Collective optical properties of silver nanoparticles organized in two-dimensional superlattices

lthis paper we describe collective properties of silver nanoparticles organ ized in two-dimensional superlattices. Our aim is to show that we can contr ol the state of organization of the silver particles deposited on the subst rate. Then the particles are found in the form of either a well-organized t wo-dimensional array of isolated particles or disordered and coalesced part icles distributed more or less randomly on the surface. The optical spectra are compared with both polarized and unpolarized light. When particles are arranged in a hexagonal array, an asymmetrical and broad peak is observed. Under p-polarized light, a new high-energy peak appears that is interprete d as a collective effect, resulting from the mutual interactions between pa rticles. We support this conclusion from numerical calculations performed o n finite-size clusters of silver spheres, where only the electrodynamic int eractions between the spheres are taken into account. With disordered and c oalesced system the high-energy peak disappears whereas a peak toward low e nergy is observed. This is attributed to coalesced particles. [S0163-1829(9 9)12415-9].