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1 Structural Color in Nature: From Phenomena to Origin

1 Structural Color in Nature: From Phenomena to Origin

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Figure 10.1 Various colorations provided by living creatures in Nature. (a) Blue Morpho butterfly. (b) Peacock. (c) Longhorn beetles Tmesisternus isabellae. (Reproduced

with permission from Ref. 81. Copyright © 2009, the Optical Society of America.) (d)

Myxomycetes Diachea leucopoda. (Reproduced with permission. Copyright the Optical

Society of America: Ref. 3.) (See insert for color representation of this figure.)

structural color in Nature is believed to originate from thin film or multilayer interference, the diffraction or scattering effect of light, or from combinations thereof.6

One typical example is the Morpho butterfly, a well-known iridescent insect living

in Central and South America. Comprehensive investigations have been devoted

to a structural analysis of Morpho butterfly wings and the theoretical explanation

of their iridescent color, with the help of advanced characterization technique.7 – 11

When observed with a microscope, both sides of the transparent epithelial membrane of their wings consist of a large number of overlapping scales having two

different scale types, known as ground scale and cover scale (Figure 10.2). The

ground scale is mainly formed by flat rectangular lamellae of size ∼50–100 mm

in width and 150–200 mm in length. These are arranged in order and attached

to the transparent epithelial membrane. These scales usually form network structures involving rows aligned parallel to each other and crosslinked by the cover





300 nm

1 μm



Figure 10.2 (a)–(c) SEM images and (d) TEM image of the wings of Morpho butterflies.

They are composed of two scales with different size, known as the ground scale and the

cover scale. Both these two scales have many equidistant ridges and are crosslinked

by ribs. (Panels (a) and (b) reproduced with permission. Copyright Wiley-VCH: Ref. 8.

Panel (c) reproduced with permission. Copyright The Royal Society: Ref. 10. Panel (d)

reproduced with permission. Copyright Nature Publishing Group: Ref. 7.)

scale on their surface. The cover scale is composed of periodical plates with a

submicron dimension, called cross ribs. The spacing between the main ridges and

cross ribs lies in the range of 0.5 to ∼5.0 mm, depending on the species of the

butterfly. Both the ground scale and cover scale have many equidistant ridges, but

the ground scale plays a key role for the origination of structural color in most

Morpho species. The brilliant blue color of the Morpho butterfly mainly originates

from these periodical, hierarchical structures on their wings, which contribute to

the diffraction and interference effect of light between air and cuticle, rather than

pigments or dyes, which selectively absorb the visible light. Nanostructures of this

type, that Nature has created, offer plenty of inspiration for the design of advanced

biomimetic materials and optical devices.





During the last decades, much effort has been directed toward the construction

of devices that can confine light with particular wavelengths and that can control

the direction of light propagation, based on the study of surface nanostructures

and control mechanisms of the Morpho butterfly and other similar living creatures. Photonic crystals (PCs) are one kind of well-known artificial material with

spatially ordered lattices that exhibit brilliant structural colors.12, 13 Due to the

periodic arrangement of the dielectric materials, a remarkable property, known as

the photonic band gap (PBG), appears. This property leads to light with certain

wavelengths or frequencies located in the PBG being prohibited from propagating

through the PCs (Figure 10.3).

The concept of the PBG in PCs is similar to the electronic band gap of a semiconductor. When electrons propagate through semiconductors, they interact with

the atoms arranged in an ordered lattice, which results in the formation of allowed

and forbidden energy states. A similar situation exists in the PCs. When photons

propagate through the PCs, they interact with the ordered periodic structures formed

by the dielectric materials. Thus, photons with certain frequencies that fall in the

PBG become prohibited by the PCs. In this way, the flow of light can conveniently

be controlled. When visible light transmits through a PC, light with certain wavelengths matched with the PBG will be reflected by the periodic structure, while

others will pass through unaffected. Distinct structural colors associated with the

reflected frequencies will be observed.

Figure 10.3 Schematic illustration of the photonic band gap in photonic crystals. When

the visible light transmits through the PCs, light with specific frequency that falls in the

PBG will be reflected by the periodical structures, while others pass through it without

being affected.



Another interesting phenomenon in PCs is the property of viewing angle

dependence, which means that different structural colors will be observed when

viewing at different orientations. This phenomenon is caused by the diversity

of lattice constants formed by the PC at different viewing angles. PCs can

mainly be divided into three types according to the arrangement of the dielectric

periodic units: one-dimensional (1D) PCs, two-dimensional (2D) PCs, and

three-dimensional (3D) PCs.


The Fabrication of Photonic Materials

Colloidal crystals composed of monodisperse silica nanospheres, polymer

nanospheres, or composite nanospheres, with the diameter ranging from several

hundred nanometers to several micrometers, are one kind of promising materials

for the fabrication of photonic materials. A variety of methods for fabricating photonic materials have been reported so far,14 – 25 including top–down micromachining,16, 17 bottom–up self-assembly,18 – 20 holographic lithography,21 – 23 laserguided stereolithography,24 and electrophoretic deposition.25 As only the 3D PCs

can meet the requirements for controlling the flow of light in all directions, the

fabrication of 3D PCs has been extensively investigated, especially those with the

PBG in the visible region.

Sedimentation is considered one of the most convenient and versatile ways to

generate 3D crystalline lattice.26 – 29 This technique incorporates several complex

processes including Brownian motion, gravitational settling, and crystallization.

The control of several parameters is necessary for, and beneficial in, the fabrication

process. These parameters include the settling velocity and the concentration, size,

and density of the colloidal crystals. Despite the simplicity of this method, it is

hard to control the morphology of the resulting structures because it involves the

formation of cracks and lattice mismatch in the epitaxial growth process between

colloidal spheres. The number of assembled layers is also not easily controllable.

The sedimentation process should, moreover, be slow enough for the crystallization

process to form a well-ordered 3D lattice. It may take several days, even several

months, to obtain a suitable result, which means it is time consuming.

To overcome these limitations, a much quicker “lifting” method has been developed for the construction of 3D crystalline lattice in recent years.30 This approach

was derived from the “vertical deposition” technique reported by Colvin and coworkers31 In this process, the substrate is first immersed in a suspension of colloidal

crystals and then lifted at a constant speed. During the lifting process, the selfassembly of colloidal crystals takes place at the air–liquid interface. The film

produced by this method has a close-packed face-centered-cubic (fcc) lattice with

the {111} facet parallel to the substrate. Due to the constant lifting speed, the

thickness of film obtained is relatively uniform; this can be conveniently modulated from single layer to multiple layers by precisely controlling the lifting speed

and the concentration of the colloidal solution. A schematic illustration of the lifting method is shown in Figure 10.4. As can be seen, colloidal crystal films with

well-controlled and highly ordered 3D periodical structures can be obtained by










Figure 10.4 (a) Schematic illustration of the lifting method. During the lifting process,

the self-assembly of colloidal crystals takes place at the air–liquid interface, due to the

capillary force and the evaporation of solvent. The lifting speed can be precisely controlled by the computer. (b) SEM images of the obtained highly ordered PC films. (c) PC

films with various distinct brilliant structural color fabricated by using colloidal crystals

with different diameters. (Reproduced with permission. Copyright the American Chemical

Society: Ref. 31.) (See insert for color representation of this figure.)

this technique. Furthermore, the band gap and structural color exhibited may be

controlled by using colloidal crystals with different diameters.


The Design and Application of Photonic Materials

As described above, photonic crystals have the ability to control the propagation

of light, whose signal can be captured and monitored through the spectrum. Taking

advantage of their optical properties, significant achievements have been made in

their practical application over the past several years.32 – 42 This section presents an

overview of the development of photonic materials. We focus on the design and

application of photonic materials in the fields of biomimetic materials, sensors,

optical devices, and display devices. Waveguide Applications In order to rigorously control the propagation of light with specific frequencies, photonic materials have become the focus

of much scientific research. However, although monodisperse colloidal crystals

naturally tend to form ordered 3D crystalline lattices during the self-assembly process, defects like vacancies, cracks, or boundaries are difficult to avoid. In fact,



they always accompany the self-assembly process. Thanks to these defects, various

different functionalities may be endowed upon PCs. For example, a point defect

in a photonic material that acts as a cavity has the ability to trap photons; a line

defect can control and direct the propagation route of photons. These defects disrupt the periodicity of the crystalline lattice and create specific optical states within

the band gap. Therefore, light coupling to these states can be localized within the

defect regions and propagate under control. When light transmits through photonic

materials, the propagation of those having frequency within the PBG can be guided

by the defects since the ordered structures around the defects block the escape of

this light. Light of other frequencies remains undisturbed and travels through the

material normally. A schematic illustration is shown in Figure 10.5.

Designing Defects in Photonic Materials. Generally, defects can be divided

into two types: intrinsic defects and extrinsic defects. Intrinsic defects always occur

during the self-assembly process of PCs. Extrinsic defects are artificially introduced

during or after the formation of PCs. By introducing defects into PCs, functional

photonic devices such as optical waveguides, switches, and microlaser devices have

been developed.43 – 45






Figure 10.5 (a) Schematic illustration of the different kinds of defect that are induced

in PCs for the fabrication of waveguide materials and (b) the propagation of photons

with the frequency located in the PBG in the photonic materials with defects. (c) SEM

image of trapezoid-shaped defects embedded in the silica colloidal crystals (Reproduced

with permission. Copyright Wiley-VCH: Ref. 50.) (d) Planar defects embedded in colloidal

crystals. (Reproduced with permission. Copyright Wiley-VCH: Ref. 52.) (e) Two direction

bend waveguide fabricated through lithography in the PCs. (Reproduced with permission.

Copyright Wiley-VCH: Ref. 45.)



A large number of methods have been developed for the introduction of

defects into PCs in recent years, including lithography, electrochemical etching,

direct writing, colloidal assembling, and layer-by-layer deposition.46 Braun

and co-workers47, 48 reported a laser-induced photopolymerization method to

form microscale line defects in a silica-based crystalline lattice. They filled the

interparticle space among the self-assembled silica spheres with a photosensitive

monomer. A laser was then used to scan the specific region, causing the

photosensitive monomer that had been infiltrated to start becoming polymerized.

After the removal of unexposed monomer, a microscale line defect was introduced

into the silica lattice.

Layer-by-layer deposition is another commonly used strategy to introduce

defects. Generally, a layer of colloidal crystal film is first deposited on the

substrate, followed by the deposition of a defect layer. Sequential growth of

the colloidal crystal layer results in a planar defect within the self-assembled

crystalline lattice. Materials such as polymers, polyelectrolyte, colloidal crystals

with different size or refractive index, are usually used to form the defect layer.

Selective doping layers of different dielectric materials also introduce disturbances

to the periodic lattice.

L´opez’s group utilized this method with the help of a chemical vapor deposition

(CVD) technique to create a homogeneous silica defect layer inside a polystyrene

opal.49 A multiple sandwich-like inverse silica opal, with a planar defect layer

embedded, was obtained by infiltrating the interparticle space of the polystyrene

spheres with silica, followed by the removal of the polystyrene spheres.

Ozin and co-workers51, 52 developed a photolithography technique to create

defects within the interior of a self-assembled lattice taking advantage of both

lithography and layer-by-layer deposition. In this process, a silica colloidal crystal

film was first deposited on the substrate. After a photoresist was applied to the film

through spin-coating, conventional photolithography was used to form defects on

the surface of the film. Then, multiple layers of silica spheres were again assembled

on the photoresist film to generate patterned defects embedded inside the silica

lattice. The removal of the silica spheres created defects embedded in the inverse

opal structures, the size and morphology of which can conveniently be modulated

during the fabrication. Zhao and co-workers also used similar approach to induce

the formation of a defect in the colloidal lattice based on photolithography.50

During the fabrication process, the thickness of the defect layer can be precisely

controlled at its introduction, to thereby control the optical properties of the system.

Furthermore, if the defect layer is composed of a responsive material such as

hydrogel or polyelectrolyte, an external stimulation will change the volume or

thickness, thereby creating a tunable optical property of the defect layer.

Applications of Photonic Materials with Defects. The fabrication of photonic

crystal fibers is one of the practical applications that arises out of the introduction

of defects into PCs.54, 55 It is achieved by introducing single defects into photonic

crystals that are used for guiding light through a 3D periodical lattice. Two main

kinds of photonic crystal fibers are involved: (1) total internal reflection photonic



crystal fibers and (2) photonic band gap fibers. The former has a core surrounded

by an array of holes. The core of the latter is usually hollow, inside of which

losses can greatly be reduced thanks to confinement of the light, thereby keeping

scattering and absorption to a rather low level.

Another application is the fabrication of laser devices, which can be obtained

when the defect layer is composed of a fluorescent laser dye. Optical investigations

by Ozin and co-workers showed that emission at the PBG is strongly prohibited and

narrow luminescence peaks appear exactly at the wavelengths of the defect transmission states.53 Thus, these waveguide photonic materials have specific properties

and promising application in optic communications, laser generation, nonlinear

devices, highly sensitive sensors, and high-power transmission.56, 57 Surface Wettability Control: Superhydrophobicity and Superhydrophilicity Besides the brilliant structural color displayed by the wings of

Morpho butterfly, another interesting phenomenon, known as surperhydrophobicity, has also drawn much attention. As described earlier, the wings of the Morpho

butterfly have scales of two different sizes: ground scales, which are responsible

for the origin of the structural color, and cover scales, which act as a waterproof

layer to prevent water from penetrating their wings. This waterproofing property

and phenomenon not only exist on the wings of butterflies but also in other living

creatures; for example, the legs of water striding insects and the leaves of lotus

and rice.58

Theoretical Investigation of Surface Wettability. Considerable efforts have been

directed toward the study of wettability on solid surfaces, in both theoretical investigations and their practical applications. As early as 1805, Young proposed that a

kind of specific energy exists between interfaces, called surface energy. Thus, the

status of a liquid on a solid surface depends on the balance of the surface tension

among the solid, liquid, and gas interface. Under ideal conditions, the contract

angle can be calculated from Young’s equation:60

cos θ =

γSV − γSL



where θ is the contact angle, and γSV , γSL , and γLV are the surface tension between

solid–vapor, solid–liquid, and liquid–vapor interface, respectively. When θ is

larger than 90◦ , the surface could be considered as hydrophobic. The surface is

hydrophilic when θ is smaller than 90◦ , as shown in Figure 10.6. It should be

noticed that Young’s equation is based on the assumption that the solid surface is

flat and only applies for the situation where γSL − γSV < γLV .

Following Young’s equation, Wenzel proposed an equation for the calculation

of the contact angle on a rough surface,61 as follows:

cos θ = r cos θ






γ LV




γ SL






Figure 10.6 The status of liquid on flat solid surface with (a) hydrophilic and (b)

hydrophobic property.

where θ is the contact angle on the rough surface, θ is the contact angle on a

flat solid surface, and r is the roughness factor, which is larger than 1. From

this equation one can conclude that when θ > 90◦ , the hydrophobic properties are

enhanced when the roughness of the hydrophobic surface is increased. When θ <

90◦ , the hydrophilic properties are enhanced when the roughness of the hydrophilic

surface is increased. This theory works well in situations where the roughness

can be infiltrated by the liquid droplets. In the case where the droplets cannot

penetrate into the roughness and bridge the surface protuberances, the droplets can

be considered to be located on a composite surface composed of solid and air, with

the contact angle calculated from the Cassie–Baxter equation:62

cos θ = f cos θ − (1 − f )


where f refers to the area fraction of the solid–liquid interface, while (1 − f )

refers to that of solid–air interface. The Wenzel and Cassie–Baxter models are

shown in Figure 10.7.

Superhydrophobic Surfaces. Generally, the wettability of droplets on a solid

surface is mainly determined by the average free energy per unit area beneath

the droplet, the roughness coefficient, and the structure of surface protuberances.

Based on our understanding of the generation of hydrophobic properties, various kinds of artificial superhydrophobic surfaces displaying both structural color





Figure 10.7 The Wenzel and Cassie–Baxter models.



and hydrophobic properties can be achieved by fabricating dual-scale roughness

structures on the substrate and taking advantage of these hierarchical structures to

increase the contact angle (as inspired by the wings of Morpho butterfly).

Sato and co-workers reported a dipping method to fabricate uniform inverse

opal films with rough surfaces.63 The suspension used for the fabrication of the

inverse opal film contained spheres of two different sizes: polystyrene spheres

with a diameter of several hundred nanometers and silica nanoparticles with a

diameter of only 6 nm. First, the opal film was created using the lifting method.

This film was then calcined at 450 ◦ C to remove the polystyrene spheres and

solidify the silica nanoparticles. A rough surface with inverse opal structures was

thereby obtained with the small silica nanoparticles filling in the voids among the

polystyrene spheres. The superhydrophobic inverse opal film was finally obtained

by modifying the surface of the film with fluoroalkylsilane. The contact angle of

this superhydrophobic inverse opal film was 155◦ . It clearly showed that rough

surfaces contribute greatly to the hydrophobic properties.

As is well known, ordered opal films usually cannot be obtained using deposition

methods involving mixtures of nanospheres with different size. But in this research,

they found that well-ordered opal films could be generated when the diameter ratio

of the small particles was less than 0.15. This finding is significant for the selfassembly of colloidal crystals and the fabrication of hierarchical structures or rough


Xu and co-workers presented a facile route to fabricate superhydrophobic surfaces via solidification of emulsion droplets that contain the colloidal nanospheres in

silicone oil.64 With the help of shearing force created by stirring, silica nanospheres

packed into beads that were generated from the emulsion droplets after the evaporation of solvent. After calcining at 450 ◦ C, steel sieves were used to narrow the

size distribution and the beads were then treated with fluoroalkylsilane to enhance

the hydrophobicity. The contact angle obtained from the rough surface formed by

these silica beads was 162◦ , indicating that the surface was superhydrophobic, as

shown in Figure 10.8.

Figure 10.8 (a) and (b) SEM images of the rough surface made of colloidal crystal

beads with dual-scale hierarchical structures. (a) Enlarged view of the rough surface; (b)

the surface of a colloidal crystal bead. The insert is the SEM image of a single colloidal

crystal bead. (c) The status of water droplet on the rough surface. The contact angle

of the water droplet measured on this surface is around 162◦ , demonstrating that the

surface shows ideal superhydrophobic property. (Reproduced with permission. Copyright

the American Chemical Society: Ref. 64.)



One important characteristic of a surperhydrophobic surface is its self-cleaning

property. On the surface of surperhydrophobic structures, the water contact angle

is greater than 150◦ . Water will therefore form spherical droplets due to the surface tension, thereby effectively minimizing the contact area of the solid–liquid

interface. As dust can easily attach to the surface of the water droplets, they will

be carried away from the surperhydrophobic surface, leading to self-cleaning. This

effect arises from the presence of hierarchical structures of different scales located

on the surface, and is known as the lotus effect.59

Superhydrophilic Surfaces. Superhydrophilicity is another important surface

characteristic, which leads to a totally different arrangement of water on the solid

surface. When water flows on a superhydrophilic surface, it can spread out on

the surface, resulting in a very low contact angle. Since titanium dioxides (TiO2 )

exhibit photocatalytic and photoinduced superhydrophilic properties, they have

been used to prepare superhydrophilic surfaces with structural color.

Sato and co-workers reported the fabrication of TiO2 inverse opal films with

superhydrophilic properties, by the use of a vertical lifting method.65 After formation of a polystyrene opal film, a hydrophilic treatment was applied to the

surface of the polystyrene using a 7% solution of polyethylene imine in ethanol.

TiO2 nanoparticles with a diameter of 15 nm were infiltrated into the voids of the

polystyrene spheres, followed by calcining at 500 ◦ C to remove the polystyrene

spheres and form TiO2 inverse opal films. Irradiation of the resulting rough surface

with UV light resulted in a contact angle of nearly 0◦ . When water was dropped

on the TiO2 inverse opal film, it infiltrated into the film due to capillary forces

and no water droplets formed on the surface. The stability of this superhydrophilic

TiO2 inverse opal films is excellent. Even after storage in the dark for a period of

several months, the contact angle can still be kept below 1◦ or 2◦ . Sensors and Bioassays As described previously, one of the important properties of the PCs is the PBG generated by well-ordered periodical structures, which has the ability to reflect light with certain wavelengths. By making use

of their optical properties, many novel kinds of sensors and bioassays have been

developed from such photonic materials.66 – 72 In these systems, PCs are usually

used to generate or transmit signals from the biological recognition event, which

brings about the possibility of fabricating simple, highly sensitive, and low-cost

sensors and bioassays.

Photonic Sensors. PC film forms the basis of one of the new type of sensors

that have been developed in recent years. Such sensors are mainly divided into two

types according to the detection method: (1) fluorescence-based PC film sensors

and (2) label-free PC film sensors.

For the former, which is widely used in scientific research, molecules are labeled

with dyes or fluorescent tags, so that the status of the targeted molecules can be

recognized by the fluorescence signals. In this situation, the PC films improve

the sensitivity of the signal through the enhancement of excitation light whose

wavelength is matched with the PBG.

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