Production of Monodisperse Gold Nanobipyramids with Number Percentages Approaching 100% and Evaluation of Their Plasmonic Properties

Gold nanobipyramids (NBPs) and nanorods (NRs) are two common types of elongated colloidal plasmonic metal nanocrystals, with their longitudinal plasmon wavelengths synthetically tunable over a wide spectral range. Au NBPs have sharper tips and narrower shape and size distributions than Au NRs. However, the number percentages of Au NBPs have been limited below approximate to 60%. Herein, a method for producing Au NBPs with number percentages approaching 100% and longitudinal plasmon resonance wavelengths synthetically tuned from approximate to 700 to approximate to 1200 nm is reported. This method relies on a stepwise combination of seed-mediated growth, Ag overgrowth, depletion force-induced self-separation, and final chemical etching of Ag. The obtained Au NBPs have the same shapes and sizes as the directly grown ones. Systematic comparisons of the plasmonic properties between the purified Au NBP and high-yield single-crystalline Au NR samples show unambiguously that Au NBPs are superior to Au NRs in terms of the plasmon peak width, refractive index sensitivity, figure of merit, two-photon photoluminescence, and surface-enhanced Raman scattering.