Coherent Light Scattering from Semicontinuous Silver Nanoshells near the Percolation Threshold

We report on measurements of visible extinction spectra of semicontinuous silver nanoshells grown on colloidal silica spheres. We find that thin, fractal shells below the percolation threshold exhibit geometrically tunable plasmon resonances. A modified scaling theory approach is used to model the dielectric response of such shells, which is then utilized to obtain the extinction cross section in a retarded Mie scattering formalism. We show that such spherical resonators support unique plasmon dynamics: in the visible there is a new regime of coherently driven cluster-localized plasmons, while crossover to homogeneous response in the infrared predicts a delocalized shell plasmon.

Figure 1

(a) Scanning electron micrograph of $1\mu \mathrm{m}$ diameter silica sphere coated with 20 nm—thick silver shell. Scale bar is 500 nm. (b) Digitally processed (see text) magnified view of region inside the square in (a). Scale bar is 200 nm. Silver appears white and dielectric inclusions are black. (c) Histogram of $p$, generated from 80 images similar to (b).

Figure 2

(a) Extinction of sparsely coated $1\mu \mathrm{m}$ diameter spheres (TEM inset) showing a dipolar plasmon resonance at 420 nm. (b) Measured extinction (dots) and fitted AK model using MG theory (line) for 70 nm thick shells (SEM inset).

Figure 3

(a) Comparison of normalized, averaged EMT (blue), and ST (red) extinctions with measured spectra (dots). Also shown is the extinction obtained using the tabulated bulk silver dielectric function (black). (b) Normalized extinction spectra of two typical thin-shell samples with same core-shell size. Inset: Spectra after subtracting computed background functions. (c) Comparison of background-subtracted, averaged extinction for EMT (blue), ST (red), and a typical data set (black). All samples had $1\mu \mathrm{m}$ diameter cores and 20 nm thick shells.

Figure 4

Calculated extinction for ST (solid line) and EMT (dashed line) for $p=0.55$, 20 nm thick silver shells on $1\mu \mathrm{m}$ diameter silica cores.