Optical characterization and modeling of nanoporous gold absorbers fabricated by thin-film dealloying

Abstract

This work studies the optical reflectance of nanoporous gold (NPG) thin films of varying pore volume fraction (PVF) synthesized by chemical dealloying of Ag-Au alloy precursors. The fabricated samples are characterized by scanning electron microscopy, and spectral hemispherical reflectance is measured with an integrating sphere. The effective isotropic optical constants of NPG with varying PVF are modeled for the wavelength range from 0.4 to 1.6 μm using the Bruggeman effective medium theory. As the thickness of the NPG thin films is more than ten times larger than the effective penetration depth, the spectral reflectance is simply modeled with the Fresnel coefficients at the interface of air and semi-infinite NPG with different incident angles and polarizations. Consistent with the modeling results, the optical measurement data shows that the spectral normal reflectance of NPG significantly decreases with larger PVF values in the near-infrared regime. On the other hand, the reflectance increases greatly only within visible range at larger oblique angles for transverse-electric polarized waves compared to transverse-magnetic waves. Moreover, the NPG samples demonstrate good thermal stability from room temperature up to 100 °C with little changes in the temperature-dependent spectral hemispherical reflectance.

In Situ Temperature Dependent Optical Characterization and Modeling of Dealloyed Thin‐Film Nanoporous Gold Absorbers

“…Detailed explanation of the integrating sphere setup and the measurement techniques can be found from ref. [ 21 ] . Figure A shows the measured near‐normal hemispherical reflectance of the NPG samples before and after heating.…”

“…Residual silver present in the samples after the dealloying process was quantified using EDS analysis. The apparent pore volume fraction (APVF) was calculated in the following manner: [ 21 ] where V pores is the volume of the air pores, V total is total volume of the sample, is the initial volume fraction of the silver in the silver–gold alloy, and is the residual silver by volume in the NPG film. The derivation of this formula is given in the Supporting Information.…”

“…The derivation of this formula is given in the Supporting Information. Note that this formula does not account for the film shrinkage associated with a certain sacrificial alloy content during dealloying, the shrinkage values are 9–17% according to literature [ 21 ] and, thus underestimating the true pore volume fraction. The pore diameter, pore area fraction, residual silver content, and APVF for all 5 fabricated NPG samples are listed in Table 1 .…”

“…However, unlike nanoparticles, nanoporous metals are only metastable and understanding its morphology evolution during sintering and laser matter interactions remains relevant for both its synthesis and applications.Room temperature surface morphology and optical properties of NPG have been extensively studied. [18][19][20][21] A recent study on NPG thin films of varying pore volume fractions at room temperature showed that spectral hemispherical reflectance significantly decreases and redshifts with larger pore volume fraction values in the visible and near-infrared regime [21] Another study found mesoporous gold films to exhibit unique optical response across all near-infrared wavelengths with non-metallic characteristics by their refractive index (n) and weak metallic character by their extinction coefficient (k). [19] For potential use in high temperature applications including thermophotovoltaics, knowledge of its porosity and surface morphology, temperature dependent optical properties and its temporal evolution is required.The optical properties of bulk metal materials at elevated temperatures have been previously studied via in situ monitoring of reflected or transmitted light.…”

Optoelectronic analysis of spectrally selective nanophotonic metafilm cell for thermophotovoltaic energy conversion

“…A bright contour band with the close-to-unity absorption due to the Fabry-Perot resonance exists selectively at the same photon energy range (0.77 ~ 0.87 eV) for both polarizations, and it does not change with the incident angles indicating a quasi-diffuse behavior. Note that the absorption band is broadened at large incidence angles only with TM waves because of the Brewster effect [34,35].…”