Synthesis and Characterization of Monodisperse Core−Shell Colloidal Spheres of Zinc Sulfide and Silica

Abstract

We report on a new type of composite particles consisting of a zinc sulfide (ZnS) core and a silica (SiO2) shell or vice versa. We developed and optimized these particles for photonic applications, because ZnS has a large refractive index and does not absorb light in the visible and both ZnS and SiO2 can be easily doped with fluorophores. Both kinds of morphologies were created using a seeded growth procedure using monodisperse seeds on which homogeneous layers with a well-defined thickness were grown. Moreover, the ZnS and SiO2 cores could be completely dissolved leaving SiO2 and ZnS shells, respectively, filled with solvent or air after drying. The particle morphology was investigated by electron microscopy. The optical properties were studied by extinction measurements and angle resolved light scattering and compared to scattering theory.

Photonic crystals of core-shell colloidal particles

“…The calculations were performed using a wavelength dependent effective refractive index. 11 In both cases, the filling fraction of bulk ZnS (in a silica matrix, n = 1.45) in the particle core or shell was 0.62. The high-index core low-index-shell particles display a maximum in the g w at γ ~ 0.70, which is about 20% larger than that of a photonic crystal of homogeneous particles with the higher refractive index.…”

“…Spherical ZnS particles can be made with a wide range of sizes (100 -1500 nm in radius) and high monodispersity. 10,11 Recently, we have shown that a silica layer can be deposited on the ZnS spheres, or vice versa, in order to change the filling fraction of the highindex material and tune the optical properties of the particles. 11 Moreover, ZnS can be doped, e.g., with manganese, 12 to induce luminescence, or, a fluorescent dye can be incorporated into the silica layer at a well-defined radial position.…”

“…10,11 Recently, we have shown that a silica layer can be deposited on the ZnS spheres, or vice versa, in order to change the filling fraction of the highindex material and tune the optical properties of the particles. 11 Moreover, ZnS can be doped, e.g., with manganese, 12 to induce luminescence, or, a fluorescent dye can be incorporated into the silica layer at a well-defined radial position. 13 In addition, pure silica or pure ZnS particles, or shells of these materials, can be used for dielectric doping of photonic crystals in order to create localized modes inside the bandgap.…”

“…The synthesis and optical characterization of the core-shell particles from ZnS and SiO 2 are described elsewhere. 11 The size and polydispersity were determined by transmission electron microscopy (TEM). The wavelength dependent effective refractive index of the ZnS was determined from extinction measurements of a dilute suspension in ethanol.…”

“…The wavelength dependent effective refractive index of the ZnS was determined from extinction measurements of a dilute suspension in ethanol. 11 Thin colloidal crystals were grown onto glass substrates using a vertical controlled drying method. 15,16 This method allows the growth of large (cm) arrays of colloidal crystals with precise control over the crystal thickness.…”