UC Berkeley

Real-space imaging provides real-space information about the properties of a sample, which in turn provides insight into the correlation between physical properties and different regions of the sample. With repeated imaging, we can record movies that reveal spatio-temporal correlations. Many processes that are important for life and in emergent technologies occur at the nanoscale, a length scale that is typically difficult for optical microscopies to access. In particular, observing dynamics in these systems proves to be challenging, where existing microscopy techniques provide different trade-offs in spatial resolution, temporal resolution, and sample damage. In this dissertation, we use low-dose scanning electron microscopy (SEM) and cathodoluminescence (CL) microscopy to elucidate dynamics in delicate materials at the nanoscale.

Chapter 2 focuses on using time-resolved cathodoluminescence (TRCL) microscopy to determine the spatial variation in the lifetime of a Mn2+ dopant in a metal halide perovskite. We observe enhanced Mn2+ luminescence at the edges of halide perovskite microplates. Using TRCL, we reveal two luminescent decay components that we attribute to two different Mn2+ populations. While each component appears to be present both near the surface and in the bulk, the origin of the intensity variation stems from a higher proportion of the longer lifetime component near the perovskite surface. We suggest that this increased CL emission is caused by an increased probability of electron-hole recombination on the Mn2+ dopant near the perovskite surface due to an increased trap concentration there.

Chapter 3 of this dissertation focuses on developments in cathodoluminescence-activated imaging by resonant energy transfer (CLAIRE) microscopy. We discuss the production of thin, free-standing scintillator imaging chips for CLAIRE imaging and demonstrate that CLAIRE is capable of imaging dynamic processes in both soft materials and with metal nanoparticle labels. We then discuss our efforts to expand CLAIRE capabilities to other samples. We developed an aqueous encapsulation scheme using multi-layer graphene to expand the compatibility of CLAIRE imaging to samples that are otherwise incompatible with the vacuum environment of the experiment. Additionally, we report preliminary steps towards using CLAIRE imaging to study photosynthetic membranes.

Chapter 4 focuses on the use of a low-dose electron beam to both drive and record dynamics in an interfacial polycrystalline colloidal monolayer. We describe the formation of these polycrystalline monolayers at the surface of an ionic liquid droplet and the incorporation of large particle dopants in the polycrystalline lattice. We demonstrate that an electron beam perturbation drives a reduction in particle density in the center of the imaging field of view due to a combination of outward particle flow and detaching from the interface and becoming immersed into the bulk ionic liquid. We find that the rate of this reduction in particle density depends on the number of large particle dopants present in the lattice and discuss possible explanations for this dependence.

Together, these experiments demonstrate the utility of low-dose SEM and CL microscopy in capturing nanoscale dynamics in a variety of samples that are not typically robust to electron beam imaging. These methods extend nanoscale imaging to materials that are not compatible with other super-resolution imaging techniques or traditional electron microscopy, providing opportunities to explore dynamics in a wide range of other samples, from soft biological materials to next generation self assembled meta-materials.