Gold-Dielectric Colloidal Core-Shell Particles for Photonic Applications

Gold 2003
Dave J. van den Heuvel, Hans C. Gerritsen, Alexander Moroz, Alfons van Blaaderen,
Abstract It was shown recently that metallo-dielectric colloidal (MDC) particles are promising candidates to construct photonic crystals with a complete band gap in the visible 1. Moreover, MDC spheres can also improve applications of photonic crystals, like ns optical switches, that do not rely on a strong index contrast. In addition, single MDC particles can enhance optical properties of dyes placed close to these particles, e. g. it has been shown that Raman scattering can be enhanced with factors of 1015 or larger caused by strong local field enhancements that are due to plasmon resonances in the metal 2.
In this contribution we will show how core-shell MDC particles can be made with tunable optical properties 3. The particles consist of a monodisperse silica core, a gold shell and an optional silica outer layer. The silica can be labeled with fluorescent dyes. The silica outer shell allows for tuning of the interparticle interaction potential through a reduction in the Van-der-Waals forces and facilitates functionalization of the particle surface; the variable gold shell around an also variable silica core makes the plasmon resonance of the particles adjustable over the whole visible and infrared region of the spectrum. The properties of the particles can be further modulated by controlled anisotropic deformation with high-energy ion irradiation. It is also possible to turn the MDC particles into hollow metal spheres. Particle characterization by SEM, TEM and extinction measurements will be discussed.
Calculations show that the spontaneous emission factor of fluorescent dye molecules placed inside the MDC particles is a strong function of radial position of the dye and strongly enhanced close to the gold shell. By single particle lifetime imaging spectroscopy it can be shown that that the lifetime of rhodamine molecules placed close to the gold shell is more than 40 % reduced compared to their lifetime in uncovered silica particles.
Finally, results on crystallization of the MDC particles into colloidal photonic crystals induced by an electric field 4 and/or the charge on the particles and the first optical measurements of these crystals will be presented. These photonic crystals were doped with dye-labeled spheres. This allows studying the effect of the photonic crystal on the lifetimes of the dye molecules.
The authors thank Carlos van Kats1 for the Scanning Electron microscopy measurements and Teun van Dillen2, Joan Penninkhof1,2 and Albert Polman2 for the Ion Beam Deformation. This work was financially supported by the Foundation for the Fundamental Research of Matter (FOM), which is part of the Netherlands Organization for Scientific Research (NOW).

(1) Moroz, A. Phys. Rev. Lett. 1999, 83, 5274.
(2) Nie, S. R.; Emroy, S. R. Science 1997, 275, 1102.
(3) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524.
(4) Dassanayake, U.; Fraden, S.; van Blaaderen, A. J. Chem. Phys. 2000, 112, 3851.
Keywords: gold shell particles, anisotropic deformation, hollow gold spheres, photonic crystals, fluorescence lifetime, colloids
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