CORRELATING THERMOELECTRIC PROPERTIES TO NANOSTRUCTURES BY ADVANCED STEM/TEM TECHNIQUES

Thermoelectrics enable direct heat-to-electricity transformation, but their performance has so far been restricted by the ambiguous understanding of the nanoscale atomic structure and its effect on carrier and phonon transport. Thanks to the recent development of aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of all nanoscale defects can now be imaged. Preliminary works link these defects to lattice thermal conductivity but they are rarely linked with carrier mobility.

In this thesis, we develop an understanding of the relationship between the nanostructure

and the properties of thermoelectric materials, especially electrical mobility, through the utilization of the versatile techniques of STEM and transmission electron microscopy (TEM). In Chapter 2, two kinds of nano-precipitates have been carefully identified to understand their effect on lattice thermal conductivity. In Chapter 3, the atomic-scale mixing of different chemical elements and the resultant subtle local strain is characterized and discussed in how they contribute to low lattice thermal conductivity. In Chapter 4, the formation mechanism of two-dimensional vdW gaps is studied. These two-dimensional vdW gaps are beneficial for thermoelectric performance. In Chapter 5, a big jump is made by linking defects and carrier scattering in the framework of quantum mechanics.

Overall, this thesis provides insights on how to better achieve and understand highperformance thermoelectric materials through a detailed microstructure investigation. The conclusion about the effect of defects and interfaces in carrier scattering can be extensively applied far beyond thermoelectric materials.

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