Structural Colors - Engineered Biomimicry - page 281

FIGURE 11.18 (a) Colored stacks of 55-nm-diameter latex spheres observed by a microscope in reflected light. To enhance
the colors, a gold-coated glass substrate was used. Each color corresponds to a stack of a specific thickness. The reference
bar is 100 μ m. Courtesy of Late Ceco Dushkin (University of Sofia, Bulgaria) [71] . Reproduced with permission of the
American Chemical Society. (b) SEM showing a tetragonal bilayer (in the center) between a hexagonal monolayer (on the
right) and a hexagonal bilayer (on the left) of latex spheres of diameter 144 nm. The reference bar is 1 μ m. Courtesy of Late
Ceco Dushkin (University of Sofia, Bulgaria) [99] . Reproduced with permission from Springer. (For interpretation of the
references to color in this igure legend, the reader is referred to the web version of this topic.)

microscopy. The scanning electron micrograph
shown in Figure 11.18 b reveals a narrow band of
tetragonal close packing between two consecu-
tive multilayers of hexagonal close packing. This
narrow band produces its own color, determined
by the effective thickness of the layer with
tetragonal structure. For example, a band of yel-
low color produced by a tetragonal four-layer
could be observed between green hexagonal tri-
layer and red hexagonal four-layer stripes. By
changing the packing geometry in an array, one
can alter the color. The stepwise change in color

with each layer in such ordered stacks could be
used for creating and recording of color images.
In contrast, latex spheres with diameters
(1.5-10 μ m) larger than the wavelength in the
visible regime self-assemble as two-dimensional
colloidal crystals and function as Bragg reflec-
tors. When illuminated with white light, they
produce iridescent colors that depend on the size
and orientation of the domains of the hexagonal
structure shown in Figure 11.19 [98, 99] . The
domain size is critical for the brightness of the
iridescence; indeed, polycrystalline films with

TABLE 11.1 Colors of dried multilayers of latex spheres of diameter (55 ± 4) nm assembled on
gold-coated glass [71] .
Number of
Layers

Layer Thickness
(nm)

Color in
Reflected Light

Number of
Layers

Thickness
(nm)

Color in
Reflected Light

1 47 Ochre 7 277 Magenta

2 85 Brown 8 316 Blue-purplish

3 124 Navy blue 9 354 Green

4 162 Sky blue 10 392 Yellow-green

5 201 Yellow 11 431 Orange

6 239 Orange 12 469 Red