Thermoelectric materials are useful for a wide range of applications like waste heat removal, solid state cooling, and power generation in space missions etc. A material's thermoelectric figure of merit (zT), which determines its performance in the applications listed above, depends on its Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) as zT=S2σT/κ. Low dimensional materials like nanowires (1D) or atomically thin films (2D) are promising as they exhibit lower thermal conductivities or higher power factors (S2σ) compared to their bulk (3D) counterparts. The thermal and electrical properties of these low dimensional materials could be further tuned by modifying their microstructure to achieve higher zTs. To achieve such material tuning in a controlled fashion, it is necessary to understand the physical mechanisms that govern the relationships between a material's microstructure and its thermoelectric properties. Reliable experimental techniques and proper interpretation of the experimental results are essential to gain insights into the physical mechanisms of interest.
This dissertation addresses the characterization of thermoelectric properties of one- and two-dimensional materials with the goal of studying the governing physical mechanisms. An experimental study on the Seebeck coefficient and electrical conductivity of atomically thin Molybdenum disulphide (MoS2), a two-dimensional semiconducting material, is presented. Seebeck coefficient and electrical conductivity are electronic transport properties. In MoS2 and atomically thin materials, the electron transport is heavily influenced by the localized states formed in their band gaps. By fitting the experimentally obtained temperature and gate voltage dependence of S and σ with a theoretical model, a determination of the nature of the localized states and the electron transport mechanism is made.
For the one-dimensional materials, the focus is on the measurement of their thermal conductivity. Most of the advances in the figure of merit were achieved in the recent past by reducing the thermal conductivity. In this light, understanding the phonon transport in the low dimensional materials gains importance. A suspended bridge measurement platform is a very commonly used technique to measure thermal conductivity of one-dimensional materials. This technique is very useful for studying the underlying fundamental transport physics as it allows measurement on an individual 1D structure, as opposed to 3ω and TDTR methods which can only measure an assembly of 1D materials. Combining this measurement technique with precise microstructure characterization using transmission electron microscopy (TEM), the influence of the microstructure on thermal transport can be deducted. In a study done previously on silicon nanowires, different but similarly made 1D structures were used for microstructure characterization and thermal measurement. This mismatch introduces an uncertainty in the correlation between microstructure and the phonon transport. In this dissertation, a modification to the usual measurement platform is presented which allows TEM imaging and thermal measurement on the same 1D structure. Furthermore, refinements to the measurement principal that have been implemented in our lab to enable measurement on much finer 1D structures are discussed in this dissertation
