In Situ and Time-Resolved Transmission Electron Microscopy of Nanoscale Processes

Observing the fast dynamics of nanoscale systems is crucial in order to understand and ultimately control their behavior. Characterizing these dynamic processes requires techniques with atomic spatial resolution and a temporal resolution that matches the timescale of the processes involved. In situ electron microscopy is a powerful technique for studying the dynamics of nanoscale systems, as it enables the direct visualization of dynamic processes with up to atomic resolution. Time-resolved electron microscopy moreover combines such experiments with the high time resolution afforded by modern laser systems, thus allowing researchers to study rapid processes on timescales down to the femtosecond range. In this thesis, both techniques are used to investigate the underlying mechanisms of processes of interest in nanoscale systems.In the first study presented in this thesis, the mechanism of the femtosecond laser-induced fragmentation of gold nanoparticles in water is investigated. The fragmentation of metallic nanoparticles in water under ultrafast laser irradiation is of interest because it produces fragments with diameters of 2-4 nm, a size range that would otherwise be difficult to synthesize. The fundamental mechanisms of this process, however, are still being debated due to a lack of direct observations. Using in situ liquid-cell transmission electron microscopy, the underlying mechanism is elucidated, which involves multiple, sequential Coulomb fission events that are also accompanied by other solution-mediated processes.The second study of this thesis describes a method to control and even eliminate specimen charging in cryo-electron microscopy. When a cryo sample is exposed to the electron beam, specimen charging can deflect the transmitted electrons and thus degrade the image quality. By heating the specimen with a laser, the charges that accumulate in the vitreous ice can be dissipated. Time-resolved experiments are used to determine the rate of charge dissipation. The results suggest that charge dissipation occurs as the sample is heated above 120 K, where the rearrangements of water molecules become possible, allowing protons trapped at defects to become mobile.In the third study, the order-disorder phase transition of single-crystal C60 is studied using time-resolved electron microscopy. Solid C60 undergoes a first-order phase transition upon heating to 260 K. This is a structural transition from the low-temperature simple cubic phase to the high-temperature face-centered-cubic phase. In this study, the C60 sample is heated with a femtosecond laser, and the evolution of the structure is tracked using selected-area diffraction. Two distinct components are revealed in the evolution of the crystal structure. The first is associated with a time constant of 1.6 ns and is ascribed to the energy redistribution between different phonon modes. The second, slower component arises from the phase transition. Its timescale is found to decrease with temperature, from 65 ns at about 290 K to 1.5 ns at about 600 K.