Experimental approach to the fundamental limit of the extinction coefficients of ultra-smooth and highly spherical gold nanoparticles

Abstract

The theoretical extinction coefficients of gold nanoparticles (AuNPs) have been mainly verified by the analytical solving of the Maxwell equation for an ideal sphere, which was firstly founded by Mie (generally referred to as Mie theory). However, in principle, it has not been directly feasible with experimental verification especially for relatively large AuNPs (i.e., >40 nm), as conventionally proposed synthetic methods have inevitably resulted in a polygonal shaped, non-ideal Au nanosphere. Here, mono-crystalline, ultra-smooth, and highly spherical AuNPs of 40-100 nm were prepared by the procedure reported in our recent work (ACS Nano, 2013, 7, 11064). The extinction coefficients of the ideally spherical AuNPs of 40-100 nm were empirically extracted using the Beer-Lambert law, and were then compared with the theoretical limits obtained by the analytical and numerical methods. The obtained extinction coefficients of the ideally spherical AuNPs herein agree much more closely with the theoretical limits, compared with those of the faceted or polygonal shaped AuNPs. In addition, in order to further elucidate the importance of being spherical, we systematically compared our ideally spherical AuNPs with the polygonal counterparts; effectively addressing the role of the surface morphology on the spectral responses in both theoretical and experimental manners.

DNA Origami‐Guided Assembly of the Roundest 60–100 nm Gold Nanospheres into Plasmonic Metamolecules

“…However, imperfect control over structural features resulting from the randomly faceted surface of 30–50 nm Au NSs was still problematic . Generally, the synthesis of relatively large Au NSs (>30 nm) depends on the so‐called seed‐growth methods; Au atoms reduced in the presence of seeds tend to grow into randomly faceted and polydispersed Au NSs due to thermodynamic effects . A few nanometers of error in plasmonic molecular assemblies such as clustered metamolecules and superlattice metamaterials is nontrivial because it can drastically change the electromagnetic coupling between Au NSs .…”

Influence of Morphological Homogeneity of Superspherical Gold Nanoparticles on Plasmonic Photothermal Heat Generation

“…1) The UV–vis absorbance spectra in Figure S10 (Supporting Information) provided A values of 0.996 for the S‐GNSs and 0.925 for the O‐GNSs. 2) The extinction coefficients of S‐GNS (44.5 nm) and O‐GNS were 9.59 × 10 9 (obtained from our previous report) and 6.9 × 10 9 (obtained from the Certificate of Analysis provided from by Nanopartz) m −1 cm −1 , respectively. 3) The laser pathway was fixed to 1 cm.…”

“…Preparation of GNS Solutions for PPT Experiments : The concentrations of each solution were exactly calculated as follows using the Beer–Lambert equationwhere A is the particle absorbance; b is the path length; and ε is the extinction coefficient. The A , ε, and b needed for the calculation were obtained as follows.…”

“…Fortunately, we have established a synthetic method for ultrasmooth and highly spherical GNPs by growing Au octahedra, then slowly etching the Au atoms at the corners and edges . These ultrasmooth and highly spherical GNPs have exhibited an unprecedented high quality with regard to several spectral measurement properties, including reproducible scattering, Fano resonance, extinction coefficients, and strongly optical magnetic polarizability . Hence, we believe that these GNPs are ideally appropriate for revealing how the morphological homogeneity (i.e., shape and size distribution) of GNPs can affect the PPT heat generation.…”

Light‐Directed Soft Mass Migration for Micro/Nanophotonics

“…Consequently, various 2D and 3D plasmonic metamolecules and optical antennas can be developed by AFM assembly . Recent advances in a chemical route for the synthesis of the roundest AuNPs have further improved the accuracy (a resolution of a few nanometers) and reliability of this AFM assembly method . Benefitting from this technological advance in AFM assembly, 32 roundest 60 nm Au nanospheres (NSs) were individually positioned at differently programmed locations.…”