Dipole-dipole plasmon interactions in gold-on-polystyrene composites.

Red-shifting of the optical absorption spectra of aggregates of gold nanoparticles by dipole-dipole interactions is of considerable interest, both for theoretical reasons and because the phenomenon can be potentially exploited in various applications. A convenient and practical way to control the effect is to assemble the aggregated ensemble of n gold nanoparticles on the outer surface of larger dielectric spheres. Here, we show by experiment and calculation how the spectra of these structures can be systematically morphed from that of isolated gold particles, through the regime of broad absorption dominated by particle-particle interactions, and finally to the limiting case of a continuous nanoshell. The experimental data were produced using the process of deposition-precipitation, which provides a facile method to decorate polystyrene microspheres with gold nanoparticles. There is no need for prior functionalization of the microsphere surface in our method of deposition-precipitation. Calculations were carried out using a code based on the discrete dipole approximation (DDA). The spectra were dominated by three effects. These were a peak absorption at about 540 nm produced by the conventional plasmon resonance of spherical gold nanoparticles, a broad absorption in the range 600-900 nm caused by diverse dipole-dipole interactions between particles, which strengthened as the number of attached gold particles increased and finally, when n was large, an absorption peak due to the onset of nanoshell-like resonances. The experimental spectra could be successfully fitted by spectra calculated using combinations of these effects.