No abstract

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a 'natural superlattice' in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered T X 2 structure which has a general formula of (M X ) 1+x (T X 2 ) n (M = Pb, Bi, Sn, Sb or a rare earth element; T = Ti, V, Cr, Nb or Ta; X = S or Se and n = 1, 2, 3). We demonstrate that one of the intercalation compounds (SnS) 1.2 (TiS 2 ) 2 has better thermoelectric properties compared with pure TiS 2 in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.

We review here recent progress on epitaxial graphene grown on a SiC substrate. Epitaxial graphene can be easily grown by heating the SiC single crystal in a high vacuum or in an inert gas atmosphere. The SiC surfaces used for graphene growth contain Si- and C-terminated faces. On the Si-face, homogeneous and clean graphene can be grown with a controlled number of layers, and the carrier mobility reaches as high as several m(2) V s(-1), although this is reduced by the presence of the substrate steps. On the C-face, although the number of layers is not homogeneous, twisted bilayer graphene can be grown, which is expected to be the technique of choice to modify the electronic structure of graphene. From the application point of view, graphene on SiC will be the platform used to fabricate high-speed electronic devices and dense graphene nanoribbon arrays, which will be used to introduce a bandgap.