a three-dimensional photonic crystal operating in the visible region

Communications

462 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0604-0462 $ 17.50+.50/0 Adv. Mater. 1999, 11, No. 6

99.27(2)�, V = 1388.6(7) �3, Z = 4, Dcalc = 2.555 g/cm3. Data collectionwas performed on a Rigaku AFC-5R diffractometer with graphitemonochromated Mo Ka radiation (l = 0.71069 �) and a rotating an-ode generator. Intensities were collected to a maximum 2y value of60.0� by the o±2y scan technique. The total number of independent re-flections measured was 4353, of which 1760 were considered to be ob-served [I > 3.00s(I)]. The structure was solved by direct methods(SHELXS86). The non-hydrogen atoms were refined anisotropically.Hydrogen atoms were included but not refined. Full-matrix least-squares refinement gave R = 0.064, Rw = 0.063.

[10] H. Kobayashi, A. Kobayashi, Y. Sasaki, G. Saito, H. Inokuchi, Bull.Chem. Soc. Jpn. 1986, 59, 301.

[11] Crystal data for the BF±4 salt: (C10H6N2S2Se4)3(BF4)1.5(CH2Cl2)0.6, Mr

= 1783.56, monoclinic, space group C2/c, a = 35.119(9), b = 12.578(7), c= 21.985(8) �, b = 99.37(3)�, V = 9581(6) �3, Z = 8, Dcalc = 2.473 g/cm3. Data collection was performed on a Rigaku AFC-7R diffracto-meter with graphite monochromated Mo Ka radiation (l = 0.71069 �)and a rotating anode generator. Intensities were collected to a maxi-mum 2y value of 60.1� by the o±2y scan technique. The total numberof independent reflections measured was 12 709, of which 2395 wereconsidered to be observed [I > 3.00s(I) ]. The structure was solved bydirect methods (SHELXS86). The sulfur and selenium atoms were re-fined anisotropically. Boron atoms were fixed. The other non-hydro-gen atoms were refined isotropically. Hydrogen atoms were includedbut not refined. Full-matrix least-squares refinement gave = 0.064, Rw

= 0.056.[12] Crystal data for the ClO±

4 salt: (C10H6N2S2Se4)3(ClO4)1.5(CH2Cl2)0.6,

Mr = 1802.53, monoclinic, space group C2/c, a = 35.308(5), b =12.600(4), c = 21.996(7) �, b = 99.36(2)�, V = 9655(6) �3, Z = 8, Dcalc =2.480 g/cm3. Data collection was performed on a Rigaku AFC-7R dif-fractometer with graphite monochromated Mo Ka radiation (l =0.71069 �) and a rotating anode generator. Intensities were collectedto maximum 2y value of 60.0� by the o±2y scan technique. The totalnumber of independent reflections measured was 12 490, of which2090 were considered to be observed [I > 3.00s(I)]. The structure wassolved by direct methods (SHELXS86). The sulfur, selenium and chlo-rine (of anions) atoms were refined anisotropically. The other non-hy-drogen atoms were refined isotropically. Hydrogen atoms were includ-ed but not refined. Full-matrix least-squares refinement gave R =0.068, Rw = 0.065.

[13] a) T. Mori, F. Sasaki, G. Saito, H. Inokuchi, Chem. Lett. 1986, 1589. b)A. Ugawa, K. Yakushi, H. Kuroda, A. Kawamoto, J. Tanaka, Chem.Lett. 1986, 1875. c) A. Ugawa, Y. Okawa, K. Yakushi, H. Kuroda, A.Kawamoto, J. Tanaka, M. Tanaka, Y. Nogami, S. Kagoshima, K. Mura-ta, T. Ishiguro, Synth. Met. 1988, 27, A407.

[14] U. Geiser, J. A. Schlueter, A. M. Kini, C. A. Achenbach, A. S. Komo-sa, J. M. Williams, Acta Crysallogr. C 1996, 52, 159.

[15] Slater-type atomic orbitals were used for the calculation of molecularorbitals. The exponent z and the ionization potential [eV] are: Se 4s,2.112, ±20.0; Se 4p, 1.827, ±11.0; Se 4d, 1.500, ±6.8; S 3s, 2.122, ±20.0; S3p, 1.827, ±11.0; S 3d, 1.500, ±5.4; N 2s, 1.950, ±26.0; N 2p, 1.950, ±13.4;C 2s, 1.625, ±21.4; 2p, 1.625, ±11.4; H 1s, 1.0 ±13.6.

A Three-Dimensional Photonic CrystalOperating in the Visible Region**

By Sang Hyun Park, Byron Gates, and Younan Xia*

This communication reports the fabrication and charac-terization of a three-dimensional (3D) photonic bandgap

(PBG) crystal that operates in the visible region of electro-magnetic radiation. The PBG crystal is a cubic-close-packed (ccp) lattice assembled from 215 nm polystyrenebeads whose surfaces have been doped with the organicdye Oil Blue N. This crystalline assembly exhibits a largePBG that extends from ~450 to ~710 nm, with a gap±mid-gap ratio of ~45 % and a maximum bandgap rejection of~26.8 dB.

Recently, PBG crystals have received considerable atten-tion due to their ability to confine and control electromag-netic (EM) waves in all three directions of space.[1] A PBGcrystal is a three-dimensionally ordered dielectric structurehaving a spatially periodic dielectric constant with a latticeparameter comparable to the wavelength of the electro-magnetic wave. Under certain conditions (e.g., with an ap-propriate symmetry and sufficient contrast between thehigh and low electric permittivity regions), a PBG crystalmay exhibit a frequency band in which electromagneticwaves are forbidden, irrespective of their directions ofpropagation in reciprocal space. Such a photonic crystalwith a complete bandgap can be used to localize electro-magnetic waves to specific areas, to inhibit spontaneousemission, and to guide propagation of electromagneticwaves along certain directions at restricted frequencies. Allof these properties are technologically important becausethey can be used, for instance, to improve the performanceof semiconductor lasers, as well as other kinds of quantumelectronic devices.[1]

It has been, however, very difficult to fabricate 3D PBGcrystals that operate at optical frequencies because conven-tional microlithographic techniques cannot be easily ap-plied to the fabrication of 3D periodic structures with fea-ture sizes comparable to the wavelength of visible light.[2]

An alternative route to 3D PBG crystals is based on crys-talline arrays of spherical particles.[3] These particles cannow be readily synthesized as monodisperse samples withprecisely controlled diameters ranging from a few nano-meters to a few hundred micrometers. They can also be as-sembled into highly ordered structures using a number ofmethods.[3,4] Silica colloids and polystyrene beads are thetwo most commonly used materials for such an application.Because the refractive-index contrast between the spheresand the matrix material (usually air or water) is relativelylow (< 2), only narrow stop bands have ever been observedin the crystalline assemblies made from these two materi-als.[3,4c] One efficient way to increase the width of the stopband (to make it overlap along all crystal directions) is toincrease the refractive-index contrast between the spheresand the matrix material. Different approaches have beendemonstrated for this purpose, for example, the use ofspherical particles (such as TiO2) with refractive indices³ 2.5,[5] or the incorporation of semiconductor nanopar-ticles (such as CdS) into the void spaces among the spheri-cal beads.[6] Here we describe another approach: namely, touse an organic dye to dope the surfaces of polystyrenebeads. The organic dye should be selected such that its ab-

±

[*] Prof. Y. Xia, Dr. S. H. Park, B. GatesDepartment of Chemistry, University of WashingtonSeattle, WA 98195-1700 (USA)

[**] This work has been supported in part by a New Faculty Award fromthe Dreyfus Foundation, a subcontract from the AFOSR MURI Cen-ter at the University of Southern California, a Royalty Research Fundfrom the UW, and start-up funds from the UW. It used the Microfabri-cation Laboratory at the Washington Technology Center (WTC). B.G.thanks the Center for Nanotechnology at the UW for a fellowship

sorption peak(s) overlap(s) with the Bragg diffraction peakof the opaline structure. Due to the resonant effect, the re-fractive-index contrast will be dramatically increased whenthe wavelength reaches the absorption peak(s) of the or-ganic dye. Therefore, a 3D crystalline lattice assembledfrom these dyed polymer beads may exhibit a wide photon-ic bandgap whose spectral position and width will be inde-pendent of the direction of light propagation.

Figure 1 shows a photograph of the experimental devicethat we have used to assemble monodisperse spherical par-ticles into the crystalline ccp structure.[4] After an aqueousdispersion (~0.05 wt.-%) of polystyrene beads (monodis-perse, Polysciences, Warrington, PA, USA) was injectedinto the cell, a positive pressure was applied through theglass tube to force the solvent (water) to flow through thechannels at the bottom of the cell. The polymer beads re-tained in the cell were assembled into a crystalline struc-ture under continuous sonication. Scanning electron mi-croscopy (SEM) and optical diffraction measurementshave confirmed that the polystyrene beads were crystal-lized into the ccp structure (similar to that of a naturalopal, with a packing density of ~74 %); the beads are inphysical contact, with the (111) face parallel to the glasssubstrates.[4c] Using this procedure, we have been able toproduce 3D opaline structures from polystyrene beads orsilica colloids (with diameters ranging from ~50 nm to~3 mm) over areas as large as ~1 cm2;[7] the thickness of theassembly could be changed as desired from ~1.2 to~50 mm.[4c]

Fig. 1. A photograph of the cell that was used to assemble polystyrene beadsinto the ccp structure. The green-colored region at the bottom of this cell isa ccp assembly crystallized from pristine polystyrene beads of 230 nm diam-eter.

One of the important features of this procedure is that itcan be applied to a wide variety of spherical particles re-gardless of their chemical compositions and/or surfaceproperties.[4] Figure 2 shows SEM images of an opalinecrystal that was assembled from 215 nm polystyrene beadsin a 12 mm thick cell over an area of ~1 cm2. The surfaces ofthe polystyrene beads had been doped with an organic dye,Oil Blue N.[8,9] Figure 2A shows the top view of a portion of

the crystalline assembly. The polystyrene beads in the toplayer form a closely packed, hexagonal array with a latticeparameter of 215 nm as measured from the SEM images.Figure 2B shows a cross-sectional SEM image of the assem-bly; this image indicates that the ccp structure extends overthe whole thickness of the cell (the total number of layers is~60) along the direction perpendicular to the surface of thesubstrate. We could precisely control the number of layersby changing the ratio between the thickness of the cell andthe diameter of the polystyrene beads.[4]

Fig. 2. SEM images of a crystalline lattice that was assembled in a 12 mmthick cell from 215 nm polystyrene beads that had been dyed with OilBlue N: A) top view; B) cross-sectional view. The sample was sputtered withgold before imaging with a field emission scanning electron microscope.

Figure 3A shows the transmission spectrum of an opalinestructure that was formed from pristine polystyrene beads(230 nm in diameter) in a 12 mm thick cell. This opalinestructure displays a stop band at 576 nm due to Bragg dif-fraction. As a result of the small contrast in the refractiveindex (in this case, polystyrene and water, n2/n1 = 1.592/1.333), this stop band is narrow (the full width at half maxi-mum, FWHM, is only ~32 nm[10]) and has a maximum at-tenuation of only ~11 dB. The position of this stop bandshifts to different wavelengths when the crystalline assem-bly is tilted relative to the incident light.[4c] These stopbands do not overlap with each other when the sample istilted by an angle of more than 5�. One way to increase therefractive-index contrast is to dope the polystyrene beads

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with an appropriate organic dye. In this case, the organicdye has a frequency-dependent refractive index, which canbe expressed as Equation 1,[11] where qe is the charge on anelectron, m is the mass of an electron, Nk is the density ofelectrons per unit volume, ok is the kth resonant frequencyof the dye molecule, and gk is the kth damping coefficientdue to optical absorption.

n 1q2

e2e0 m k

Nko2

ko2 igko

(1)

When the frequency (o) of light approaches any one ofthe resonant frequencies, the refractive index of the mol-ecule increases dramatically. This increase in refractive-in-dex contrast will generate a wide PBG in the crystalline as-sembly.

Blue dyes are known to have absorption peak(s) in therange of 570±670 nm,[12] which covers the Bragg diffraction

peak of an opaline structure assembled from polystyrenebeads with diameters ranging from 230 to 270 nm.[13] Figur-e 3B shows the transmission spectrum of a dilute dispersionof polystyrene beads (215 nm in diameter, ~0.01 wt.-%,length of cell ~1 cm) whose surfaces have been doped withOil Blue N.[8,9] Figure 3C shows the transmission spectrumof an opaline structure that was assembled in a 12 mm thickcell from the blue-dyed polystyrene beads. The light was un-polarized and propagated along the [111] direction of theccp assembly (or the L-point of the reciprocal lattice).[3a]

The drop-off in the intensity of the transmitted light is dueto a combination of the crystalline structure of the assemblyand the doping of the polystyrene beads with the appropri-ate organic dye. The intensity of transmitted light (Itrans)can be expressed as Equation 2, where Iinc is the intensity ofthe incident light, and ±(Idiff + Iabs) is the attenuation in in-tensity due to Bragg diffraction and dye absorption.

Itrans = Iinc ± (Idiff + Iabs) (2)

The contribution due to dye absorption can be removedby using an amorphous sample (formed from the sameblue-dyed polystyrene beads and of approximately thesame thickness as the above crystalline sample) as a ref-erence. In this case, the recorded spectrum (Fig. 3D) canbe considered as the difference in light attenuation due toBragg diffraction (for the ordered sample) and destructivescattering (for the disordered sample); almost no dye ab-sorption should be involved. As shown in Figure 3D, thereis a very strong drop-off in the transmittance that extendsfrom ~450 to ~710 nm (Dl/l » 45 %), and the strongest at-tenuation is about 26.8 dB.[14]

We studied the angular dependence of the photonic band-gap by tilting the sample with different angles (y) relative tothe incident light (Fig. 4A). Figure 4B shows the transmis-sion spectrum of the opalline assembly that was recorded atroom temperature and at y = 0�. Figures 4C and 4D givetransmission spectra recorded at room temperature and at y= 20� and y = 40�, respectively. When the angle (y) increasesfrom 0� to 40�, the width of the photonic bandgap changesvery little. The relative insensitivity of the peak position tothe tilting angle suggests that there might be a PBG that ex-tends throughout the Brillouin zone for the present PBGcrystal. We also heated the sample to elevated temperaturesto change the packing density of the opaline assembly tolarger than 74 %.[3h,6,15] At temperatures close to the glasstransition temperature Tg of polystyrene (~90 �C), the poly-mer beads deformed to form sintered (slightly shrunk) sam-ples with packing density close to 100 %. Figures 4E and 4Fshow the transmission spectra of an opaline assembly thathas been heated to two different temperatures. SEM studiesshow that the packing density of the opaline structurechanges from ~74 % at room temperature to ~82 % at 60 �Cand 96 % at 90 �C. In this thermal sintering process, thephotonic gap of the crystalline structure also changed very

Fig. 3. The transmission spectra of A) a crystalline assembly that was formedin a 12 mm thick cell from 230 nm pristine polystyrene beads, B) an aqueousdispersion of 215 nm polystyrene beads dyed with Oil Blue N, C) a crystal-line assembly (12 mm thick) formed from the 215 nm polystyrene beadsdoped by Oil Blue N, and D) a crystalline assembly as in (C), but with anamorphous sample as a reference. The amorphous sample was formed fromthe same blue-dyed polystyrene bead as in (C) and had approximately thesame thickness.

little, because the Bragg peaks of samples heated to differ-ent temperatures are expected to be still in the region cov-ered by the absorption of the organic dye.

Fig. 4. A) The scheme used to measure the angular dependence of photonicbandgap structures. B±D) The transmission spectra of a ccp assembly(12 mm thick) that was crystallized from 215 nm blue-dyed polystyrenebeads at y = 0�, 20�, and 40�, respectively. E,F) The transmission spectrumof a ccp assembly that had been heated to 60 �C and 90 �C, respectively, for~6 h. These spectra were recorded without using an amorphous sample as areference.

Photonic bandgap crystals are usually fabricated from di-electric materials with positive, real, and frequency-inde-pendent dielectric constants. A number of experiments andtheoretical calculations have demonstrated that materials(such as metals and polar semiconductors) with frequency-dependent dielectric constants can also be used to con-struct PBG crystals.[16] The results from these studies sug-gest that the dispersion curves for electromagnetic wavespropagating through such systems might exhibit interestingfeatures.[17,18] The system described in this report providesanother class of material that can be used to effectivelyconstruct PBG crystals. A complete theoretical analysis ofthe current system would be useful for the design and fabri-cation of new 3D PBG crystals in the future.

In summary, we have fabricated and characterized a 3DPBG crystal that exhibits a wide PBG in the visible region.The fabrication method that we used is relatively simple andis potentially useful for applications that require 3D PBGcrystals over areas as large as several square centimeters.The system demonstrated here provides a plausible ap-proach for investigating the impurity modes by replacingthe polystyrene beads with scatterers having the same sizebut different dielectric strength. This system also offers flex-

ibility in designing new PBG crystals with lattice parameterstailored for use in specific spectral regions. For example, byusing properly dyed polystyrene beads with a diameter of~500 nm, photonic crystals with operating frequenciesaround 1.5 mm (the most commonly used wavelength in op-tical communication) can also be readily fabricated.

Received: October 12, 1998Final version: December 11, 1998

±[1] Recent reviews: a) J. D. Joannopoulos, P. R. Villeneuve, S. Fan, Nature

1997, 386, 143. b) E. Yablonovitch, J. Opt. Soc. Am. B 1993, 10, 283.[2] See, e.g., a) S. Fan, P. R. Villeneuve, R. D. Meade, J. D. Joannopoulos,

Appl. Phys. Lett. 1994, 65, 1466. b) C. C. Cheng, A. Scherer, J. Vac.Sci. Technol. B 1995, 13, 2696. c) S. Noda, N. Yamamoto, A. Sasaki,Jpn. J. Appl. Phys. 1996, 35, L909. d) T. F. Krauss, R. M. D. L. Rue, S.Brand, Nature 1996, 383, 699. e) A. Rosenberg, R. J. Tonucci, E. A.Bolden, Appl. Phys. Lett. 1996, 69, 2638. f) J. S. Foresi, P. R. Ville-neuve, J. Ferrera, E. R. Thoen, G. Steinmeyer, S. Fan, J. D. Joanno-poulos, L. C. Kimerling, H. I. Smith, E. P. Ippen, Nature 1997, 390,143. g) S. Y. Lin, J. G. Fleming, D. L. Hetherington, B. K. Smith, R.Biswas, K. M. Ho, M. M. Sigalas, W. Zubrzycki, S. R. Kurtz, J. Bur,Nature 1998, 394, 251.

[3] Recent studies: a) I. I. Tarhan, G. H. Watson, Phys. Rev. Lett. 1996, 76,315. b) W. L. Vos, M. Megens, C. M. van Kats, P. Bösecke, J. Phys.:Condens. Matter 1996, 89, 503. c) W. L. Vos, R. Sprik, A. van Blaade-ren, A. Imhof, A. Lagendijk, G. H. Wegdam, Phys. Rev. B 1996, 53,16 231. d) J. H. Holtz, S. A. Asher, Nature 1997, 389, 829. e) W. L. Vos,M. Megens, C. M. van Kats, P. Bösecke, Langmuir 1997, 13, 6004. f)H. Miguez, C. López, F. Meseguer, A. Blanco, L. Vàzquez, R. Mayo-ral, M. Ocana, V. Fornes, A. Mifsud, Appl. Phys. Lett. 1997, 71, 1148.g) R. Mayoral, J. Requena, J. S. Moya, C. López, A. Cintas, H. Miguez,F. Meseguer, L. Vàzquez, M. Holgado, A. Blanco, Adv. Mater. 1997, 9,257. h) J. S. Moya, J. Requena, A. Mifsud, V. Fornes H. Miguez, F. Me-seguer, C. Lopez, A. Blanco, J. S. Moya, J. Requena, A. Mifsud, V. For-nes, Adv. Mater. 1998, 10, 480.

[4] a) S. H. Park, D. Qin, Y. Xia, Adv. Mater. 1998, 10, 1028. b) S. H. Park,Y. Xia, Adv. Mater. 1998, 10, 1045. c) S. H. Park, Y. Xia, Langmuir,1999, 15, 266.

[5] J. E. G. J. Wijnhiven, W. L. Vos, Science 1998, 281, 802.[6] See, e.g., a) Y. A. Vlasov, V. N. Astratov, O. Z. Kaminov, A. A. Ka-

plyanskii, V. N. Bogomolov, A. V. Prokofiev, Phys. Rev. B 1997, 55R,13 357. b) V. N. Bogomolov, S. V. Gaponenko, I. N. Germanenko,A. M. Kapitonov, E. P. Petrov, N. V. Gaponenko, A. V. Prokofiev,A. N. Ponyavina, N. I. Silvanovich, S. M. Samoilovich, Phys. Rev. E1997, 55, 7619.

[7] B. Gates, D. Qin, Y. Xia, Adv. Mater. 1999, 11, this issue, p. 466.[8] a) G. Pan, R. Kesavamoorthy, S. A. Asher, J. Am. Chem. Soc. 1998,

120, 6525. b) P. A. Rundquist, S. Jagannathan, R. Kesavamoorthy, C.Brnardic, S. Xu, S. A. Asher, J. Chem. Phys. 1991, 94, 711.

[9] The dyed polystyrene beads (cat. #15 706) were obtained from Poly-sciences. The polystyrene beads were doped with ~4 % (wt) of OilBlue N; the dye only penetrated ~10 % into the outside surfaces of thepolystyrene spheres.

[10] The FWHM was calculated using a standard method commonly usedin spectroscopy and chromatography: A. Braithwaite, F. J. Smith,Chromatographic Methods, 5th ed., Blackie Academic & Professional,New York, 1986, p. 403.

[11] R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures onPhyscis, Vol. 1, Addison-Wesley, Menlo Park, CA 1977, pp. 31±38.

[12] D. D. Ebbing, General Chemistry, 4th ed., Houghton Mifflin, Boston,MA 1993, p. 1009.

[13] For normal incidence, the position of the Bragg peak can be calculatedas: lmax = 2(2/3)1/2Dnc, where D is the diameter of the polystyrenebeads and nc is the averaged refractive index of the crystal. Becausethe refractive index of dyed polystyrene beads is a little bit higher thanthat of pristine polystyrene beads, we expect that the crystal of 215 nmdyed beads will have a Bragg diffraction peak at a position similar tothat of the crystal formed from 230 nm pristine beads.

[14] The edges of the PBG are indicated in the figures; they were taken asthe point at which the transmittance started to drop dramatically. Thedecibel value was calculated as 10 log(I710 nm/I600 nm).

[15] S. Mazur, R. Beckerbauer, J. Buckholz, Langmuir 1997, 13, 4287.

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[16] See, e.g., A. R. McGurn, A. A. Maradudin, Phys. Rev. B 1993, 48,17 576. M. M. Sigalas, C. T. Chan, K. M. Ho, C. M. Soukoulis, Phys.Rev. B 1995, 52, 11 744. E. R. Brown, O. B. McMahon, Appl. Phys.Lett. 1995, 67, 2138. J. S. McCalmon, M. M. Sigalas, G. Tuttle, K.-M.Ho, C. M. Soukolis, Appl. Phys. Lett. 1996, 68, 2759.

[17] V. Kuzmiak, A. A. Maradudin, Phys. Rev. B 1997, 55, 7427.[18] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, P. A. Wolff, Nature

1998, 391, 667.

Assembly of Nanoparticles intoOpaline Structures over Large Areas**

By Byron Gates, Dong Qin, and Younan Xia*

This paper describes a method that has been used to as-semble nanoparticles into three-dimensionally orderedstructures over relatively large areas (~0.5 cm2). Thesecrystalline lattices of nanoparticles exhibit collective physi-cal behaviors such as Bragg diffraction in the ultraviolet(UV) region.

Monodispersed spherical nanoparticles with well-definedsizes can now be readily prepared in large quantities usingprocedures based on controlled synthesis or nucleation,and confined growth or polymerization.[1] Typical examplesinclude dendrimers,[2] quantum dots,[3] colloids of metals,[4]

colloids of silica,[5] and polymeric latex particles.[6] Theability to assemble these particles into three-dimensionallyordered structures is directly useful in many areas. For ex-ample, such crystalline lattices can be used as sacrificialtemplates to produce highly ordered porous membranes;[7]

and they can be used as model systems to study phase tran-sition and the tunneling or transportation of electrons be-tween nanoparticles with a wide range of different proper-ties.[8] The three-dimensional (3D) periodicity of theselattices also makes them particularly useful as diffractiveelements in the fabrication of optical or electro-optical de-vices.[9] Natural opal, for instance, is opalescent because itconsists of a cubic-close-packed (c.c.p.) lattice (the so-called opaline structure) of spherical particles of silica(150±400 nm in diameter) with a periodicity comparable tothe wavelength of visible light.[10] Synthetic opals as-sembled from mesoscale (100 nm±10 mm in diameter)spheres of polymers or silica have also been widely used inthe fabrication of photonic bandgap (PBG) structures.[11]

Solvent evaporation, sedimentation, and self-assemblybased on electrostatic repulsion are the three most success-

ful methods for producing three-dimensionally orderedstructures from meso- and nanoscale particles.[12±14] How-ever, sedimentation requires powerful centrifuges and longperiods of time in order to sediment completely particlessmaller than 100 nm.[13] Assembly based on electrostatic in-teractions is capable of producing regular arrays of nano-particles over relatively large areas, but the particles in thearrays are usually separated by a certain distance (in mostcases, more than several hundred nanometers) because ofrepulsive interactions between the particles.[14] Self-assem-bly using properly designed chemical linkers has recentlybeen demonstrated by several groups, but the domain sizesof the assemblies produced using this approach are still toosmall to be useful in practical applications.[15] Recently, wehave demonstrated a convenient and practical method thatuses self-assembly under confinement to form crystallinelattices of mesoscale particles over areas as large as ~1 cm2,with well-controlled numbers of layersÐfrom one up to atleast sixty.[16] Here we extend this procedure to the nano-meter scale, with monodispersed polystyrene beads and sil-ica colloids (50 and 100 nm in diameter) as examples todemonstrate the concept.

Figure 1 shows the schematic procedure.[16] After anaqueous dispersion of monodispersed particles(~0.05 wt.-%) was injected into the cell (fabricated by

±

[*] Prof. Y. Xia, B. GatesDepartment of Chemistry, University of WashingtonSeattle, WA 98195-1700 (USA)

Dr. D. QinCenter for Nanotechnology, University of WashingtonSeattle, WA 98195-2140 (USA)

[**] This work was supported in part by a New Faculty Award from theDreyfus Foundation, a subcontract from the AFOSR MURI Center atthe University of Southern California, a Royalty Research Fund andstart-up funds from the University of Washington. It used the Micro-fabrication Laboratory at the Washington Technology Center. B.G.thanks the Center for Nanotechnology for a fellowship, and Dr. S. H.Park for his technical assistance.

Fig. 1. Schematic outline of the experimental procedure. Aqueous disper-sions of particles were injected into the cell through the rubber tube using asyringe. A dispersion of 480 nm polystyrene beads was first injected to forma barrier for the assembly of nanoscale particles.

a three-dimensional photonic crystal operating in the visible region

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