Thermoelectric Properties of Small Bandgap Semiconductor Nanowires: The Effects of 1D Electron Subband Formation and Phonon Boundary Scattering

Nanomaterials such as nanowires or nanoribbons offer a promising platform for efficient thermoelectric energy conversion applications, owing to their greatly reduced thermal conductivity and potentially high power factor due to strong one-dimensional (1D) quantum confinement effects. While the thermal conductivity of nanomaterials is widely found to be suppressed, the 1D electronic quantum confinement effect tuned power factor remains elusive in experiment. Small diameter nanowires of InAs or InSb provides an opportunity to observe such 1D quantum confinement effect, thanks to their high electron mobility and small effective mass (i.e. long electron wavelength/large 1D subband spacing). We present a study of the thermoelectric properties of InAs nanowires where the gate was used to control the electrons' Fermi level. At temperatures below about 100K, oscillations in the thermopower and power factor due to the formation of 1D electron sub-bands are observed in semiconductor nanowires for the first time. We also discuss the limiting factors in the pursuit of 1D confinement enhanced thermoelectric performance in semiconductors and point out that disorder induced 1D energy level smearing sets the current bottleneck for achieving 1D quantum effect enhanced figure of merit or power factor at practical temperatures (e.g. 300K). We further discuss a recent study of Bi₂Se₃ nanoribbon's thermal conductivity by the three-omega method. The as measured thermal conductivity for a single Bi₂Se₃ nanoribbon is nearly two orders of magnitude smaller than the bulk value, which is attributed to enhanced phonon boundary scattering in nanostructured materials. This work is done together with M.R. Sakr, J.M. Kinder, M. J. MacDonal, D. Liang, G.D. Li, R. L.J. Qiu at CWRU and Y. Tian and H.J. Gao at Chinese Academy of Sciences. The authors acknowledge NSF CAREER Award (DMR-1151534), ACS PRF (48800-DNI10) and the NSF of China for support of this research.