Size-Modulation of Plasmonic Nanorings Obtained by the Self-Assembly of Gold Nanoparticles and Block Copolymers

Metal nanoparticles exhibit interesting optical properties due to the collective excitation of conduction electrons called the plasmon. Within appropriate metal nanostructures, cooperative plasmon modes appear and the resonance plasmon frequency is modified. This article reports a simple method for the formation of such structures, in the form of self-assembled nanorings. Rings of alkanethiol-capped gold nanoparticles are obtained by the Langmuir–Blodgett technique and a block copolymer (PS-b-P2VP) template. With this approach, organized nanoparticle arrangements covering a large surface area are obtained. Furthermore, geometric parameters such as ring diameter, ring-to-ring separation, and ring width can be systematically varied by the addition of homopolymer or in situ nanoparticle regrowth. Optical extinction spectra recorded for the nanoparticle rings depend both on ring diameter and particle size. In particular, after in situ particle regrowth, the plasmon extinction spectrum exhibits a red-shift that increases with ring diameter. Theoretical spectra generated with the discrete dipole approximation indicate that this spectral shift can be attributed to plasmon coupling that extends over an increasing number of particles as the ring is enlarged.