Time-Resolved Transmission Electron Microscopy: Instrumentation and Applications

In recent decades, in situ electron microscopy has attracted considerable attention. The method enables real time observations of the transformations of nanoparticles under various stimuli with atomic spatial resolution. Time-resolved electron microscopy adds a new dimension, time, to such experiments, making it possible to study fast and ultrafast processes on their natural timescales. This thesis presents the work devoted to the development and characterization of a time-resolved transmission electron microscope, as well as multiple studies that demonstrate its capabilities to perform various in situ experiments at timescales from minutes to hundreds of femtoseconds. In Chapter 2, the transformation of a conventional transmission electron microscope into a time-resolved instrument is discussed. The required modifications include the installation of several optical elements inside the device. Some of them aimed to direct a laser beam to a sample, other to focus a laser beam into the electron gun to generate electron pulses. Our optical scheme allows us to generate high brightness electron pulses from the tungsten filament, or pulses with a high number of electrons from the extractor electrode. The energy spread, duration of pulses, and instantaneous brightness of the electron beams generated by the nanosecond and femtosecond UV lasers were characterized. Thus, a wide range of experiments can be carried out with the instrument developed in our group. Chapter 3 presents an in situ electron microscopy study of Coulomb fission of plasmonic nanoparticles under femtosecond laser irradiation. Gold nanoparticles encapsulated in a silica shell fission by ejecting gold clusters of 1-2 nm diameter. Continued exposure to the laser beam leads to the coalescence of the ejected particles into a second core within seconds. Under ultrafast pulses, the core ionizes, emitting the electrons along the laser polarization direction. The resulting uneven charge distribution in the silica shell then causes the progeny particles to be ejected in the same direction. This chapter demonstrates that the combination of ultrafast laser systems with a transmission electron microscope allows in situ observations of complex laser-driven processes with high spatial resolution. Chapter 4 presents a time-resolved electron microscopy study that elucidates the morphological dynamics of so-called jumping nanodroplets that were created by melting triangular gold nanoprism on a graphene surface with a laser pulse. A nanoscale movie was recorded that shows the shape evolution of the droplets as they dewet and jump off the substrate. Inhomogeneities of the graphene surface lead to variations in the dewetting trajectories. Surprisingly, some droplets adopt cylindrical or dumbbell-shaped geometries after they jump, suggesting that they spin with considerable angular momentum.