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Elucidating Heterogeneities and Dynamic Processes at the Nanoscale with Cathodoluminescence and Cathodoluminescence-Activated Microscopies


Super-resolution imaging has revolutionized how the structure of biological systems are observed at the nanoscale. Yet, observing dynamic processes in biology with high temporal and spatial resolution remains a significant challenge. Additionally, elucidating the nanoscale structure and dynamics in functional materials, particularly in optoelectronics, would greatly aid in the development of more efficient devices, such as solar cells and light-emitters. Unfortunately, most super-resolution microscopy platforms are designed for imaging biological samples and are incompatible with complex, functional materials. To extend super-resolution imaging to capture both biological dynamics and nanoscale material properties, we have developed cathodoluminescence imaging with low electron exposure (CILEE) and cathodoluminescence-activated imaging by resonance energy transfer (CLAIRE). Both imaging methods use cathodoluminescence (CL) microscopy to achieve nanoscale spatial resolution. The main drawback of CL microscopy is damage caused by the relatively high energy electron beam. In CILEE, the electron beam dose is significantly reduced to image samples only moderately robust to the electron beam. In CLAIRE, more fragile samples can be imaged by placing a thin scintillator film between the sample and electron beam. When excited by a focused electron beam, the scintillator film acts as a nanoscale optical excitation source, providing contrast based on interactions between luminescent dopant atoms in the sctillator and the adjacent sample in the near field. In this dissertation, the development of CILEE and CLAIRE are outlined, as well as many examples of uncovering new nanoscale phenomena with both imaging platforms.

Part I of this dissertation, which includes Chapters 2-5. focuses on using CILEE to elucidate the nanoscale structure and dynamic properties of lead halide hybrid perovskites, which are promising materials for optoelectronics. Using CILEE, we reveal a surprising degree of heterogeneity at the surface of hybrid perovskite thin films that differs greatly from the more homogeneous environment found in the bulk. Our CILEE study suggests that solar cells composed of a hybrid perovskite active layer can improve in efficiency by decreasing the heterogeneity through synthetic approaches. We also use CILEE to investigate the process by which mixed halide hybrid perovskites phase separate upon photoexcitation, a process that severely limits solar cell efficiency. Through a combination of CILEE and multiscale modeling, we find that phase separation is driven by polaronic strain in the lattice. Our results represent a new type of nanoscale phase transformation that is unique to hybrid materials. The emergence of CILEE as new approach to non-invasive super-resolution imaging has led to a greater understanding of the complex structure and dynamics in hybrid perovskite materials.

Part II of this dissertation, which includes Chapters 6-11, introduces CLAIRE as a new super-resolution imaging platform designed to image soft materials, such as organic or biological samples. In this dissertation, we describe the production of thin, free-standing scintillator films for CLAIRE and the incorporation of these scintillator films into a functional imaging device. We demonstrate that CLAIRE is capable of imaging soft materials and dynamic processes. The capability of CLAIRE to image biological samples with endogenous chromophores, such as photosynthetic membranes, is also demonstrated.

Together, CILEE and CLAIRE extend non-invasive super-resolution optical imaging to new classes of soft materials that are incompatible with current super-resolution optical imaging approaches and traditional electron microscopy. These new nanoscale imaging methods provide promising opportunities to visualize biological dynamics at high spatial and temporal resolution and to interrogate the nanoscale optical properties of functional optoelectronic materials to understand their fundamental properties, leading to higher efficiency devices.

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