Optical Response of Metallic Nanoparticle Heteroaggregates with Subnanometric Gaps

The optical response of metallic nanoparticle heteroaggregates with well-defined, subnanometric interparticle gaps is studied both theoretically and experimentally for clusters formed by nanoparticles of different size and/or different material (Au and Ag). The optical spectra of the aggregates can be understood in terms of excitation of chain-like plasmon modes, which are associated with short and long linear chains that constitute the cluster internally. The dependence of the optical properties of the aggregates on the size of the metallic nanoparticles, the interparticle gaps, and the dielectric environment is revealed in model electromagnetic calculations. A rigid molecular linker, cucurbit[5]uril, is used to experimentally form Au heteroaggregates of different sized particles, as well as heteroaggregates of silver and gold. It is shown that in both types of heteroaggregates, the ratio of the two different nanoparticles provides a versatile and easily controllable way to tailor the dominant long-wavelength chain mode. Experimental tracing of the optical response of such heteroaggregates in time during aggregation supports the formation of long-wavelength modes, which can be attributed to excitation of long chains inside the clusters.