Light Splitting in Nanoporous Gold and Silver

Abstract

Nanoporous gold and silver exhibit strong, omnidirectional broad-band absorption in the far-field. Even though they consist entirely of gold or silver atoms, these materials appear black and dull, in great contrast with the familiar luster of continuous gold and silver. The nature of these anomalous optical characteristics is revealed here by combining nanoscale electron energy loss spectroscopy with discrete dipole and boundary element simulations. It is established that the strong broad-band absorption finds its origin in nanoscale splitting of light, with great local variations in the absorbed color. This nanoscale polychromaticity results from the excitation of localized surface plasmon resonances, which are imaged and analyzed here with deep sub-wavelength, nanometer spatial resolution. We demonstrate that, with this insight, it is possible to customize the absorbance and reflectance wavelength bands of thin nanoporous films by only tuning their morphology.

Unveiling Nanometer Scale Extinction and Scattering Phenomena through Combined Electron Energy Loss Spectroscopy and Cathodoluminescence Measurements

“…In the color maps, the color and intensity of each pixel can be related to the energy and amplitude of a locally measured resonance. 42 These maps clearly show that two main excitations, located at the tips (lower energy excitation, yellowish color) and at the sides (higher energy excitation, greenish color) of the nanoprism, are measured in EELS, whereas only the low energy one is observed in CL. It is worth noting that such maps can be generated on the fly right after acquisition without resorting to the more lengthy procedures required for a quantitative analysis described below.…”

Surface Plasmon Damping Quantified with an Electron Nanoprobe

“…(c) Overlaid, colour-coded EELS maps from the three dominant plasmon modes, with in the legend their Q and T 2 values. On the EELS maps, the STEM annular dark field image of the nanocross is projected in grayscale40. Locations I–IV are indicated on the map, 1–2 nm off the tip of the arms.…”

Using corrosion to fabricate various nanoporous metal structures

“…Electron tomography which holds great promise for uncovering complex nanostructures because of its excellent spatial resolution down to the subnanometer scale, has recently been successfully applied to investigate the internal structure of nanoporous metals [20,[41][42][43]. For example, Chen and coworkers [20] used a computer controlled STEM equipped with a high-tilt specimen holder to investigate the morphology of np-Au.…”