Advances in Fundamental Single Molecule Studies with Surface-Enhanced Raman Spectroscopy

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Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for characterizing molecular systems. It combines the chemical selectivity of vibrational spectroscopy with plasmonic signal enhancement to achieve the ultimate limit of detection--a single molecule. By overcoming the effects of ensemble averaging, single molecule SERS (SMSERS) probes distributions in molecular interactions and dynamics. Deeper insight into phenomena normally obscured by ensemble averaging is attainable, including the principle mechanisms of SERS, the behavior of single molecules (SM) on plasmonic surfaces, and site-specific chemistries. The research reported in this dissertation contributes to the advancement of the fundamental understanding of SMSERS. First, an introductory review of SMSERS is provided, encompassing the early development and current status of the technique. Second, a critical analysis of the isotopologue and bianalyte SM proofs is presented. This includes experimental considerations and the proposal of more rigorous thresholds for reliably verifying SM detection. Third, the SM capability of discrete Ag nanopyramids fabricated by nanosphere lithography is proven. These nanoparticle arrays are a reproducible alternative to the commonly used, chemically-synthesized nanoparticles which are inherently polydisperse in shape, size, and aggregation state. In addition, they provide the first example of SMSERS without nanogaps. Fourth, the potential use of SM anti-Stokes SER scattering to measure temperatures of plasmonic junctions is investigated. It was demonstrated that SM temperature measurements with SERS are complicated by uneven enhancement of the anti-Stokes and Stokes SER scattering. Necessary considerations for reliably measuring temperatures with SERS are covered. Fifth, the generality of the relative intensity fluctuations between vibrational modes in SMSERS is explored. It was found that the signal fluctuations are not the result of surface diffusion and are wavelength dependent, occurring when excited on molecular resonance. Finally, this dissertation concludes with a review of recent applications and a discussion on future directions for SMSERS. All of the aforementioned research studies have contributed to the central goal of transitioning SERS into a robust technique for studying SM chemistry, allowing us to resolve the complexity of molecular interactions.

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  • 02/19/2018
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