Optical properties of silver nanocrystals self-organized in a two-dimensional superlattice: Substrate effect

Experimental and calculated reflectivity spectra of silver nanocrystals self-assembled in compact hexagonal networks on various substrates [highly oriented pyrolytic graphite (HOPG), gold, silicon, and ${\mathrm{Al}}_{0.7}{\mathrm{Ga}}_{0.3}\mathrm{As}]$ are compared. The calculated spectra are deduced from a model based on a mean-field theory and taking into account the electromagnetic resonances for p (parallel) and s (perpendicular) polarizations. From experimental and calculated spectra, it is concluded that the major change in the responses is due to the refractive index of the substrate. Hence, the optical properties of coated silver nanocrystals organized in a hexagonal network do not depend on the substrate used. That is to say no specific interactions between nanocrystals and substrate take place. Collective optical properties due to the formation of a film made of 5-nm silver nanocrystals are observed. This is pointed out by the appearance of an additional resonance at a higher energy than that of the plasmon resonance of isolated nanocrystals.