| Surface enhanced Raman spectroscopy (SERS) was originally discovered in the 1970s with the observation that organic molecules adsorbed onto a metal surface exhibit greatly enhanced Raman scattered light intensities when illuminated with a laser source. Enhancements of approximately 10 6 over regular Raman scattering have been commonly observed and proposed applications of SERS-active sensors exist over a wide range of fields, including chemical analysis, healthcare, food safety and national security, spurring an intense scientific interest in the area. More recently, observations of single- molecule SERS have demonstrated enhancement factors greater than 10 13 at random 'hot spots', but so far, these enhancement factors are poorly understood due to lack of reproducibility and lack of methodical characterization of such spots. Theoretical calculations have shown that the dominant field enhancements are specifically localized in the crevices between metal nanoparticles and are strongly dependent on particle morphology, excitation wavelength and, perhaps above all, particle-particle coupling. The focus of this thesis is to address experimentally theoretical predictions by fabricating SERS configurations and to make definitive measurements of the SERS magnitude at interparticle hot spots. In this work, metal nanoparticles have been directed to form ordered arrays exclusively of metal nanoparticle dimers with control over orientation, size and interparticle spacing. In order to achieve unprecedented control of the material and geometric variables, elastomeric substrates were used to change particle-particle distance while holding all other physical parameters constant. This fundamental new approach to hot spot creation has opened doors to a new family of SERS substrates, where the turning on/off of a hot spot is as easy as flipping a switch. Most recently, I have demonstrated the feasibility of this approach with long nanorods that show an outstanding theoretical SERS match with the characteristic polarization dependence expected of such nanostructures. Additionally, this thesis demonstrates the feasibility of creating SERS-active dimers over a large area using a capillary force deposition technique which has further been used to compare the SERS enhancement factors derived from dimers to those of longer linear nanoparticle chains, ultimately demonstrating the practicality of the dimer configuration over more complex nanostructures. |
