electrochemically grown photonic crystals
Current Opinion in Colloid & Interface Science 7 Ž2002. 116᎐123
Macroporous materialsᎏelectrochemically grown photonic crystals P.V. Braun a,b,c , P. Wiltzius a,b,d,U b
a Department of Materials Science and Engineering, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA Beckman Institute for Ad¨ anced Science and Technology, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA c Materials Research Laboratory, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA d Department of Physics, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA
Abstract Colloidal crystallization has been explored for several years as a fabrication method for photonic crystals. While macroporous materials grown with silica or polymer colloids might exhibit pleasing opalescence, truly novel photonic behavior such as photonic band-gaps, is expected only for very high index of refraction contrast systems. This can possibly be achieved through electrodeposition of semiconductors, polymers, or metals in the interstitial space of self-assembled colloids. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Colloidal self-assembly; Photonic crystals; Electrodeposition; Photonic band-gap materials; Macroporous; Microperiodic
1. Introduction In recent years interest in microperiodic threedimensional structures has grown tremendously due to the exciting possibilities that such materials might have, in particular, in the area of photonics w1 x. Such three-dimensional structures, often termed photonic crystals, are the extension of the well-known dielectric stack into three-dimensions. While we marvel at the beautiful colors of naturally occurring opals, which stem from diffraction of white light by planes of highly ordered sub-micron silica spheres, our tools and techniques to build such objects in the laboratory are still very limited. A particularly interesting class of optical structures are so-called photonic band-gap materials. For example, a microperiodic material made of low refractive index spheres arranged in a face-centered-cubic array 䢇
U
Corresponding author. Tel.: q1-217-244-8373; fax: q1-217244-0987. E-mail address: [email protected] ŽP. Wiltzius..
in a high index of refraction matrix, with a lattice constant on the order of the wavelength of light Žvisible or infrared., could be such a photonic bandgap material w2x. Similar to how a dielectric stack has a stop-band for light in a given frequency range, this material would not allow light in a given frequency range to travel through in any direction. In essence, it would be an omnidirectional, perfectly lossless mirror. Adding line defects and cavities to such a material would allow manipulation of light in a new, compact way, opening up the field of microphotonics w1 x. Layer-by-layer fabrication of photonic crystals using state-of-the-art VLSI tools, e.g. deep UV photolithography, CVD, chemical᎐mechanical polishing, has been demonstrated w3x, but formidable processing difficulties limit the formation of large area and truly threedimensional structures. Self-assembled colloids are natural candidates for the construction of photonic crystals. Good crystal quality is achieved only with colloids that have very low size polydispersity Ž- 5%., limiting the materials choice to either SiO 2 or polymers, both of which have
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a fairly low index of refraction of approximately 1.5. This has led researchers to take a two-stage approach. In a first step the microperiodic structure is assembled using colloids. In a second step, this structure is used as a template to build an inverse structure with a higher index of refraction material. A variety of techniques including CVD w4x and imbibing with nanoparticles w5x have been demonstrated for the infilling of colloidal templates, but for a variety of reasons these routes may not be suitable for many applications. A very promising method for replication with a high-dielectric material is electrodeposition ŽFig. 1.. The potential for high refractive index materials, large area structures, and the complete infilling of thick, three-dimensional colloidal templates, as well as the low processing cost of electrodeposition has led to the interest in this area. In the following paragraphs we will review three different classes of materials that have been electrodeposited into self-assembled colloidal crystals, namely semiconductors, polymers, and metals.
2. Recent developments 2.1. Semiconductor electrodeposition Semiconductors are interesting candidates for photonic crystals primarily because of their high refractive indices and generally robust nature. For example, CdS has a refractive index of 2.5, and materials such as GaP, Si and Ge have indices of 3.4, 3.5 and 4.0, respectively. However, routes to create periodic macroporous structures from such materials are limited because of their very high melting points and low solubility in common solvents. To date, the II᎐VI semiconductors CdS and CdSe w6 ,7x, and ZnO w8x have been electrochemically grown through colloidal templates, resulting, after dissolution of the template, in macroporous semiconductor films. For all systems a conducting oxide film on glass was used as the substrate. The macroporous CdS films were generated by galvanostatic deposition through the interstitial space of a colloidal crystal formed from 1 m SiO 2 spheres, and both CdS and CdSe macroporous films were generated by potentiostatic deposition through a colloidal template generated from 466 nm polystyrene spheres ŽFig. 2.. Following electrodeposition, the SiO 2 and polystyrene colloidal templates were removed with aqueous HF and toluene, respectively. Because of the high rigidity of the semiconductor network, contraction upon removal of the template was limited to a few percent at most. The fine and gross morphologies of the electrodeposited semiconductors are presented in Fig. 3. The macroporous ZnO films were formed through 䢇
Fig. 1. Generalized procedure for creating three-dimensionally periodic macroporous materials through colloidal templating of electrodeposition. Monodispersed colloids sediment onto a conducting substrate, self-assembling into a crystal. The sample may be dried and sintered before electrolyte is added. A counter-electrode allows electrodeposition of the desired material Žsemiconductor, polymer and metal. into the interstitial space. In a final step, the electrolyte and the templating colloid are removed. In the case of polymeric colloids, this can be done either by heat treatment at elevated temperature or dissolution with a solvent. For silica colloids, aqueous HF is effective for dissolving the template.
potentiostatic deposition through a colloidal crystal formed from 368 nm polystyrene spheres, and the spheres were removed with toluene. Careful control of the electrodeposition conditions was found to be
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compared to conventional polymers or inorganic materials; their optical properties can be electrochemically modulated, fine control over properties can be obtained through organic chemistry and, in many cases, they are mechanically flexible. Electrochemical growth of conducting polymers is a fairly well developed field, and many procedures for the growth of solid films have been published w10x. There are, however, only a few reports on the growth of porous conducting polymer films. Fibers of polypyrrole, polyŽ3-methylthiophene. and polyaniline were formed in the early 1990s by electrodeposition from the appropriate monomer solution through a porous membrane w11x. The first example of the electrochemical deposition of a conducting polymer
Fig. 2. Schematic representation of the experimental set-up for the potentiostatic deposition of CdSe through the interstitial space of a colloidal crystal.
necessary; if the electrodeposition was performed at a potential less negative than y1.0 V vs. AgrAgCl large crystalline grains of ZnO formed, which disrupted the structure of the colloidal template. Through the use of a deposition potential more negative than y1.0 V, the formation of large grain ZnO was suppressed, and the colloidal template was not disrupted. All the electrodeposited semiconductor films are reported to be opalescent, however, detailed optical spectroscopy has yet to be performed. Real progress in optically interesting materials may await the electrochemical deposition of materials such as GaP, Ge and Si which, because they have refractive indices greater than three, may result in materials with three-dimensional photonic band-gaps. Routes to the electrodeposition of such materials have been demonstrated w9x, but problems, such as hydrogen gas evolution and the generally harsh conditions, will need to be solved before successes in these areas are likely. 2.2. Polymer electrodeposition Electrodeposition of conducting polymers Želectropolymerization. through self-assembled colloidal crystals, followed by removal of the colloidal template is a promising route to optically active macroporous materials. Several significant advancements over the last few years have begun to demonstrate the potential of conducting polymer-based microperiodic photonic structures. Inherently, because of the low refractive index of polymeric materials, it is quite unlikely that a three-dimensional photonic band-gap material will result from a polymer-based photonic crystal, however, conducting polymers have advantageous properties as
Fig. 3. SEM images of potentiostatically deposited CdSe Ža., and galvanostatically deposited CdS Žb., Žc. after removal of the polystyrene colloidal template. In the overdeposited system Žb., the overlying solid CdS film can be clearly seen on the right side of the micrograph. The apparent lack of periodic pore structure in the underdeposited system is not due to disorder in the colloid, but rather because the nodular surface of the semiconductor cuts through multiple lattice planes of the template.
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around a colloidal template was in 1992 where polypyrrole was grown around latex particles w12x. However, no attempt was made to remove the colloidal particles, and the optical properties of the resulting films were not measured. It is only in the past few years that researchers have explored the possibility of the templated growth of conducting polymers for photonic applications. To date, there have been three reports on colloidal templating of conducting polymers, all of which have followed the general procedure of Ž1. colloidal crystal formation on a conducting substrate, Ž2. electrochemical deposition from solution, and Ž3. dissolution of the colloidal template with an appropriate solvent. In the first example, polypyrrole was grown potentiostatically from a solution of pyrrole in acetonitrile through a colloidal crystal comprised of SiO 2 spheres with a mean diameter of 238 nm, assembled on F-doped SnO 2 coated glass, followed by the removal of the colloidal template with aqueous HF w13 x. Very shortly after that, macroporous polypyrrole, polyaniline and polybithiophene films were potentiostatically polymerized through a colloidal crystal assembled from 500 and 750 nm polystyrene spheres, using as the substrate gold-coated glass. The polystyrene template was then removed with toluene w14x. In the most recent example, polypyrrole and polythiophene macroporous films were potentiostatically grown through colloidal crystals assembled from 150 and 925 nm polystyrene spheres, respectively, on indium tin oxide-coated glass in a similar fashion as the previous report, however, in this case, the polystyrene was removed with tetrahydrofuran w15x. In this last report, preliminary optical characterization was performed, and a weak dip in transmittance that appears to be correlated to the periodic structure was observed. One significant issue for electrodeposited macroporous polymers is the contraction of the period structure upon removal of solvent. This was most clearly observed in the polystyrene templated systems, where significant contraction, ranging from 13 to 40%, was observed for the macroporous polypyrrole and polyaniline. However, very little contraction was observed in polystyrene templated macroporous polybithiophene, or when SiO 2 spheres were used as the template. The primary difference is that organic solvents are used to remove the polystyrene spheres and an aqueous HF solution is used to remove the SiO 2 spheres. This would suggest that the organic solvent softens the electrodeposited polymer allowing it to contract, but there also may be systems similar to polybithiophene where contraction of the macroporous matrix does not occur. This is less of a problem for macroporous metals and semiconductors. As first reported by Yanagida et al. w13 x, and
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Fig. 4. SEM images of Ža. the silica template, and the macroporous polypyrrole films formed by electrodeposition through this template at a potential of Žb. 0.55, Žc. 0.75 and Žd. 0.85 V vs. AgrAgCl. The insets show the interconnect diameter corresponding to each SEM image w; 50 nm Žb.; ; 75 nm Žc.; and ; 90 nm Žd.x.
confirmed by Caruso et al. w15x, through careful control of electrodeposition conditions, the size of the holes between the macropores can be regulated, which may be important for control of photonic structures ŽFig. 4.. In general, as the applied potential is increased, the diameter of the interconnecting holes increases. Because all the films have been studied in their dry state, it is still unclear what size the holes are before removal of the colloidal template, or if the size changes upon dissolution of the template and drying. 2.3. Electrodeposition of metals Metallic macroporous ordered replicas of colloidal assemblies are of potential interest for a wide range of applications including filtration, separation, and
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catalysis. In addition, they might have interesting electrical, magnetic or optical properties. The tools and techniques for electrochemically plating metals have been well established for thin films and even bulk materials w16x. It is thus fairly straightforward to develop recipes to backfill the interstitial space of a colloidal self-assembled crystal with almost any metal. The focus of this article is photonic crystals and the majority of the work in this field has, thus far, been concentrated on dielectric structures. Materials of choice are those with a high index of refraction and a low optical absorption coefficient Žsee paragraphs above.. Bulk metals have large imaginary parts of their dielectric constants and hence, readily absorb light. When the metallic structures become small enough, however, strong optical resonances associated with plasmon frequencies of the conduction electron in the metals can lead to qualitatively new phenomena. A well-known example of this is the red color of a nanosized dispersion of gold colloid. A more recent manifestation of unexpected behavior is the anomalously high transmission of small holes Ž200 nm. in thin metallic films w17x. Theoretical calculations w18 ᎐20x on ordered three-dimensional arrays of metallodielectric spheres show that these are promising for the construction of full photonic band-gap materials in the visible part of the optical spectrum. The advantage of metallodielectric structures over purely dielectric structures is that it should be easier to achieve a full band-gap in the visible. A full band-gap in the visible is exceedingly difficult, if not impossible to create with purely dielectric structures since there are very few dielectric materials with an index of refraction greater than three, and very low absorption in the visible. This has led to the development of synthesis routes to produce metallo-dielectric colloidal core-shell particles with sizes in the sub-micron range w21,22x, and metallic shell thicknesses or cores that are small enough to show the resonance effects. In the remainder of this section we describe several recent advancements in building metallo-dielectric structures by replicating colloidal assemblies with electrochemically grown metal. Vos et al. w23 x made replicas of colloidal crystals made from silica Žradius s 113 nm. and polystyrene Žradius s 322 nm.. They potentiostatically deposited gold in these opals from a HAuCl 4 solution. Prior to electrodeposition, the silica spheres were sintered through a heat treatment at 600 ⬚C. After the deposition of the gold the template was removed by etching the silica with aqueous HF. The polystyrene spheres were removed by combustion at 450 ⬚C. The results can be seen in Fig. 5. The templated spherical voids in the gold replica can be seen clearly. In the case of the silica template, there is good fidelity between the 䢇
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original colloidal assembly and the replica. There are no dimensional changes between the dried, sintered colloid and the final replica, although some cracking is observed during the original drying and sintering process. This indicates that the electrochemically formed gold is dense and structurally robust. This is a definite improvement over other methods of producing macroporous structures with high-dielectric materials, e.g. liquid-phase or sol᎐gel chemistry w24,25x, and infiltration with nanosized particles w5 x. Polystyrene colloidal assemblies produced less promising results ŽFig. 5b.. While the gold deposited in a layered fashion on the polystyrene spheres, it is speculated that lack of cohesion between the spheres led to the destruction of long-range order in the replica. Xu et al. w26x described methods to electrodeposit gold and nickel into periodic structures made of 300 nm diameter silica spheres. They also relied on a sintering process of the silica at 120 ⬚C for 2 days, followed by 750 ⬚C for 4 h. The electrodeposition was done galvanostatically in commercially available nickel and gold plating solutions. The silica was again removed using aqueous HF. The final thickness of the metal replica was ; 100 m. The spherical ‘floret’-like appearance could be due to incomplete filling during electroplating or the malleability of gold, which could lead to some restructuring during drying. A similar effect was observed during electroless plating in a colloidal structure reported by Jing et al. w27x. Nickel is reported to be more robust leading to replicas with greater fidelity. Luo et al. w28x reported good results on Ni and Pt replication of colloids. The pore size varied between 90 and 210 nm depending on the size of the original polystyrene spheres. They also measured the internal surface area Ž6.2 m2rg for Ni and 3.6 m2rg for Pt., and uniform pore size ŽNi, 142 " 7 nm; Pt, 125 " 5 nm.. Since the original sphere size was 210 nm, significant shrinkage occurs in these cases. The film thicknesses obtained were adjusted from several micrometers to tens of micrometers, with typical growth rates of 100 nmrmin. These authors also report electrodeposition of a SnCo alloy from a commercially available SnCo solution bath. Changing the composition of the alloy allows control over the hardness, chemical and optical properties. A group at the University of Southampton w29x prepared structured macroporous palladium, platinum and cobalt films using polystyrene latex sphere templates. Fig. 6 shows a variety of different films. While the quality of these films appears reasonable, it should be noted that they are typically only a few sphere layers thick. A qualitatively different approach was reported by Lellig et al. w30x. They used a relatively low volume
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Fig. 5. Ža. SEM image of a crystal of air spheres Žradius s 111 nm. in gold, made with a silica template. The inset is a Fourier transform of the SEM image. Žb. SEM of macropores Žradius s 322 nm. in gold, made with a latex template. The structure has short-range order, but no long-range order. Reproduced with permission from Wiley.
fraction of highly charged colloidal crystals that had been immobilized in a polyacrylamide hydrogel as a template. The hydrogel composite was polymerized on a stainless steel electrode, and silver was deposited in the interstices from an aqueous AgNO3 solution. Drying of these samples led to shrinkage, but SEM revealed that large areas of the silver sample preserved the long-range order of the original colloidal crystal. Due to the low volume fraction of the polymeric colloid, the template was not continuous and could not be removed, contrary to the cases described above where the colloidal spheres were arranged in a hardsphere touching configuration. This is an obvious disadvantage if the final product is supposed to have very large dielectric constant contrast. Other applications, e.g. catalysis, would also require an interconnected internal void space.
3. Conclusion Recent work has demonstrated that electrodeposition through the interstitial space of highly-ordered colloidal crystals has promise for creating photonic crystals comprised of a diverse set of materials including semiconductors, metals and polymers. The resulting three-dimensionally macroperiodic materials have been formed with close-packed macropores ranging in diameter from 100 nm to a few microns, giving the potential to modulate light ranging from deep UV to the infrared. However, there are still many important problems to overcome before this approach to photonic structures comes to fruition. The first is the perfection of the self-assembled template. Although this review concentrates on electrochemical routes to templating of materials with colloidal crystals, much
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Fig. 6. SEM micrographs of macroporous films of platinum, palladium and cobalt electrochemically deposited at 0.10, 0.25 and y0.90 V vs. SCE, respectively, through templates formed from 400 nm Ža᎐c. and 700 nm Žd. polystyrene spheres pre-assembled on gold surfaces. Ža. Platinum film, deposition charge of y2.00 C cmy2 ; Žb. cobalt film, deposition charge of y1.40 C cmy2 ; Žc. cross-sectional image of a platinum film, deposition charge of y1.50 C cmy2 ; and Žd. palladium film, deposition charge of y1.15 C cmy2 .
significant work remains to be done on creating colloidal templates with sufficient short- and long-range order. Assuming such perfect templates can be assembled, the electrodepostion conditions will have to be regulated to create macroporous structures with a high degree of structural order. This includes, for example, the size of the interconnects between the macropores. For polymeric materials at least, initial results on the effect of electrodeposition potential on interconnect diameter are promising. Just as critical is the refractive index of the electrodeposited material. Under ideal conditions, for a complete photonic band-gap, a refractive index contrast between the material and the voids of at least three is required. So far, it has not been demonstrated that any of the systems can meet this requirement. There is, however, an extraordinarily diverse set of materials that can be electrodeposited, and if high refractive index materials such as Si or Ge can be electrochemically templated with colloidal crystals, this requirement will be met. In addition, composite structures such as metallodielectric systems may have strong photonic responses. Mechanically flexible polymeric structures, with optical properties that can be modulated electrochemically may find application as switchable optical elements. The future indeed is bright for electrochemically grown photonic crystals. References and recommended reading 䢇 䢇䢇
of special interest of outstanding interest
w1x Joannopoulous JD, Meade RD, Winn JN. Photonic crystals:
molding the flow of light. Princeton: Princeton University Press, 1995. This is a general introduction to the field of Microphotonics, which is easily accessible to readers at the undergraduate level. Concepts of one-, two- and three-dimensional photonic crystals and full band-gap materials are well explained. Also included is a description of defects and waveguides. w2x Busch K, John S. Photonic band gap formation in certain self-organizing systems. Phys Rev E 1998;58:3896᎐3908. w3x Lin SY, Fleming JG, Hetherington DL et al. A three-dimensional photonic crystal operating at infrared wavelengths. Nature 1998;394:251᎐253. w4x Blanco A, Chomski E, Grabtchak S et al. Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres. Nature 2000;405: 437᎐440. w5x Vlasov YA, Yao N, Norris DA. Synthesis of photonic crystals for optical wavelengths from semiconductor quantum dots. Adv Mater 1999;11:165᎐169. w6x Braun PV, Wiltzius P. Electrochemically grown photonic 䢇 crystals. Nature 1999;402:603᎐604. This is the first paper in which replication of a colloidal crystal using electrodeposition is reported. Both CdS and CdSe replicas were constructed with greater robustness and a higher index of refraction than had been achieved with other methods. w7x Braun PV, Wiltzius P. Electrochemical fabrication of 3-D microperiodic porous materials. Adv Mater 2001;13:482᎐485. w8x Sumida T, Wada Y, Kitamura T, Yanagida S. Macroporous ZnO films electrochemically prepared by templating of opal films. Chem Lett 2001;1:38᎐39. w9x Pandey RK, Sahu SN, Chandra S. Handbook of semiconductor electrodeposition. New York: Marcel Dekker, 1996. w10x Gurunathan K, Vadivel Murugan A, Marimuthu R, Mulik UP, Amalnerkar DP. Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices. Mater Chem Phys 1999;61:173᎐191. w11x Martin CR. Nanomaterialsᎏa membrane based synthetic approach. Science 1994;266:1961᎐1966. 䢇
P.V. Braun, P. Wiltzius r Current Opinion in Colloid & Interface Science 7 (2002) 116᎐123 w12x Koopal CGJ, Feiters MC, Nolte RJM, Deruiter B, Schasfoort RBM. 3rd generation amperometric biosensor for glucosepolypyrrole deposited within a matrix of uniform latex particles as mediator. Bioelectrochem Bioenerg 1992;29:159᎐175. w13x Sumida T, Wada Y, Kitamura T, Yanagida S. Electrochemi䢇 cal preparation of macroporous polypyrrole films with regular arrays of interconnected spherical voids. Chem Commun 2000;1613᎐1614. The novelty of this paper is two-fold. It is the first replication of a colloidal self-assembled crystal using electrodeposition of a polymer, polypyrrole. In addition, the macropores of the void space were interconnected with size-controlled holes. The size of the holes could be adjusted by changing the deposition conditions. w14x Bartlett PN, Birkin PR, Ghanem MA, Toh CS. Electrochemical syntheses of highly ordered macroporous conducting polymers grown around self-assembled colloidal templates. J Mater Chem 2001;11:849᎐853. w15x Cassagneau T, Caruso F. Semiconducting polymer inverse opals prepared by electropolymerization. Adv Mater 2002;14:34᎐38. w16x Schlesinger M, Paunovic M. In: Schlesinger M, Paunovic M, editors. Modem electroplating, 4th ed. New York: John Wiley, 2000. w17x Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998;391:667᎐669. w18x Moroz A. Photonic crystals of coated metallic spheres. Euro䢇 phys Lett 2000;50:466᎐472. Most of the photonic crystal research focuses on purely dielectric structures. In order to get a full band-gap in a three-dimensional material, the required index of refraction contrast is very large Že.g. n ) 3.0 for an fcc lattice of air spheres with the high index material in the interstitial space.. While this is achievable in the near infrared with semiconductors, it is probably not possible to find an appropriate material in the visible part of the spectrum. Moroz shows a different approach by calculating the band structure of metallodielectric materials. Here, plasmon resonances play an important role and keep the losses typically associated with metals under control. w19x Moroz A. Three-dimensional complete photonic band-gap structures in the visible. Phys Rev Lett 1999;83:5274᎐5277. w20x Zhang WY, Lei XY, Wang ZL et al. Robust photonic band gap from tunable scatterers. Phys Rev Lett 2000;84: 2853᎐2856.
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w21x Oldenburg SJ, Averitt RD, Westcott SL, Halas NJ. Nanoengineering of optical resonances. Chem Phys Lett 1998;288: 243᎐247. w22x Graf C, van Blaaderen A. Metallodielectric colloidal coreshell particles for photonic applications. Langmuir 2002;18: 524᎐534. w23x Wijnhoven JEGJ, Zevenhuizen SJM, Hendriks MA, Van䢇 maekelbergh D, Kelly JJ, Vos WL. Electrochemical assembly of ordered macropores in gold. Adv Mater 2000;12:888᎐890. This is the first paper of several in a short time interval reporting replication with a metal. Electrodeposition of metals has, of course, been known for a long time, although for the particular application of photonics, these metal films would have to be very thin and very well controlled in their morphology. It is not clear at this point whether metal deposition will be a successful method to build metallodielectric structures. There are certainly other applications, e.g. catalysis, filtration, electronic or magnetic materials, where this method could prove very useful. w24x Holland BT, Blanford CE, Stein A. Synthesis of macroporous minerals with highly ordered three-dimensional arrays of spheroidal voids. Science 1998;281:538᎐540. w25x Wijnhoven JEGJ, Vos WL. Preparation of photonic crystals made of air spheres in titania. Science 1998;281:802᎐804. w26x Xu L, Zhou WL, Frommen C, Baughman RH, Zakhidov AA, Malkinski L, Wang JQ, Wiley JB. Electrodeposited nickel and gold nanoscale meshes with potentially interesting photonic properties. Chem Commun 2000;997᎐998. w27x Jiang P, Cizeron J, Bertone JF, Colvin VL. Preparation of macroporous metal films from colloidal crystals. J Am Chem Soc 1999;121:7957᎐7958. w28x Luo Q, Liti Z, Li L, Xie S, Kong J, Zhao D. Creating highly ordered metal, alloy, and semiconductor macrostructures by electrodeposition, ion spraying, and laser spraying. Adv Mater 2001;13:286᎐289. w29x Bartlett PN, Birkin PR, Ghanem MA. Electrochemical deposition of macroporous platinum, palladium and cobalt films using polystyrene latex sphere templates. Chem Commun 2000;1671᎐1672. w30x Lellig C, Hartl W, Wagner J, Hempelmann R. Immobilized highly charged colloidal crystals: new route to three-dimensional mesoscale structured materials. Angew Chem Int Ed 2002;41:102᎐104.
Macroporous materialsᎏelectrochemically grown photonic crystals P.V. Braun a,b,c , P. Wiltzius a,b,d,U b
a Department of Materials Science and Engineering, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA Beckman Institute for Ad¨ anced Science and Technology, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA c Materials Research Laboratory, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA d Department of Physics, Uni¨ ersity of Illinois at Urbana, Champaign, IL 61801, USA
Abstract Colloidal crystallization has been explored for several years as a fabrication method for photonic crystals. While macroporous materials grown with silica or polymer colloids might exhibit pleasing opalescence, truly novel photonic behavior such as photonic band-gaps, is expected only for very high index of refraction contrast systems. This can possibly be achieved through electrodeposition of semiconductors, polymers, or metals in the interstitial space of self-assembled colloids. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Colloidal self-assembly; Photonic crystals; Electrodeposition; Photonic band-gap materials; Macroporous; Microperiodic
1. Introduction In recent years interest in microperiodic threedimensional structures has grown tremendously due to the exciting possibilities that such materials might have, in particular, in the area of photonics w1 x. Such three-dimensional structures, often termed photonic crystals, are the extension of the well-known dielectric stack into three-dimensions. While we marvel at the beautiful colors of naturally occurring opals, which stem from diffraction of white light by planes of highly ordered sub-micron silica spheres, our tools and techniques to build such objects in the laboratory are still very limited. A particularly interesting class of optical structures are so-called photonic band-gap materials. For example, a microperiodic material made of low refractive index spheres arranged in a face-centered-cubic array 䢇
U
Corresponding author. Tel.: q1-217-244-8373; fax: q1-217244-0987. E-mail address: [email protected] ŽP. Wiltzius..
in a high index of refraction matrix, with a lattice constant on the order of the wavelength of light Žvisible or infrared., could be such a photonic bandgap material w2x. Similar to how a dielectric stack has a stop-band for light in a given frequency range, this material would not allow light in a given frequency range to travel through in any direction. In essence, it would be an omnidirectional, perfectly lossless mirror. Adding line defects and cavities to such a material would allow manipulation of light in a new, compact way, opening up the field of microphotonics w1 x. Layer-by-layer fabrication of photonic crystals using state-of-the-art VLSI tools, e.g. deep UV photolithography, CVD, chemical᎐mechanical polishing, has been demonstrated w3x, but formidable processing difficulties limit the formation of large area and truly threedimensional structures. Self-assembled colloids are natural candidates for the construction of photonic crystals. Good crystal quality is achieved only with colloids that have very low size polydispersity Ž- 5%., limiting the materials choice to either SiO 2 or polymers, both of which have
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a fairly low index of refraction of approximately 1.5. This has led researchers to take a two-stage approach. In a first step the microperiodic structure is assembled using colloids. In a second step, this structure is used as a template to build an inverse structure with a higher index of refraction material. A variety of techniques including CVD w4x and imbibing with nanoparticles w5x have been demonstrated for the infilling of colloidal templates, but for a variety of reasons these routes may not be suitable for many applications. A very promising method for replication with a high-dielectric material is electrodeposition ŽFig. 1.. The potential for high refractive index materials, large area structures, and the complete infilling of thick, three-dimensional colloidal templates, as well as the low processing cost of electrodeposition has led to the interest in this area. In the following paragraphs we will review three different classes of materials that have been electrodeposited into self-assembled colloidal crystals, namely semiconductors, polymers, and metals.
2. Recent developments 2.1. Semiconductor electrodeposition Semiconductors are interesting candidates for photonic crystals primarily because of their high refractive indices and generally robust nature. For example, CdS has a refractive index of 2.5, and materials such as GaP, Si and Ge have indices of 3.4, 3.5 and 4.0, respectively. However, routes to create periodic macroporous structures from such materials are limited because of their very high melting points and low solubility in common solvents. To date, the II᎐VI semiconductors CdS and CdSe w6 ,7x, and ZnO w8x have been electrochemically grown through colloidal templates, resulting, after dissolution of the template, in macroporous semiconductor films. For all systems a conducting oxide film on glass was used as the substrate. The macroporous CdS films were generated by galvanostatic deposition through the interstitial space of a colloidal crystal formed from 1 m SiO 2 spheres, and both CdS and CdSe macroporous films were generated by potentiostatic deposition through a colloidal template generated from 466 nm polystyrene spheres ŽFig. 2.. Following electrodeposition, the SiO 2 and polystyrene colloidal templates were removed with aqueous HF and toluene, respectively. Because of the high rigidity of the semiconductor network, contraction upon removal of the template was limited to a few percent at most. The fine and gross morphologies of the electrodeposited semiconductors are presented in Fig. 3. The macroporous ZnO films were formed through 䢇
Fig. 1. Generalized procedure for creating three-dimensionally periodic macroporous materials through colloidal templating of electrodeposition. Monodispersed colloids sediment onto a conducting substrate, self-assembling into a crystal. The sample may be dried and sintered before electrolyte is added. A counter-electrode allows electrodeposition of the desired material Žsemiconductor, polymer and metal. into the interstitial space. In a final step, the electrolyte and the templating colloid are removed. In the case of polymeric colloids, this can be done either by heat treatment at elevated temperature or dissolution with a solvent. For silica colloids, aqueous HF is effective for dissolving the template.
potentiostatic deposition through a colloidal crystal formed from 368 nm polystyrene spheres, and the spheres were removed with toluene. Careful control of the electrodeposition conditions was found to be
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compared to conventional polymers or inorganic materials; their optical properties can be electrochemically modulated, fine control over properties can be obtained through organic chemistry and, in many cases, they are mechanically flexible. Electrochemical growth of conducting polymers is a fairly well developed field, and many procedures for the growth of solid films have been published w10x. There are, however, only a few reports on the growth of porous conducting polymer films. Fibers of polypyrrole, polyŽ3-methylthiophene. and polyaniline were formed in the early 1990s by electrodeposition from the appropriate monomer solution through a porous membrane w11x. The first example of the electrochemical deposition of a conducting polymer
Fig. 2. Schematic representation of the experimental set-up for the potentiostatic deposition of CdSe through the interstitial space of a colloidal crystal.
necessary; if the electrodeposition was performed at a potential less negative than y1.0 V vs. AgrAgCl large crystalline grains of ZnO formed, which disrupted the structure of the colloidal template. Through the use of a deposition potential more negative than y1.0 V, the formation of large grain ZnO was suppressed, and the colloidal template was not disrupted. All the electrodeposited semiconductor films are reported to be opalescent, however, detailed optical spectroscopy has yet to be performed. Real progress in optically interesting materials may await the electrochemical deposition of materials such as GaP, Ge and Si which, because they have refractive indices greater than three, may result in materials with three-dimensional photonic band-gaps. Routes to the electrodeposition of such materials have been demonstrated w9x, but problems, such as hydrogen gas evolution and the generally harsh conditions, will need to be solved before successes in these areas are likely. 2.2. Polymer electrodeposition Electrodeposition of conducting polymers Želectropolymerization. through self-assembled colloidal crystals, followed by removal of the colloidal template is a promising route to optically active macroporous materials. Several significant advancements over the last few years have begun to demonstrate the potential of conducting polymer-based microperiodic photonic structures. Inherently, because of the low refractive index of polymeric materials, it is quite unlikely that a three-dimensional photonic band-gap material will result from a polymer-based photonic crystal, however, conducting polymers have advantageous properties as
Fig. 3. SEM images of potentiostatically deposited CdSe Ža., and galvanostatically deposited CdS Žb., Žc. after removal of the polystyrene colloidal template. In the overdeposited system Žb., the overlying solid CdS film can be clearly seen on the right side of the micrograph. The apparent lack of periodic pore structure in the underdeposited system is not due to disorder in the colloid, but rather because the nodular surface of the semiconductor cuts through multiple lattice planes of the template.
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around a colloidal template was in 1992 where polypyrrole was grown around latex particles w12x. However, no attempt was made to remove the colloidal particles, and the optical properties of the resulting films were not measured. It is only in the past few years that researchers have explored the possibility of the templated growth of conducting polymers for photonic applications. To date, there have been three reports on colloidal templating of conducting polymers, all of which have followed the general procedure of Ž1. colloidal crystal formation on a conducting substrate, Ž2. electrochemical deposition from solution, and Ž3. dissolution of the colloidal template with an appropriate solvent. In the first example, polypyrrole was grown potentiostatically from a solution of pyrrole in acetonitrile through a colloidal crystal comprised of SiO 2 spheres with a mean diameter of 238 nm, assembled on F-doped SnO 2 coated glass, followed by the removal of the colloidal template with aqueous HF w13 x. Very shortly after that, macroporous polypyrrole, polyaniline and polybithiophene films were potentiostatically polymerized through a colloidal crystal assembled from 500 and 750 nm polystyrene spheres, using as the substrate gold-coated glass. The polystyrene template was then removed with toluene w14x. In the most recent example, polypyrrole and polythiophene macroporous films were potentiostatically grown through colloidal crystals assembled from 150 and 925 nm polystyrene spheres, respectively, on indium tin oxide-coated glass in a similar fashion as the previous report, however, in this case, the polystyrene was removed with tetrahydrofuran w15x. In this last report, preliminary optical characterization was performed, and a weak dip in transmittance that appears to be correlated to the periodic structure was observed. One significant issue for electrodeposited macroporous polymers is the contraction of the period structure upon removal of solvent. This was most clearly observed in the polystyrene templated systems, where significant contraction, ranging from 13 to 40%, was observed for the macroporous polypyrrole and polyaniline. However, very little contraction was observed in polystyrene templated macroporous polybithiophene, or when SiO 2 spheres were used as the template. The primary difference is that organic solvents are used to remove the polystyrene spheres and an aqueous HF solution is used to remove the SiO 2 spheres. This would suggest that the organic solvent softens the electrodeposited polymer allowing it to contract, but there also may be systems similar to polybithiophene where contraction of the macroporous matrix does not occur. This is less of a problem for macroporous metals and semiconductors. As first reported by Yanagida et al. w13 x, and
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Fig. 4. SEM images of Ža. the silica template, and the macroporous polypyrrole films formed by electrodeposition through this template at a potential of Žb. 0.55, Žc. 0.75 and Žd. 0.85 V vs. AgrAgCl. The insets show the interconnect diameter corresponding to each SEM image w; 50 nm Žb.; ; 75 nm Žc.; and ; 90 nm Žd.x.
confirmed by Caruso et al. w15x, through careful control of electrodeposition conditions, the size of the holes between the macropores can be regulated, which may be important for control of photonic structures ŽFig. 4.. In general, as the applied potential is increased, the diameter of the interconnecting holes increases. Because all the films have been studied in their dry state, it is still unclear what size the holes are before removal of the colloidal template, or if the size changes upon dissolution of the template and drying. 2.3. Electrodeposition of metals Metallic macroporous ordered replicas of colloidal assemblies are of potential interest for a wide range of applications including filtration, separation, and
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catalysis. In addition, they might have interesting electrical, magnetic or optical properties. The tools and techniques for electrochemically plating metals have been well established for thin films and even bulk materials w16x. It is thus fairly straightforward to develop recipes to backfill the interstitial space of a colloidal self-assembled crystal with almost any metal. The focus of this article is photonic crystals and the majority of the work in this field has, thus far, been concentrated on dielectric structures. Materials of choice are those with a high index of refraction and a low optical absorption coefficient Žsee paragraphs above.. Bulk metals have large imaginary parts of their dielectric constants and hence, readily absorb light. When the metallic structures become small enough, however, strong optical resonances associated with plasmon frequencies of the conduction electron in the metals can lead to qualitatively new phenomena. A well-known example of this is the red color of a nanosized dispersion of gold colloid. A more recent manifestation of unexpected behavior is the anomalously high transmission of small holes Ž200 nm. in thin metallic films w17x. Theoretical calculations w18 ᎐20x on ordered three-dimensional arrays of metallodielectric spheres show that these are promising for the construction of full photonic band-gap materials in the visible part of the optical spectrum. The advantage of metallodielectric structures over purely dielectric structures is that it should be easier to achieve a full band-gap in the visible. A full band-gap in the visible is exceedingly difficult, if not impossible to create with purely dielectric structures since there are very few dielectric materials with an index of refraction greater than three, and very low absorption in the visible. This has led to the development of synthesis routes to produce metallo-dielectric colloidal core-shell particles with sizes in the sub-micron range w21,22x, and metallic shell thicknesses or cores that are small enough to show the resonance effects. In the remainder of this section we describe several recent advancements in building metallo-dielectric structures by replicating colloidal assemblies with electrochemically grown metal. Vos et al. w23 x made replicas of colloidal crystals made from silica Žradius s 113 nm. and polystyrene Žradius s 322 nm.. They potentiostatically deposited gold in these opals from a HAuCl 4 solution. Prior to electrodeposition, the silica spheres were sintered through a heat treatment at 600 ⬚C. After the deposition of the gold the template was removed by etching the silica with aqueous HF. The polystyrene spheres were removed by combustion at 450 ⬚C. The results can be seen in Fig. 5. The templated spherical voids in the gold replica can be seen clearly. In the case of the silica template, there is good fidelity between the 䢇
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original colloidal assembly and the replica. There are no dimensional changes between the dried, sintered colloid and the final replica, although some cracking is observed during the original drying and sintering process. This indicates that the electrochemically formed gold is dense and structurally robust. This is a definite improvement over other methods of producing macroporous structures with high-dielectric materials, e.g. liquid-phase or sol᎐gel chemistry w24,25x, and infiltration with nanosized particles w5 x. Polystyrene colloidal assemblies produced less promising results ŽFig. 5b.. While the gold deposited in a layered fashion on the polystyrene spheres, it is speculated that lack of cohesion between the spheres led to the destruction of long-range order in the replica. Xu et al. w26x described methods to electrodeposit gold and nickel into periodic structures made of 300 nm diameter silica spheres. They also relied on a sintering process of the silica at 120 ⬚C for 2 days, followed by 750 ⬚C for 4 h. The electrodeposition was done galvanostatically in commercially available nickel and gold plating solutions. The silica was again removed using aqueous HF. The final thickness of the metal replica was ; 100 m. The spherical ‘floret’-like appearance could be due to incomplete filling during electroplating or the malleability of gold, which could lead to some restructuring during drying. A similar effect was observed during electroless plating in a colloidal structure reported by Jing et al. w27x. Nickel is reported to be more robust leading to replicas with greater fidelity. Luo et al. w28x reported good results on Ni and Pt replication of colloids. The pore size varied between 90 and 210 nm depending on the size of the original polystyrene spheres. They also measured the internal surface area Ž6.2 m2rg for Ni and 3.6 m2rg for Pt., and uniform pore size ŽNi, 142 " 7 nm; Pt, 125 " 5 nm.. Since the original sphere size was 210 nm, significant shrinkage occurs in these cases. The film thicknesses obtained were adjusted from several micrometers to tens of micrometers, with typical growth rates of 100 nmrmin. These authors also report electrodeposition of a SnCo alloy from a commercially available SnCo solution bath. Changing the composition of the alloy allows control over the hardness, chemical and optical properties. A group at the University of Southampton w29x prepared structured macroporous palladium, platinum and cobalt films using polystyrene latex sphere templates. Fig. 6 shows a variety of different films. While the quality of these films appears reasonable, it should be noted that they are typically only a few sphere layers thick. A qualitatively different approach was reported by Lellig et al. w30x. They used a relatively low volume
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Fig. 5. Ža. SEM image of a crystal of air spheres Žradius s 111 nm. in gold, made with a silica template. The inset is a Fourier transform of the SEM image. Žb. SEM of macropores Žradius s 322 nm. in gold, made with a latex template. The structure has short-range order, but no long-range order. Reproduced with permission from Wiley.
fraction of highly charged colloidal crystals that had been immobilized in a polyacrylamide hydrogel as a template. The hydrogel composite was polymerized on a stainless steel electrode, and silver was deposited in the interstices from an aqueous AgNO3 solution. Drying of these samples led to shrinkage, but SEM revealed that large areas of the silver sample preserved the long-range order of the original colloidal crystal. Due to the low volume fraction of the polymeric colloid, the template was not continuous and could not be removed, contrary to the cases described above where the colloidal spheres were arranged in a hardsphere touching configuration. This is an obvious disadvantage if the final product is supposed to have very large dielectric constant contrast. Other applications, e.g. catalysis, would also require an interconnected internal void space.
3. Conclusion Recent work has demonstrated that electrodeposition through the interstitial space of highly-ordered colloidal crystals has promise for creating photonic crystals comprised of a diverse set of materials including semiconductors, metals and polymers. The resulting three-dimensionally macroperiodic materials have been formed with close-packed macropores ranging in diameter from 100 nm to a few microns, giving the potential to modulate light ranging from deep UV to the infrared. However, there are still many important problems to overcome before this approach to photonic structures comes to fruition. The first is the perfection of the self-assembled template. Although this review concentrates on electrochemical routes to templating of materials with colloidal crystals, much
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Fig. 6. SEM micrographs of macroporous films of platinum, palladium and cobalt electrochemically deposited at 0.10, 0.25 and y0.90 V vs. SCE, respectively, through templates formed from 400 nm Ža᎐c. and 700 nm Žd. polystyrene spheres pre-assembled on gold surfaces. Ža. Platinum film, deposition charge of y2.00 C cmy2 ; Žb. cobalt film, deposition charge of y1.40 C cmy2 ; Žc. cross-sectional image of a platinum film, deposition charge of y1.50 C cmy2 ; and Žd. palladium film, deposition charge of y1.15 C cmy2 .
significant work remains to be done on creating colloidal templates with sufficient short- and long-range order. Assuming such perfect templates can be assembled, the electrodepostion conditions will have to be regulated to create macroporous structures with a high degree of structural order. This includes, for example, the size of the interconnects between the macropores. For polymeric materials at least, initial results on the effect of electrodeposition potential on interconnect diameter are promising. Just as critical is the refractive index of the electrodeposited material. Under ideal conditions, for a complete photonic band-gap, a refractive index contrast between the material and the voids of at least three is required. So far, it has not been demonstrated that any of the systems can meet this requirement. There is, however, an extraordinarily diverse set of materials that can be electrodeposited, and if high refractive index materials such as Si or Ge can be electrochemically templated with colloidal crystals, this requirement will be met. In addition, composite structures such as metallodielectric systems may have strong photonic responses. Mechanically flexible polymeric structures, with optical properties that can be modulated electrochemically may find application as switchable optical elements. The future indeed is bright for electrochemically grown photonic crystals. References and recommended reading 䢇 䢇䢇
of special interest of outstanding interest
w1x Joannopoulous JD, Meade RD, Winn JN. Photonic crystals:
molding the flow of light. Princeton: Princeton University Press, 1995. This is a general introduction to the field of Microphotonics, which is easily accessible to readers at the undergraduate level. Concepts of one-, two- and three-dimensional photonic crystals and full band-gap materials are well explained. Also included is a description of defects and waveguides. w2x Busch K, John S. Photonic band gap formation in certain self-organizing systems. Phys Rev E 1998;58:3896᎐3908. w3x Lin SY, Fleming JG, Hetherington DL et al. A three-dimensional photonic crystal operating at infrared wavelengths. Nature 1998;394:251᎐253. w4x Blanco A, Chomski E, Grabtchak S et al. Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres. Nature 2000;405: 437᎐440. w5x Vlasov YA, Yao N, Norris DA. Synthesis of photonic crystals for optical wavelengths from semiconductor quantum dots. Adv Mater 1999;11:165᎐169. w6x Braun PV, Wiltzius P. Electrochemically grown photonic 䢇 crystals. Nature 1999;402:603᎐604. This is the first paper in which replication of a colloidal crystal using electrodeposition is reported. Both CdS and CdSe replicas were constructed with greater robustness and a higher index of refraction than had been achieved with other methods. w7x Braun PV, Wiltzius P. Electrochemical fabrication of 3-D microperiodic porous materials. Adv Mater 2001;13:482᎐485. w8x Sumida T, Wada Y, Kitamura T, Yanagida S. Macroporous ZnO films electrochemically prepared by templating of opal films. Chem Lett 2001;1:38᎐39. w9x Pandey RK, Sahu SN, Chandra S. Handbook of semiconductor electrodeposition. New York: Marcel Dekker, 1996. w10x Gurunathan K, Vadivel Murugan A, Marimuthu R, Mulik UP, Amalnerkar DP. Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices. Mater Chem Phys 1999;61:173᎐191. w11x Martin CR. Nanomaterialsᎏa membrane based synthetic approach. Science 1994;266:1961᎐1966. 䢇
P.V. Braun, P. Wiltzius r Current Opinion in Colloid & Interface Science 7 (2002) 116᎐123 w12x Koopal CGJ, Feiters MC, Nolte RJM, Deruiter B, Schasfoort RBM. 3rd generation amperometric biosensor for glucosepolypyrrole deposited within a matrix of uniform latex particles as mediator. Bioelectrochem Bioenerg 1992;29:159᎐175. w13x Sumida T, Wada Y, Kitamura T, Yanagida S. Electrochemi䢇 cal preparation of macroporous polypyrrole films with regular arrays of interconnected spherical voids. Chem Commun 2000;1613᎐1614. The novelty of this paper is two-fold. It is the first replication of a colloidal self-assembled crystal using electrodeposition of a polymer, polypyrrole. In addition, the macropores of the void space were interconnected with size-controlled holes. The size of the holes could be adjusted by changing the deposition conditions. w14x Bartlett PN, Birkin PR, Ghanem MA, Toh CS. Electrochemical syntheses of highly ordered macroporous conducting polymers grown around self-assembled colloidal templates. J Mater Chem 2001;11:849᎐853. w15x Cassagneau T, Caruso F. Semiconducting polymer inverse opals prepared by electropolymerization. Adv Mater 2002;14:34᎐38. w16x Schlesinger M, Paunovic M. In: Schlesinger M, Paunovic M, editors. Modem electroplating, 4th ed. New York: John Wiley, 2000. w17x Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998;391:667᎐669. w18x Moroz A. Photonic crystals of coated metallic spheres. Euro䢇 phys Lett 2000;50:466᎐472. Most of the photonic crystal research focuses on purely dielectric structures. In order to get a full band-gap in a three-dimensional material, the required index of refraction contrast is very large Že.g. n ) 3.0 for an fcc lattice of air spheres with the high index material in the interstitial space.. While this is achievable in the near infrared with semiconductors, it is probably not possible to find an appropriate material in the visible part of the spectrum. Moroz shows a different approach by calculating the band structure of metallodielectric materials. Here, plasmon resonances play an important role and keep the losses typically associated with metals under control. w19x Moroz A. Three-dimensional complete photonic band-gap structures in the visible. Phys Rev Lett 1999;83:5274᎐5277. w20x Zhang WY, Lei XY, Wang ZL et al. Robust photonic band gap from tunable scatterers. Phys Rev Lett 2000;84: 2853᎐2856.
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w21x Oldenburg SJ, Averitt RD, Westcott SL, Halas NJ. Nanoengineering of optical resonances. Chem Phys Lett 1998;288: 243᎐247. w22x Graf C, van Blaaderen A. Metallodielectric colloidal coreshell particles for photonic applications. Langmuir 2002;18: 524᎐534. w23x Wijnhoven JEGJ, Zevenhuizen SJM, Hendriks MA, Van䢇 maekelbergh D, Kelly JJ, Vos WL. Electrochemical assembly of ordered macropores in gold. Adv Mater 2000;12:888᎐890. This is the first paper of several in a short time interval reporting replication with a metal. Electrodeposition of metals has, of course, been known for a long time, although for the particular application of photonics, these metal films would have to be very thin and very well controlled in their morphology. It is not clear at this point whether metal deposition will be a successful method to build metallodielectric structures. There are certainly other applications, e.g. catalysis, filtration, electronic or magnetic materials, where this method could prove very useful. w24x Holland BT, Blanford CE, Stein A. Synthesis of macroporous minerals with highly ordered three-dimensional arrays of spheroidal voids. Science 1998;281:538᎐540. w25x Wijnhoven JEGJ, Vos WL. Preparation of photonic crystals made of air spheres in titania. Science 1998;281:802᎐804. w26x Xu L, Zhou WL, Frommen C, Baughman RH, Zakhidov AA, Malkinski L, Wang JQ, Wiley JB. Electrodeposited nickel and gold nanoscale meshes with potentially interesting photonic properties. Chem Commun 2000;997᎐998. w27x Jiang P, Cizeron J, Bertone JF, Colvin VL. Preparation of macroporous metal films from colloidal crystals. J Am Chem Soc 1999;121:7957᎐7958. w28x Luo Q, Liti Z, Li L, Xie S, Kong J, Zhao D. Creating highly ordered metal, alloy, and semiconductor macrostructures by electrodeposition, ion spraying, and laser spraying. Adv Mater 2001;13:286᎐289. w29x Bartlett PN, Birkin PR, Ghanem MA. Electrochemical deposition of macroporous platinum, palladium and cobalt films using polystyrene latex sphere templates. Chem Commun 2000;1671᎐1672. w30x Lellig C, Hartl W, Wagner J, Hempelmann R. Immobilized highly charged colloidal crystals: new route to three-dimensional mesoscale structured materials. Angew Chem Int Ed 2002;41:102᎐104.
