Understanding Transport and Interaction of Microscopic Energy Carriers at the Single-Mode Level: Computation and Experiment

Department of Mechanical Engineering & Materials Science Seminar

Understanding Transport and Interaction of Microscopic Energy Carriers at the Single-Mode Level: Computation and Experiment
Dr. Bolin Liao
Division of Chemistry & Chemical Engineering
California Institute of Technology

Solid-state energy converters, including thermoelectric, photovoltaic and photocatalytic devices, hold great promise of providing sustainable clean energy and addressing global challenges such as climate change and air/water pollution. One common feature of these devices is that their performance is largely determined by the transport and interaction processes of microscopic energy carriers, mainly electrons, phonons and photons. It is being increasingly recognized that a thorough understanding of these processes is the key to the ultimate performance of a wide range of energy materials. In particular, the information on how an individual mode/state transports and interacts with others is highly desirable to guide rational design and engineering of energy materials, whereas how to obtain this information remains an outstanding challenge given the required extreme spatial and temporal resolutions. In this talk, I will first report our recent development and application of DFT-based first-principles simulation tools to understand phonon-phonon and electron-phonon interactions mode-by-mode. One specific example is a surprising finding that electron-phonon interaction can significantly reduce phonon lifetime, and thus the thermal conductivity, in heavily-doped semiconductors. To verify this prediction, we developed a three-pulse photoacoustic spectroscopic method to experimentally quantify how a single phonon mode is damped by photoexcited free electrons. In the second half of the talk, I will introduce the ultrafast electron beam, which combines the femtosecond time resolution and nanometer spatial resolution, as an ideal probe to reach the single-mode-level detection of transport and interaction. As a demonstration, I will present striking visualization of photocarrier dynamics in hydrogenated amorphous silicon and black phosphorus using the scanning ultrafast electron microscope, and discuss the findings that can only be revealed by ultrafast spatial-temporal imaging, such as the spontaneous electron-hole separation after photo-excitation in amorphous silicon. To conclude, I will share my thoughts on the outlook of combining first-principles simulation and ultrafast optical/electron spectroscopy and imaging to eventually "see" how solid-state energy materials work at the most fundamental level.

Bio: Bolin Liao is currently a Kavli Nanoscience Institute Prize Postdoctoral Fellow at the California Institute of Technology. Bolin obtained his Ph.D. in Mechanical Engineering from MIT in March 2016, advised by Prof. Gang Chen. His Ph.D. thesis focused on developing theoretical, computational and experimental tools to understand transport and interaction of microscopic energy carriers in thermoelectric materials. At Caltech, Bolin is working in Prof. Ahmed Zewail's lab, where spectroscopic and imaging tools based on ultrafast electron beam are being developed. Bolin is the leading author of 9 peer-reviewed research papers, and is a recipient of the Wunsch Foundation Silent Hoist and Crane Award for Outstanding Thesis from MIT Mechanical Engineering, and the Chinese Government Award for Outstanding Students Abroad.

Host: Udo Schwarz
Monday, January 23, 2017
Mason Lab 107 2:30pm - 3:30pm
Refreshments 2:15pm