Reinvestigation of the thermal properties of single-crystalline SnSe

Abstract

The simple binary SnSe has been recently proposed as a prospective candidate for thermoelectric applications due to its exceptionally low lattice thermal conductivity. However, the thermal transport in single crystals was found to be significantly lower than in polycrystalline samples despite the presence of grain boundary scattering in the latter. In order to better understand the origin of this issue, we report here on a detailed characterization of the thermoelectric properties of a vertical-Bridgman-grown single-crystal of SnSe along the a, b, and c crystallographic axes in a wide range of temperatures (5–700 K). We find that the thermal conductivity features a pronounced Umklapp peak near 12 K whose magnitude depends on the crystal orientation. Unlike prior reports, our results evidence a significant anisotropy between the a, b, and c directions with lattice thermal conductivity values reaching 1.2, 2.3, and 1.7 W m−1 K−1 at 300 K, respectively. While the fundamental reasons behind these differences remain unclear, our results indicate that the intrinsic lattice thermal conductivity of single-crystalline SnSe is likely significantly higher than previously thought.

High-performance SnSe thermoelectric materials: Progress and future challenge

“…These results were consistent with early studies performed on single crystal SnSe (1.9 W m -1 K -1 at 300 K perpendicular to the c-axis) [247]. Besides, the single crystal SnSe with a relative density of 99 % demonstrated that the values along the a-, b-, and c-axes were all ~1.0 W m -1 K -1 at 700 K [246], which were higher than the reported results of ~0.24, 0.34 and 0.31 along the a-, b-, and c-axes, respectively [61]. In this respect, the reported single crystal with a low calculated ρ should be at least "SnSe crystal".…”

“…At the same time, experiments based on a vertical-Bridgman-grown single crystal SnSe [246] with a high density (ρ = 6.10 ± 0.05 g cm -3 ) showed that the intrinsic κ of single crystal SnSe is likely significantly higher than that of the reported undoped SnSe single crystals with a density of ~88 % [61]. The single crystal SnSe with a relative density of 99 % showed that the κ values along the a-, b-, and c-axes were 1.2, 2.3, and 1.7 W m -1 K -1 at 300 K, respectively [246], which were higher than the values of 0.46, 0.70 and 0.67 along the a-, b-, and c-axes [61]. These results were consistent with early studies performed on single crystal SnSe (1.9 W m -1 K -1 at 300 K perpendicular to the c-axis) [247].…”

Ideal Strength and Deformation Mechanism in High-Efficiency Thermoelectric SnSe

“…[25][26][27][28][29] Possible reasons for the unreproducible thermal conductivity data were discussed in both in detail and were related to surface oxidation, low density, large number of intrinsic defects, etc. 23,24 Based on our results, the lowest ideal strength was found along the (100) plane, which could lead to formation of cracks and hence lower the density, which may partially account for the (controversial) ultralow thermal conductivity of SnSe crystals.…”

Evidence for hard and soft substructures in thermoelectric SnSe

“…There is recent controversy regarding the intrinsic thermal conductivity in SnSe, 15,16 with a new single crystal study suggesting that j lat % 2 W m À1 K À1 within the tightly bound rocksalt layers and $1 W m À1 K À1 in the perpendicular direction (300 K values). 17 In polycrystalline samples, a range of j lat values have been reported, [18][19][20][21][22] with some suggestion that the disorder is able to reduce j lat . 21,23 The measured j lat values have also been reported to be sensitive to oxidation.…”