Transport Phenomena down to Sub-nm Scale : Effect of Material Structure

1. Phonon and energy transport across interfaces

Interface energy transport sustained by phonons is a critical process that determines the heat dissipation in micro/electronics, and also can be used to characterize the local interface structure and bonding strength. Also this process plays a critical role in the heat conduction capability of nanostructured materials, nanocomposites, and nanofluids. Due to the great varieties and complicated structures of interfaces, experimental characterization is always far more challenging than computer simulations while experimental characterization always prompts discoveries and new understandings. Our research in this area has been extremely strong and various breakthroughs have been made, including the first nm~scale temperature differential probing across an interface, interface optical interference, stress buildup, and pioneering solid-liquid interface thermal conductance measurement.

2. Charge and energy transport by electrons

Charge and energy transport by electrons in metals and semiconductors could be highly correlated/conjugated. The relation for metals can be described by the Wiedemann-Franz law while the Lorenz number could be strongly structure-dependent. Our work has led to the discovery of significantly increased Lorenz number in nm-thick nanocrystalline metals, linear correlation between phonon and electron transport in carbon materials, phonon softening, and electron hopping in organic materials.

3. Phonon transport in materials

This is a very broad topic and has been studied by experiments and computer modeling world-wide. Our work in this area covers both computer modeling and high-level experimental characterization. We have achieved structure tailoring of ultra-high molecular weight (UHMW) polyethylene (PE) fiber and found the first ultra-high thermal conductivity sustained by highly aligned amorphous molecular chains. Also we reported the first very high thermal conductivity of poly(3-hexylthiophene) films and studied its anisotropic thermal conductivities.

4. Hot carrier transport in semiconductors

Hot carrier transport is a critical process in semiconductors and 2D materials under photon excitation. It is conjugated with phonon transport, and is very difficult to single out and characterize. We have done very frontier work in this area, including distinguishing  phonon and hot carrier transport, measuring hot carrier diffusivity, and determining electron-hole radiative recombination.