CFN Colloquium

Imaging soft materials by transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM) remains a challenge due to modest electron contrast and sensitivity to the electron beam. Recent advances in instrumentation for electron microscopy have aimed to push the resolution limit, leading to remarkable instruments capable of imaging at 0.5 Å. But, when imaging soft materials, the resolution is often limited by the amount of dose the material can handle rather than the instrumental resolution. Despite the strong constraints placed by radiation sensitivity, recent developments in electron microscopes have the potential to advance soft matter electron microscopy. For example, we have used monochromatated sources for spectroscopy and imaging based on differences in the valence electronic structure of conjugated polymers, multi-modal electron tomography to map nanoscale inhomogeneity in the density of membranes used for reverse osmosis, and direct electron detectors to minimize the required dose and enable imaging of lattice planes in crystalline polymers. The combination of multiple techniques, such as nanodiffraction with focused probe imaging (4DSTEM), can reveal correlations in orientation and crystalline domains of organic molecules. These advances in electron microscopy highlight the potential for broad impact in a variety of material classes, including for solutions and aggregates, where cryogenic TEM (cryo-EM) can enable unique insights into assembly processes. We have used cryo-EM to reveal how the structure of mixed ionic-electronic polymers emerges from solution aggregates, and how the nanoscale structure of reverse osmosis membranes swell in water. Altogether, the field of soft matter electron microscopy is poised to make substantial advances in the near future.