The size and shape-dependent plasmon absorption of metal nanoparticles is due to the collective oscillation of electrons within the particle. For gold particles these absorption bands span the visible spectrum. Mie theory provides an exact solution for the surface plasmon resonance of spheres. However, for non-spherical particles, the optical spectra have to be calculated numerically using, for example, the Discrete Dipole Approximation (DDA). Within DDA, the target particle is divided into N polarizable point dipoles and the response of each dipole to an electromagnetic field calculated. DDA thus requires knowledge of the exact particle morphology.

Recently, single particle measurements utilising dark-field microscopy have been employed to avoid the effects of polydispersity on the measured ensemble surface plasmon resonance, allowing direct measurement of the homogeneous line width and position of the surface plasmon band of a single metal nanoparticle. The recently reported Focussed Ion Beam (FIB) Registration method allows the optical spectrum of a particle with known size and shape to be determined.1

Here, we present the first theoretical and experimental study of the effects of particle size, morphology and roughness on the optical properties of the same individual metal nanoparticles studied by dark field microscopy and SEM. Knowledge of the particle size and shape allows the construction of a target for DDA calculations, and the calculations can be compared to a single nanoparticle of exactly same size and shape. From the information garnered via SEM and TEM studies, a target consisting of ~ 20,000 dipoles was constructed as a model for the particle. We demonstrate that the surface plasmon peak position is extremely sensitive to tiny changes in morphology and surface roughness.

1) Novo, C., Funston, A. M., Pastoriza-Santos, I., Liz-Marzán, L. M., Mulvaney, P., Angew. Chemie. Int. Ed., 2007, 46, 3517-3520.

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