Plasmonic Substrates for Mechanistic Investigations of Surface-Enhanced Raman Spectroscopy

For many of Surface-Enhanced Raman Spectroscopy (SERS) applications, the enhancing substrate must exhibit a number of critical properties that include low-cost, robustness, and reproducibly high enhancement over large areas. Fabrication of optimized plasmonic substrates in the form of immobilized nanorod assemblies (INRA) for SERS is presented in this dissertation. Included are high-resolution images of the surface nanostructures, along with a mechanistic description of their growth. It is shown that by varying the fabrication method, the localized surface plasmon resonance (LSPR) is tuned between 330 nm and 1840 nm. Also presented are predicted optimal microsphere sizes for the commonly used SERS laser wavelengths of 532 nm, 633 nm, 785 nm, and 1064 nm. The work demonstrates quality performance of large-area, broad-plasmon substrates on large ensembles of non-resonant surface-bound molecules.;The SERS fundamental enhancement factor of AgINRA substrates is investigated as a function of both the dielectric sphere diameter (310 nm to 780 nm) and the input laser wavelength (532 nm to 1064 nm) with a technique called plasmon-sampled Surface-Enhanced Raman excitation spectroscopy (PS-SERES). Tuning the LSPR in this manner allowed an interrogation of the electromagnetic enhancement (EM) effect as a function of λmaxi separation (Δλ, see Figure 1.9) from the ideal EM-predicted spectral wavelength.;Higher enhancement factors (EFs) were measured as the plasmon resonance and excitation wavelength's relative separation was optimized and both moved toward the infrared region, ultimately eclipsing the 108 mark. This is the highest EF to date measured on this type of large-area substrate. The enhancement factors reported here are the result of efficient coupling between free space photons and the surface plasmon states in the metal INRA substrate.;Normal and Surface-Enhanced Raman spectra for a set of substituted benzenethiols were measured experimentally and calculated from static polarizability derivatives determined with Time-Dependent Density Functional Theory (TDDFT). Both silver and gold cluster-thiolate complexes were studied to investigate how the chemical enhancement varies with substituent. The experimental relative peak intensities and positions are well matched by their theoretical counterparts. It is found that the experimental enhancement varies by ∼10x with chemical substitution; with stronger electron donating groups on benzene leading to higher enhancements.;Finally, an application is demonstrated with the development of near-infrared Surface-Enhanced Raman spectroscopy (NIR-SERS) for the identification of eosin Y, an important historical dye. NIR-SERS benefits from the absence of some common sources of SERS signal loss including photobleaching and plasmonic heating, as well as an advantageous reduction in fluorescence, which is beneficial for art applications. This work also represents the first rigorous comparison of the enhancement factors and the relative merits of two plasmonic substrates utilized in art applications; namely citrate-reduced silver colloids and metal Immobilized Nanorod Assembly (INRA) substrates.