Development of new methods for the synthesis of plasmonically-active precious metal rods and shells
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The ability to synthesise metal nanoparticles with various geometries has vastly improved in recent years. The plasmon resonance, the mechanism responsible for the optical response of metal nanoparticles, is highly sensitive to their geometry. This is the primary reason for the current interest in developing syntheses that produce a distinct geometry. In contrast, polydisperse samples of nanoparticles have relatively poorly defined plasmon resonances. Although nanospheres are still the most common geometry of metal nanoparticle synthesised, there is rapidly increasing interest in nanorods and nanoshells on account of their more flexible optical response. Therefore, developing a reliable synthesis for nanorods and nanoshells has been a target of much recent research. Gold is the most popular metal for the synthesis of plasmonically active nanoparticles. In this thesis I present a development of synthesis methods for plasmonically active nanoparticles and a characterisation of the resulting products. In my work I have synthesised gold nanorods, a mixed dispersion of gold nanorings and hollow gold nanoparticles, silver nanorods and platinum nanospheres. To characterise these nanoparticles I have used a range of techniques including UV-Vis-NIR spectrometry, SEM, TEM, cryo-TEM, SAXS and electrodynamics simulations. Early in my work I recognised that gold nanorods provided the best opportunities to achieve large scale applications. Some significant drawbacks in the existing methods of synthesis were identified, such as the inefficient reaction of gold. This realisation led me to focus the majority of my efforts on improving the understanding of the mechanisms involved in the synthesis of gold nanorods and, in particular, on the all-important transition from spherical seed particle to anisotropic rod. The nearest competitor to nanorods, with respect to applications, is nanoshells and so I have also compared these two geometries in the literature review. From the exhaustive work presented in this thesis I present a set of optimum conditions for the synthesis of gold nanorods. Evidence for the disproportionation of gold (I) bromide as the mechanism of gold metal formation in the gold nanorod synthesis is presented. I also show that it is necessary to sacrifice control of the aspect ratio of the nanorods produced in order to improve the efficiency of the reaction. I use a coreductant to show that the formation of nanorods is dependent on the effectiveness of the reductant that is present after the addition of the gold nanoparticle seeds. It is also apparent that it is possible to achieve a range of aspect ratios as well as particle dimensions by varying the amount of seed particles added to the growth solution. I have used a range of experimental techniques including cryo-TEM, SEM, UV-Vis spectroscopy and small angle X-ray scattering to probe the physical dimensions and optical properties of gold nanorods at various stages of their growth and from this I have developed a new growth model. Simulations of the optical properties of the intermediate nanoparticle geometries observed support this new growth model.
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