Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Physics & Astronomy

First Advisor

Zahra Fakhraai

Second Advisor

Nader Engheta

Abstract

Plasmonic nanoparticles are a powerful and versatile tool for molecular sensing, drug delivery, and cancer treatment. When exposed to incident light, these nanoparticles have greatly increased far-field scattering and near-field enhancement. Spiky gold nanoshells are a recently developed class of nanoparticles composed of sharp gold spikes decorating a polystyrene core. Spiky nanoshells are synthesized using the templated surfactant-assisted seed growth method, which enables extensive control of the nanoparticle morphology. Here, it is shown that these particles have a tailorable far-field resonance, extremely uniform single-particle surface enhanced Raman scattering, and modal interference in dark-field microscopy measurements. Finite-difference time-domain simulations are performed to determine the morphological features which control these unusual behaviors. Additionally, a T-matrix method was developed to use finite-difference time-domain simulations to analyze mode mixing in these particles. These studies show that the lengths of spikes are critical in determining the far-field scattering peak. Additionally, simulation of the electric field near the particle surface show that the near-field Raman surface enhancement is dominated by the quadrupole modes, resulting in Quadrupole Enhanced Raman Scattering. Due to the large number of spikes, the near-field enhancement is relatively insensitive to variations in individual spikes, resulting in emergent homogeneity in optical properties due to heterogeneity in the structure. The disorder induced asymmetry of the spiky nanoshell enables mode-mixing between the dipole and quadrupole modes, which is observed experimentally in dark-field measurements and predicted theoretically in a T-matrix analysis of finite-difference time-domain simulations. This mode mixing was found to be of the order of 5% between the quadrupole and dipole modes. Such mode mixing is responsible for the broadening of the quadrupole modes towards the infrared and for the activation of all six quadrupole moments, partially explaining how heterogeneity can result in reliable and robust near-field enhancement.

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