Interplay between Point Defects and Thermal Conductivity of Chemically Synthesized Bi₂Te₃ Nanocrystals Studied by Positron Annihilation

Abstract

In this work, Bi2Te3 nanocrystals were synthesized via a hydrothermal method. They were treated by spark plasma sintering (SPS) at 350 °C and further annealed between 350 and 500 °C. The crystal structures and morphologies of these annealed Bi2Te3 samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) measurements. SEM observation indicates an obvious increase of particle size with increasing annealing temperature, but the grain size estimated from HRTEM observation and the broadening of X-ray diffraction lines show little change in the annealing temperature range between 350 and 500 °C. Positron annihilation lifetime measurements reveal vacancy defects in all of the samples, which exist most probably in the grain boundary region. The average positron lifetime shows a monotonic decrease from 301 to 273 ps with increasing annealing temperatures up to 500 °C. Detailed analysis of the positron lifetime indicates decrease of vacancy concentration after annealing. Meanwhile, the lattice thermal conductivity of the Bi2Te3 nanocrystals increases with increasing annealing temperature. The electrical resistivity and Seebeck coefficient have also some changes for the annealed samples. The intimate correlation between vacancy defects and lattice thermal conductivity confirms that reduction of thermal conductivity in Bi2Te3 nanocrystals is due to phonon scattering by vacancy defects rather than grain size effects.