Insights on Plasmon-Driven Chemical and Photophysical Processes Using Surface Enhanced Raman Spectroscopy: From the Steady State to the Ultrafast

Abstract

Plasmonic materials can interact with light in unique ways to produce energy-rich nanoscale environments. This energy can be harvested by nearby molecules to drive chemical reactions in ways that were previously not possible. However, due to the small size and fast timescale of plasmon decay, the processes governing plasmon-driven chemistry are difficult to study and therefore poorly understood. In my thesis work, I have used surface-enhanced Raman spectroscopy (SERS) and other analytical tools to better understand the latent mechanisms involved in these light-driven processes, revealing insights into spatial control, charge transfer, and new applications with these exciting materials. In this thesis, we will first discuss my SERS investigations of plasmon-driven chemistry in the steady state, where we observed a plasmon-mediated methyl rearrangement of N-methylpyridinium to 4-methylpyridine. In this work, we highlight both the ability of plasmonic materials to drive complex/selective reactions and identify potential methodsfor their simple optical control. In later work, we will explore my application of ultrafast SERS in the first direct observation of plasmon-mediated electron transfer from a molecular perspective. Importantly, by correlating ultrafast SERS with spectroelectrochemical techniques, we were able to quantify the electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules on timescales relevant to plasmon decay. Finally, we examine my investigation of a continuous wave upconversion process with plasmonic copper selenide nanocrystals. Our initial studies suggest that a plasmon-driven thermal mechanism likely plays a role in this unexpected photoluminescence. Lastly, I will describe a number of prospective directions for research on plasmonic systems. We will consider potential routes toward better understanding plasmon-driven chemistries by applying more robust statistical analyses, developing SERS imaging techniques, and investigating the contrasting impacts of pulsed and continuous wave irradiation on plasmon excitation. In conjunction with ultrafast SERS, these characterization techniques have realistic potential to inform the design of future plasmonic photocatalysts.