Date of Award

12-2009

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Dr. Ramakrishna Guda

Second Advisor

Dr. Asghar Kayani

Third Advisor

Dr. Alvin Rosenthal

Fourth Advisor

Dr. Yirong Mo

Keywords

Plasmonics, LSPR, hot spots, mie theory, discrete dipoleapproximation, nanoparticles

Abstract

Localized surface plasmons resonances (LSPRs) in metallic nanoparticles (NPs) arise from the interactions between incident light and conduction electrons and have attracted enormous research interest in recent years both for their fundamental nature as well as applications in interdisciplinary areas of sciences such as biological imaging, plasmonic photo-thermal therapy, photovoltaics, and plasmonic sensors. LSPRs are strongly localized and depend on the shape, size, the composition of the NPs, the polarization direction of the incident light, refractive index (RI) of the surrounding medium as well as on the chemical environment that surrounded NPs. Although significant research has progressed both theoretically and experimentally, several questions need to be answered, including regarding the quantum size effects and the effect of the surface passivating layer on the LSPR. The research work carried out in this study is to understand the plasmonic properties of metal nanoparticles in different environments.

Firstly, we introduced a package written in MATLAB to describe discrete dipole approximation (DDA) method for computing optical properties of arbitrary-shaped NPs. This is the first package written in MATLAB language that implicates Biconjugate Gradient and one-dimensional Fast Fourier Transform techniques to reduce the computational cost of the DDA calculation significantly. This study also represents an algorithm to run the DDA in graphics processing units, which reduces the computational time by almost one order of magnitude. One aspect of LSPR that did not attract much research attention is for plasmons of small plasmonic NPs in size range from 2 to 10 nm. In this study, we investigated the influence of the size, RI of the medium, and chemical ligand effect on the plasmonic properties of the spherical quantum-sized silver NPs using both quantum and classical models. Also, ligand and size effect on the electron-phonon (e-p) relaxation dynamics of the thiolate-protected gold (Au) clusters was unraveled. From the studies, it was shown that aromatic passivating ligands deaccelerate e-p relaxation dynamics of the Au clusters more in comparison to an aliphatic ligand.

Binding molecules on the surface of the NPs change the RI of the surrounding medium that results in shifting LSPR wavelength. This plasmonic shift can be used to detect biological molecules. In this study, we explore the LSPR sensitivity of six different hollow-Au nanoshells (sphere, disk, rod, ellipsoid, rectangular block, and prism). The results show that the shell thickness affects the LSPR sensitivity. Also, the study demonstrates that the LSPR sensitivity of the rod shape and rectangular block nanoshells are higher than other structures. In an effort to understand the plasmonic properties of embedded metal nanoparticles, LSPR properties of Au, Ag, and Cu NPs in silica matrix are studied using the DDA. The results show that rod-shaped NPs have higher extinction and produce stronger field enhancement in comparison to the spherical ones. The study also demonstrates that embedded Ag NPs have stronger plasmonic properties than Au and Cu NPs.

Access Setting

Dissertation-Open Access

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