Mechanochromic Fibers with Structural ColorHoupu Li, Xuemei Sun,* and Huisheng Peng*[a]

1. Introduction

Nature is full of colors, which make our life vigorous and beau-

tiful. Some of them come from dyes or pigments, such aschlorophyll in leaf or hemoglobin in our blood, whereas many

others come from structural colorations, such as those in opal,the wings of a butterfly, the iridescence of a bubble, and the

plumage of a peacock. Unlike the pigments that absorb light

by transitions of electrons, structural colors originate from theBragg scattering of microstructures in materials such as period-

ically stacked micro-fibrils, particles or slices in nature.[1, 2] Manyefforts have been devoted to develop artificial materials with

structural coloration. Among them, photonic crystals (PhC) rep-resent one of the most attractive materials for structural colors

by controlling and manipulating the flow of light. PhC materi-

als have a structure of periodically distributed dielectrics withthe period in the scale of the optical wavelength and display

colors by Bragg scattering.[3, 4] Photonic band gaps (PBGs) thatprohibit certain light waves to travel in the PhC would be

formed.[5, 6] PhCs can have one-dimensional (1D), two-dimen-sional (2D) or three-dimensional (3D) PBG structures(Figure 1).[7] The 1D PhCs are recognized as an alternative

repeat stacking of two layered materials with different dielec-

tric constants, forming a PBG in only one dimension.[8] The 2D

PhCs have periodic structure in two directions similar to a hon-eycomb and therefore form PBGs in two dimensions.[9, 10] The

mostly studied 3D PhCs display periodic structures in three di-mensions.[11] 3D PhCs can further be divided into different

types by the construction. The simplest is the stacked micro-

structures, especially microspheres, similar to the structure ofnatural opals and thus named as opal-structure. Another type

is the infiltrated opal structure, in which a second phase isgenerally infiltrated into the voids among arranged micro-

spheres to increase the refractive index.[12, 13] The matrix can bea polymer, a gel, or an inorganic component.[14] Besides the in-

filtration methods, core–shell-structured polymer particles pro-

duce stable PhCs by cross-linking the soft shell intoa matrix.[15, 16] Furthermore, after forming the matrix, the micro-

spheres can be removed by solvent or acid, producing anotherPhC structure named inverse opal structure.[12, 17, 18]

The color displayed by PhCs can be described according toBragg’s law [Eq. (1)]:

ml ¼ 2dn sin q ð1Þ

where m is an arbitrary integer named order of diffraction, l is

the wavelength, d is the spacing between two neighboring

planes in the lattice, n is the refractive index of the material,and q is the glancing angle between the incident light and the

diffraction crystal plane.

The color of PhCs can be controlled by varying the distanceof two neighboring lattice planes (d) when the refractive index

and observing angle are fixed. To this end, various responsivePhCs have been developed to change colors under various

stimuli such as solvent, heat, electricity, light, magnetism, andstrain.[11] Most responsive PhCs were achieved by incorporating

responsive polymers.[14, 19–21] Among them, mechanical stimula-

tion represents a straightforward and effective way to changethe color of PhCs compared with the slow response to solvent,

heat, and light—and the inconvenience of electric or magneticfields. Mechanochromic PhCs can be easily performed for color

changes that are promising in a variety of fields such as smartdisplays, strain sensing, and fingerprint identification. Recently,

Responsive photonic crystals have been widely developed to

realize tunable structural colors by manipulating the flow of

light. Among them, mechanochromic photonic crystals attractincreasing attention due to the easy operation, high safety and

broad applications. Recently, mechanochromic photonic crystal

fibers were proposed to satisfy the booming wearable smart

textile market. In this Concept, the fundamental mechanism,

fabrication, and recent progress on mechanochromic photoniccrystals, especially in fiber shape, are summarized to represent

a new direction in sensing and displaying.

Figure 1. Structure diagrams of: a) 1D, b) 2D, and c) 3D PhCs. Reprinted withpermission from Ref. [7]. Copyright 2013, Wiley-VCH.

[a] H. Li, Dr. X. Sun, Prof. H. PengState Key Laboratory of Molecular Engineering of PolymersDepartment of Macromolecular Scienceand Laboratory of Advanced MaterialsFudan University, Shanghai 200438 (China)E-mail : sunxm@fudan.edu.cn

penghs@fudan.edu.cn

ChemPhysChem 2015, 16, 3761 – 3768 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3761

ConceptsDOI: 10.1002/cphc.201500736

mechanochromic PhCs in a fiber shape have attracted consid-erable attentions as they may satisfy the booming wearable

smart textile market. Herein, the recent progress on fiber-shaped mechanochromic PhCs is carefully described to high-

light their necessity and importance.

2. Mechanochromic Photonic Crystals

In the field of mechanochromic PhCs, 1D materials with bilayer

polymers have been reported, whereas nearly no 2D ones

have been studied—possibly due to the difficulty in prepara-tion. In contrast, 3D mechanochromic PhCs with self-assem-

bled microspheres have been greatly explored. Generally, me-chanochromic PhCs are required to be stretchable to complete

the chromatic transitions under the change of strains. 1D me-chanochromic PhC films were made by stacking bilayer elastic

films of different refractive indices such as polydimethylsil-oxane (PDMS) and polystyrene–polyisoprene copolymers (PSPI)

(Figure 2 a).[22] The resulting multilayer film with alternatePDMS and PSPI layers showed a reversible chromatic transition

between red and blue under the varying strain due to thechange of the film thickness. 1D PhC films had also been pre-

pared from the bilayers during polymerization of the hydro-gel.[23]

The most popular way to prepare 3D mechanochromic PhCs

is the self-assembly of microspheres. For the obvious require-ment of elasticity, infiltrated opal and inversed opal structuresare widely used, in which elastic polymers such as PDMS, poly-urethane, and polyacrylate are used as the matrix. For the one-step crosslinking process, the hard core should be monodis-perse while the soft shell could be melted and cross-linked to

form an elastomer matrix.[15, 16] For example, microspheres are

Figure 2. Preparation methods for mechanochromic PhCs. a) Stacking the bilayer polymer film for 1D PhCs. Reprinted with permission from Ref. [22] . Copy-right 2010, OSA. b) Assembly of core–shell polymer microspheres for 3D PhCs. Reprinted with permission from Ref. [24] . Copyright 2013, American ChemicalSociety. c) Filling an elastic polymer to ordered microspheres for 3D PhCs. d) Template method for 3D inverse opal PhCs. e) Structure of metastable colloidalcrystalline arrays and their change under pushing and pulling. Reprinted with permission from Ref. [35] . Copyright 2014, Wiley-VCH. f) 3D PhC films fromliquid-crystalline polymer to change colors under stretching. Reprinted with permission from Ref. [31]. Copyright 2015, Nature publication group.

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Concepts

prepared with a PS/PMMA hard core and a poly(ethyl acrylate)(PEA) soft shell connected by poly(ethyl acrylate-co-allyl meth-

acrylate) via emulsion polymerization (Figure 2 b).[15, 24] The re-sulting PhC film by melt compression of these spheres dis-

played color changes under compressing and stretching. An-other method is a two-step infiltration process that includes

assembling hard microspheres and filling the space with elasticpolymers, followed by swelling to enhance the mechanochro-

mic capability (Figure 2 c).[25] For instance, PS colloidal crystals

are embedded into a PDMS matrix, and the swollen film exhib-its a reversible color change between red and green understretching and releasing.[26] An inverse opal structure can beformed by removing the microspheres (Figure 2 d).[27] The re-

flected wavelength of the inverse opal film, with a pore diame-ter of 350 nm, was shifted from 700 to 600 nm due to the mor-

phology change of the pore from sphere to ellipsoid. The

porous materials could be deformed more easily than the em-bedded opal structure and thus could be used for color finger-

printing.Mechanochromic PhCs can also be prepared from non-close-

packed colloidal particles. If charged colloidal nanoparticles aredispersed in a solution with few ions, interparticle repulsion

can make the nanoparticles form an ordered but non-close-

packed colloidal crystal array structure.[28] In this case, thevolume fraction of the microspheres is as low as 17 %, much

lower than 74 % of the closely packed microspheres.[29] Themechanochromic photonic gel can also simultaneously consist

of ordered and disordered do-mains of assembled micro-

spheres. For instance, metastable

crystalline colloidal arrays of SiO2

are fixed in a polymer gel

through photo-polymerization(Figure 2 e).[30] The resulting pho-

tonic gel is composed of bothordered and amorphous stackedSiO2 microspheres, and the poly-

mer gel could be further swollenby solvents for a higher elasticitybefore polymerization. The colorcan be changed from green to

blue under pulling and green tored under pushing. Different

from using lattice of micro-spheres, a gel film of liquid-crys-talline polymers also displays

color changes from red to greenand blue under increasing

strains. When a polymer crystal-lite is used, the PhC indicates

a similar microstructure to the

stacked fibers in nature (Fig-ure 2 f).[31]

A change in the interlayer dis-tance can cause a chromatic

transition. Two main kinds ofstrains can induce such a dis-

tance change, that is, normal strain, which occurs along thestress, and shearing strain, which occurs perpendicularly to the

stress. Normal strain is typically applied perpendicularly to thediffraction crystal plane of the PhCs, where the d changes inthe same direction and shows the same ratio as the appliedstrain.[7] A vivid example was made by putting the PhC in

a wedge-shaped space. It showed chromatic transitions start-ing from red to blue and covered the whole visible spectrawith the increasing compression.[32] For the shearing strain,

when the PhC is stretched to be parallel to the diffraction crys-tal plane, the change of d is perpendicular to the appliedstrain. Moreover, the change of d should have a different ratiofrom the applied stain but with a certain proportion, that is,Poisson ratio. The color change caused by normal and shearingstrains can be simplified under certain conditions and are de-

scribed by Equations (2) and (3), respectively:

lðeÞ ¼ l0ð1þ eÞ ð2ÞlðeÞ ¼ l0ð1¢ ueÞ ð3Þ

Here u is the Poisson ratio, e is the applied strain, and l0

and l(e) are the reflection wavelengths at relaxed and strained

states, respectively. Accordingly, both perpendicular compres-sion to the diffraction crystal plane and parallel stretching will

cause a blue-shift color. In most cases, Poisson ratios of mecha-nochromic PhCs were calculated to be 0.22-0.29, much smaller

Figure 3. Schematic diagram, microstructure, and photographs of the 3D mechanochromic PhCs. a) Rearrange-ment of microspheres under stretching. Reprinted with permission from Ref. [16] . Copyright 2007, AmericanChemical Society. b) Deformation of microspheres under compression. Reprinted with permission from Ref. [24] .Copyright 2013, American Chemical Society

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Concepts

than 0.5 for typical elastic polymer materials.[25, 33] A possiblereason lies that the PhCs share a periodic structure for multi-

component materials compared with homogeneous materials.Two types of lattice changes occur in 3D mechanochromic

PhCs during deformation, namely, the rearrangement of hardspheres from densely packed FCC cubic to other lattice or dis-

order structures[34, 35] and shape transition of the buildingspheres from spherical to ellipsoidal.[15, 36] Basically, bothchanges depend on the building materials and the deforma-

tion conditions. The matrix material must be soft to performstretchability. When the spheres are rigid like silica and PS, thedeformation mainly occurs in the soft matrix material while thespheres re-arrange. For example, in a core-shell bead systemwith hard PS core and soft PEA shell, the soft PEA matrix de-formed while the PS spheres in the (111) plane switched from

hexagon to square patterns under stretching, corresponding

to a blue-shift color change (Figure 3 a).[15, 16] When the buildingspheres are softer such as polymer above glass transition tem-

perature, they will change into ellipsoids to absorb morestrains rather than re-arranging. For example, when the same

PhC system from PS/PEA core-shell spheres were compressedabove the glass transition tem-

perature of PS, the PS core de-

formed from sphere to ellipsoid,and the color changed from

dark color to bright red andgreen with the increasing com-

pression (Figure 3 b).[15] For an in-verse opal structure, the pores

were changed from circular to

elliptical shape in the cross sec-tion.[27, 36]

3. MechanochromicPhotonic Crystal Fibers

Although the fiber shape is

a common architecture withbroad application, artificial me-chanochromic PhCs in fibershape have not appeared until

recently, possibly due to the dif-ficulty in the preparation. PhC

fibers used in the field of com-munications can be synthesizedby drawing stacked and fused

silica capillaries through a glasstechnology. The diameter of the

capillaries would shrink from mil-limeters to micrometers during

the drawing process to form

a highly regular honeycomb-likePhC lattice due to a balanced

surface tension. The fiber hada solid or hollow core where

light could be confined by thePhC layer around the core.[37–39]

PhC fibers can also be formed by electrophoretic deposition ofmicrospheres on conductive carbon fibers[17, 40] or through self-

assembly of microspheres onto glass fibers in microspace.[41]

However, those PhC fibers are limited for lattice changing due

to the stiffness of glass or carbon fiber, thus losing the mecha-nochromic capability. Currently, mechanochromic PhC fibers

are generally prepared by three methods, rolling a PhC filmonto rigid fiber substrate followed by removal of the fiber

core, depositing a PhC layer on elastic fiber substrate, and

melting extrusion from the core–shell microsphere.[42–45]

As previously mentioned, a 1D PhC film can be fabricated byalternatively stacking two dielectric materials. Therefore, a 1DPhC fiber can be obtained by rolling up the bilayer film.[46] A

thin bilayer film composed of PDMS and PSPI was prepared viaspin coating and then rolled onto a glass fiber with the assis-

tance of a liquid (Figures 4 a–c). The mechanochromic PhC

fiber was then produced by etching away the glass fiber. Itchanged colors from red to blue with a spectral blue-shift of

over 200 nm under the strain of 200 % (Figures 4 d and 4 e).The sensitivity (the change in reflection peak over the elonga-

tion) was typically ~1 nm/ %.[46] In this case, the interlayer

Figure 4. Mechanochromic PhC fiber by rolling a bilayer polymer film. a) Schematic of the rolling technique of thebilayer film. Reprinted with permission from Ref. [22] . Copyright 2010, OSA. b) SEM image of the cross-section.c) Periodic structure from a cross-sectional view. d) Reflection spectra with increasing strains from one to eight.e) Optical micrographs with increasing stains. Reprinted with permission from Ref. [46] . Copyright 2013, Wiley-VCH.

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space of the PhC would alwaysbe the thickness of rolled film,

guaranteeing the periodic struc-ture. The Poisson ratio of the

stretchable multilayer fiber wascalculated to be ~0.46, similar to

the film of the two constituentmaterials which have compara-

ble Poisson ratios. Therefore, the

thickness change was uniformand the reflection intensity re-

mained constant during stretch-ing. However, due to the refrac-

tion from the curved surface,these fibers showed multiplepeaks in the spectrum, reducing

the purity of color. For instance,their UV-visible spectra displayed

a triple-peak reflection (Fig-ure 4 d). Moreover, the use offlexible and elastic fiber corewould be a better alternative to

save the removal step of rigid

fiber core.Mechanochromic PhC fibers

were also developed by coatingmechanochromic 3D PhC layers

onto elastic fibers. However, theassembly of microspheres was

different in curved surfaces of

fibers compared to flat planarsubstrates. Electrophoretic depo-

sition was thus employed to effi-ciently deposit charged micro-

spheres onto conductive fibers such as metal and carbonfibers, but the resulting structural colored fibers were not

stretchable.[17, 45] To this end, an elastic conductive fiber was re-

alized by winding aligned carbon nanotube (CNT) sheets ontoPDMS fiber. PS microspheres were electrophoretically deposit-ed on the elastic conductive fibers to form PhCs.[44] The mecha-nochromic fibers were finally obtained by further filling and

swelling elastic PDMS into the interval space of PS micro-spheres (Figure 5 a). Note that the synthesis of PS micro-

spheres, aligned CNTs and PDMS has been well addressed, sothis method can be efficient for a large-scale production. Theresulting fibers possessed a unique core-sheath structure (Fig-

ures 5 b and 5 c) with elastic matrix content up to 98 %, whichendowed the PhC fiber a decent elasticity and homogeneous

elongation to produce color changes. They exhibited brilliantcolor changes and obvious reflection peak shifts under strains

from 0 to 30 %, that is, from red to green and from green toblue for the fibers derived from 200 and 240 nm PS micro-spheres, respectively (Figures 5 d–f). The Poisson ratio of the

fiber was calculated to be ~0.455, within the strain of 30 %,which is much higher than the one reported for the film and

close to that of the bare PDMS elastomer (i.e. 0.5). The mecha-nochromic properties showed high average sensitivity

(~2.8 nm/ %), reversibility, and stability for thousands of cycles,which surpassed those of the related films. Although the mi-

crospheres in these fibers were generally orderly arranged to

form localized PhC segments, those at the edge of segmentwere arranged disorderly due to the curved surface of fiber

substrate. Therefore, more efforts are required to optimize thepreparation process to obtain a uniform microsphere layer.

3D PhC fibers were also realized wholly from microspheresby melt-extruding core–shell polymer spheres (Figures 6 a and6 b).[42] The soft shell would melt under the heat, enabling thehard cores to arrange under the shearing force caused by theextrusion; the hard cores form a uniform matrix after cooling

down. The fiber diameters could be controlled by the die aper-ture (Figures 6 c and 6 d). These fibers could change colors

from red to green under different stains with tunable reflectionspectra across the visible region (Figures 6 e and 6 f). The fiber

was expected to show a Poisson ratio comparable to that of

3D PhC films composed of closely packed hard spheres andsoft spacer, that is, in the range of 0.22–0.29. The sensitivity of

this mechanochromism was typically between 2.5 and 3.0 nm/%. Since the introduced shearing force for the formation of

PhC fibers comes from the flowing, which typically occurrednear the surface, the periodic structure should be formed

Figure 5. Core–sheath structured mechanochromic PhC fiber from PS microspheres and a PDMS matrix. a) Sche-matic illustration of the preparation process. b, c) Schematic illustration and optical micrograph of the fiber cross-section. The green layer contains arranged PS microspheres. d) Reflection spectra of the fiber under increasingstrains. e, f) Photographs of the fiber in the relaxed and stretched states, with colors changing from green to blue.Reprinted with permission from Ref. [44] . Copyright 2015, Wiley-VCH.

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Concepts

mainly near the fiber surface. Therefore, the interior of thefiber was not homogeneous but showed a series of different

colored circular zones (Figure 6 c). This phenomenon could bereduced by decreasing the diameter of the fibers (Figure 6 d),

but the rate of tuning colors with strains would also be de-

creased due to a more effective cross-linking and less viscoe-lastic effect. Although these fibers, to a certain extent, can becontinuously produced, the preparation of core-shell micro-spheres limits the generalization in preparing the desired fiber

material.Fundamentally, the three methods for mechanochromic PhC

fibers were developed from the techniques of PhC films. Therolled PhC fiber was derived from the stacked bilayer PhC film(Figure 2 a), and the rolled structure ensured that each layer

had exactly the same thickness, guaranteeing the regularity ofthe PhC structure.[22] The elasticity comes from the feature of

the dielectric film. The infiltrated PhC fiber employed themature electrophoretic deposition of microspheres onto differ-

ent substrates to offer a good regularity,[47] which was further

protected by the infiltration of PDMS (Figure 2 c). The elasticityand mechanochromism come from the elastic PDMS as both

fiber substrate and filling matrix. The extruded fiber stemmedfrom the melt-compression method, utilizing a shearing force

in the procedure of extrusion to bring regularity to the micro-spheres (Figure 2 b).[15] The soft shell was crosslinked to immo-

bilize the cores and providegood elasticity. It should be no-ticed that PhC fibers involvingmicrospheres suffer a loss of re-

flection intensity under deforma-tion, that is, the peaks of the re-

flection spectra decreased withincreasing strain, although they

recovered upon the release ofthe strain. This phenomenonmay be produced by the un-

evenness elongation and trans-formation of microspheres from

FCC to other lattices or amor-phous structures at higher

strains. As a result, these fibers

exhibited colors of low bright-ness and broad reflection spec-

tra under high strains. This prob-lem also exists in PhC films and

is still not solved yet.[8, 13] ThePhC fiber rolled from the bilayer

polymer film showed the best

spectral peaks at high strains, al-though this is the most difficult

method regarding the prepara-tion, for example, they show in-

tensities as high as those of therelaxed state.[46] On the other

hand, both fibers from micro-

sphere structures display a me-chanochromic sensitivity above

2.5 nm/ %, whereas the PhC with rolled bilayer structure offera sensitivity of only 1 nm/ %. Different PhC systems may be

used to meet requirements for different applications.

4. Summary and Outlook

In this Concept, recent progress on mechanochromic PhCs

with a fiber shape is highlighted to represent the booming de-velopment in sensing and displaying. Mechanochromic PhC

fibers only appeared a few years ago, still calling for furtherperfection on preparation, structure, performance, and applica-

tion.The preparation of PhC fibers is different from other forms,

such as cubes and films, although they share the mechanisms

for color appearance and transition. Unlike the flat surface ofcommon PhC gels or films, PhC fibers have a curved surface,

making it more difficult for the microspheres to form a continu-ous ordered arrangement. Though modern industry provides

large amounts of fibers, the large-scale production of uniformPhC fibers is still challenging. For the current methods, thefibers rolled from bilayer polymer films show the best mecha-

nochromic ability. However, the procedure is complicated andit remains difficult to produce continuous fibers. The electro-

phoretic deposition and extrusion methods are compatible fora continuous preparation, but the mechanochromic per-

Figure 6. Mechanochromic PhC fiber by melt-extruding core–shell microspheres. a) Structure of core–shell micro-spheres. b) Extrusion process for fiber fabrication. c, d) Cross-sections of fibers with different diameters. e) Reflec-tion spectra of the fiber under increasing strains. f) Corresponding optical micrographs of the fiber under increas-ing strains, with color changes from red to green. Reprinted with permission from Ref. [42] . Copyright 2011, OSA.

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Concepts

formance needs to be further enhanced. Other methods suchas stacking and drawing, usually applied to obtain silica PhC

fibers, may also be promising for the preparation of mechano-chromic fibers, if the elasticity problem can be solved through

material design.[38]

Various other structures can be incorporated for high-per-

formance PhC fibers by learning from mechanochromic films,such as inverse opal,[18] non-close packing, and liquid crystalstructures. In addition, it should be noted that the observed

colors are varied in the radial direction of the fiber due to thecurved surface. This iridescent phenomenon is well recognizedfor the highly ordered PhCs. To this end, PhC fibers with amor-phous microstructure arrays are promising due to the angle-in-dependent feature. The amorphous microstructure arrays havebeen rarely studied and the mechanism is not fully understood

yet.[48] More attention should be paid to this direction.

The form of the fiber offers many unique and promising ap-plications. Firstly, they can be used as colored fibers to make

fabrics for displays, avoiding using organic dyes or pigments.Secondly, they can change colors under different strains and

can be used for sensing visually. Thirdly, since mechanochrom-ism comes from a change in the lattice parameters, the color

transition can be used to detect structure changes and strain

distributions in materials. After a careful design, they mayserve as visualized pressure sensing and health monitoring de-

vices, for example, for the in situ detection of blood pressure.In particular, they can be combined with widely explored fiber-

shaped electronic devices such as solar cells and light emittingdevices.[49, 50] PhCs were found to improve planar solar cells

and light-emitting devices,[51] so it is expected that PhC fibers

can be also integrated into fiber-shaped optoelectronic devicesfor more functionalities and better performances. To summa-

rize, fiber-shaped PhCs represent a new and encouraging di-rection in displaying, sensing, and many other fields.

Acknowledgements

This work was supported by the NSFC (21225417, 51403038),

MOST (2011CB932503), STCSM (15XD1500400), the China Post-doctoral Science Foundation (2047M560290, 2015T80391), the

Changjiang Chair Professor Program, and the Program for Out-standing Young Scholars from the Organization Department of

the CPC Central Committee.

Keywords: displays · fibers · mechanochromic materials ·photonic crystals · structural color

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Manuscript received: August 28, 2015Accepted Article published: September 30, 2015Final Article published: October 30, 2015

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