Colloidal Crystals Made of Polystyrene Spheroids: Fabrication and Structural/Optical Characterization

This paper describes the fabrication and characterization of colloidal crystals constructed from spheroidal building blocks with well-controlled major-to-minor ratios. Such a crystalline lattice was fabricated by infiltrating an opaline lattice of monodispersed polystyrene spheres with an elastomer precursor such as poly(dimethylsiloxane) (or PDMS), followed by thermal curing and stretching of the composite film at a temperature higher than the glass transition temperature of polystyrene. In this process, the polystyrene spheres were transformed into spheroids through viscoelastic deformation, while the long-range order of this three-dimensional lattice was essentially preserved. Because of the low contrast in the refractive index, the colloidal crystal fabricated in the present work exhibited a stop band (rather than a complete band gap) in the optical regime. The position of this stop band was determined by the diameter of the polystyrene spheres and the elongation ratio of the elastomeric composite. When the crystalline lattice was made of 240-nm polystyrene spheres, the stop band shifted from 594 to 522 nm, as the PDMS film was stretched by an elongation of 130%, and further down to 470 nm, as the incident angle was changed from 0 to 45°. These spectroscopic measurements were in good agreement with the partial band structures calculated using the plane-wave-expansion-method (PWEM). In addition to their use as a model system to investigate the dependence of photonic band structures on the shape (or symmetry) of lattice points, the nonspherical system described here also provides a potentially useful approach to fine-tuning the optical properties of colloidal crystals.