IOP PUBLISHING NANOTECHNOLOGY
Nanotechnology 19 (2008) 095302 (7pp) doi:10.1088/0957-4484/19/9/095302
Light-emitting nanocasts formed frombio-templates: FESEM andcathodoluminescent imaging studies ofbutterfly scale replicasJ Silver1, R Withnall1, T G Ireland, G R Fern and S Zhang
Wolfson Centre for Materials Processing incorporating The Centre for Phosphor and DisplayMaterials, Brunel University, Uxbridge UB8 3PH, UK
E-mail: [email protected] and [email protected]
Received 18 July 2007, in final form 2 January 2008Published 11 February 2008Online at stacks.iop.org/Nano/19/095302
AbstractNanocasts comprising of red-light-emitting cubic Y2O3:Eu phosphors were made from butterflywing scale bio-templates. We report herein the first cathodoluminescent images made fromsuch nanocasts and show that valuable insights into the nature of the internal structure of thecasts can be gained by the use of this technique. The casts faithfully reproduced the finesub-micrometre size detail of the scales, as was made evident by both FESEM andcathodoluminescent images that were collected from the same sample areas using a hyphenatedFESEM-CL instrument. There was excellent agreement between the FESEM andcathodoluminescent images, the image quality of the latter indicating that the Eu3+ activatorions were evenly dispersed in the Y2O3:Eu phosphor on a sub-micrometre scale. The casts weremade by infilling the natural moulds with a Y2O3:Eu precursor solution that was subsequentlydried and fired to convert it into the phosphor material. This method provides a simple, low costroute for fabricating nanostructures having feature dimensions as small as 20 nm in size, and ithas the potential to be applied to other metal oxide systems for producing nano-andmicro-components for electronic, magnetic or photonic integrated systems.
1. Introduction
Most butterfly wings display vibrant, highly saturated coloursthat are generated by the structure of the scales fixed to thewing of the insect [1] and are used as communication signalsfor attraction, repulsion or camouflage. Some butterflies havedeveloped exceptionally striking iridescent colours such as theblue Morpho [2]. Other butterflies exhibit varying shades ofblack, brown or a range of spectral colours (in some casesgoing beyond the visible into the ultraviolet), in addition todisplaying optical polarization effects [1, 3]. As well as thestructural origin of colour in butterflies, a less visually strikingsource of colour is from pigments, which are less visible andlead to fewer optical effects [4]. The distinction betweenstructural colour and colour originating from pigments anddyes is evident in butterfly wings. These often have vibrant
1 Authors to whom any correspondence should be addressed.
colours that do not fade over time, in contrast to the coloursof many pigments and dyes which fade at rates that depend ontheir colour fastness. This is because the strong colouration ofa butterfly wing arises from light interference that is caused byits periodic structure, in contrast to the less visible colour of apigment or dye molecule that is due to selective absorption ofvisible light [5–11].
The origin of the iridescence is in the interaction of lightwith the nanostructured butterfly wing scales [12–16]. Thenaturally evolved structures that make up the nanostructuresof the butterfly wing scales can be thought of as part of aclass of materials that are given the modern name ‘photonicbandgap’ (PBG) materials [12–16]. These materials, whenman-made, are composites that have a periodically modulateddielectric constant with a periodicity of the order of opticalwavelengths. They have the ability to inhibit the propagationof light over particular wavelength bands and directions,
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Nanotechnology 19 (2008) 095302 J Silver et al
creating ‘stop bands’. The propagation of light in theseperiodically structured materials has been likened to theconduction properties of electrons in semiconductor crystalswhere interference results from the diffraction of electronwaves by the crystal lattice [14, 15]. The periodic structuregenerates Bragg diffractions where light wave propagationin one or more directions is forbidden for certain bands offrequencies. These stop bands can be omnidirectional forsome three-dimensional periodic dielectric structures, if thephotonic bandgap inhibits light propagation in all directions.The position of the stop band can be customized to appear ina desired wavelength region by varying the repeat distance ofthe template or changing the refractive index of the dielectricmaterial [14–16].
Photonic crystals were envisaged as long ago as 1946when Purcell predicted that the probability of spontaneousemission at radio frequencies could be influenced by thesurrounding environment [17]. He suggested that a resonantoscillator could be created by dispersing small metallicparticles in a nuclear magnetic material, thus inhibiting thespontaneous emission from nuclear spin levels.
It was not until 1972 that further studies in this areawere reported (by Bykov) on the production of a photonicbandgap by a periodic dielectric lattice with pitches smallerthan the radiation wavelength. Such a lattice would inhibitthe spontaneous emission of optical radiation by embeddedatoms and molecules [18, 19]. Zengerle presented work onoptical Bloch waves in singly and doubly periodic planarwaveguides in 1980 [20, 21]. The relevance of theseoptical systems was not acknowledged at the time and it wasnot until 1987 when Yablonovitch and John independentlypublished their work on these systems that widespread interestwas generated [12, 13]. They suggested that photoniccrystals with a three-dimensional periodicity could be tunedto frequency regions where electromagnetic wave propagationis forbidden (stop bands) in all directions, thus opening acomplete photonic bandgap (PBG). These materials, theybelieved, would have great potential for cavity quantumelectrodynamics. The ultimate aim was to produce thesematerials with a significant extended three-dimensional sub-structure with periodic nanoscale lattice parameters withdimensions comparable to optical wavelengths. Applicationsof these materials have centred on the exploitation andmanipulation of their light propagation and spontaneousemission properties [14–16].
Yablonovitch et al fabricated the first PBG crystal in1991 [22]. The crystal had a diamond lattice structurecomposed of millimetre-sized holes that were drilled intoa transparent material. The crystal was found to inhibitfrequencies in the microwave region. Since then researchershave developed many techniques to fabricate PBG crystalsto generate bandgaps at shorter wavelengths in a wide rangeof materials from plastics to silicon [23–41]. The greatestchallenge is to construct three-dimensional PBG crystals withisotropic three-dimensional bandgaps operating at the visiblewavelengths of the electromagnetic spectrum [14–16].
Colloidal suspensions comprising spheres of polystyrene(PS) [42, 43] or silica (SS) [44, 45] have been used to form
three-dimensional periodic templates by self-assembly. Thesetemplates are called bare or synthetic opals [46, 47]. Thevoids between the spheres are infilled with a high refractiveindex material. The spheres are then eliminated by thermaltreatment in the case of PS or etching the SS with hydrofluoricacid [38, 39, 41]. The resulting PBG crystal is composedof periodic highly refractive index material and air voids, thecrystal being known as an inverse opal. The depth of the stopband depends on: (a) the ordering of the inverse crystal (defectsand small domains are detrimental) and (b) maximizing therefractive index contrast between the periodic material and thevoids [14–16].
The definitive aim of this field is to produce materials withan omnidirectional PBG having a sub-structure on a periodicsub-micrometre scale comparable to optical wavelengthdimensions. If large crystals having omnidirectional PBGs inthe visible region can be fabricated commercially (althoughthe synthesis would need to be both relatively simple andcost effective), the new photonic devices such as tight-angleoptical fibres, super-prisms and photonic transistors should beattainable in the near future.
Many materials have been used to form inverse PBGcrystals, some of which have contained luminescent materialssuch as dyes and lanthanide ions [48–52]. The incorporation ofoptical materials inside the PBG crystal would allow the effectof band structure on the emission properties to be studied. Wehave pioneered and reported on the synthesis of novel PBGphosphor-containing materials [38, 39, 41, 53, 54] in which thelight-emitting properties of the phosphor can be controlled byproducing the materials as periodic micro-structures throughthe use of templates. These studies have demonstrated how theemission signature of a phosphor can be manipulated by thepresence of the PBG. The excited state lifetime of the activatorcan be controlled by designing and tailoring the host lattices, sothat their PBGs are tuned to the desired wavelength region. Atthis point in time such ‘opal-like’ type structures are not mass-producible, though they can be easily made out of inorganicphosphor materials in the laboratory, as we have previouslydemonstrated [38, 39, 41, 53, 54].
The thrust of the work is twofold. First, it is toexplore the feasibility of producing casts of nanostructures thathave known photonic properties using inorganic metal oxides.Second, it is to find ways for the eventual mass fabrication ofthree-dimensional photonic bandgap (PBG) structures.
To address these twin aims we have turned our attentionto studying naturally occurring photonic structures. NaturalPBG structures are found in semi-precious opal gemstones [55]and minerals such as labradorite [56]. In the animal kingdomthey are found in butterfly wings [5–11], as well as in thecat tapetum [57], the scales of snakes and fish [11], birdfeathers [11], the hairs of the sea mouse [58] and abalone shells(paua) [59], to mention just a few examples. As previouslymentioned, butterfly wing scales are highly structured photonicmaterials and these were chosen as moulds in this work tofabricate replica structures out of phosphor materials.
Butterfly wings produce colour by diffraction associatedwith the nanostructure of their scales. There are two typesof scales, namely cover and ground; these alternate and
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partially overlap each other and are attached to both sides ofthe transparent epithelial membrane forming the wing [1–10].The cover scales are primarily responsible for the majorityof the colour in butterfly wings and are present in differentproportions within each species. Each scale is attached tothe epithelial membrane by a single stem [1]. The shape ofa scale is elongated, and it varies in size between species.Individual scales are approximately 50–100 µm in width and150–200 µm in length [60]. Butterfly scales manifest twotypes of shape at their terminus, one being dentated and theother rounded. Dentated scales are generally brown in colour,although it is known that some are coloured. Rounded scalesare usually coloured and typical for male specimens. The basicarchitecture of the scales resembles a flattened hollow sac,the bottom of the sac (lower lamina) being fairly featurelesswhile the top (upper lamina) has a structure similar to adiffraction grating [1]. The top of the scale has evenly spacedlongitudinal ridges with transverse ribs forming a net-likestructure supported from below by pillars or trabeculae. Thespacing between the longitudinal ridges and transverse ribsvaries between 0.8–2.0 µm and 0.1–0.8 µm, respectively. Thevoid between the top and bottom of the scale may have one ofa number of filling media, e.g. an amorphous distribution ofmaterial such as pigments, alternating layers of air and chitin,hexagonal close-packed structures or it may just be empty [1].The longitudinal ridges usually have an overlapping slantedlamella morphology pointing in a downwards direction alongthe side of the ridge. At the bottom of the ridge are founda series of small vertical ridges forming the micro-ribs. Across section through some scales reveals a Christmas-tree-like nanostructure shape, the vertical ridge supporting branch-like lamellae creating a series of air–cuticle layers that areresponsible for the iridescent colouration [1, 4–8]. The amountof layers determines the degree of colouration from the scale.The interior of the scale at the base may contain isolated ordiffused areas of pigment granules [1].
Recently we have reported on the fabrication of the firstnanostructured materials cast from the scales of the Morphopleides butterfly [61]. These were in fact phosphor-basedmaterials. The nanostructures that formed the scales of thebutterfly were intimately penetrated by an air-stable europium-doped yttrium oxide (Y2O3:Eu3+) precursor solution. Theprecursor solution was developed to facilitate a simpler methodof infilling photonic bandgap (PBG) bare opal templates thanthe complicated methods used before that time [2]. Theprecursor solution was applied to the butterfly wing (where itpenetrated the nanostructures of the wing) then allowed to dry.The butterfly wing was annealed in a furnace for 30 min at atemperature of 700 ◦C. During the annealing of the resultingsolids the chitin and other material, such as pigment granules,forming the butterfly scales was eliminated, leaving a cast ofthe butterfly scale structure that was composed of the resultingphosphor material. Scanning electron microscopy (SEM) andphotoluminescence spectroscopy were used to confirm that thecasts of the butterfly scales were faithful replicas of the originalscales and that the material was indeed cubic Y2O3:Eu3+.
To investigate the integrity of the phosphor material form-ing the nanostructure of the butterfly scale casts further elec-
tron microscopy investigations were undertaken in combina-tion with cathodoluminescent imaging, all measurements be-ing performed on samples positioned in a FESEM vacuumchamber (FESEM-CL). These investigations were undertakento observe how evenly distributed the Eu3+ activator ions werewithin the finite spaces of the nanostructures forming the but-terfly scale casts. The study was also undertaken in order to re-examine the efficiency of the infilling procedure. This methodof bio-templating to produce nanostructured materials may beapplied to other metal oxide systems.
2. Experimental details
2.1. Materials
Yttrium and europium oxides (99.99%, Ampere Industries,France), nitric acid (Merck, AnalaR) and absolute ethanol(Merck) were used for the phosphor synthesis. All materialswere used without further purification. The butterfly thatwas used for this study is the Sericinus montelus (male)species which belongs to the Papilionidae (Swallowtails andBirdwings) Family and occurs in Sichuan Province, China.
2.2. Method
Precursor Y2O3:Eu3+ phosphor solutions were prepared bythe novel method described elsewhere [41]. The infilledand inverse butterfly Y2O3:Eu3+ phosphor structures wereprepared as follows. A whole butterfly wing (approximately2 cm by 3 cm) was placed between two quartz plates containinga film of precursor Y2O3:Eu3+ phosphor solution, and theslides were then pressed together under a 2 kg weight for aperiod of 30 min. The slides and impregnated section of wingwere then placed in a drying cabinet for 1 h at a temperature of80 ◦C. When the precursor Y2O3:Eu3+ solution was observedto become opaque it was placed in a furnace at a temperatureof 100 ◦C. The temperature of the furnace was programmed toincrease at a rate of 10 ◦C min−1 until it reached a maximum of700 ◦C. It was held for 30 min at that temperature then cooledslowly to room temperature. After the slides had cooled, theywere gently separated and the infilled butterfly Y2O3:Eu3+
phosphor material imprint resulted, with the bulk phosphormaterial covering the remaining area on both slides. With a lowmagnification dissecting microscope the extent of the infilledscales could be clearly seen and areas large enough (up to 1 cmin length) for FESEM characterization were gently removed.
2.3. Instrumentation
FESEM images were originally studied in-house using a ZeissSupra VP instrument to ascertain the quality of the butterflyscale casts. Upon confirmation that the details of the butterflyscales had been faithfully reproduced in the nanocasts, a JEOLFESEM in conjunction with a Gattan CCD detector was usedto collect both SEM and CL images.
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(a) (b)
2 µm 1 µm
Figure 1. (a) FESEM image of two scale sections of a natural butterfly wing, one showing a dentated scale terminus (scale bar is 2 µm), and(b) a higher magnification study of one of the scale sections shown in (a) having a scale bar of 1 µm.
(a)
(c) (d)
(b)
100 nm 200 nm
1 µm 1 µm
Figure 2. FESEM (Zeiss Supra) studies of butterfly scale casts formed from Y2O3:Eu3+: (a) detail of scale cast showing the fine replication ofstructure (scale bar is 100 nm), (b) scale cast with a thicker deposition of Y2O3:Eu3+ (scale bar is 200 nm), (c) cast section of a dentated scaleterminus (scale bar is 1 µm) and (d) cast of a dentated scale tip (scale bar is 1 µm).
3. Results and discussion
Figure 1 presents FESEM images of the butterfly scales beforeinfilling with the Y2O3:Eu3+ phosphor precursor solution.
In figure 2 the FESEM images acquired using theZeiss Supra instrument are shown. In all the images thebutterfly scale nanocasts are formed from Y2O3:Eu3+ phosphormaterial. An extremely good replication of the fine detail ofthe original scale structures presented in figure 1 can clearly beseen. In particular, excellent reproductions of the longitudinalridges with overlapping slanted lamellae running along thetop of the ridge, transverse ribs and micro-ribs running downthe sides of the longitudinal ribs indicate that the Y2O3:Eu3+precursor solution had permeated the fine structures of thescales.
The JEOL FESEM instrument fitted with the Gattan CCDdetector was used to collect FESEM images of the Y2O3:Eu3+
butterfly scale casts, which are shown in figure 3 (the view istaken from a position almost directly over the top of a scale).Figure 3(a) shows that the longitudinal ridges and transverseribs have again been faithfully replicated. The longitudinalridges are approximately 0.9 µm apart (centre to centre) andthe transverse ribs spanning the ridges have spaces betweenthem in the region of 0.15–0.50 µm. The longitudinal ridgesand transverse ribs form a cavity and, in the case of this scalecast, they all have different dimensions. There is no floorat the bottom of the cavity, but instead the opening forms awindow into the lower reaches of the scale cast. Runningalong the longitudinal ridges, the termini of the overlappingslanted lamellae that point in a downwards direction along theside of the ridge on the original scale can be observed. Themicro-ribs running down the sides of the longitudinal ridgesare clearly defined. They are most evident on the first and
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(a) (b)
0.5 µm0.5 µm
Figure 3. FESEM-CL study of a butterfly scale cast formed from Y2O3:Eu3+: (a) SEM image and (b) CL image. The scale bar is 0.5 µm inboth cases.
(a) (b)
0.5 µm 0.5 µm
Figure 4. FESEM-CL study of another butterfly scale cast formed from Y2O3:Eu3+: (a) SEM image and (b) CL image. The scale bar is0.5 µm in both cases.
second ridges. Also, small holes, each with a circular rim, canbe seen between the fourth and fifth longitudinal ridges. Theseholes are very probably fumaroles formed where escapinggases (formed in the production of the phosphor casts) issuefrom these structures. A CL image of the Y2O3:Eu3+ materialforming the scale cast is presented in figure 3(b) in which allthe features present in figure 3(a) can be seen, especially themicro-ribs on the first and second longitudinal ridges. The CLimage is weaker in intensity than figure 3(a) and this is to beexpected as in an FESEM the field emission process producesmany back-scattered or secondary electrons yielding a veryintense image, whereas the CL image is produced by photonsemitted by Eu3+ activator ions after they have been excited inthe Y2O3 lattice by the incident electrons, thus forming a muchweaker, more diffuse image.
Figure 4 displays an FESEM study of another butterflyscale taken using a similar magnification but observed froma different angle than that shown in the previous figure. Thetermini of the overlapping slanted lamellae running along thelongitudinal ridges are again observed along with some micro-ribs. In figure 4(a), there appears to be a higher proportion offumarole type structures present. The cavities formed by thetransverse ribs spanning the longitudinal ridges have similar
dimensions with very smooth walls that have curved internalcorners and edges. These cavities also have smooth floors, afeature that is not observed in the scale cast shown in figure 3,as the floors are in the shaded (darkened) areas.
The fracture in figure 4 across the scale cast allows across-sectional view that reveals detail from the interior regionsbetween the cavities. In a longitudinal direction runningunderneath the ridges and between the rows of cavities a veryfine honeycomb-like structure is apparent. This honeycomb-like structure is present underneath some of the cavities (seerows one and three) and absent under others (see row two).The spaces between the cavities underneath the transverse ribsare very confined and there does not appear to be any roomavailable for the honeycomb-like structures, except where theedges curve inwards forming the floor of the cavities. Thehoneycomb-like structures have spherical voids approximately40–50 nm in diameter which are randomly distributed forminga tightly packed assembly. The honeycomb-like structure doesnot display a close-packed assemblage of face centred cubicbubbles or spheres as in the iridescent scales of Callophrysrubi reported by Ghiradella [1] and Morris [62], that had adiameter of approximately 260 nm which is far larger thanthe voids reported here. For these reasons it is suggested
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(a) (b)
0.2 µm 0.2 µm
Figure 5. Higher magnification FESEM study of the butterfly scale cast formed from Y2O3:Eu3+ shown in figure 4: (a) SEM image and(b) CL image. The scale bar is 0.2 µm in both cases.
that the honeycomb-like structures are probably the inverseof pigment granules. The fine structure of the honeycomb-like structures is indicative of the excellent penetration by theY2O3:Eu precursor solution into all areas of the butterfly wing.
The CL image of the Y2O3:Eu3+ phosphor materialcomprising the scale cast is presented in figure 4(b) anddisplays all the major features shown in (a) except that someof the fine detail of the honeycomb-like structure is lost. Thiscould be due to the much smaller dimensions of the phosphormaterial forming the walls of the voids. These are in the regionof 20 nm in thickness and they are probably composed of muchsmaller nanocrystallites, whereas the main structure of thescale cast is formed from larger bulk-like Y2O3:Eu3+ phosphormaterial. It has been shown that the cathodoluminescentintensity of Y2O3:Eu3+ phosphor nanoparticles with 20–40 nmdiameters is around 60% of the intensity of bulk commercialphosphor particles [63].
In figure 5 a higher magnification FESEM image ispresented of the top left corner of the scale cast shown infigure 4, and this shows details of the region of honeycomb-like phosphor material. A comparison of figures 5(a) (FESEMimage) and (b) (CL image) does indeed demonstrate thatboth images display similar structures and they differ only incontrast. Therefore the distribution of the Eu3+ activator ionsis shown to be evenly dispersed, on a sub-micrometre scale,within the yttrium oxide phosphor material that forms all thestructures shown in the butterfly scale cast.
At this point some discussion of the infilling process isnecessary. In our previous work [41] the infilling of thephotonic lattices formed from either polystyrene nanospheresor silica nanospheres was achieved by the infilling (phosphorprecursor) solution filling the voids between the nanospheres,as it could not penetrate the surfaces of the spheres. Inthe present case the infilling (phosphor precursor) solutionpenetrates into all of the major organic structures, which aredehydrated due to the butterfly being stored as a specimen fora long period of time. Thus in the present case a positivereproduction of the structure is the result of the firing process,whereas in the previous case (infilling the photonic lattice [41])an inverse lattice (a negative reproduction) is formed.
The fact that the nanocasts reproduced the originalstructures so accurately can be attributed to the highconcentration of the phosphor precursors in the infillingsolution [41]. These solutions are viscous at the time of useso that no concentration gradients can be set up in any filledspaces.
The size of sample that can be prepared in this way doesnot seem to be a problem. We were able to do entire butterflywing samples and were only restricted by the size of wingavailable in the species. We believe that in principle any dried,absorbent sample could be cast by our method.
It is difficult to estimate the shrinkage that takes place inthe firing process, as there is no cracking or obvious distortionin the final structures. By comparing measurements we believethat very little shrinkage has taken place, certainly much lessthan 5%.
This particular butterfly is mainly white and we found noevidence of light manipulating structures, though we wouldhave expected to see such structures on the FESEMs of thewing scales, if they were present.
Though no such structures were obvious we seem tohave infilled between structures that we believe are pigmentgranules that are very small (see figure 5) and much smallerthan the wavelengths of UV or visible light. It is to benoted that the chitin that makes up the natural structures willhave a refractive index in the range 1.55–1.66, whereas theY2O3:Eu3+ phosphor material that makes up the positive casthas a refractive index varying from 1.80 to 2.00, dependingon wavelength. This would modify any photonic propertiespresent due to structure, but we found no evidence for sucheffects using optical microscopy.
4. Conclusions
It has been demonstrated by FESEM herein that the finenanostructures present within butterfly wing scales have beenfaithfully replicated using a precursor Y2O3:Eu3+ phosphorsolution. The stable Y2O3:Eu3+ precursor solution is formedby volume reduction of the rare earth metal nitrate solutionsuntil they become viscous; this allows a simple infillingprocedure of the butterfly scale structures to be achieved
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without the need for complicated experimental conditions orequipment. The infilling is facilitated by the dehydratedstate of the butterfly caused by its desiccation while ondisplay. The subsequent annealing of the infilled butterflyscales simultaneously converts the precursor solutions to theluminescent phosphor oxide and eliminates the materialsforming the wing scales. Cathodoluminescent imagesfurther demonstrate the faithful reproduction of the scalestructures formed from Y2O3:Eu3+ due to the homogeneousluminescence from the low concentration of europium activatorions (2.0 mol%). This method allows a simple and cost-effective route to the fabrication of complex nanostructureswith features down to approximately 20 nm dimensions insize. The macro-size of the sample does not seem to be aproblem; certainly sample areas of a few square centimetrescan be produced by this method. Although we did notfind any photonic structures in the butterfly scales we haveclearly demonstrated that this method provides the necessaryresolution to see such structures if they are present. Thefabrication method may be applied to other metal oxidesystems that could eventually lead to the production ofoptical waveguides and splitters, nanoelectronic circuits orcomponents as building blocks for nanoelectronic, magnetic orphotonic integrated systems.
Acknowledgments
We would like to express gratitude to A Yarwood of JEOL,Welwyn Garden City, UK for providing access to the JEOLFESEM instrument that was fitted with a Gattan CCD detector.
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