(A) study on the characterization of thermoelectric properties of low dimensional nanostructured semiconductors

Abstract

Thermoelectric (TE) materials are of increasing interest as environmental friendly and sustainable energy conversion, which are able to convert heat into electricity, and vice versa. Despite those materials have been studied as promising new energy source for recent decades, the energy conversion efficiency still remains as an obstacle to propel commercialization. To tackle this, various efforts have been conducted to enhance their energy conversion efficiency, ZT=$S^2\sigma$T/K (S: Seebeck coefficient, $\sigma$: electrical conductivity, T: operating temperature, and k: thermal conductivity). Over the past two decades, two different approaches have been developed to search for the next generation of thermoelectric materials: one is finding and using new families of bulk thermoelectric materials with complex crystal structures, and the other is synthesizing and using lowdimensional thermoelectric materials systems. There is also much interest in the accurate measurement of thermoelectric properties as the studies on nanostructured materials for the enhancement of ZT are advanced.

We studied on the measurement method using the platform and design of platform that matches the size of the structure for the reliable measurement of nanostructures. Homemade microfabricatied thermoelectric properties measurement platform [MTMP] for the nanowire, nanosheet, and nano patterned 3-D structure was fabricated. And devised a manipulation system that uses a tungsten tip and AAO template with van der waals force allows easy transfer sample to platform.

In order to reduce the loss of electrical transport by contact resistance, appropriate ohmic contact conditions are established and on the bridge type MTMP for thermal conductivity measurement. Thermal conductivity was measured about the heat passed by nanostructured material through the heat dissipation (Fourier heat transport) for the simple and reliable measurement.

This dissertation is organized in the following manner. Chapter 1, the introduction, discusses the motivation for current research. Chapter 2 introduces the synthesis of undoped and Si-doped InAs NWs to investigate the effect of carrier concentration and the lattice thermal conductivity on ZT. All TE parameters of S, σ, and k with temperature on the microfabricated TE measurement platforms (MTMPs) were measured and photoinduced thermoelectric properties of InAs NWs was evaluated in Chapter 3. Micromanipulation and thermoelectric characteristic of of 2-D black phosphorus nanosheets are presented in Chapter 4. Also enhancement of thermoelectric properties of black phosphorus nanosheets by gating effect was discussed. Finally, Chapter 5 summarizes the conclusions of this work and the insight gained from performing these experimental investigations and points out the future work that may contribute to the development of nanotechnology.