Abstract:
Femtosecond time-resolved transmission electron microscopy (TEM) has a time resolution ten orders of magnitude better than that of conventional TEM. Instead of using thermionic electrons in TEM, laser-driven electron pulses allow various modes of detection such as imaging, diffraction, and spectroscopy, all with unprecedented spatiotemporal resolution - sub-nanometer and femtosecond. In this presentation, I will discuss the development of 4D Ultrafast EM and summarize the up-to-date accomplishments that represent its broad capabilities in chemistry, materials science, and biology. As a first example, I will present the critical role of 4D Ultrafast EM in investigating the crystallization dynamics of matter. It could observe in space and time the ephemeral nucleation of liquid-to-crystal phase transitions of titanium dioxide by probing a series of electron diffraction snapshots. Interestingly, it was found that the temporal behavior of crystallization exhibits unique two-step dynamics, with a robust plateau that extends over a microsecond. Such behavior reflects the presence of intermediate structure(s) that precede the ordered crystalline state. Secondly, I will introduce how this technique provides a dynamic probe for the active sites of photocatalytic materials and visualizes the femtosecond atomic movement at the titanium active center of a single-site photocatalyst. These findings contribute fundamental insights for developing advanced photocatalysts and suggest broad ranges of applications.