Nanostructure, excitations, and thermoelectric properties of Bi ₂Te ₃-based nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi ₂Te ₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi ₂Te ₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al ₂O ₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb ₂Te ₃ and Bi ₂Te ₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb ₂Te ₃ films revealed low charge carrier density of 2.6 × 10 ¹⁹ cm ^-3, large thermopower of 130 μV K ^-1, and large charge carrier mobility of 402 cm ² V ^-1 s ^-1. Bi ₂(Te _0.91Se _0.09) ₃ and (Bi _0.26Sb _0.74) ₂Te ₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50 nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi ₂Te ₃ nanomaterials are discussed.