Transport of phonons and electrons in thermoelectric materials and graphene

Abstract

Understanding transport of phonons and electrons plays a critical role in developing energy conversion and information devices. Thermoelectric materials, which directly convert heat to electricity or vice versa, require both extremely low thermal conductivity and high thermoelectric power factor. However, a good understanding of low thermal conductivity is still lacking even for several good thermoelectric materials that have been studied over several decades. For the information devices, graphene has recently drawn much attention for various applications including high speed transistors due to its high electron mobility and high thermal conductivity. However, the graphene's high thermal conductivity has yet to be fully understood. There have been many studies based on diffusive-ballistic phonon transport, but no conclusive explanation for the graphene's high thermal conductivity has been drawn. In this thesis, we investigate the transport of phonons and electrons in thermoelectric materials and graphene using both first principles calculations and experimental characterizations. We start by studying phonon transport in Bi and Bi-Sb alloys using first principles calculations. A notable observation from this calculation is that a strong long-range interaction exists in Bi and Sb along a specific crystallographic direction. We further show that this long-range interaction is also found in other good thermoelectric materials, and is a key to understanding their low thermal conductivity. The long-range interaction is explained with resonant bonding which many good thermoelectric materials commonly share. The particularly strong resonant bonding in group IV-VI materials leads to the low thermal conductivity through the long-range interaction and resulting softening of optical phonons that strongly scatter acoustic phonons. We study electron transport in thermoelectric materials with two-dimensional discontinuities, such as grain boundaries. We set up an experimental system to measure thermo- and galvano-magnetic electron transport coefficients of a Bi₂Te₂.₇Se₀.₃ nanocomposite sample to examine the electron filtering effect by many grain boundaries in the nanocomposite. The experimental results indicate that the nanocomposite sample exhibits the electron filtering effect and it would be possible to increase the thermoelectric power factor by engineering the potential barrier of grain boundaries. While thermoelectric applications require materials with low thermal conductivity, electronic and optoelectronic devices often require high thermal conductivity. Graphene is attractive for these applications because of its unique electrical, optical, and thermal properties. We use first-principles calculations to reveal that the phonon transport in graphene is not diffusive unlike many threedimensional materials, but is hydrodynamic due to graphene's two-dimensional features. The hydrodynamic phonon transport is demonstrated through a drift motion of phonons, phonon Poiseuille flow, and second sound, all of which are not possible in both diffusive and ballistic phonon transport.