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Single-wall carbon nanotubes (SWCNTs) and Bi2Te3 nanoplates are very promising thermoelectric materials for energy harvesting. When these two materials are combined, the resulting nanocomposites exhibit high thermoelectric performance and excellent flexibility. However, simple mixing of these materials is not effective in realizing high performance. Therefore, we fabricated integrated nanocomposites by adding SWCNTs during solvothermal synthesis for the crystallization of Bi2Te3 nanoplates and prepared flexible integrated nanocomposite films by drop-casting. The integrated nanocomposite films exhibited high electrical conductivity and an n-type Seebeck coefficient owing to the low contact resistance between the nanoplates and SWCNTs. The maximum power factor was 1.38 μW/(cm K2), which was 23 times higher than that of a simple nanocomposite film formed by mixing SWCNTs during drop-casting, but excluding solvothermal synthesis. Moreover, the integrated nanocomposite films maintained their thermoelectric properties through 500 bending cycles.

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Thin film thermoelectric generators are expected to be applied as power supplies for various Internet of Thing devices owing to their small size and flexible structure. However, the primary challenges of thin film thermoelectric generators are to improve their thermoelectric performance and reduce their manufacturing cost. Hence, Bi2Te3 thin films were deposited using direct current magnetron sputtering, followed by heat treatment at 573 K with different temperature increase rates ranging from 4 to 16 K/min. The in-plane Seebeck coefficient and electrical conductivity were measured at approximately 293 K. The in-plane thermal conductivity was calculated using the models to determine the power factor (PF) and dimensionless figure of merit (ZT). The temperature increase rate clearly affected the atomic composition, crystal orientation, and lattice strains, but not the crystallite size. The PF and dimensionless ZT increased as the temperature increase rate increased. The highest PF of 17.5 µW/(cm·K2) and ZT of 0.48 were achieved at a temperature increase rate of 16 K/min, while the unannealed thin film exhibited the lowest PF of 0.7 µW/(cm·K2) and ZT of 0.05. Therefore, this study demonstrated a method to enhance the thermoelectric performance of Bi2Te3 thin films by heat treatment at the appropriate temperature increase rate.

Two-dimensional bismuth telluride (Bi 2 Te 3 ) nanoplates have great potential for thermoelectrics and topological insulators. The material performance increases as the nanoplate size decreases. However, the initial stage of the crystal growth of the nanoplates has not been significantly investigated. The Bi 2 Te 3 nanoplates are prepared by solvothermal synthesis and the phase transition based on the screw dislocation-driven spiral growth of the nanoplates is investigated. The optimal synthesis conditions are first determined by controlling the Te concentration in the precursor solution. The spiral-grown nanoplates are collected from the synthesized products. The diameter of the critical nucleus is calculated to be in the range of 5.0-7.2 nm from the step width in the spiral. Subsequently, the solvothermal synthesis is implemented by changing the synthesis time from 15 to 1200 min under the optimal conditions of the precursor solution. Crystals are not grown in the solution at 15 min. At 25 min, an intermediate phase of Bi 2 TeO 5 with an approximate grain size of 5.0 nm is formed, which corresponds to the calculated diameter of the critical nucleus. Another intermediate phase of Te is formed, and the Bi 2 Te 3 nanoplates with a lateral size of 300 nm grow slowly at the expense of Bi 2 TeO 5 and Te.