Colloidal crystals
are excellent model systems to study
self-assembly
and structural coloration because their periodicities coincide with
the wavelength range of visible light. Different assembly methods
inherently introduce characteristic defects and irregularities, even
with nearly monodisperse colloidal particles. Here, we investigate
how these imperfections influence the structural coloration by comparing
two techniques to obtain colloidal crystals. 3D colloidal crystals
produced by convective assembly are well-ordered and periodically
arranged but show microscopic cracks. (2+1)D colloidal crystals fabricated
by stacking individual monolayers show a decreased hexagonal order
and limited crystal registration between single monolayers in the z-direction. We investigate the optical properties of both
systems by comparing identical numbers of layers using correlative
microspectroscopy. These measurements show that the less ordered (2+1)D
colloidal crystals exhibit higher reflected light intensities. Macroscopic
reflection integrating all angles shows that the reflected light intensity
levels out with an increasing number of layers, whereas incoherent
scattering increases. Although both types of colloidal crystal show
similar angle-dependent color shifts in specular reflection, the less-ordered
structure of the (2+1)D colloidal crystal scatters light within a
larger angular range under diffusive illumination. Our results suggest
that structural coloration is surprisingly robust toward local defects
and irregularities.
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