Plasmon Coupling in Clusters Composed of Two-Dimensionally Ordered Gold Nanocubes

Gold nanocubes are assembled into clusters of varying numbers and ordering on indium tin oxide substrates. The plasmon coupling in the clusters is studied with both dark-field imaging and finite-difference time-domain calculations. Generally, as a cluster becomes larger and more asymmetric, it exhibits more scattering peaks towards longer wavelengths. The coupling of the vertically oriented dipole in the nanocube with its image dipole in the substrate generates two scattering peaks. One is fixed in energy and the other red-shifts with increasing cluster size. The coupling of horizontally oriented dipoles among different nanocubes produces multiple scattering peaks at lower energies. Their positions and intensities are highly dependent on the number and ordering of nanocubes in the cluster. Au nanocubes in the clusters are further welded together by thermal treatment. The scattering peaks of the thermally treated clusters generally become sharper. The lower-energy scattering peaks arising from dipolar oscillations are red-shifted.