Synthesis and thermoelectric properties of a new family of misfit layered compounds: lead selenide-tungsten selenide

This dissertation describes a general synthesis approach using the Modulated Elemental Reactants technique for the preparation of new families of misfit layered compounds with incommensurate structures and demonstrates its effectiveness through the preparation of a large family [(PbSe)0.99]m(WSe 2)n (1 ≤ m, n ≤ 8) of ultra low thermal conductivity misfit layered compounds. The new compounds [(PbSe)0.99]m (WSe2)n were synthesized by annealing reactant precursors containing m layers of alternating elemental Pb and Se followed by n layers of alternating elemental W and Se in which the thickness of each pair of elemental layers was calibrated to yield a structural layer of rock salt PbSe and hexagonal WSe2. The compounds are kinetically trapped by the similarity of the composition profiles and modulation lengths in the precursor and the targeted compounds. The structural evolution from initial reactant of layered elements to crystalline misfit layered compounds was tracked using X-ray diffraction (XRD). The crystal structures of new compounds were probed using both X-ray techniques and STEM. The c-axis is perpendicular to the substrate and changes regularly as m or n varied. The in-plane lattice parameters did not change as the individual layer thicknesses were increased, and a misfit parameter of 0.99, which is much smaller than other known misfit layered compounds, was calculated.;[(PbSe)0.99]m(WSe2)n compounds possess the extraordinarily low cross-plane thermal conductivities, ranging from 0.06 to 0.14 W/m·K, and very low in-plane thermal conductivities about 0.4 W/m·K. The low thermal conductivities in [(PbSe)0.99 ]m(WSe2)n could be attributed to the rotational (turbostratic) disorder of the a-b planes in the c-lattice direction, which is supported by STEM images and XRD. The low thermal conductivities would make [(PbSe)0.99]m(WSe2)n compounds attractive materials for thermoelectric applications.;(PbSe)0.99]m[WSe2]n materials exhibit semiconducting behaviors of the electrical properties. The effects of the post-growth annealing under controlled chalcogenide vapor pressures on the semiconducting [(PbSe)0.99]l(WSe2) l samples were explored with the aim to control and optimize their electrical properties. The best ZT value of [(PbSe)0.99]l(WSe 2)l is estimated to be ∼0.09 at the room temperature. The electronic band structure for the [(PbSe)0.99]l(WSe 2)l compound will be discussed.;This dissertation includes both my previously published and my co-authored materials.