Ultrasmooth, Highly Spherical Monocrystalline Gold Particles for Precision Plasmonics

Abstract

Ultra-smooth, highly spherical monocrystalline gold particles were prepared by a cyclic process of slow growth followed by slow chemical etching, which selectively removes edges and vertices. The etching process effectively makes the surface tension isotropic, so that spheres are favored under quasi-static conditions. It is scalable up to particle sizes of 200 nm or more. The resulting spherical crystals display uniform scattering spectra and consistent optical coupling at small separations, even showing Fano-like resonances in small clusters. The high monodispersity of the particles we demonstrate should facilitate the self-assembly of nanoparticle clusters with uniform optical resonances, which could in turn be used to fabricate optical metafluids. Narrow size distributions are required not only to control the spectral features, but also the morphology and yield of clusters in certain assembly schemes. KEYWORDS:Gold nanospheres, plasmonics, monodisperse, chemical etching, Fano-like resonance. TOC Graphic 3Accepted version of ACS Nano 7 (12): 11064 (2013) In equilibrium, a nanoscale crystal adopts a polyhedral morphology to minimize its surface free energy. As a result, metallic nanoparticles grown near equilibrium form facets. [1][2][3] Although the plasmon resonances and ease of functionalization 4 make such particles promising 1 for bottom-up assembly of optical resonators 5,6 and isotropic metamaterials, 7 it is difficult to make structures whose optical properties are reproducible from one particle or one multiple-particle cluster to the next since the resonances are often sensitive to features such as sharp corners, 8 number of facets, 9 roughness, 10 and overall size and shape. 11 Moreover, the interparticle optical coupling varies significantly with nanometer-scale changes in gap distance 12 and orientation. 13 The ideal particle for self-assembly of plasmonic structures is therefore not a polyhedron, but a spherical crystal without facets or grain boundaries. However, producing such particles is a materials challenge, since spherical crystals are not stable under any growth conditions. Here we show that a cyclic process of slow growth followed by slow chemical etching, which selectively removes edges and vertices, results in ultra-smooth, highly spherical monocrystalline gold particles. The etching process, which is functionally similar to (but chemically different from) that used to make monocrystalline silver nanospheres for surface-enhanced Raman spectroscopy, 14,15 effectively makes the surface tension isotropic, so that spheres are favored under quasi-static conditions. The resulting spherical crystals display uniform scattering spectra and consistent optical coupling at small separations, even showing Fano-like resonances 16 in small assemblies. The cyclic process we demonstrate could be extended to other metals and, because it is scalable up to particle sizes of 200 nm or more, might be used to create strongly scattering particles for sensors, 17,18 electromagnetic resonators, 7 and oth...

Clusters of polyhedra in spherical confinement

“…The imposed structures are a set of dense motifs that are robust against changes in particle shape. This result has implications for experimental applications in which the fabrication of highly spherical particles is difficult or undesirable, as in the case of several plasmonic applications mentioned earlier (31,33,77).…”

“…In the plasmonics community, for example, recent efforts have focused on the manufacture of highly spherical metallic nanoparticles for the production of plasmonic nanoclusters with consistent and reproducible structure (77). However, faceted geometries are thermodynamically preferred over spherical geometries during the metallic nanoparticle growth process (77,107), which complicates the production of spherical metallic nanoparticles.…”

“…In the plasmonics community, for example, recent efforts have focused on the manufacture of highly spherical metallic nanoparticles for the production of plasmonic nanoclusters with consistent and reproducible structure (77). However, faceted geometries are thermodynamically preferred over spherical geometries during the metallic nanoparticle growth process (77,107), which complicates the production of spherical metallic nanoparticles. We showed here that a host of sphere cluster geometries, including among many others the optically interesting four-particle tetrahedron (27) and 13-particle centered icosahedron (108), are in fact robust against changes in particle shape.…”

Influence of pH-control in phosphoric acid treatment of zinc oxide

“…When used as a cosmetic pigment, both the particle shape and size distribution of the phosphate particles are important. Homogeneous spherical particles are expected to spread well on the skin [9], however overly small particles are unsuitable because the pigments may enter the pores of the skin. Generally, sub-micrometer pigments are used.…”

Structural characterization of electric-field assisted dip-coating of gold nanoparticles on silicon

“…Several techniques have been used to control the growth of metal nano-particles (NP), for instance a slow growth followed by slow chemical etching method is used to produce GNP with a highly spherical shapes for precision plasmonics. 15 Energy dispersive X-ray spectroscopy (EDX) is also used to characterize the film of nano-and microstructure. Fig.7 shows the EDX analysis of the samples.…”