Galvanic Replacement Reaction as a Route to Prepare Nanoporous Aluminum for UV Plasmonics

There is a growing interest in extending plasmonics applications into the ultraviolet region of the electromagnetic spectrum. Noble metals are commonly used in plasmonic, but their intrinsic optical properties limit their use above 350 nm. Aluminum is probably the most suitable material for UV plasmonics, and in this work we fabricated substrates of nanoporous aluminum starting from an alloy of Al2Mg3. The porous metal is obtained by means of a galvanic replacement reaction. Such nanoporous metal can be exploited to achieve a plasmonic material suitable for enhanced UV Raman spectroscopy and fluorescence. Thanks to the large surface to volume ratio, this material represents a powerful platform for promoting interaction between plasmonic substrates and molecules in the UV.


Supporting Note #1 -Chemical Dealloying of melted Al2 Mg3
In order to compare the results obtained from GRR, chemical dealloying in acetic acid 1M (in Methanol) has been performed on the same starting alloy. Different durations of the dealloying have been tested: 30 minutes, 3 hours and 7 hours. The results, in terms of morphology and composition are reported in Fig. S1 and in Table S1. As can be observed the complete removal of Mg is not possible and the oxide level is high (the high level of oxidation makes also hard to collect a high resolution SEM micrograph in the case of the longest dealloying - Fig. S1C).   The XPS analysis (Fig. S3) is affected by the superficial oxidation of the sample, that occurs due to its exposure to the air. After a first XPS measurement, the sample was sputtered inside the XPS machine chamber, to unveil the not-oxidized material under the surface, but apparently the sputtering time was insufficient, yielding an oxidation value that is just slightly lower than the one measured for the as prepared GRR sample.

Supporting Note #3 -Dielectric Constants from Kramers-Kronig relationship
The Kramers-Kronig (KK) relations can be used to derive various expressions connecting the real and imaginary parts of different optical parameters. In the present case, since the only experimental observable was the reflectivity of the NPA films, we employed the KK relations to derive the phase shift ( ) of an electromagnetic wave due to the reflection on the NPA surface. In formulas Recalling that the measured reflectivity is defined by the square modulus of the complex reflection coefficient And the Fresnel equations We can find an expression for Finally, to derive the dielectric function we use = √.
Since the integral in the first equation is performed over the entire frequency range and we are able to measure the reflectivity in a defined frequency window, we have to extrapolate the data in the uncovered range. For the low energy extrapolation, the spectra are extrapolated by the Hagen-Rubens [1], as expected for metallic samples. For the high energy extrapolation, the reflectance spectra are extrapolated as constant up to 25eV (200000 cm -1 ) and above this energy according to the behaviour of free electrons (about ω -4 ). It is important to notice from the integral that regions such as ≪ or ≫ has minor contributions to ( ).

Supporting Note #4 -Fluorescence measurements data
The following table reports the experimental values obtained from fluorescence measurements. The enhancement (E) is calculated with respect to the same measurements performed on a Silicon substrate where no enhancement is expected. The concentration is calculated as described in Method section.

Supporting Note #5 -Additional numerical simulations
Penetration of the norm of the electric field along the vertical direction orthogonal to the surface  . Comparison in penetration depth as predicted with two different calculation approaches. The norm of the electric field is plotted along a vertical line through the NPA region cross section (red arrow). The case of nanoporous and flat geometry are compared at two wavelengths, 260 nm and 350 nm. In both cases, the nanoporous geometry predicts larger penetrations of the electric field in the NPA. Here the penetration (δ and δ for the two numerical approaches) is assumed, for simplicity, the distance from the surface at which the field is decreased to 10% of its incident value.

Thermal response of the NPA substrate
To calculate the approximated thermal response of the NPA substrate, we have mapped the 2D absorption, , at = 260 nm calculated following Joule dissipation, = • , on a 2D axisymmetric geometry which allows us to introduce the realistic circular shape of the impinging beam. The beam size (radius) utilized during the experiment is = 0.5 and the input power is = 25 . We have utilized the available SEM images to reproduce the 3D heat propagation in a 2D axisymmetric geometry. While this approach fully reproduces the porous features only within vertical sections, we believe it represents a fair starting point for thermal evaluations. A full 3D calculation would require many different SEM tomographic sections. In Fig. S5, we report the electric field distribution, heat dissipation and temperature profile of the calculate NPA structure. A final temperature of ~315 K is calculated. The irregularities in the temperature maps are due to the different values of the air ( = 0.025 /( • )) and aluminium ( = 238 /( • )) thermal conductivities which alternate within the structure, according to the same mapping procedure utilized for the structure's optical properties (see equation for ε in the main text).