Multimode Resonances in Silver Nanocuboids

A rich variety of dipolar and higher order plasmon resonances have been predicted for nanoscale cubes and parallopipeds of silver, in contrast to the simple dipolar modes found on silver nanospheres or nanorods. However, in general, these multimode resonances are not readily detected in experimental colloidal ensembles, due primarily to the usual variation of size and shape of the particles obscuring or blending the individual extinction peaks. Recently, methods have been found to prepare silver parallopipeds with unprecedented shape control by nucleating the silver onto a tightly controlled suspension of gold nanorods (Okuno, Y.; Nishioka, K.; Kiya, A.; Nakashima, N.; Ishibashi, A.; Niidome, Y. Uniform and Controllable Preparation of Au-Ag Core-Shell Nanorods Using Anisotropic Silver Shell Formation on Gold Nanorods. Nanoscale 2010, 2, 1489-1493). The optical extinction spectra of suspensions of such monodisperse particles are found to contain multiple extinction peaks, which we show here to be due to the multimode resonances predicted by theoretical studies. Control of the radius of the nanoparticle edges is found to be an effective way to turn some of these modes on or off, These nanoparticles provide a flexible platform for the excitation, manipulation, and exploration of higher order plasmon resonances.