Synthesis, modelling and characterisation of gold nanoparticle colloidal crystals
- Publication Type:
- Thesis
- Issue Date:
- 2008
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Three-dimensional, micron-sized colloidal crystals comprised of gold nanospheres
have been synthesised directly from a gold nanoparticle/methyl methacrylate (MMA)
colloid by application of a 514 nm laser at 480 mW. An array of colloidal crystals can be
created by translation of the glass substrate under the laser beam, after two minutes of
irradiation at each site. Control experiments and calculations show that plasmon-induced
localised heating of the gold nanoparticles contributes to the rapid formation of colloidal
crystals.
The effects of particle order and disorder on the optical response of three-dimensional
structures containing 15 nm diameter gold nanospheres are investigated using the T-matrix
technique. Calculations were performed on structures containing up to 163 particles. The
ordered structures produce an additional extinction peak that is not present in the disordered
structures. The position of this additional peak depends upon the inter-particle spacing. In
the disordered structure this peak is therefore missing because the inter-particle spacing is
not well-defined.
The optical response of a simplified array of a one-dimensional chain of 15 nm
diameter gold nanospheres in the regime where the near-fields of the particles are coupled
is investigated using the T-matrix technique. Calculations are performed with chains up to
150 particles in length and with an inter-particle spacing between 0.5 and 30 nm. For
wavevectors perpendicular to the chain axis and longitudinal polarisation the extinction
peak red-shifts as the inter-particle spacing is reduced. The magnitude of the peak-shift is
inversely proportional to the inter-particle spacing, a result that is consistent with the Van
der Waals attraction between two spheres at short range. For a fixed particle gap the
extinction peak tends towards an asymptotic value with increasing chain length, with the
asymptotic value determined by the inter-particle spacing.
A nanoshell geometry that produces maximum absorption efficiency is investigated
using a formulation of Mie theory. The calculated surface heat flux under sunlight (800
W/m²) and laser (50 kW/m²) irradiation is used to determine the temperature of the
nanoshell using a convective heat transfer model. For irradiation by sunlight, the resultant
heat flux is optimised for an 80 nm diameter nanoshell with an aspect ratio of 0.8, while for
irradiation by laser the maximum heat flux is found for 50 nm nanoshells, but with an
aspect ratio of 0.9.
A direct comparison between the absorption efficiencies of geometrically varying
nanoshells and nanorods is performed using a formulation of Mie theory and the Discrete
Dipole Approximation (DDA) technique, respectively. The absorption efficiency produced
by nanorods far exceeds that produced by nanoshells for a constant volume of gold.
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