Improving Thermoelectric Properties of Chemically Synthesized Bi2Te3-Based Nanocrystals by Annealing

The power factors of chemically synthesized Bi2Te3 and Bi0.5Sb1.5Te3 nanocrystals (NCs) were improved up to 2.4 and 7.8 mu W cm(-1) K-2, respectively, which are significantly higher than previously reported values for chemically synthesized Bi2Te3 NCs and even comparable to the recently reported highest power factor of 5 mu W cm(-1) K-2 for Bi2Te3 NCs consolidated by spark plasma sintering. This improvement was achieved by annealing the NCs under argon protection, and the crystal structures and morphologies of these annealed NCs were characterized via XRD, SEM, and TEM measurements. The temperature-dependent thermoelectric properties of these modified NCs were explored on cold-pressed pellets of NCs. Improvement of the thermoelectric performances of the pellets resulted primarily from an increase in electrical conductivity (sigma), while only weakly increasing the lattice thermal conductivity (kappa(L)), which was still kept lower than bulk values. Hall carrier concentration studies suggest that the improvement of the electrical conductivity is caused primarily by modification of the charge carrier mobility rather than the carrier concentration. A mechanism is proposed to explain a large increase of electrical conductivity by annealing related to a decrease of activation energy for the mobility after the removal of organic capping ligands through annealing.