Unusual extinction spectra of nanometer-sized silver particles arranged in two-dimensional arrays

Two-dimensional arrays of 1000 Angstrom silver particles were prepared by adsorption from colloidal suspensions onto glass slides derivatized with (3-mercaptopropyl)trimethoxysilane. By variation of the concentration of the colloidal suspensions, slides with different interparticle distances were obtained, and their extinction spectra were measured in water. A blue shift (up to 90 nm) and sharpening of the peak corresponding to the plasmon resonance was observed as the distance between particles decreased to a value comparable to, or less than, their diameter. The angular dependence of the extinction spectra was studied for s- and p-polarized light. The data are interpreted to result from coherent coupling of the plasmon resonances in closely spaced silver particles upon irradiation with light.