Abstract: We have developed a synthetic approach to new materials that uses composition control on an Angstrom length scale to control solid-state reaction pathways, leading to the self-assembly of new nanostructured compound by avoiding compounds on equilibrium phase diagrams. One class of new compounds we are currently investigating consists of two or more compounds with different crystal structures that are interleaved. These misfit layered compounds can be naturally occurring nanostructured solids and have been reported to have low thermal conductivities on the order of 0.8 Wm-1K-1 and unoptimized figures of merit for thermoelectric performance, ZT, as large as 0.3. We have recently reported thermal conductivity in [(PbSe)1.00]m[MoSe2]n and [(PbSe)0.99]m[WSe2]n misfit compounds as low as 0.06 Wm-1K-1. Here we describe annealing treatments of these misfit compounds in fixed chalcogen partial vapour pressures and demonstrate that samples equilibrate with the dominant source of vapour, resulting in controlled carrier concentrations. The thermal stability of these materials allows annealing times and temperatures in excess of 24 hours and 500° C to be used without destroying the layered structure. Holding the sample at constant temperature while changing the reservoir's annealing temperature provides an independent means to control chalcogen partial pressure and hence carrier concentrations. We present data showing the convergence of electrical properties for isostructural samples on annealing. In addition to control of carrier concentrations, the annealing treatments dramatically improve the carrier mobility. We report electrical resistivity, Seebeck coefficients and carrier concentrations as a function of annealing conditions.
We believe the ability to prepare entire families of new nanostructured compounds and equilibrating them to control carrier concentrations permits a new "thin film metallurgy" or "nanochemistry" in which nanostructure and composition can both be used to tailor physical properties.
Hosts: Eric Garfunkel and Martha Greenblatt