Microscopie électronique ultrarapide en transmission

After the setup of the laboratory infrastructure and the delivery of the equipment in 2014, the work on the project UTEM was first devoted to the setup and optimization of the ultrafast electron microscope. A major part of the optical setups, the tests, and the optimization of the instrument had to be done by us. The UTEM can be operated in the stroboscopic mode, where reversible phenomena are studied, as well as in the single-shot mode for studying irreversible transitions. For the first time, a microscope that is optimized for both modes is now available. It is a characterization tool working with high temporal as well as high spatial resolution. In the stroboscopic mode, temporal, spatial, and energy resolutions of 1 ps, 0.2 nm, and 0.7 eV can be reached, respectively. Under realistic conditions, tradeoffs have to be found. In the single-shot mode working with intense 7 ns pulses, spatial resolutions of tens of nanometers are reached. For the first time, electron energy-loss spectroscopy (EELS) with single nanosecond electron pulses became possible and allows the quantitative analysis of nanoparticles with nanosecond resolution. In the second project period, studies on dedicated materials systems are carried out. For allowing access for external users, the microscope was integrated into the national microscopy network METSA. The study of irreversible chemical reactions in nanoparticles has given highly useful information about fast reaction kinetics. In the reduction reaction of oxide nanocrystals, short-lived transition states were detected and fast reaction speeds in nanocrystals at high temperature were measured. Another project focused on the laser-induced amorphization of nanocrystalline metal-carbon systems. In a project supported by METSA, laser-induced morphologial changes of spin-crossover materials and local plasmonic heating effects were studied with nanosecond resolution.