Modeling the optical properties of noble metal nanoparticles, nanoparticle arrays, and molecule-nanoparticle complexes

The optical properties of noble metal nanoparticles have been widely used in the design of chemical and biological sensors. The efficiency of this design relies heavily on understanding the interaction of light with nanoparticles, the interparticle coupling of nanoparticles in arrays, and the interaction of nanoparticles with adsorbed molecules. In this thesis, I present theoretical modeling of these interactions and of the optical properties that result from them.;The extinction spectra of individual silver spheres of different sizes have been studied by both exact Mie theory and a quasi-static treatment. The goal here is to emphasize the usefulness of simple analytical theories for simple shapes (spheres and spheroids) in providing qualitative insight about the effect of particle size. The extinction spectra of one-dimensional linear chains and two-dimensional arrays of silver spheres have been studied using both T-matrix and coupled dipole (CD) methods. The main focus here is to determine the influence of interparticle spacings and array structures on extinction spectra, but the effect of particle size is also considered. The CD approach is shown to capture most of the array effects for spherical particles of 30 nm radius or smaller, and a semi-analytical model is proposed to interpret the simulated results using simple concepts. Both the CD and the semi-analytical approaches have then been applied to model arrays of spheroids, and the simulated spectral variations are in good qualitative agreement with experimental data on arrays of cylinders.;The correlation between enhanced Raman scattering and molecule-nanoparticle interactions has been studied using a recently developed time-dependent density functional theory method, based on a short-time approximation. The method is tested first to simulate both normal and resonance Raman scattering spectra of the uracil molecule, and then employed to study the enhanced Raman scattering of the pyridine-Ag20 complex, the pyrazine-Ag20 complex, and the Ag20-pyrazine-Ag20 junction structures. Enhancements of 105--106 are found for all three model systems, which are comparable to findings on nanoparticles. However, the relative importance of different enhancement mechanisms is found to be different for each system. Moreover, a combination of information about the vibrational motion and the local chemical environment provides a simple picture of why certain vibrational modes are enhanced more than others.