Gold Nanocube-Nanosphere Dimers: Preparation, Plasmon Coupling, and Surface-Enhanced Raman Scattering

We explore plasmon coupling and surface-enhanced Raman scattering (SERS) from nanogaps defined by different surface shapes. To this end, we assemble heterodimers where gold nanocubes (AuNCs, edge length 84 nm) are linked to gold nanospheres (AuNSs, diameter 55 nm) through their vertices or edges via 1,8-octanedithiol (C8DT). Regioselective disintegration of trimethylammonium bilayers on the vertices and edges of AuNCs using acetonitrile permits the formation of AuNC-AuNS dimers in high yield. Strong plasmonic interactions between the AuNCs and AuNSs produce the longitudinal plasmon coupling band at 790 nm, significantly red-shifted from the surface plasmon resonance band of the isolated AuNCs (lambda = 581 nm) or AuNSs (lambda = 534 nm). Localized electric fields confined to the nanogaps between the AuNCs and AuNSs also generate a strong SERS signal. We observe the Raman spectrum for C8DT from the AuNC-AuNS dimers with a 1 x 1010 enhancement factor (EF), which is much larger than that for nanoassemblies consisting of only AuNSs, such as core-satellites (EF = similar to 10(8)) and clusters (EF = similar to 10(7)). Comparison with finite difference time domain simulations reveals the nature of the plasmon coupling and the local field enhancement in the AuNC-AuNS dimers.