Structural Colors - Engineered Biomimicry - page 282

Identical spheres of diameters between 10 nm
and 1 μ m can be readily assembled into colloi-
dal crystals and synthetic opals that possess
three-dimensional order. The difference between
colloidal crystals and synthetic opals is in the
volume fraction of the constituent spheres. Col-
loidal crystals contain less than 10% v/v con-
stituent spheres, which are highly charged and
arranged on face-centered-cubic (fcc) lattice.
Synthetic opals have a cubic close-packed struc-
ture (also on an fcc lattice) with a packing den-
sity close to 74%, similar to that of natural opal
[102] . In addition, inverse opals can be fabricated
by templating various kinds of precursors
against crystalline arrays of colloidal spheres.
Nowadays, the process of production of large
quantities of exceedingly uniform spheres from
silica and polymer colloids of different chemical
composition (e.g., polystyrene and polymethyl
methacrylate) is routine [102] . Silica colloids
represent one of the best-characterized inor-
ganic systems used for production of monodis-
perse spheres. The self-assembly of colloidal
spheres is a well-developed technique [102, 103] ,
and is both a faster and a cheaper approach for
the fabrication of high-quality three-dimen-
sional structures than the common microfabrica-
tion techniques (deposition, photolithography,
etching, and doping) that require a clean room
[104, 105] . Nanopatterning becomes easy to
implement, as does the fabrication of lithogra-
phy masks, templates to make three-dimen-
sional macroporous materials, and several
photonic devices--including photonic crystals
and diffraction gratings [102] .

FIGURE 11.19 Optical image of a two-dimensional
crystal of diameter 1.4 cm, formed by self-assembly of latex
microspheres of diameter 1.696 μ m on a transparent sub-
strate [99] . Courtesy of Late Ceco Dushkin (University of
Sofia, Bulgaria). Reproduced with permission from Springer.

very small domains appear white. Although the
interference colors from the nanospheres can be
viewed only under an optical microscope, the
diffraction colors from microspheres are observ-
able with an unaided eye. A dry two-dimen-
sional crystal of latex microspheres is
environmentally stable and exhibits colors for
several years.
The process of two-dimensional crystallization
of latex nanospheres on a substrate involves the
evaporation of water from an aqueous suspension
of monodisperse latex spheres of diameter a small
fraction of visible wavelengths, as well as with
microspheres of diameter ranging from 1.5 μ m to
10 μ m [71, 98-100] . This technique, however, does
not allow for controllable growth of a single layer
of particles over large areas.
In the convective-assembly technique [101] , a
glass substrate is pulled out from an aqueous
suspension of monodisperese polystyrene
spheres at an appropriate angle and with a
proper speed to allow the particles to self-
assemble on a regular two-dimensional lattice
over a large area. This technique is applicable to
a broad range of particle sizes (from about
100 nm to several micrometers in diameter),
since each size results in a differently textured
surface.

11.5.2 Bioinspired and Biomimetic
Reproduction of the Morpho Blue
The early attempts to artificially reproduce the
Morpho blue employed self-assembled struc-
tures comprising microspheres and nanospheres
[71, 98-101] . The convective-assembly technique
was used to produce two-dimensional arrays of
polystyrene spheres with diameter comparable