| The work presented in this dissertation is centered on experimental efforts to maximize the sensitivity of surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) toward their application in ultrasensitive nanoscale chemical analysis. The work focuses on fundamental studies of the plasmonic properties of silver nanostructures, and how those properties affect the SERS enhancement arising from these structures. The first part of the work describes an effort toward exploring the wavelength of highest SERS enhancement for nanoparticle arrays fabricated using nanosphere lithography (NSL). It is concluded that the specific relationship between the incident laser wavelength and the wavelength of the localized surface plasmon resonance is a key feature of the SERS enhancement. It is then demonstrated that the SERS enhancement factor of these nanoparticle arrays is not very sensitive to the height of the particles, when they are excited at the proper laser wavelength. These experiments indicate that the SERS-active properties of NSL-derived nanoparticles are not dominated by the nanoparticle corners and edges. The photoreduction of a self-assembled monolayer on a SERS substrate is studied, and exploited for the development of a new technique for imaging areas of intense SERS enhancement (hotspots) with very high spatial resolution. A new type of SERS substrate, Ag nanostructures coated with a very thin alumina layer by atomic layer deposition, is examined and shown to be promising for a variety of SERS studies. TERS, with a silver-coated AFM tip, is demonstrated for self-assembled monolayers adsorbed to thin metal films, and enhancement factors >104 are achieved. Additionally, TERS is used to study tip-molecule interactions, and it is employed for the first time to create junctions with Ag nanoparticles on a surface. Very large TERS enhancement is observed when the tip is brought into contact with an array of silver nanoparticles. Finally, some efforts towards applying SERS as a rapid, sensitive, and inexpensive sensing modality are presented. |
