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

Abstract

In this work, Bi 2 Te 3 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 Bi 2 Te 3 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 Bi 2 Te 3 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 Bi 2 Te 3 nanocrystals is due to phonon scattering by vacancy defects rather than grain size effects.

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

“…The specific contribution of vacancies to the reduction of κ ph is often unaccounted or mistakenly attributed to grain boundaries. He et al recently studied the relation between point defects, grain boundaries, and the reduction κ ph in Bi 2 Te 3 nanocrystals by means of thermal conductivity, electron microscopy, and positron annihilation measurements . It is instructive to note that the κ ph of Bi 2 Te 3 nanocrystals increases with an increasing annealing temperature but the grain size barely changes upon annealing (Figure c).…”

“…c) Temperature dependent lattice thermal conductivity of Bi 2 Te 3 nanocrystals annealed at different temperatures (The inset presents the grain size of Bi 2 Te 3 nanocrystals annealed at different temperatures). d) Variations of positron lifetime τ 1 , τ 2 , intensity I 2 , and average lifetime τ av as a function of annealing temperature …”

“…However, the routine powder metallurgy methods used to refine grain size often involve deformation processes that create vacancies (c.f. Section ) . The specific contribution of vacancies to the reduction of κ ph is often unaccounted or mistakenly attributed to grain boundaries.…”

“…A good example is the reduction of κ ph in Bi 2 Te 3 nanocrystals (cf. Section ), in which the deformation‐induced vacancies dominate over grain boundaries.…”

A comprehensive review on Bi2Te3‐based thin films: Thermoelectrics and beyond

“…[ 12–14,27–29 ] So far, three important aspects regarding the transport properties could be identified in Bi 2 Te 3 ‐based thin films. (1) Atomic defect engineering: defect formation energy calculations and advanced experimental characterizations have revealed [ 101,119–123 ] that vacancies at the anion sites (V Se and V Te ) and anti‐site defects at both anion and cation sites (Bi Te , Sb Te , and Te Bi ) have the lowest formation energies and thus are the dominant atomic defects in thin Bi 2 Te 3 ‐based films. The manipulation of atomic defects and their density is crucial for the optimization of the PF of the films, as will be clarified in Sections 3 and 4.…”