Enhanced thermoelectric properties of solution grown Bi ₂Te _3-xSe _x nanoplatelet composites

We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi ₂Te _3-xSe _x) nanoplatelet (NP) composites synthesized by the polyol method. Variation of the Se composition within NPs is demonstrated by X-ray diffraction and Raman spectroscopy. While the calculated lattice parameters closely follow the Vegard's law, a discontinuity in the shifting of the high frequency (E _g ² and A _1g ²) phonon modes illustrates a two mode behavior for Bi ₂Te _3-xSe _x NPs. The electrical resistivity (ρ) of spark plasma sintered pellet composites shows metallic conduction for pure Bi ₂Te ₃ NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (κ) for all NP composites is much smaller than the bulk values and is dominated by microstructural grain boundary scattering. With temperature dependent electrical and thermal transport measurements, we show that both the thermoelectric power S (-259 μV/K) and the figure of merit ZT (0.54) are enhanced by nearly a factor of 4 for SPS pellets of Bi ₂Te _2.7Se _0.3 in comparison to Bi ₂Te ₃ NP composites. Tentatively, such an enhancement of the thermoelectric performance in nanoplatelet composites is attributed to the energy filtering of low energy electrons by abundant grain boundaries in aligned nanocomposites.