New Compounds Consisting of Turbostratic Intergrowths: Ultra-low Thermal Conductivities and Tunable Electric Properties
- PDF / 3,308,409 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 30 Downloads / 203 Views
New Compounds Consisting of Turbostratic Intergrowths: Ultra-low Thermal Conductivities and Tunable Electric Properties Matt Beekman, Daniel B. Moore, Ryan Atkins, Colby Heideman, Qiyin Lin, Krista Hill, Michael Anderson, and David C. Johnson Department of Chemistry, University of Oregon, Eugene, OR, 97404 U.S.A. ABSTRACT A recently discovered synthetic route to new kinetically stable [(MSe)y]m[TSe2]n layered intergrowths has been applied to prepare several different compositions (M = Pb or Sn, T = Ta, Nb, Mo, or W) with m = n = 1, in thin film form. Scanning transmission electron microscopy and synchrotron X-ray diffraction show the nanostructure of these materials is characterized by a combination of in-plane component crystallinity with misregistration and rotational misorientation between adjacent layers. Extremely low cross-plane thermal conductivity as low as 0.1 W m-1 K-1 are attributed to the turbostratic nanostructure. By appropriate choice of M and T, we demonstrate that a range of electrical transport properties are possible, from metallic to semiconducting. Annealing (PbSe)0.99WSe2 and (PbSe)1.00MoSe2 specimens in a controlled atmosphere of PbSe or WSe2 is observed to systematically influence carrier properties, and is interpreted in terms of reduction of the concentration of electrically active defects. Considering these observations and the large composition and structural space that can be explored in such [(MSe)y]m[TSe2]n intergrowths, these materials are of interest for further investigation as potential thermoelectric materials. INTRODUCTION Nanostructured materials offer distinct opportunities to achieve enhanced thermoelectric (TE) performance, resulting from the ability to effectively decouple electrical and thermal transport properties [1]. Development of new types of materials in which composition and structure can be controlled with nanoscale precision, such that the mechanisms for TE enhancement can be studied and potentially applied, is therefore of particular interest. Toward this aim, we have recently proposed a simple design principle to guide discovery of novel nanostructured materials based on layered intergrowths: if a stable interface between two materials exists, intergrowths formed by interleaving them on a nanometer length scale should be at least metastable. Guided by this principle, we have used the method of modulated elemental reactants (MER) [2] to prepare new layered intergrowth compounds based on monoselenides and transition metal diselenides [4-6], [(MSe)y]m[TSe2]n (M = Pb, Sn, Ce, or Bi, T = Ti, V, Nb, Mo, or Ta), where m and n denote the number of contiguous layers of the respective constituent in the repeating unit of the intergrowth. The parameter y reflects the lack of an epitaxial relationship between the substructures of the intergrowth components, i.e., the (in general) incommensurate relationship, or “misfit” between the in-plane MSe and TSe2 lattice parameters. The stability of these new metastable compounds was predicted based on the known existence of the analo
Data Loading...
