivity. b, Heat capacity. c, Electronic thermal conductivity. d, The ratio of lattice thermal conductivity (κlat) to total thermal conductivity (κtot). The Lorenz number (L) used for obtaining κlat (κlat = κtot − σLT, where σ is the electrical conductivity and T is absolute temperature) was approximately equal to 1.5 × 10−8 V2 K−2 since the undoped SnSe is a non-degenerate semiconductor; the ratio of κlat to κtot indicates that κtot is dominated by phonon transport.

Source data

Extended Data Figure 7 High-resolution TEM images of single-crystal SnSe along four zone axes, and SAD along a low-order zone axis (in insets).

a, Along the [100] direction. b, Along the [201] direction. c, Along the [211] direction. d, Along the [021] direction. e, Electron diffraction pattern along the [001] direction. f, Electron diffraction pattern along the [010] direction. The combination of all six figures provides strong evidence of the single-crystal orthorhombic layered structure of SnSe.

Extended Data Figure 8 Simulated SADs for SnSe.

a, Room-temperature (RT; Pnma) phase; b, high-temperature (HT; Cmcm) phase. B, zone axis. Note is used here to indicate −1.

Extended Data Figure 9 Differential thermal analysis of SnSe.

DTA measurements showing two heating and cooling cycles. DTA results indicate that SnSe appears to melt congruently; one endothermic peak is observed at 881 °C on the heating curve and one exothermic peak is observed at 868 °C on the cooling curve. Two heating–cooling cycles indicate the same melting and crystallization point for a given sample, consistent with high purity. Heat flow is represented as a μV signal, plotted on the y axis.

Source data

Extended Data Figure 10 Reproducibility, and thermoelectric properties as a function of temperature, for seven samples of SnSe crystals along the b axis.

a, Electrical conductivity. b, Seebeck coefficient. c, Power factor. d, Total thermal conductivity. e, ZT; error bars are ±15%.

Source data

Extended Data Table 1 Transverse (TA/TA’) and longitudinal (LA) Debye temperatures (Θ), phonon velocities (v) and Grüneisen parameters (γ) along a, b and c axes in the low-temperature SnSe phase.
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Zhao, LD., Lo, SH., Zhang, Y. et al. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 508, 373–377 (2014). https://doi.org/10.1038/nature13184

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