Morphological and interaction effects on the surface plasmon resonance of metal nanoparticles

Abstract

The influence of the morphology and separation of Ag nanoparticles (NPs) on the position of the surface plasmon resonance has been studied. The NPs have been produced on amorphous a-Al₂O₃ surfaces by means of alternate pulsed laser deposition in a layered structure. The films finally produced consist of five layers containing the NPs with spacing layers of a-Al₂O₃, the space between the NPs also being filled-in with a-Al₂O₃. The in-plane dimensions of the NPs have been varied in the 2–12 nm range while their shape changes from that of spheres to that of oblate ellipsoids. The in-depth separation of the layers containing the NPs was varied in the 8–33 nm range. Above an average in-plane dimension threshold of 4.6 nm, the optical absorption of these nanostructured films exhibits two resonances that shift red and blue in respect to that observed in smaller NPs. The results clearly show that this threshold is well above that at which the NPs become oblate spheroids and it is instead close to the threshold for in-plane coalescence among adjacent NPs. The results also show that multipolar electromagnetic interactions among NPs can be neglected for separations larger than 4.4 ± 0.7 nm but they become important for coalesced NPs.