Transmission electron microscopy of conjugated polymers using energy-filtering and phase contrast enhancement

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PART 1: NON-TECHNICAL SUMMARYThe properties of various polymeric materials depend on the structure at the nanoscale. As a consequence, characterizing the arrangement and order of polymers at nanometer length scales is crucial for the continued development of new materials. Much progress in this area is due to the application of advanced microscopy techniques (transmission electron microscopy) to complex polymeric systems in order to resolve the structure at very high resolutions. New developments in electron microscopy instrumentation are pushing the limits of what is possible, but these advances have not been tailored for the microscopy of polymers, which are very sensitive to electron beams and undergo rapid damage. This project will develop electron microscopy tools to image the structure of electrically conducting polymers used in a variety of emerging electronic applications, such as transistors and solar cell devices. By demonstrating new approaches for resolving the morphology with sub-nanometer resolution, this research will advance the fundamental science of how the chemical composition of polymers leads to the microstructure and aid in the development of polymers for flexible electronics applications. Furthermore, integrated educational and research activities will also include recruitment and retention of students from underrepresented groups. The investigators involved in this project will further support diversity in engineering education by offering mentorship and research experiences to undergraduate students. PART 2: TECHNICAL SUMMARYThis project will develop advanced electron microscopy tools for the study of polymeric materials. New methods to generate contrast in the electron microscope are warranted because of the complexity of the microstructure of conjugated polymer systems. The proposed work begins by examining radiation damage in the transmission electron microscope using two observables: diffraction intensities and low-loss electron energy-loss spectroscopy. These measurements will guide further experiments and establish limits of imaging modalities. Low-loss energy-filtered transmission electron microscopy will generate high-resolution images of long-range order in conjugated polymers. Phase plates will enhance contrast in both bright-field and focused-probe modes to enable imaging of polymer/fullerene mixtures, polymer blends, and block copolymers used in photovoltaics. Furthermore, charge transport heterogeneity will be examined using low-loss spectrum imaging to see deviations in density of states under bias, and by imaging local variations in electric fields that result from charge heterogeneity using a phase plate. Overall, this proposal will demonstrate the utility of monochromated electron sources and phase plates for imaging of conjugated polymers. Educational objectives are motivated by the need to enhance diversity in engineering. A number of specific programs will be implemented encompassing recruitment, retention, and mentoring of underrepresented groups.