The unique characteristics of localized plasmon resonances in metal nanocrystals include greatly enhanced optical cross-sections and concentration of electromagnetic energy near the metal surface. In particular, colloidal Au nanobipyramids (NBPs) are an intriguing type of metal nanocrystals owing to their attractive, excellent plasmonic properties. Compared with other colloidal Au nanocrystals, Au NBPs are nearly monodisperse and therefore their inhomogeneous spectral broadening is suppressed. In addition, the electric field enhancement exhibited by Au NBPs is much stronger than that of Au nanorods (NRs) with equivalent sizes owing to their much sharper tips. Such strong field enhancements make Au NBPs highly preferable as building units to design structures and devices for various high-performance plasmonic applications. However, it has been a great challenge to produce Au NBPs in high yields, which were typically limited below ~50%. This prevents the full utilization of Au NBPs in various plasmonic applications.We have developed a method for the production of Au NBPs with yields up to 100% and longitudinal plasmon wavelengths tunable from ~650 nm to ~1350 nm by combining seed-mediated growth, Ag overgrowth into long nanorods, and depletion force-induced purification. The Ag segments on highly purified Au/Ag hetero-nanorods were etched away chemically to give Au NBPs having nearly the same shape and size as those obtained from the seed-mediated growth. We compared the extinction cross-sections of Au NBPs and NRs both experimentally and theoretically. Au NBPs were found to have larger extinction cross-sections than Au NRs. Single-particle dark-field scattering measurements indicated that Au NBPs possessed narrower FWHM values. We also compared the refractive index sensitivities and surface-enhanced Raman scattering between Au NBPs and NRs. Our results showed unambiguously that Au NBPs exhibit much better plasmonic properties than Au NRs. Au NBPs are therefore expected to bring largely improved performances in various plasmon-based applications.

Monodisperse gold nanobipyramids with yields approaching 100% and their plasmonic properties

247th National Meeting (2014)

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