Ultrafast spectroscopy beyond the diffraction limit: elucidating charge and lattice interactions with individual grain boundaries

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Fundamental understanding and effective engineering of electrical and thermal transport processes in materials is important, as the efficiency of our energy infrastructure and the reliable performance of electronic devices, the transportation and the power systems are significantly affected by these processes. Polycrystalline materials consisting of microscale crystal grains separated by grain boundaries are an important class of materials widely used in thin-film solar cells, high-performance thermoelectric modules and electronic and photonic devices. How electrical and thermal energy is transferred across the grain boundaries strongly impacts the macroscopic transport properties of polycrystalline materials and device performance. Currently, a detailed understanding of these microscopic processes is still lacking. The overarching goal of this project is to develop theoretical and experimental techniques to elucidate local energy transport processes across individual grain boundaries. Specifically, the research team will use ultrafast optical and electron spectroscopies to examine how these processes are affected by the morphology, structure and disorder of the grain boundaries. This project supports educational activities to advocate renewable energy technologies to K-12 and undergraduate students through hands-on class projects and short classes. To promote diversity in the renewable energy workforce, the research team also provides research opportunities to high school and undergraduate researchers from underrepresented minority communities. A thorough understanding of the structure-property relationship of functional materials with complex microstructures has been a longstanding goal for modern materials science. Grain boundaries are among the most common microstructures with significant influence on the macroscopic properties. Despite the paramount importance of grain boundaries in functional materials, most previous studies have focused on the average effect of a macroscopic ensemble of grain boundaries and/or under static and equilibrium conditions. This project aims to address this challenge by combining state-of-the-art first-principles simulations with ultrafast optical and electron spectroscopy with high spatial-temporal resolutions. Specifically, the research team will examine individual grain boundaries in multicrystalline silicon, polycrystalline perovskites and nanostructured thermoelectric materials regarding their local interactions with electrons and phonons and their impact on the macroscopic electrical and thermoelectric transport properties. This project provides a systematic understanding of how individual grain boundaries affect local electron and phonon transport properties, especially under dynamic and non-equilibrium conditions. This knowledge can enable transformative opportunities to build functional materials with a bottom-up approach, "one grain at a time".This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.