Synthesis, modelling and characterisation of gold nanoparticle colloidal crystals
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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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