Biomedical Engineering Reference
In-Depth Information
A positive replica can be made by infilling the
void regions of the negative replica and remov-
ing the material of the negative replica
[84]
, but
the delicate nature of butterfly wings has made
it difficult to implement this step.
a few times, resulting in the formation of a one-
dimensional photonic crystal. Although the
Langmuir-Blodgett method is simple, a major
shortcoming until recently has been the
restricted range of materials for which it can be
used.
A related technique is layer-by-layer assembly
[53, 114]
, which is based on the alternating
adsorption of positively and negatively charged
species from aqueous solutions and can also be
used to create one-dimensional photonic crystals.
The roughness, thickness, and porosity of a mul-
tilayer film can be controlled at the molecular
level by adjusting experimental parameters such
as pH and ionic strength. Compared to the Lang-
muir-Blodgett method, layer-by-layer assembly
is generally much simpler and faster, is more ver-
satile, and usually results in more stable films
[115]
. The same procedure can be implemented
even faster using inkjet printing of cationic and
anionic polymers alternately
[116]
.
A host of methods, collectively called
physical
vapor deposition
[53, 79]
, are commonly used to
deposit thin films. In these methods, discussed in
Chapter 15, a collimated vapor flux is generated
from a solid target--by heating it, by passing a
current through it, or by bombarding it with elec-
trons or ions--in a low-pressure chamber. The
collimated vapor flux arrives normally on a sub-
strate and condenses as a dense thin film. When
targets of two different solid materials are used
alternately, a one-dimensional photonic crystal
can be formed on the substrate. With proper
selection of the two materials and the thicknesses
of their layers, this periodically multilayered
structure can display very vivid colors
[117]
.
If the vapor flux arrives at the substrate
obliquely, the density of the film is reduced. The
film is an array of parallel tilted nanowires and
can be considered optically as an effectively
homogeneous biaxial dielectric continuum
[118]
.
Rotation and/or rocking of the substrate during
deposition provides these nanowires with a
shape. Such a film is called a
sculptured thin film
(STF)
[79]
, which can be considered optically as
11.5.4 Structural Colors by Thin-Film
Technologies
As mentioned in Section
11.4.4
, the exocuticle
of many scarab beetles has a layered helicoidal
morphology and displays iridescent color when
viewed with the light of one circular polariza-
tion state but not of the other. Simpler periodi-
cally multilayered structures in nature display
structural color without discrimination of the
circular polarization state of the incident light.
A way to realize structural colors of this type is
by depositing periodic arrangements of homo-
geneous layers of two different materials on a
substrate. The refractive index of one of the two
layers in the unit cell should be higher than that
of the other. Deposition can be carried out using
a variety of thin-film technologies.
The Langmuir-Blodgett method is used for
the deposition of molecular monolayers and
multilayers
[53, 113]
. The process of building a
Langmuir-Blodgett multilayer consists of peri-
odically dipping a substrate into an aqueous
solution of amphiphilic molecules. Each amphi-
phlic molecule has a head and a tail. A layer is
deposited during each dip. However, the mol-
ecules deposited during the removal step have
their heads oriented toward the substrate,
whereas the molecules deposited in the immer-
sion step are oriented with their tails facing the
substrate. The result is that the heads in the
latest layer adhere to the heads in the previ-
ously deposited layer during the upstroke, and
the tails of the latest layer stick to the tails in the
previously deposited layer during the down-
stroke. Several layers of one type of molecule
may be followed by several layers of another
type of molecule, and then the cycle is repeated
