(Invited) Role of Anharmonicity on Thermoelectric Properties of Fully Dense Single-Crystalline Snse

Abstract

Thermoelectric (TE) materials enable conversion of “waste” heat to electricity, leading to environmentally friendly solutions for power generation applications. The thermal-to-electrical energy conversion efficiency of a TE material is largely determined by its dimensionless figure-of-merit, ZT = S2σT/κ that consists of interrelated material properties, viz., the Seebeck coefficient (S), electrical conductivity (σ), and the thermal conductivity (κ). Recently, single-crystalline SnSe has attracted much attention due of its record high figure-of-merit ZT ≈ 2.6, [1] that was debated in subsequent reports due to the low mass density of these single crystals. [2] We performed structural, electrical, and thermal transport measurements on high-quality fully dense SnSe single crystals over a wide temperature range along the major crystallographic directions, that yielded a maximum ZT ~ 1 along one of the major crystallographic directions.[3] It is well known that phonon anharmonicity is one of the contributing factors that yields a remarkably low κ in binary SnSe. As an attempt to understand the role of phonon anharmonicity, we performed a combined temperature-dependent polarized Raman spectroscopic and heat-capacity study on fully dense single-crystalline SnSe, which revealed that the anharmonicity in SnSe is driven by soft optical modes in its bc plane. [4] This study provides a deeper understanding of the role of phonon-phonon scattering and anharmonicity leading to reported thermal transport properties of SnSe. [1] Zhao, L.-D., Lo, S.-H., Zhang, Y., Sun, H., Tan, G., Uher, C., Wolverton, C., Dravid, V. P., Kanatzidis, M. G. Ultralow Thermal Conductivity and High Thermoelectric Figure of Merit in SnSe Crystals, Nature 2014, 508 (7496), 373–377. [2] Wei, P. C., Bhattacharya, S., He, J., Neeleshwar, S., Podila, R., Chen, Y. Y., Rao, A. M., The Intrinsic Thermal Conductivity of SnSe, Nature 2016, E1–E2. [3] Wei, P.C., Bhattacharya S., Liu Y-F., Liu F., He J., Tung Y-H., Yang C-C., Hsing C-R., Nguyen D.L., Wei C-M., Chou M-Y., Lai Y-C., Hung T-L, Guan S-Y., Chang C-S., Wu H-J., Lee C-H., Li W-H., Hermann R. P., Chen Y-Y., Rao A.M., Thermoelectric figure-of-merit of fully dense single crystalline SnSe, ACS Omega, 2019, 4 (3), 5442–5450. [4] Liu F., Parajuli P., Rao R., Wei P.-C., Karunarathne A., Bhattacharya S., Podila R., He J., Maruyama B., Priyadarshan G., Gladden J. R., Chen Y.-Y., Rao A.M., Phonon Anharmonicity in single-crystalline SnSe, Phys. Rev. B 2018, 98, 224309.