Characterization of bismuth telluride two-dimensional nanosheets for thermoelectric applications

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Solid-state thermoelectric devices are compact, scalable, quiet, and environmentally friendly, which are widely used as thermal engines or refrigerators. Bismuth telluride (Bi2Te3) and other V-VI group chalcogenides are known as one of the best thermoelectric materials specifically for applications in a temperature environment from room temperature to 300 ℃. Recently, the unique topological surface states were discovered in Bi2Te3 family materials, and these novel surface states are arisen from a strong spin-orbit coupling in topological insulators. Topological surface states are protected against time-reversal perturbations (i.e., non-magnetic impurities or surface defects), making the electronic transport essentially dissipation-less. Such unique transport behavior with zero energy loss provides new opportunities to enhance thermoelectric properties. Although the promise in thermoelectric properties of topological insulators have been shown in theoretical reports, there is a lack of experimental investigations for a better understanding of their basic properties. This research work focuses on the characterizations of fundamental properties of Bi2Te3 two-dimensional (2D) nanosheets. Samples were prepared via respective solvothermal synthesis and van der Waals epitaxy. The charged surface properties of Bi2Te3 2D nanosheets were investigated using kelvin probe force microscopy. The measured electrical potential difference between aminosilane self-assembled monolayer and Bi2Te3 nanosheet surfaces is found to be ∼650 mV, which is larger than that (∼400 mV) between the silicon oxide substrate and Bi2Te3 nanosheet surface. The elastic properties of Bi2Te3 2D nanosheets (i.e., Young’s modulus and prestress) were acquired by analyzing the thickness dependence of 2D nanosheet deformations creating by atomic force microscopy tips. The Young's modulus by fitting linear elastic behaviors of 26 samples is found only 11.7–25.7 GPa, significantly smaller than the bulk in-plane Young's modulus (50–55 GPa). Furthermore, the thermoelectric properties of Bi2Te3 2D nanosheets were characterized in the cryostat system at a temperature range of 20-400 K. The results reveal that electrical conductivity of 2D nanosheets decreases with increasing temperature and thickness, while the measured Seebeck coefficient does not show a strong thickness dependence and the value is smaller than bulk Bi2Te3. These fundamental properties would help improve the basic understanding of topological surface states towards practical applications.

Electronic Thesis or Dissertation
Chemical engineering