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Visible Frequency Thin Film Photonic Crystals from Colloidal Systems of Nanocrystalline Titania and Polystyrene Microspheres
Author(s) -
Subramania Ganapathi,
Constant Kristen,
Biswas Rana,
Sigalas Mihail M.,
Ho Kaiming
Publication year - 2002
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2002.tb00284.x
Subject(s) - materials science , nanocrystalline material , refractive index , polystyrene , photonic crystal , colloidal crystal , ceramic , grain size , crystallization , micrometer , visible spectrum , optics , composite material , nanotechnology , colloid , optoelectronics , chemical engineering , polymer , physics , engineering
This work describes a simple and novel ceramic processing technique to form periodic ordered structures in ceramic materials with a uniform pore size distribution. This material shows photonic gaps at visible/near‐IR wavelengths. Monodisperse colloidal polystyrene microspheres are self‐organized into a crystalline structure of close‐packed spheres in a suspension of nanocrystalline titania. The nanoparticle titania fills the intersphere region simultaneously during colloidal crystallization. Removal of the polystyrene microspheres by calcination at a temperature of 520°C results in a periodic porous structure with a high refractive index background material. Crystals having ordered regions, a few millimeters across with typical grain sizes of 50–70 μm, are grown as thin films on substrates including glass and silicon. Optical reflectivity measurements indicate peaks at the stop band wavelengths that scale with the pore size. Visual inspection and optical microscopy reveal uniform colored regions for crystals with periodicity comparable to visible wavelengths. Despite the presence of cracks resulting from drying and heat treatment as well as numerous grain boundaries, optical characterization clearly demonstrates a photonic band gap. Reflectance peaks due to a pseudogap can be shifted by application of high pressure. In the following sections we will describe the experimental procedure and discuss optical reflectance and transmission measurements that can reveal information about the crystals, namely, the lattice constant, the refractive index, and the filling fraction of the background material.