Colloidal crystals with tunable colors and their use as photonic papers

This article describes the fabrication and characterization of colloidal crystals whose stop bands could be varied through the application of a liquid. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene beads with a liquid prepolymer to poly(dimethylsiloxane), followed by thermal curing. When a liquid (e.g., a silicone fluid, hexane, or octane) capable of swelling the elastomer matrix was applied to the surface of this crystal, the lattice constant and thus the wavelength of Bragg-diffracted light was increased. For instance, the color of light diffracted from a colloidal crystal made of 175 nm polystyrene beads could be varied from violet to green, orange, and red simply by swelling it with different solvents. On the basis of this mechanism, we further demonstrated a photonic paper/ink system where color patterns could be conveniently generated on the surface of a thin film of colloidal crystal by writing with a Pilot pen, by screen printing, or by microcontact printing with an elastomer stamp. To fully illustrate the potential of this paper/ink system, we have demonstrated the fabrication of photonic papers as large as 75 cm(2) in area, supported on both rigid substrates and flexible Mylar films. By judicially choosing the diameter of the polystyrene beads, it was also possible to adjust the color initially displayed by a photonic paper to any wavelength within the spectral region from ultraviolet to near-infrared. As a result, the photonic papers could be fabricated to appear as colorless while the written patterns displaying a shiny color, or vice versa.