Cubic architectures on the nanoscale: The plasmonic properties of silver or gold dimers and the catalytic properties of platinum-silver alloys
Bordley, Justin Andrew
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This thesis explores both the optical and catalytic properties of cubic shaped nanoparticles. The investigation begins with the sensing capabilities of cubic metal dimers. Of all the plasmonic solid nanoparticles, single Ag or Au nanocubes exhibit the strongest electromagnetic fields. When two nanoparticles are in close proximity to each other the formation of hot spots between plasmonic nanoparticles is known to greatly enhance these electromagnetic fields even further. The sensitivity of these electromagnetic fields as well as the sensitivity of the plasmonic extinction properties is important to the development of plasmonic sensing. However, an investigation of the electromagnetic fields and the corresponding sensing capabilities of cubic shaped dimers are currently lacking. In Chapters 2-5 the optical properties of cubic dimers made of either silver or gold are examined as a function of separation distance, surrounding environment, and dimer orientation. A detailed DDA simulation of Au–Au and Ag-Ag dimers oriented in a face-to-face configuration is conducted in Chapter 2. In this Chapter a distance dependent competition between two locations for hot spot formation is observed. The effect of this competition on the sensing capabilities of these dimers is further explored in Chapters 3 and 4. This competition originates from the generation of two different plasmonic modes. Each mode is defined by a unique electromagnetic field distribution between the adjacent nanocubes. In Chapter 4 the maximum value of the electromagnetic field intensity is investigated for each mode. Notably the magnitude of the electromagnetic field is not directly proportional to its extinction intensity. Furthermore, the sensitivity of a plasmonic mode does not depend on its extinction intensity. The sensitivity is rather a function of the magnitude of the electromagnetic field intensity distribution. Also, the presence of a high refractive index substrate drastically affects the optical properties and subsequent sentivity of the dimer. In Chapter 5 the sensing properties of a cubic dimer is investigated as a function of orientation. As the separation distance of the nanocube dimer is decreased the orientation of the dimer drastically affects its coupling behavior. The expected dipole-dipole exponential coupling behavior of the dimer is found to fail at a separation distance of 14 nm for the edge-to-edge arrangement. The failure of the dipole-dipole coupling mechanism results from an increased contribution from the higher order multipoles (eg. quadrupole-dipole). This behavior begins at a separation distance of 6 nm for the face-to-face dimer. As a result, the relative ratio of the multipole to the dipole moment generated by the edge-to-edge dimer must be larger than the ratio for the face-to-face orientation. In the last section of this thesis the catalytic properties of cubic nanoparticles composed of a platinum-silver alloy are investigated. The catalytic activity and selectivity towards a given reaction is intimately related to the physical and electronic structure of the catalyst. These cubic platinum-silver alloys are utilized as catalysts for the oxygen reduction reaction (ORR). A maximum enhancement in the specific activity (3.5 times greater than pure platinum) towards the ORR is observed for the cubic platinum-silver alloy with the lowest platinum content. This activity is investigated as a function of the physical structure of a cubic shaped catalyst as well as the electronic modifications induced by the formation of a platinum-silver alloy.