Keynote Speech: Nanostructure thermoelectrics

Thermoelectric materials reusing waste heat have drawn much attention for overcoming the energy problem. In the development of thermoelectric material, there has been a crucial bottleneck for high thermoelectric conversion efficiency: intercorrelated relationship among the three properties of electrical conductivity, Seebeck coefficient, and thermal conductivity. Many researchers have proposed nanostructuring approaches such as nanocomposite, low- dimensional structure and complex structure, to increase the thermoelectric conversion efficiency. Recently, thin film thermoelectric material is attracting much attention as one- chip stand-alone power source of IoT sensor. Until now, we have studied thin film thermoelectric materials composed of eco-friendly light elements using nanostructures. Here, we present 3 methodologies for realizing high-performance thermoelectric materials. (1) Epitaxial connected Si nanodots exhibited anomalously-low thermal conductivity beating amorphous limit. (2) In Si films including Ge nanodots, thermal conductivity was reduced while maintaining electrical conductivity, demonstrating the independent control of phonon and electron. (3) ZnO nanowire embedded-structure exhibited higher thermoelectric power factor than that of conventional ZnO films. The thermoelectric power factor enhancement was brought by Seebeck coefficient enhancement due to smooth transport of higher energy carrier than energy barrier height through the nanowires/films interfaces. These successes largely accelerate the development of thermoelectric materials.