Mechanical and Civil Engineering Seminar

Ultrafast measurements are increasingly used for investigations of thermal transport, including thermal properties at the nanoscale and thermal transport across interfaces. With the time resolution of the order of tens of femtosecond (10^-15 s) from commercially available femtosecond laser sources, it becomes possible to directly access energy transport among fundamental energy carriers, including photons, electrons, phonons, and excitons. From a microscopic viewpoint, energy transport and conversion are determined by interactions among these energy carriers, which often occur at a time scale of femtoseconds to picoseconds (10^-12 s). Recently, there are renewed interests due to the discovery of much improved transport properties and energy conversion efficiency in nanomaterials such as quantum dots, nanowires, and superlattices. In this talk, I will discuss investigations of energy transfer and conversion using ultrafast laser spectroscopy. We develop experimental techniques to investigate interactions among energy carriers, with the aim of discovering new energy coupling channels to facilitate or inhibit energy transport and discovering new applications for energy conversion and utilization. In nanoscale photovoltaic materials, e.g., quantum dots, it is possible that quantized phonon vibration states lead to decreased interactions between electrons and phonons, therefore, increasing the probability for harvesting energy from hot electrons before it is converted to heat. This charge transfer depends on the morphology of quantum dots as well as the interface between quantum dots and their surrounding materials. We also investigate phonon vibrations of THz (10¹² Hz) frequencies in thermoelectric materials. In nanoscale thermoelectric materials, interfaces, boundaries, and impurities are engineered to produce extra phonon scattering channels, which reduce thermal conductivity and increase thermoelectric efficiency. We employ coherent phonon spectroscopy to shed light on the role of phonon scattering vs. coherence of phonon vibrations and the resulting lattice thermal conductivity. Potential applications of nanoscale thermoelectric materials for waste heat recovery will also be discussed.