Grain boundary re-crystallization and sub-nano regions leading to high plateau figure of merit for Bi₂Te₃ nanoflakes

Nanoengineering is an effective strategy to strengthen phonon scattering, reduce lattice thermal conductivity and boost thermoelectric material performance. However, nanostructure features are generally in the size of similar to 10 nm, and the fine control of nanostructure characteristics down to the sub-nano level (below several nanometers) remains as a key challenge. Here, we demonstrate that solvothermally synthesized Bi2Te3 can re-crystallize preferentially at the grain boundaries to form sub-nano boundary regions with width <2 nm via the optimization of sintering conditions. The optimized formation process of these sub-nano boundary regions can induce synergistic effects, including strengthened mid- to short-wavelength phonon scattering, weakened lattice/carrier scattering, carrier concentration optimization, weakened band degeneracy, as well as the optimized bipolar effect. Finally, a wide plateau figure of merit (zT) of >1.2 (from similar to 323 to similar to 423 K) and a high average zT of similar to 1.18 (from 303 to 473 K) have been achieved in the Bi2Te3 pellet sintered at 593 K. This study not only reveals the formation mechanism of sub-nano boundary regions but also demonstrates that these sub-nano boundary regions and their formation process can effectively induce synergistic effects contributing to high thermoelectric performance, and guide the design of high-performance thermoelectric materials.