Asymmetric Seed Passivation for the Synthesis of Bowl-Like Plasmonic Nanostructures

Symmetry dominates the structure and function of the universe. Symmetry dictates the properties of many nanomaterials and nanostructures with low, but still defined, symmetries often display markedly different properties compared to their higher symmetry counterparts. An excellent example of symmetry-dependent properties can be found in the optical properties of Au cuboctahedra of <i>O_h</i> symmetry which display a single, dipolar resonance and Au nanorods with <i>D_nh</i> symmetry which display two dipolar plasmon modes corresponding to the long and short axes of the rods. While numerous routes have proven experimentally promising at systematically reducing NP symmetry, use of surface protecting groups such as silica or collapsed polymer shells have shown promise at restricting growth on NP seeds. Herein, poly(styrene-b-polyacrylic acid) (PSPAA) is used to asymmetrically passivate cubic Au seeds through competition with CTAB ligands. The asymmetric passivation via collapsed PSPAA caused only select vertices and faces of the Au cubes to be available for deposition of new material (<i>i.e., </i>Au, Au-Ag alloy, and Au-Pd alloy) during their overgrowth. The resulting NPs form nanobowl-like structures with some degree of branching, being reduced symmetry guided by the asymmetric seed passivation. Through experiment and simulation, the eminent optoelectronic properties of this class of nanomaterials are probed, with applications such as Surface Enhanced Raman Scattering (SERS) being employed to elucidate the structures’ viability as nanosensing platforms.