In Situ 4D STEM with an Ultrafast Detector to Study Phenomena in Liquids

Ultrafast direct electron cameras create the opportunity for <i>in situ</i> 4D STEM experiments at (spatial) frame rates of order 10 frames per second (fps). We have used this capability to study phenomena in highly supercooled liquids near the glass transition, where dynamics are slow. We have used the scattering angle dependence of various elemental partial structure factors to measure the composition-dependent relaxation times from <i>in situ</i> 4D STEM. These data are otherwise accessible only through expensive and inaccessible radiotracer diffusion experiments. We have also used <i>in situ</i> 4D STEM to observe two-stage, non-classical nucleation in a metallic supercooled liquid. Instead of forming a crystal embryo with an interface with the liquid, the system first forms a nanoscale, disorder precursor particle. The crystal then form within the disordered precursor and grows to consume it before growing into the liquid. We believe this is the first experimental observation of non-classical nucleation of an inorganic crystal from a liquid of the same composition. Both of these experiments generate multi-TB scale datasets, which require efficient, out-of-memory lazy processing to analyze.