Colloidal
Photonic Crystals toward Structural Color
Palettes for Security Materials
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Abstract
Self-assembly of monodisperse colloidal
particles into regular
lattices has provided relatively simple and economical methods to
prepare photonic crystals. The photonic stop band of colloidal crystals
appears as opalescent structural colors, which are potentially useful
for display devices, colorimetric sensors, and optical filters. However,
colloidal crystals have low durability, and an undesired scattering
of light makes the structures white and translucent. Moreover, micropatterning
of colloidal crystals usually requires complex molding procedures,
thereby limiting their practical applications. To overcome such shortcomings,
we develop a pragmatic and amenable method to prepare colloidal photonic
crystals with high optical transparency and physical rigidity using
photocurable colloidal suspensions. The colloidal particles dispersed
in a photocurable medium crystallized during capillary force-induced
infiltration into a slab, and subsequent photopolymerization of the
medium permanently solidifies the structures. Furthermore, conventional
photolithography enables micropatterning of the crystal structures.
The low index contrast between particles and matrix results in high
transparency of the resultant composite structures and narrow reflection
peaks, thereby enabling structural color mixing through the overlapping
of distinct layers of the colloidal crystals. Multiple narrow peaks
in the spectrum provide high selectivity in optical identification,
thereby being potentially useful for security materials