Developments in time-resolved microscopic characterization of excited state dynamics in semiconducting materials

Abstract

Due to their versatile properties, nanocrystal solids and lamellar materials are at the forefront of practical and theoretical research advancements. These materials are essential for applications such as solar cells, photocatalysts, LEDs, artificial photosynthetic devices, and quantum communication technologies. The utility of these materials hinges on the longevity and transport of light-induced high-energy states within them. These energetic states must last and be transported efficiently to emit light, enter an electronic circuit, or facilitate a desired chemical reaction. The material properties influencing the excited state lifetime and transport include ligand properties in nanocrystals and internal electric fields in lamellar materials. In perovskite nanoparticle films, ligands protect the active material from degradation and extend the lifetime of excited states but can inhibit transport. The structure of layered materials can induce internal electric fields that enhance photocatalytic activity. Time-resolved microscopic techniques provide new insights into the excited state dynamics in these material classes. Chapter Two presents a comparative study of ligand systems using pump-probe microscopy, finding that short, conjugated chains with aromatic rings facilitate faster diffusion and excited state decay than long aliphatic chains, suggesting that conjugated ligands may lower transport barriers. Chapter Three explores excited state lifetime imaging of bismuth oxybromide microplatelets, finding that enhanced charge separation extends further from facets than can be explained by surface- mediated phenomena alone. Defects in the crystal lattice, which increase in density as a function of distance from the center, are proposed in Chapter Three to contribute strongly to the enhanced charge separation. To quantify the impact of noise on diffusion measurements, Chapter Four reports simulation results across a broad parameter space and introduces newly released open- source software to help researchers evaluate their accuracy and precision. This work underscores the power of time-resolved microscopic techniques in characterizing excited state dynamics, contributes to standards for measuring and reporting excited state diffusion in semiconducting materials, and provides valuable insights into the complex relationships between structural characteristics and photophysical behaviors in semiconductors.