Metal Nanoprisms: Synthesis, Optical Properties, Assembly

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Metal nanoprisms are a novel class of anisotropic nanomaterials with unique optical properties dependent on their size, shape, and composition. Silver nanoprisms are of particular interest because their surface plasmon resonance bands are readily tunable in the visible and near-IR region. The objectives of this thesis research focus on photochemical synthesis of silver nanoprisms, formation mechanism, optical properties, and assembly strategies. Chapter two presents a pH-controlled method for the synthesis of silver nanoprisms through single beam excitation. This development substantially simplifies the prism synthesis by eliminating the need for secondary irradiation, which was previously used to suppress prism fusion. With appropriate pH regulation, one can achieve excellent control over the nanoprism edge length and the corresponding plasmon bands. Chapter three describes a way of assembling silver nanoprisms on amine-terminated substrates. The prism-coated glass substrates exhibit reversible optical changes when repeatedly cycled between the dry and wet states. This optical phenomenon is attributed to the high sensitivity of the prism dipole plasmon band to the environmental dielectric constant Chapter four introduces a novel strategy for coating silver nanoprisms with silica by using 16-mercaptohexadecanoic acid (MHA) as a mediation layer. The resulting prismatic core-shell nanostructures exhibit improved stability and surface functionality as compared with bare silver nanoprisms Chapter five demonstrates the significance of plasmon excitation in the photomediated growth of triangular silver nanoprisms by using gold nanoparticles as plasmonic seeds and reaction labels. This approach yields a novel core-shell nanostructure that contains a spherical gold core with a triangular silver prism shell. The architecture of these structures is tunable by controlling excitation wavelengths and the diameter of the gold core. In chapter six, the photomediated growth mechanism of silver nanoprisms is investigated. This photochemical process is driven by silver redox cycles involving reduction of silver cations by citrate on the silver particle surface and oxidative dissolution of small silver particles facilitated by BSPP. The plasmonic silver particles serve as photocatalysts and mediate the Ag+ reduction by citrate under plasmon excitation. These relatively small silver particles and BSPP act like a buffer in the photoreaction to keep the silver ion concentration constant (~20 μM), which facilitates the Ag deposition on the silver particle surface under photochemical control. Moreover, we have discovered that dipole plasmon excitation favors selective formation of prismatic silver nanostructures and sharp tip triangular nanoprism growth, while quadrupole excitation leads to truncated prism growth.

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  • 08/30/2018
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