Disordered stoichiometric nanorods and ordered off-stoichiometric nanoparticles in n-type thermoelectric Bi₂Te₂.₇Se₀.₃

Abstract

N-type Bi₂Te₂.₇Se₀.₃ bulk thermoelectric materials with peak ZT values up to ∼1 were examined by transmission electron microscopy and electron diffraction. Two nanostructural features were found: (i) a structural modulation of ∼10 nm, which consisted of nanorods with crystalline and nearly amorphous regions, having the rod axes normal to (0,1,5)-type planes, and wave vector normal to (1,0,10)-type planes and (ii) non-stoichiometric ordered Bi-rich nanoparticles. The presence of the structural modulation was not influenced by the ion milling energy or temperature in this study while the non-stoichiometric ordered nanoparticles were only observed when ion milling at low temperatures and low energy was used. It is proposed that both the structural modulation of ∼10 nm and the presence of non-stoichiometric nanoparticles are responsible for the low lattice thermal conductivity (∼0.6 W/mK) of the Bi₂Te₂.₇Se₀.₃ bulk thermoelectric materials studied.