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Supporting information for

Optical Plasmons of Individual Gold Nanosponges

Cynthia Vidal1, Dong Wang

2, Peter Schaaf

2, Calin Hrelescu

1*, and Thomas A. Klar

1

1 Institute of Applied Physics, Johannes Kepler University Linz, 4040 Linz, Austria

2 Institute of Materials Engineering and Institute of Micro- and Nanotechnologies MacroNano®,

Technische Universität Ilmenau, 98693 Ilmenau, Germany

* calin.hrelescu@jku.at

KEYWORDS plasmonics, mesoporous gold nanoparticles, localized surface plasmon resonance,

nanoporosity, metamaterial.

In

Histograms and Autocorrelation Functions of Nanosponges on a Silicon Wafer

Figures S1 (a) show the histogram of the nanosponges’ diameter on a silicon wafer and the

autocorrelation function indicating the average distance between each nanosponge for two

samples fabricated with 4 nm Au/8 nm Ag (upper plots) and 6 nm Au/12 nm Ag (lower plots).

The fixed Au/Ag ratio (1/2) insures a similar average ligament size between the two samples.

The different Au and Ag thicknesses influence the average particle diameter and particle to

particle distance on each sample. The nanosponges in Figure 2 (b) and (c) were taken from the

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sample fabricated with 4 nm Au/8 nm Ag and the ones in Figure 2 (a) and (d) from the sample

fabricated with 6 nm Au/12 nm Ag. For the data corresponding to the particles in Figure 1, see

Figures 3 and 4 in 1.

Figure S1. (a) Histogram of nanosponge diameter distribution. (b) Radially averaged

autocorrelation function of the induced particles fabricated with 4 nm Au/8 nm Ag (upper

graphs) and 6 nm Au/12 nm Ag (lower graphs). Arrows indicate the corresponding characteristic

nanosponge spacing for the nanosponges considered in Figure 2.

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Unpolarized White-Light Scattering Spectra of Single Nanosponges

In Figure S2 we present additional experimental data for a different nanosponges batch than

the nanosponges presented in the Figure 2 in the main text. In order to correlate the morphology

of the individual nanosponges with their scattering spectra, we performed the same set of

characterization as described in the main text. Figures S2 (a)-(e) show the SEM images (left) of

spheroidal Au nanosponges and their corresponding unpolarized scattering spectra (right). The

presented nanosponges have similar dimensions (between 190 nm and 200 nm in diameter) to the

nanosponges presented in the main text.

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Figure S2. (a)-(f) SEM images of ca. 200nm nanosponges (left) and corresponding unpolarized

scattering spectra (right). The nanosponges measure (a) 190 nm, (b) 195 nm, (c) 175 nm, (d)

190 nm and (e) 200 nm in diameter.

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Polarization Dependent Scattering Spectra of Single Nanosponges

Figures S3 (a1) and (b1) repeat the SEM images and the corresponding polarization dependent

scattering spectra (a2) and (b2) as in Figure 3 in the main text. Figure S3 (a3) and (b3) show the

polarization dependent scattering spectra for more polarization directions than given in the main

text. The double arrows in the SEM images represent the 0° polarization axis. The polarizer axis

was rotated clockwise in 20° steps. In Figures S3 (c) and (d), additional examples with diameters

of 170 nm and 195 nm, respectively, are presented.

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Figure S3. SEM images (column 1) and polarization dependent scattering spectra (column 2

and 3). (a1,2) and (b1,2) as in Figure 3, (a3) and (b3) including more polarization directions. (c)

and (d) are additional examples.

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Polarization dependent and unpolarized scattering spectra of single nanosponges

Figure S4 shows the polarization dependent scattering spectra of the nanosponges in Figure 3

with their corresponding unpolarized scattering spectra as well as the scattering spectra obtained

by summing all the polarization scattering spectra. The scattering spectra obtained by summing

up the polarization spectra show maxima at the same spectral position as the unpolarized spectra

However, the relative intensities of the different maxima are more pronounced for the summed

spectra than in the case of the unpolarized spectra. This is due to depolarization effects induced

by the high numerical aperture dark field condenser used for the measurements, i.e. in each

polarization dependent spectrum there is the same contribution originating from the relatively

strong longitudinal (z) polarization.2 Thus, summing up all the polarization dependent intensity

spectra leads to artificially multiplying the same contribution by the number of polarization

dependent spectra. In contrast, unpolarized detection registers the square of the interfering fields

of all polarizations, which is different from the sum of the intensities of different polarizations.

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Figure S4. Single nanosponges from Figure 3, their corresponding polarization dependent

scattering spectra (left), unpolarized scattering spectra (middle) and scattering spectra obtained

by summing all the polarization contributions (from 0° to 180° in steps of 20° i.e. 10 spectra)

(right).

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Numerical Modeling of Single Nanosponges

Figure S5 shows the calculated unpolarized scattering spectra of single model nanosponges.

The nanosponges were modelled as gold half-spheres of 195 nm in diameter on an ITO substrate.

Each nanosponge has its own, individual distribution of air spheres of 24 nm in diameter. In

Figure S5 (a) black spectrum, the unpolarized spectrum of a plain (without air spheres) Au half

sphere of 195 nm in diameter on ITO is shown for comparison. For the unpolarized spectra, the

individual scattering cross sections were calculated for a normal incidence, plane wave excitation

and different incident polarizations from 0°-165° (in steps of 15°) and summed up. The colored

spectra in Figure S5 (a)–(f) correspond to individual model nanosponges of 195 nm in diameter,

where the distribution of the air spheres throughout the whole gold half-sphere is different in

each case, given by the random generator. The calculated spectra are in accordance to the

experimental findings displayed in Figure 2 in the main text and in Figure S2.

Figure S6 shows the calculated polarization dependent scattering spectra as in Figure 4 in the

main text for more incident polarization directions. As in the main text, for the Figure S4 (a), the

air spheres are distributed randomly throughout the whole gold half-sphere mimicking a

nanosponge. In Figure S6 (b) the air spheres are distributed only on the surface of the gold half-

sphere forming surface dents, while the interior is solid gold. In Figure S6 (c), only a porous

interior was mimicked, the air spheres are distributed only in the interior of the gold half-sphere

while the surface is smooth. An additional example for a different random distribution of the air

spheres but the same nanosponge diameter (195 nm) is shown in Figure S7.

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Figure S5. Calculated unpolarized scattering spectra of single model nanosponges of 195 nm in

diameter (colored lines). In each of the nanosponges, (a) till (f), the distribution of the air spheres

throughout the whole gold half-sphere is different. The calculated unpolarized scattering spectra

of a gold half-sphere of 195 nm in diameter without air spheres is displayed as the black

spectrum in (a). Comparison of the unpolarized spectrum (orange) and polarized spectra (c)

(magnified six times). (f) Comparison of the unpolarized spectrum (violet) and polarized spectra

(magnified six times).

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Figure S6. Schematics of the model nanosponges (left) and calculated scattering spectra (i.e.

scattering cross section) from Figure 4, with more different incident polarizations (15° steps)

(right). (a) Nanosponge, (b) nanoparticle with surface dents but with solid interior and

(c) nanoparticle with porous interior and a smooth golden surface .

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Figure S7. Additional example of calculated polarized scattering spectra for (a) Nanosponge,

(b) nanoparticle with surface dents but with solid interior and (c) nanoparticle with porous

interior and a smooth golden surface, where the distribution of the air spheres differs from the

one presented in Figure S4. As in Figure S4 the diameter of the gold half-sphere was 195 nm.

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(1) Wang, D.; Schaaf, P. Nanoporous Gold Nanoparticles. J. Mater. Chem. 2012, 22, 5344.

(2) Bahlmann, K.; Hell, S. W. Electric Field Depolarization in High Aperture Focusing

with Emphasis on Annular Apertures. J. Microsc. 2000, 200, 59–67.