Optical properties of noble-metal nanoparticles coated with a dye J-aggregate monolayer

We report experimental and theoretical studies of the optical properties of composite nanoparticles consisting of a noble-metal (Ag or Au) core and a cyanine dye J-aggregate shell. Using transmission electron microscopy, the particles have been shown to be spherical in shape and 4 - 10 nm in size. The crystal structure of their core has been determined by 1-keV electron diffraction. The absorption spectra of such nanoparticles in aqueous suspension show two peaks, one due to the plasmon resonance in the metallic core, and the other to electronic excitations in the J-aggregate shell. These results are interpreted in a model based on the expression for the absorption cross section of small particles, using polarisability calculations for two concentric spheres with allowance for the size effect on the dielectric function of the metallic core. The dielectric function of the J-aggregates is derived from experimental data using Kramers - Kronig relations. The shape and position of the peaks in the absorption spectra of the hybrid nanoparticles are strongly dependent on their geometry and the optical properties of the core and shell materials. This offers the possibility of controlling the optical properties of composite materials based on such nanoparticles.