No abstract

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 ≈60%. Herein, a method for producing Au NBPs with number percentages approaching 100% and longitudinal plasmon resonance wavelengths synthetically tuned from ≈700 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.

Gold nanocrystals and nanoassemblies have attracted extensive attention for various applications, including chemical and biological sensing, solar energy harvesting, and plasmon-enhanced spectroscopies, due to their unique plasmonic properties. It is of great importance to prepare shape-controlled Au nanocrystals with high monodispersity over a large range of sizes. In this work, Au nanospheres with sizes ranging from 20 nm to 220 nm are prepared using a simple seed-mediated growth method aided with mild oxidation. As-prepared Au nanospheres are remarkably uniform in size. The resultant Au nanospheres of different sizes are ideal building blocks for constructing plasmonic nanoassemblies. Core/satellite nanostructures are assembled out of differently sized Au nanospheres with molecular linkers. The core/satellite nanostructures show a red-shifted plasmon resonance peak in comparison to that of the Au cores, which is consistent with the results calculated according to Mie theory. As predicted by fi nite-difference time-domain simulations, the assembled core/satellite nanostructures exhibit strongly enhance Raman signals. This facile growth of Au nanospheres and assembly of core/satellite nanostructures are expected to facilitate the design of new nanoassemblies with desired plasmonic properties and functions.Adv. Optical Mater. 2014, 2, 65-73 66 wileyonlinelibrary.com

Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, e.g., plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry-Pérot interference, and Rayleigh-Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine, and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved.

Moderate-refractive-index dielectric nano-spheres are found to possess strong electric and magnetic dipole resonances in the visible region. Owing to the overlap of the electric and magnetic dipole resonances, moderate-refractive-index dielectric nanospheres exhibit directional forward scattering at the strongest scattering peak. Such directional scattering is experimentally observed on colloidal Cu2O nanospheres, which are readily prepared through wet-chemistry methods.