Enhanced Thermoelectric Properties of Solution Grown Bi2Te3-xSex Nanoplatelet Composites

We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi2Te3-xSex) 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(lg)(2)) phonon modes illustrates a two mode behavior for Bi2Te3-xSex NPs. The electrical resistivity (rho) of spark plasma sintered pellet composites shows metallic conduction for pure Bi2Te3 NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (kappa) 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 mu V/K) and the figure of merit ZT (0.54) are enhanced by nearly a factor of 4 for SPS pellets of Bi2Te2.7Se0.3 in comparison to Bi2Te3 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.