- Understanding the UV-Visible-NIR scattering of non-spherical metallic nanoparticles. SW Prescott and P Mulvaney. 19th Conference of the European Colloids and Interfaces Society, Geilo, Norway, September 2005.
The unusual optical properties of some nanoparticles (e.g. gold nanoparticles) are well known, while recent synthetic advances have provided a new range of particle morphologies and correspondingly different scattering spectra. Recent experiments have demonstrated the synthesis of non-spherical gold and silver nanoparticles. Particle morphologies that have been synthesised include hemispherically capped cylinders (nanorods), cubes, triangular prisms and hexagonal prisms [1-4]. The techniques utilised for these syntheses include various electrochemical reactions, the use of particular surfactant systems, and the addition of trace amounts of other metals. These novel structures have interesting optical properties, with the scattering behaviour in the UV-visible-NIR region depending on the orientation of the nanoparticle with respect to the incident light, as well as the polarisation of the incident light; for example, aligned gold nanorods illuminated with polarised white light appear either red or blue depending on the direction of polarisation. While understanding the scattering spectra of these nanoparticles is non-trivial, it is an essential step in the development of optical applications for these materials. Approaches to simulating the UV-visible-NIR scattering spectra of a range non-spherical metallic nanoparticles will be discussed, with a view to understanding the relationships between particle shape and experimental spectra. Of particular interest is the application of numerical approaches such as the Discrete Dipole Approximation . It will be seen that the Discrete Dipole Approximation approach readily reproduces the major features of the scattering spectra for a range of particle morphologies at least qualitatively. In the case of gold nanorods, the wavelength at which the maximum extinction occurs can be accurately predicted for both the transverse and longitudinal scattering modes, with the longitudinal mode being tuneable based on the aspect ratio of the rods. The scattering behaviour of other particle morphologies (triangular prisms, hexagonal prisms etc) and approaches for improving the quantitative agreement between experimentally determined and simulated spectra will also be discussed.
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