Morphological and interaction effects on the surface plasmon resonance of metalnanoparticles

Abstract

The influence of the morphology and separation of Ag nanoparticles (NPs) on the position ofthe surface plasmon resonance has been studied. The NPs have been produced on amorphousa-Al2O3 surfaces by means of alternate pulsed laser deposition in a layered structure. The filmsfinally produced consist of five layers containing the NPs with spacing layers ofa-Al2O3,the space between the NPs also being filled-in witha-Al2O3.The in-plane dimensions of the NPs have been varied in the 2–12 nm range while theirshape changes from that of spheres to that of oblate ellipsoids. The in-depth separation ofthe layers containing the NPs was varied in the 8–33 nm range. Above an average in-planedimension threshold of 4.6 nm, the optical absorption of these nanostructured filmsexhibits two resonances that shift red and blue in respect to that observed insmaller NPs. The results clearly show that this threshold is well above that atwhich the NPs become oblate spheroids and it is instead close to the threshold forin-plane coalescence among adjacent NPs. The results also show that multipolarelectromagnetic interactions among NPs can be neglected for separations larger than4.4 ± 0.7 nmbut they become important for coalesced NPs.