Development of new methods for the synthesis of plasmonically-active precious metal rods and shells
- Publication Type:
- Thesis
- Issue Date:
- 2011
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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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