Twin-driven thermoelectric figure-of-merit enhancement of Bi2Te3 nanowires

Abstract

Thermoelectric figure-of-merits (ZT) are enhanced or degraded by crystal defects such as twins and excess atoms that are correlated with thermal conductivity (k) and carrier concentration (n). For Bi₂Te₃, it is unclear whether the crystal defects can enhance ZT without a degradation in the thermopower factor. In the present study, n-type Bi₂Te₃ nanowires (NWs) are electrochemically synthesized to have twin-free (TF) or twin-containing (TC) microstructures with a ZT of 0.10 and 0.08, respectively, at 300 K. The ZTs of TF and TC NWs remarkably increase up to 0.21 and 0.31, when heat-treatments cause n-reduction and twins induce phonon scattering, as follows: first, the enhancement of the Seebeck coefficient from −70 to −98 μV K⁻¹ for TF NWs and from −57 to −143 μV K⁻¹ for TC NWs, by virtue of n-reduction; secondly, twin-driven k-reduction from 1.9 to 1.4 W m⁻¹ K⁻¹ of TC NWs, while the k of TF NWs increases from 2.3 to 2.6 W m⁻¹ K⁻¹ due to the enhanced carrier mobility. The lattice thermal conductivities of TC NW are lowered from 1.1 to 0.8 W m⁻¹ K⁻¹ by phonon scattering at twins. Density functional theory calculations indicate that twins do not significantly influence the Seebeck coefficient of Bi₂Te₃. It is strongly recommended that twins be incorporated with an optimum carrier concentration to enhance the ZT of Bi₂Te₃.