Absorption properties of the composite silver/dye nanoparticles in colloidal solutions

The results of the experimental and theoretical studies of the optical absorption properties of the composite metal/organic nanoparticles in colloidal solutions are reported. The nanoparticles consist of Ag-core coated with the J-aggregate of 3,3′-disulfopropyl-5,5′-dichlorothiacyanine sodium salt (TC). The absorption spectrum of the composite Ag/TC nanoparticles exhibits two peaks at λ = 395 and 480 nm, which originally come from the localized surface plasmon resonance of Ag nanoparticles and the electronic excitation of TC-dye, respectively. To explain the results obtained we calculate the extinction cross-section of such composite material on the basis of a model for the polarizability of two concentric spheres by taking into account complex frequency-dependent dielectric functions of Ag and the TC-dye and size-dependent effects. The model provides a good description of the main features in the measured absorbance. It is shown that for the core–shell nanoparticles under investigation the peak positions and their intensities are strongly dependent on their geometrical size and the thickness of the J-aggregate coating. This opens a possibility to controllable driving of the absorption properties of such composite materials.