Date on Master's Thesis/Doctoral Dissertation

8-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Zamborini, Francis

Committee Member

Grapperhaus, Craig

Committee Member

Baldwin, Richard

Committee Member

Powe, Aleta

Committee Member

Sumanasekera, Gamini

Subject

Gold; Nanoparticles; Nanostructured materials

Abstract

With the development of a strategy to correlate the dark-field light scattering spectra of individual nanostructures with scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the same nanostructures, we were able to investigate several interesting optical properties of Au nanoplates (NPs) and asymmetrically-coupled Au nanospheres (NSs) attached to Au NPs with a high level of control. The light scattering spectra of the NP/NS coupled structures depend strongly on the location of NS attachment on the NP. Attachment of multiple NSs at the edge/vertex sites leads to a unique synergistic effect. In contrast to the uniform distribution of NSs, asymmetric distributions of multiple NSs attached to the sides of a NP result in complex, broadened, multi-peaked spectra with larger plasmonic shifts. Simulations using the discrete dipole approximation (DDA) method verified all of the experimental results. The positive shift in the dipolar plasmon mode of the NP/NS assembly relative to the original NP increases with increasing NS size for those attached on the side of the NP in the order of 9±2 nm, 24±4 nm, and 98±16 nm for the 13, 24, and 51 nm average diameter NSs, respectively. For a NS attached to the top terrace of a NP, the shift in the dipolar plasmon mode is 1±1 nm, 3±1 nm, and 14±4 nm for the 13, 24, and 51 nm NS, respectively, and the spectra become more broad. The attachment of a Au NS to a hexagonal or circular Au NP through a cysteamine (Cys) linker shows different light scattering properties compared to attachment through 4-aminothiophenol (4-ATP). The shorter length of Cys leads to stronger dipolar plasmon coupling along the long axis of the NP/NS structure. This leads to a larger red-shift compared to linking with 4-ATP. The geometric shape of the NPs dramatically affects their sensitivity to refractive index changes in the environment and sensitivity to the attachment of a Au NS. The sensitivity of λmax to a change in the refractive index of the environment followed the order of triangles > hexagons > circles. This research provides new fundamental information and a better understanding of shape-dependent optical properties and plasmon coupling of asymmetric metallic nanostructures with potential use in three-dimensional spatial sensing and other plasmonic applications.

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