We investigate the lower limit to the lattice thermal conductivity of Bi2 Te3 and related materials using thin films synthesized by the method of elemental reactants. The thermal conductivities of single layer films of (Bi0.5 Sb0.5)2 Te3 and multilayer films of (Bi2 Te3) m (TiTe 2) n and [(Bix Sb1-x) 2 Te3] m (TiTe2) n are measured by time-domain thermoreflectance; the thermal conductivity data are compared to our prior work on nanocrystalline Bi2 Te3 and a Debye-Callaway model of heat transport by acoustic phonons. The homogeneous nanocrystalline films have average grain sizes 30<d<100 nm as measured by the width of the (003) x-ray diffraction peak. Multilayer films incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of Bi2 Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, the thermal conductivity of Bi2 Te3 approaches the predicted minimum thermal conductivity of 0.31 W/m K. The dependence of the thermal conductivity on grain size is in good agreement with our Debye-Callaway model. The use of alloy (Bi,Sb) 2 Te3 layers further reduces the thermal conductivity of the nanoscale layers to as low as 0.20 W/m K.
ASJC Scopus subject areas
- Physics and Astronomy(all)