Speaker
Description
Elastic mean free paths of medium-energy electrons are in the range of several to a few hundred nanometers depending on atomic numbers and densities of the materials probed. Consequently, electron diffraction in reflection geometry has natural surface sensitivity or even specificity for materials and interfacial research. In this talk, I discuss how reflection ultrafast electron diffraction (UED) directly probes unique dynamical behavior at materials surfaces, of buried interfaces, in stacked layered heterostructures, and across multi-component junctions following photoexcitation. The out-of-plane dynamics information obtained is an integral and indispensable part to fully understand photoinduced behavior over a photocycle, together with electronic dynamics shown by femtosecond optical spectroscopy and in-plane structural dynamics studied using transmission UED or microscopy. Examples are given to provide atomic-level understanding about ultrafast electronic–lattice correlation in anisotropic layered materials, motion-based coupling across van der Waals (vdW) gaps for energy transport, as well as the impact of chemical interactions on the behavior of supporting substrate surfaces. Taking these studies together, I also provide the technical validation for the use of reflection UED and address the long-standing concern related to the effects of laser-induced surface transient electric fields.
The support from the National Science Foundation (CHE-2154363) and the Welch Foundation (E-2263) is acknowledged.