Homogeneous, Core-Shell, and Hollow-Shell ZnS Colloid-Based Photonic Crystals Ian D. Hosein and Chekesha M. Liddell* Department of Materials Science and Engineering, Cornell UniVersity, Ithaca, New York 14853 ReceiVed September 4, 2006. In Final Form: NoVember 23, 2006 Ordered ZnS-based colloidal crystals from homogeneous, core-shell, and hollow building blocks were prepared via electrosteric colloid stabilization combined with a convective assembly technique. The polyelectrolyte stabilized colloids assembled into face-centered cubic arrays with the (111) face perpendicular to the substrate. Structure- property correlations were made using scanning electron microscopy, scanning transmission electron microscopy, and UV/visible/near-IR spectroscopy. Multilayer film growth, with film thickness of several micrometers, was achieved. Optical spectra showed (111) stopgaps along with pronounced higher order peaks. The spectral position of the photonic stopgap can be predicted using a volume average refractive index and the Maxwell-Garnett formula for the homogeneous and core-shell particles, respectively. This work holds the promise of harnessing ZnS for optical property engineering and enhanced photonic band gap materials. Introduction Photonic crystals (PCs) are periodic dielectric structures that tunably inhibit the propagation of light of specific frequency and direction. 1,2 This optical phenomenon is expected to advance technologies in areas such as optical circuits, chemical and biological sensing, photovoltaics, and high reflectivity coatings. Colloid-based photonic crystals are particularly attractive due to the low cost of large area deposition, the diversity of colloid chemistries, and the ease of three-dimensional fabrication using self-assembly methods. Colloidal crystals have been prepared from both polymer and silica spheres, using several techniques including convective assembly, confinement-assisted assembly, and sedimentation. 3 Crystals of these materials possessed incomplete photonic band gaps, 4 but can be used as templates for the infiltration of higher refractive index materials to obtain complete band gaps (inverted opal structures). 5 Modifying the distribution of dielectric materials by incor- porating certain core-shell (i.e., high-index core, low-index shell) architectures into PCs more than doubles the photonic band gap width compared to the traditional inverted opal structure. 6,7 Numerical calculations have also shown that in addition to the complete 8-9 band gap, finely tuned 2-3 pseudogap (stopband) features can be achieved, for example by infiltrating the interstitial pores between hollow ZnS shells of precisely adjusted thickness with a high refractive index medium such as InAs. 8 ZnS colloids of both homogeneous and hollow-shell morphol- ogy have been utilized as building blocks for PCs in the present work. The material possesses a high refractive index 9 (cubic ZnS n 589 ≈ 2.36) and low adsorption in the visible regime. 10 ZnS can also be doped with manganese to induce photoluminescence for optically active photonic materials. 11 Additionally, arrays and crystals of homogeneous ZnS colloids may be useful in nanofabrication for the submicron placement of materials such as carbon nanotubes, semiconductor nanowires, and nanobelts. The metal catalysts needed to grow the electronic materials are often sulfurphilic, and their attachment to self-assembled ZnS colloids could promote more uniform and controllable substrate coverage. However, fabricating colloidal crystals using the self- assembly of ZnS-based colloids is challenging. The high density of the particles causes rapid settling from suspensions. Also, the limited particle stability in aqueous suspensions 12,13 leads to particle aggregation. 14 Velikov et al. demonstrated that a stabilizing shell of silica can be deposited on ZnS colloids in an ethanol solution through the slow hydrolysis of TEOS with ammonia. 15 Thin-film colloidal crystals of the stabilized particles on glass substrates were produced using a vertical deposition technique. 16 Although silica core-ZnS shell particles were synthesized by the same group, 15 assemblies of the particles into colloidal crystals were not reported. Polystyrene (PS)-core-ZnS-shell particles were also function- alized with mercaptoacetic acid and redispersed in a basic potassium hydroxide solution in order to increase the electrostatic charge on the ZnS shell surface. 17 The stabilized particles were convectively assembled at room temperature between two glass slides, slowly withdrawn from the particle suspension at 23 μm/ s. This method produced monolayers with only very limited local order. Stabilization using small molecule adsorption may not have induced repulsive interactions strong enough for the * Corresponding author. E-mail: cliddell@ccmr.cornell.edu; address: 128 Bard Hall, Cornell University, Ithaca, NY 14853; phone: (607) 255-0159, fax: (607) 255-2365. (1) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059-2062. (2) John, S. Phys. ReV. Lett. 1987, 58, 2486-2489. (3) Lopez, C. AdV. Mater. 2003, 15, 1679-1704. (4) Moroz, A.; Sommers, C. J. Phys.: Condens. Matter 1999, 11, 997-1008. (5) Blanco, A.; Chomski, E.; Grabtchak, S.; Isbate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437-439. (6) Busch, K.; John, S. Phys. ReV.E 1998, 58, 3896-3908. (7) King, J. S.; Gaillot, D.; Yamashita, T.; Neff, C.; Graugnard, E.; Summers C. J. Mater. Res. Soc. Symp. Proc. 2005, 846. (8) Yi, G.; Yang, S. J. Opt. Soc. Am. 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Langmuir 2001, 17, 903-907. 2892 Langmuir 2007, 23, 2892-2897 10.1021/la062592q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007