Effects of the Number of Satellites on Surface Plasmon Coupling of Core-Satellite Nanoassemblies

Abstract

Noble metal nanoparticles have attracted much attention due to their unique optical properties, represented by surface plasmon resonance (SPR). The resonance frequency of the surface plasmon varies with the size, shape, material, and local environment of the nanoparticles. Another appealing means to tune the optical response of the nanoparticle systems is assembling nanoparticles, instead of modifying individual nanoparticles. Coupling of surface plasmons of nanoparticles in close proximity shifts the SPR band of the assembly from that of the individual nanoparticles. The shift naturally depends on the extent of the coupling, and thus on the detailed structure of assemblies such as interparticle distance, allowing for an infinitely fine control of the optical properties. Therefore, assembling nanoparticles in a controlled fashion is of paramount importance. Recently, we developed a novel method to prepare coresatellite nanoassemblies with well-defined and easily controllable structural parameters in a colloidal dispersion state. Selective desorption of large gold nanoparticles (AuNPs) against small AuNPs from glass substrates by sonication led to the production of ultrapure nanoassemblies. The nanoassemblies consist of large core AuNPs decorated by small satellite AuNPs with a distance defined by self-assembled monolayers (SAMs) of alkanedithiol linkers. Different lengths of the alkanedithiol linkers allowed us to measure the surface plasmon couplings between the AuNP core and satellites as a function of the interparticle distances. In complex nanoclusters, the surface plasmon coupling also depends on the number of nanoparticles that constitute the nanoclusters. For example, the surface plasmon coupling band continuously red-shifts as more nanoparticles are assembled in one direction. This is intuitively understood as the red-shifting longitudinal surface plasmon mode supported by elongated nanostructures. Contrary to linear assemblies, core-satellite nanoassemblies can be regarded as multiples of a size-asymmetric heterodimer consisting of a core and a satellite. In this simplified picture, the addition of satellites should not influence the surface plasmon coupling frequencies, although it will increase the intensity of the coupling. In this respect, it is intriguing to experimentally explore the surface plasmon coupling of the core-satellite nanoassemblies as a function of the number of satellites. The new assembly method we developed enables facile control over the number of satellites in the nanoassembly. The following process was used for the production of coresatellite nanoassemblies: (1) surface functionalization of glass substrates with amine, (2) adsorption of core AuNPs (50 ± 5 nm) on the glass substrate, (3) thiol-functionalization of the adsorbed core nanoparticles using 1,10-decanedithiol (DDT) SAMs, (4) adsorption of satellite AuNPs (13 ± 1 nm), and (5) selective desorption of the core-satellite nanoassemblies into ethanol by sonication. The immersion time in step 4, allowed for the satellite AuNPs in solution to interact with the core AuNPs on the glass substrate, determines the number of satellites covering the core nanoparticles (Figure 1(a)). Transmission electron microscopy (TEM) images of more than 100 final core-satellite