Mat. Res. Soc. Symp. Vol. 636 © 2001 Materials Research Society
Fabrication of Micro- and Nanostructures with Monodispersed Colloidal Spheres as the Active Components
Byron Gates, Brian Mayers, Zhi-Yuan Li, and Younan Xia* Department of Chemistry, University of Washington, Seattle, WA 98195-1700
[email protected] ABSTRACT Monodispersed colloidal spheres with dimensions in the range of 100 nm to 10 µm can be used as building blocks to fabricate highly ordered 3D micro- and nanostructures. For example, they can be self-assembled into closely packed lattices, which can be subsequently used as templates to generate 3D porous structures. Here we present the recent progress in our group regarding this approach. INTRODUCTION Crystalline arrays of monodispersed colloidal spheres have found a number of applications in the fabrication of micro- and nanostructured materials. For example, spherical colloids have been extensively exploited as the building blocks to fabricate long-range ordered, 3D structures through processes such as crystallization[1] or template-directed synthesis.[2] These periodic lattices have been pursued for the fabrication of photonic bandgap (PBG) crystals that are characterized by a spatially periodic variation of high and low dielectric regions.[3] Such a crystal is capable of manipulating photons in all three dimensions of space: for example, to block the propagation of photons or confine photons to a specific area at restricted frequencies. A number of approaches have been successfully demonstrated by various groups to fabricate these crystals for use in different spectral regions.[1,2,4] Spherical colloids have been used as building blocks to generate periodic lattices with well-defined and highly ordered structures. These crystalline arrays of colloids have been found to exhibit stop bands with the midgap position easily controlled by changing the size of the colloidal spheres. The optical properties of these lattices are dependent on the structure and long-range order of the crystalline array. Understanding the influence of bulk and point defects will be important to fabricating and controlling the optical properties of PBG crystals.[5] This paper presents our recent studies on the defects within crystalline lattices of colloidal particles with an analysis by transmission spectroscopy and scanning electron microscopy (SEM). Theoretical predictions are also presented along with a synthetic approach to fabricating PBG crystals with the appropriate structure to obtain a complete bandgap. EXPERIMENT Crystalline lattices with micro- and nanostructured periodicity were selfassembled from polystyrene (PS) colloids (Bangs Laboratories, Fishers, IN; Polysciences, Warrington, PA) using a procedure demonstrated by our group.[6] The crystalline arrays D9.15.1
obtained by this method usually have a cubic-close-packed (ccp) structure with the ABC stacking sequence perpendicular to the substrates between which the crystal is grown. Monodispersed colloidal suspensions of 230-nm PS colloids were used to form host lattices to study the influence of defects on the structural and optical properties of the crystalline lattice. Dopants were introduced into the aqueous dispersions prior to their organization into 3D arrays. Monodispersed samples of 155-nm PS colloids were mixed in known ratios with the host particles. The crystalline arrays were analyzed using UVvisible spectroscopy (HP-8453) and field emission scanning electron microscopy (JEOL6300F). Three-dimensional porous membranes were fabricated by templating sol-gel precursors against the crystalline arrays of colloids.[2i] Inorganic oxide membranes were prepared from sol-gel precursors (e.g., tetraethylorthosilicate for silica membranes) dissolved in isopropanol. The precursor solution was injected into a dried crystalline lattice of PS colloids by capillary action. This precursor hydrolyzed within the lattice upon exposure to atmospheric moisture. Ceramic membranes were obtained by etching the polystyrene spheres in toluene for ~2 hours. RESULTS We have successfully self-assembled a variety of colloidal particles with different sizes and surface properties. We changed the dimensions of the crystal by controlling the design of our confinement cells, in an effort to fabricate photonic crystals with a midgap position at wavelengths in the range from visible to near infrared. Figure 1 shows the results of monodispersed colloids self-assembled into crystalline lattices. Figure 1A shows a digital image of a confinement cell containing three sizes of colloidal particles, each separately crystallized in close-packed arrays (Figure 1B) by a sequential injection scheme. The three crystalline lattices have unique optical properties with a midgap position dependent on the size of the colloidal particle. Independent transmission spectra are shown in Figure 1C for the different regions showing a red shift in midgap position with increased particle diameter. The packing density and structure of these ccp lattices could also be further controlled by sintering the sample at elevated temperatures,[6d] and changing the interactions between charges on the surface of the colloidal particles.[6e]
Figure 1. (A) Photograph of the confinement cell used to assemble PS colloids into a series of ccp lattices. (B) SEM image with a close-up view of the (111) crystalline plane of a close-packed array of 220-nm PS colloids. (C) UV-visible transmittance spectra for three crystalline arrays from colloidal particles with diameters of 206, 220 and 270-nm. D9.15.2
Figure 2A shows the UV-visible transmission spectra of a series of 3D crystalline lattices fabricated from colloidal dispersions of 230-nm PS colloids into which different amounts of 155-nm PS colloids have been added. The spectra show the stop band at ~573 nm does not change appreciably in position with increased doping up to 17% of the host lattice. A decrease from ~9.8 to ~3.0 dB is observed in the maximum attenuation of this pseudo bandgap with increased doping levels. Simultaneously, the transmittance at wavelengths shorter than the midgap significantly decreased. The influence of the doping on the bandgap properties of these crystalline arrays could be understood through analysis of the change in defects within the lattice. Doped crystals were analyzed using SEM, comparing the crystals with increasing levels of dopant. Figure 2B and 2C compare the structures of two crystalline arrays. The SEM image in Figure 2B shows the (111) plane of a photonic crystal doped with 155-nm PS at 1.4%. Long-range order is preserved in this 3D lattice at this low level of doping. Higher levels of dopants presented point defects and also had a more profound influence on the long-range order within the 3D lattice (Figure 2C). This figure shows dislocations and slight variance in domain orientations with a 9.1% doping of the host colloids. Further increases in the doping level were accompanied by a higher density of dislocations and subsequently smaller domain sizes.
Figure 2. (A) UV-visible transmittance spectra of a series of photonic crystals that were assembled from dispersions of 230-nm PS colloids doped with increasing percentages (017%) of 155-nm PS colloids. The midgap attenuation at ~573 nm, represented by the number in parenthesis, decreased with increasing level of doping. (B, C) SEM images of the (111) planes of two photonic crystals of 230-nm PS colloids doped with 1.4% (B) and 9.1% (C). The arrows in (B) and (C) point to defects induced by the 155-nm PS colloids. Crystalline lattices can be easily self-assembled from colloidal particles with an angle dependent midgap position predicted by the Bragg equation, but do not have a complete photonic bandgap.[1a] Theoretical predictions have been used to predict the appropriate structure and refractive index contrast necessary for a photonic crystal to have a complete bandgap. We have used the Plane-Wave-Expansion-Method (PWEM) to calculate photonic band structure for different crystalline lattices with an opaline structure.[7] Figure 3A shows the band structure calculated for an opaline lattice of silicon with a refractive index contrast (n0/n1) of 3.49. The figure predicts the photon propagation in three-dimensions through the photonic crystal, plotted in reduced frequency units over all reciprocal space. Overlapping bands predict an incomplete photonic bandgap and a material characterized by pseudo bandgaps. An alternative structure with a complete photonic bandgap is an inverse opal (crystalline porous D9.15.3
membrane). Figure 3B shows the photonic band structure for an inverse opal of silicon (n1/n0 = 3.49). A complete bandgap extends over all the reciprocal space for this structure.
Figure 3. Photonic band structures calculated by Plane-Wave-Expansion-Method (A) for an opal of silicon colloids with a 74% filling fraction of silicon in an fcc lattice and (B) for an inverse opal with an fcc lattice of air (74% filling fraction) in a silicon matrix. Pseudo bandgaps exist in (A) and a complete photonic bandgap exists in (B) as indicated by the bar extending over all reciprocal space. Templating with liquid precursors against the crystalline lattice of colloidal particles is an effective method of generating inverse opals. After curing the liquid precursor into a solid, the colloidal particles are selectively removed through a thermal combustion or chemical etching. Figure 4A shows the SEM image of an inverse opal of silica fabricated from sol-gel precursors templated against 220-nm PS colloids. An inverse opal of a higher refractive index material, titania, is shown in Figures 4B and 4C, templated against 480-nm PS colloids. These inverse opals have a periodicity determined by the diameter of the colloidal particles. Templating with the appropriate precursor materials, we believe this is a viable fabrication route to inverse opals with higher refractive indicies for use as photonic bandgap crystals in the UV to NIR regime. Aside from photonic applications the formation of mesostructured materials with controlled dimensions, periodicity, and chemical structure have applications from microfluidics to catalytic supports.
Figure 4. SEM images of ceramic inverse opals. (A) The top surface of a silica membrane fabricated by templating against a crystalline array of 220-nm PS colloids. (B, C) Titania inverse opal fabricated by templating with sol-gel precursors against a crystalline array of 480-nm PS colloids. A high magnification image (C) shows windows into neighboring void spaces. D9.15.4
CONCLUSIONS In summary, we have presented a convenient method for self-assemblying mesoparticles into highly ordered, 3D crystalline lattices. The assembly process is fast, versatile, and has allowed one to incorporate dopants to study their influence on the photonic and structural properties of these crystalline arrays. It also offers a useful approach to the fabrication of inverse opal structures. Both steps are important in fabricating photonic bandgap crystals exhibiting complete bandgaps and a range of other useful properties. In addition, this approach is an effective route to uniform composite or porous materials with desired periodicity and chemical composition. ACKNOWLEDGEMENTS This work has been supported in part by a National Science Foundation (NSF) Career Award (DMR-9983893), a subcontract from the AFOSR MURI Center at the University of Southern California, a Research Fellowship from the Alfred P. Sloan Foundation, a New Faculty Award from the Dreyfus Foundation, and startup funds from the UW. B.G. would like to thank the Center for Nanotechnology at the UW for an IGERT Fellowship Award funded by the NSF (DGE-9987620). REFERENCES 1. See, for example, (a) a recent review: Y. Xia, B. Gates, Y. Yin, and Y. Lu, Adv. Mater. 12, 693, (2000). (b) O. D. Velev, A. M. Lenhoff, and E. W. Kaler, Science 287, 2240, (2000). (c) H. Miguez, F. Meseguer, C. Lopez, A. Blanco, J. S. Moya, J. Requena, A. Mifsud, and V. Fornes, Adv. Mater. 10, 480, (1998). (d) W. L. Vos, R. Sprik, A. van Blaaderen, A. Imhof, A. Lagendijk, and G. H. Wegdam, Phys. Rev. B 53, 16231, (1996). (e) J. H. Holtz, and S. A. Asher, Nature 389, 829, (1997); (f) I. I. Tarhan, and G. H. Watson, Phys. Rev. Lett. 76, 315, (1996). 2. See, for example, (a) A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S. W. Leonard, C. Lopez, F. Meseguer, H. Miguez, J. P. Mondia, G. A. Ozin, O. Toader, and H. M. van Driel, Nature 405, 437, (2000). (b) O. D. Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, Nature 389, 447, (1997). (c) Yu. A. Vlasov, N. Yao, and D. J. Norris, Adv. Mater. 11, 165, (1999). (d) J. E. G. J. Wijnhiven, and W. L. Vos, Science 281, 802, (1998). (e) B. T. Holland, C. F. Blanford, and A. Stein, Science 281, 538, (1998). (f) P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, and G. D. Stucky, Science 282, 2244, (1998). (g) A. A. Zakhidov, R. H. Baughman, Z. Iqbal, C. Cui, I. Khayrullin, S. O. Danta, J. Marti, and V. G. Ralchenko, Science 282, 897, (1998). (h) G. Subramanian, V. N. Manoharan, J. D. Thorne, and D. J. Pine, Adv. Mater. 11, 1261, (1999). (i) B. Gates, Y. Yin, and Y. Xia, Chem. Mater. 11, 2827, (1999). (j) G. Subramania, K. Constant, R. Biswas, M. M. Sigalas, and K.M. Ho, Appl. Phys. Lett. 74, 3933, (1999). (k) O. D. Velev, P. M. Tessier, A. M. Lenhoff, and E. W. Kaler, Nature 401, 548, (1999). (l) P. Jiang, J. Cizeron, J. F. Bertone, and V. L. Colvin, J. Am. Chem. Soc. 121, 7957, (1999). D9.15.5
3. (a) J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, Nature 386, 143, (1997). 4. See, for example, (a) S. Noda, N. Yamamoto, and A. Sasaki, Jpn. J. Appl. Phys. 35, L909, (1996). (b) 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, and J. Bur, Nature 394, 251, (1998). (c) M. Campbell, D. N. Sharp, M. T. Harrison, R. G. Denning, and A. J. Tuberfield, Nature 404, 53, (2000). (d) S. Noda, K. Tomoda, N. Yamamoto, and A. Chutinan Science 289, 604, (2000). 5. See, for example, (a) special issue in IEEE J. Lightwave Technology 17, 1931, (1999). (b) J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals, Molding the Flow of Light, (Princeton, 1995), pp.78-93. 6. (a) S. H. Park, D. Qin, and Y. Xia, Adv. Mater. 10, 1028, (1998). (b) S. H. Park, and Y. Xia, Langmuir 15, 266, (1999). (c) B. Gates, D. Qin, and Y. Xia, Adv. Mater. 11, 466, (1999). (d) B. Gates, S. H. Park, and Y. Xia, Adv. Mater. 12, 653, (2000). (e) B. Gates, and Y. Xia, Adv. Mater. 12, 1329, (2000). 7. (a) Z.-Y. Li, J. Wang, and B.-Y. Gu, Phys. Rev. B 58, 3721, (1998). (b) K. Busch, and S. John, Phys. Rev. E 58, 3896, (1998). (c) M. Doosje, B. J. Hoenders, and J. Knoester, J. Opt. Soc. Am. B 17, 600, (2000).
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