Thermoelectric properties of SnSe and SnSe2 single crystals

Thermoelectric materials can serve for conversion between thermal and electrical energy. In the search for thermoelectric materials, layered SnSe and ${\mathrm{SnSe}}_{2}$ are promising candidates. We have successfully synthesized SnSe and ${\mathrm{SnSe}}_{2}$ single crystals by the modified Bridgman method and studied their thermoelectric properties: thermopower $(S)$, thermal conductivity $(\ensuremath{\kappa})$, and electrical conductivity $(\ensuremath{\sigma})$ in the temperature range between 2 and 400 K, which are absent in the literature. The kink observed in the thermopower corresponds to the metallic-nonmetallic crossover temperature for both SnSe and ${\mathrm{SnSe}}_{2}$, reflecting their inherent electronic nature. Compared with ${\mathrm{SnSe}}_{2}$, >100 K, we find that SnSe exhibits higher electrical conductivity, higher thermopower, and lower thermal conductivity, thus resulting in the higher figure of merit. Hall effect measurements reveal that the Hall mobility in SnSe is an order higher than that in ${\mathrm{SnSe}}_{2}$, advancing its thermoelectric performance. These experimental results are supported by first-principles calculations, which indicate that the inequivalent Sn-Se bonding lengths help improve the figure of merit of SnSe.