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Timestamp: 2019-04-24 04:54:57+00:00

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The spectral and angular radiation properties of gold-silica-gold multilayer nanoshells are investigated using Mie theory for concentric multilayer spheres. The spectral tunability of multilayer nanoshells is explained and characterized by a plasmon hybridization model and a universal scaling principle. A thinner intermediate silica layer, scaled by particle size, red shifts the plasmon resonance. This shift is relatively insensitive to the overall particle size and follows the universal scaling principle with respect to the resonant wavelength of a conventional silica-gold core-shell nanoshell. The extra tunability provided by the inner core further shifts the extinction peak to longer wavelengths, which is difficult to achieve on conventional sub-100 nm nanoshells due to limitations in synthesizing ultrathin gold coatings. We found multilayer nanoshells to be more absorbing with a larger gold core, a thinner silica layer, and a thinner outer gold shell. Both scattering intensity and angular radiation pattern were found to differ from conventional nanoshells due to spectral modulation from the inner core. Multilayer nanoshells may provide more backscattering at wavelengths where silica-gold core-shell nanoshells predominantly forward scatter.
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Fig. 1. Geometries of (a) silica-gold core-shell conventional nanoshells, and (b) gold-silica-gold multilayer nanoshells.
Fig. 2. Calculated spectra of CNS and MNS with various inner core radii while the silica and outer radius remain the same (Media 1). The red shift of MNS from CNS is indicated by lambda shift.
Fig. 3. Mie calculation results for wavelength shifts from the MNS (R 1/R 2/R 3) vis-NIR plasmon resonance peak relative to that of CNS (R 2/R 3) with the same silica and outer gold radius versus intermediate silica thickness scaled by particle size of MNS. t=(R 2-R 1)/R 3, α=972.43, β=13.48.
Fig. 4. Calculated extinction spectra of R20/30/35 nm MNS immersed in various media with distinct refractive indices.
Fig. 5. Scattering-to-extinction ratio at plasmonic resonant wavelengths of MNS with (a) inner gold core radius kept at 10 nm and varying the silica and outer gold layer thickness, (b) silica layer thickness kept at 10 nm and varying the inner gold core radius and outer gold layer thickness, (c) outer gold shell kept at 20 nm and varying the inner gold core radius and silica layer thickness. Absorption-to-extinction ratio (d) at plasmonic resonant wavelengths of MNS with the overall diameter and outer gold shell thickness fixed and varying the inner gold core radius and silica layer thickness.
Fig. 6. Angular radiation pattern of R90/125/140 nm MNS (blue) and R125/140 nm CNS (green) at (a) 550 nm, (b) 755 nm, (c) 1145 nm, (d) 1270 nm and (e) scattering spectra of MNS (blue) and CNS (green) (Media 2). Nanoshells are located in the center of the plot, and the incident wave enters from the bottom (180°).
Fig. 7. (a) Extinction efficiency spectra for Rx/100 nm CNS with varying silica core radius x. (b) Anisotropy factor plot for corresponding CNS at different wavelengths. (c) Extinction efficiency spectra for Rx/85/100 nm MNS with varying gold core radius x. (d) Anisotropy factor plot for corresponding MNS at different wavelengths.
Table 1. Surface plasmon resonance peak shift of R20/35/50 nm (MNS1), R30/35/50 nm (MNS2), and R35/50 nm (CNS) in different dielectric media (λ p 2>λ p 1).
Surface plasmon resonance peak shift of R20/35/50 nm (MNS1), R30/35/50 nm (MNS2), and R35/50 nm (CNS) in different dielectric media (λ p 2>λ p 1).

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