Optical characterization of nanofabricated silver films: Surface plasmon resonance and surface-enhanced spectroscopy

The optical extinction of two distinct kinds of nanofabricated silver films will be examined: silver film over polystyrene nanosphere films (AgFONs), and periodic particle arrays of silver (PPAs). In the first section of this document we study the optical extinction of AgFONs as a function of nanosphere size and external dielectric medium. We show that these films are excellent substrates for surface-enhanced Raman spectroscopy (SERS) and that they provide additional insight into the electromagnetic mechanism of SERS.;Silver PPAs are used to examine the effect of particle size, shape, and external dielectric medium on the surface plasmon resonance (SPR) spectrum measured via optical extinction spectroscopy. By tuning the size and shape of the nanoparticles, wavelength tunability of the SPR throughout the entire visible region and into the mid-IR region of the spectrum is demonstrated. Spatially-resolved UV-vis extinction measurements show that defect sites present in the film play a negligible role in determining the SPR spectrum. We find that there is a linear relationship between the peak wavelength of the SPR and the refractive index of the external dielectric medium.;Using a numerical implementation of electrodynamic theory known as the Discrete Dipole Approximation (DDA) we are able to quantitatively model the SPR spectra of silver nanoparticles using only the shape, size, and dielectric properties of the nanoparticle, and no adjustable parameters. The shape and size of the nanoparticles are measured with AFM, and the values for the bulk dielectric constants of silver are obtained from the literature.;We have compared the use of silver PPAs to silver island films as substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Large PPAs have a discrete, localized surface plasmon resonance in the mid-infrared region, whereas island films have a broad, non-discrete surface plasmon resonance. We find that when the different surface area coverages by metal in the two different films are taken into account, the PPAs give rise to approximately the same SEIRA enhancement as the island films.