Sonochemically produced ZnS-coated polystyrene core-shell particles for use in photonic crystals

Zinc sulfide films were grown on carboxyl-modified polystyrene microspheres (PS-CO2) through sonochemical deposition in an aqueous bath containing zin c acetate and sulfide, released through the hydrolysis of thioacetamide. Th e resulting particles were "optically hollow", due to a large refractive in dex contrast between the core and shell materials. Continuous, uniform film s were obtained after 3-4 h and reached a maximum thickness of 70-80 nm aft er 13 h of growth, as characterized by transmission electron microscopy (TE M). Aggregation was minimized by subsequent modification of the core-shell particles with mercaptoacetic acid to increase their surface charge and pro duce good colloidal suspensions. Oscillations in the optical spectra of dil ute suspensions of the particles were indicative of interference patterns a s expected from Mie light scattering calculations. X-ray diffraction (XRD) patterns match the zinc blende structure of ZnS and indicate a compression in the crystal lattice (alpha = 5.305 +/- 0.037 Angstrom), as compared to t he bulk material (alpha = 5.406 Angstrom). Hollow ZnS shells were formed by annealing the core-shell particles in a thermal gravimetric analysis (TGA) oven, at 400 degreesC. A 34% weight loss was observed upon heating, a valu e approximately equal to that of the polystyrene core. The hollow shells re mained intact and readily resuspended in water. Both core-shell and hollow ZnS particles self-assemble to form well-ordered, hexagonal close-packed la yers.