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Novel Network Polymer for Templated Carbon Photonic Crystal Structures Mark W. Perpall,† K. Prasanna U. Perera,† Jeff DiMaio,‡ John Ballato,‡ Stephen H. Foulger,‡ and Dennis W. Smith, Jr.*,† Department of Chemistry and School of Materials Science and Engineering, Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson University, Clemson, South Carolina 29634 Received February 11, 2003. In Final Form: June 11, 2003 Inverse opaline photonic crystal structures are created from carbon precursor polymer networks derived from the thermal Bergman cyclopolymerization of bis-ortho-diynyl arene (BODA) monomers. A new hydroxyfunctional BODA monomer was prepared that exhibited excellent compatibility with silica opal templates. Monomer melt infiltration of the template, in situ thermal polymerization, and pyrolysis, followed by removal of the silica with HF affords a carbon inverse opal structure that conserves the original dimensions of the template. The photonic crystal was characterized by the reflectance spectra, and the refractive index of the carbon was estimated. The functionality of the carbon opal as a sensor element was demonstrated with water/acetonitrile mixtures and reveals a bandstop shift of 13 nm over a refractive index change of 0.011.
Introduction The fabrication of photonic bandstop materials, also known as photonic or electromagnetic crystals, is a rapidly growing field primarily because of their potential applications in optical communication devices and optical sensor technologies.1 Visible photonic crystals are broadly defined as materials that possess spatial periodicities in their dielectric structure that are on the same order of magnitude as the visible wavelengths. This periodicity can permit light to be diffracted depending on the symmetry of the periodicity and the magnitude of the refractive index difference comprising the periodic dielectric structure.2-4 Artificial silica opals based on the self-assembly of solgel prepared particles have been employed to produce a photonic bandstop.5-7 Self-assembled systems in general have potential cost and rapid production advantages over nanolithographic methods. To achieve an inverted opal structure or to further modify the refractive index contrast, the opals can be infiltrated with precursor materials and the silica phase removed by chemical means. Alternatively, sol-gel syntheses can be used to produce the inverse opal media templated by polymer spheres that can be removed thermally.7,8 Recently, silicon-infiltrated silica templates have been used to create a photonic crystal with a complete * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, Clemson University. ‡ School of Materials Science and Engineering, Clemson University. (1) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (2) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Yablonovitch, E.; Gmitter, T. J. Phys. Rev. Lett. 1989, 63, 19501953. (4) Doosje, M.; Hoenders, B. J.; Knoester, J. J. Opt. Soc. Am. B 2000, 17 (4), 600-606. (5) Park, S. H.; Gates, B.; Xia, Y. Adv. Mater. 1999, 11, 462-466. (6) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khyrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897901. (7) Subramanian, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261-1265. (8) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848.
three-dimensional bandstop near the 1550-nm optical communications wavelength.9 Many reports of inorganic inverse opals exist with much focus on TiO2 as a result of its high refractive index.10 Aqueous suspensions of selfassembled charged polymeric particles or crystalline colloidal arrays have also been used as templates to form photonic crystal polymer networks.11 Although amorphous carbon generally is not considered an optical material of much value given its high absorption, the resultant high refractive index (∼2.1) has led to some interest in its use in structures where surface chemistry and reflection are the operative phenomena. Carbon-based structures, including photonic crystals, suffer from the limited availability of suitable and easily fabricated organic precursors. Zakhidov et al.6 have demonstrated the use of phenolic resins and hydrocarbon chemical vapor deposition (CVD) to prepare inverse carbon opals. Phenolic resin precursor methods are limited, however, because of the large shrinkage of the carbon phase during pyrolysis, resulting in multistep (template-cure-etch-pyrolyze) processes in addition to extremely long cure times (1 week).6,12 Here, we report a new low-shrinkage precursor (1) for inverse carbon opal synthesis, which undergoes relatively rapid in situ templating pyrolysis in one step. Hydroxy-terminated acetylenic monomer 1 (Scheme 1) was prepared in three steps from the corresponding commercial bisphenol. Bis-ortho-diynyl arene (BODA) monomers of this type have been employed as processable, highyield (75-80%), and, thus, low-shrinkage glassy-carbon precursors for microfabrication.13 New BODA monomer (9) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miquez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; Van Driel, H. M. Nature 2000, 405, 437-439. (10) Blanford, C. F.; Yan, H.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13 (6), 401-407. Turner, M. E.; Trentler, T. J.; Colvin, V. L. Adv. Mater. 2001, 13 (3), 180-183. Yang, P.; Rivzvi, A. H.; Messer, B.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Adv. Mater. 2001, 13 (6), 427-431. (11) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W., Jr.; Ballato, J. Langmuir 2001, 17, 6023-6026. Foulger, S. H.; Jiang, P.; Lattam, A.; Smith, D. W., Jr.; Ballato, J.; Dausch, D.; Grego, S.; Stoner, B. Adv. Mater. 2003, 15, 685-689. (12) Jenkins, G. M.: Kawamura, K. Polymeric carbonsscarbon fibre, glass, and char; Cambridge University Press: Cambridge, MA, 1976. (13) Shah, H. V.; Brittain, S. T.; Huang, Q.; Hwu, S.-J.; Whitesides, G. M.; Smith, D. W., Jr. Chem. Mater. 1999, 11 (10), 2623-2625.
10.1021/la034237v CCC: $25.00 © 2003 American Chemical Society Published on Web 08/07/2003
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Scheme 1. Bergman Cyclopolymerization of BODA Monomers to Polynaphthalene Networks and Pyrolysis to Glassy Carbon
Scheme 2. Fabrication of the Carbon Inverse Opal with Optical Images of the Silica Template (A), Pyrolyzed Carbon/Silica Composite (B), and the Resulting Carbon Opal (C).
1 and the in situ templating pyrolysis method allow the fabrication of inverse carbon opals with relative ease compared to CVD methods and with much greater fidelity to the template than previous polymeric precursor routes.6 Results and Discussion The thermal polymerization of BODA monomers is a versatile route to polyarylene networks,13-15 as well as glassy carbon microstructures from high-yield processable precursors.13,15 Tetrafunctional BODA monomers utilize the Bergman cyclization16 to form branched, processable intermediates prior to polynaphthalene network formation. Linear polyarylenes, such as polyphenylenes and polynaphthalenes, are more difficult to process and are typically formed by polycondensation reactions that can be problematic in extremely small-scale processing. BODA chemistry enables the formation of soluble reactive oligomeric precursors that are easily processed into micro-13 or, as is demonstrated here, nanoscale structures. Once cured to 450 °C, these highly cross-linked structures can then be pyrolyzed at 900-1000 °C to form glassy (14) Smith, D. W., Jr.; Babb, D. A.; Snelgrove, R. V.; Townsend, P. H.; Martin, S. J. J. Am. Chem. Soc. 1998, 120, 9078-9079. (15) Shah, H. V.; Babb, D. A.; Smith, D. W., Jr. Polymer 2000, 41, 4415. (16) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25-31. Bharucha, K. N.; Marsh, R. M.; Minto, R. E.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 3120.
carbon structures that maintain the same shape and approximately the same size as the template.13 In general, BODA monomer synthesis is amenable to a wide variety of functional groups.17 Initial carbon opal preparation attempts with phenyl-terminated monomer 2 resulted in the hydrophobic beading of the monomer and oligomers on the surface of the template. However, hydroxy-functional monomer 1 (Scheme 1) has shown compatibility with sol-gel precursors18 and, therefore, was an obvious choice for coating and infiltrating silicate structures (Scheme 2). Scheme 2 illustrates the process for fabricating the inverse carbon opal in addition to optical images of the surface at different stages. Silica opals were prepared from silica nanospheres (ca. 213-nm average diameter by scanning electron microscopy, SEM) by a previously reported sol-gel route.19,20 Monomer 1 was simply melted in contact with the washed silica template followed by cure and pyrolysis to glassy carbon. Treatment with aqueous HF resulted in the complete removal of the silica template. The excellent space-filling capability of the (17) Perera, K. P. U.; Krawiec, M.; Smith, D. W., Jr. Tetrahedron 2002, 58, 10797-10203. (18) Perera, K. P. U.; Smith, D. W., Jr. Polym. Mater. Sci. Eng. 2001, 85, 365. (19) Ballato, J.; James, A. J. Am. Ceram. Soc. 1999, 82, 2273-2275. (20) Ballato, J. J. Opt. Soc. Am. B 2000, 17, 219-225.
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Figure 1. Normalized reflectance spectra of the air-filled carbon opal relative to the starting silica opal template. Figure 3. Attenuation of the carbon opal bandstop versus the aqueous acetonitrile solution composition (water, n ) 1.330 at 20 °C; acetonitrile, n ) 1.341 at 20 °C).
Figure 2. Scanning electron micrograph of the carbon inverse opal (25 K magnification with 150 K magnification in the inset).
monomer and oligomers suggests that the process may be suitable for rapid in situ templating pyrolysis at the nanoscale. The reflectance spectra of the opal template compared to the inverse carbon opal illustrates the difference in the bandstops of the two structures (Figure 1). Figure 2 depicts the SEM images of the carbon opal where the center-tocenter spacing of the voids was measured to be ca. 233 nm. This represents an average pore diameter that is in good agreement with the template size.20 The bandstop at the L point can be predicted from the formula21
λmax ) 2x(2/3)dnaverage
(1)
where d is the center-to-center distance of two adjoining spheres in the opal structure and naverage is the average refractive index as calculated by eq 2. Because the scale of the composite is on the scale of the wavelength of light, the composite is seen to have one composite refractive index, naverage. This is described by the effective medium theory.21 2 ) naverage
∑ni2Vi
(2)
The close-packed opal and inverse opal structures are amenable to this type of analysis because both λmax and d are directly measured from reflectance measurements (Figure 1) and scanning electron micrographs (Figure 2), (21) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266-273.
respectively. The volume fractions of carbon and the voids are estimable from the geometry of the face-centered cubic structure of the template. Therefore, assuming that the inverse opal volume fraction ) 0.26 (void fraction ) 0.74), d ) 233 nm as measured by SEM, and λmax ) 535 nm as shown in the reflectance spectra (Figure 1), and using 1.00 as the refractive index of air, the refractive index of the glassy carbon is calculated to be 2.18. This analysis assumes a complete filling of the matrix with polymer/ glassy carbon. Typically, the carbon refractive index is difficult to measure by classical techniques because of the great optical loss, yet reported values of refractive index for graphite, calculated from the measured optical dielectric constant, are between 2.04 ( 0.04 and 2.15 ( 0.04 for the basal plane.22 The index of refraction of glassy carbon is variable on the basis of the amount of aromaticity (i.e., polarizability) of the carbon. The estimated index of refraction for our opal carbon is reasonably within this range yet below the accepted index value of 2.8 required to open a complete photonic band gap in an inverse opaline structure.3 Nevertheless, a clear photonic bandstop is accessible and can be reproducibly measured. Because the wavelength of the bandstop depends on both the indeces of refraction of the carbon and that which fills the voids, the reflected wavelength can be tailored by filling the open voids with varying refractive index materials.23-29 A simple demonstration of this phenomenon was accomplished by filling the voids with a series of acetonitrile (n ) 1.341 at 20 °C) and water (n ) 1.330 at 20 °C) solutions to observe the change in the wavelength with the change in the refractive index (Figure 3). The opal amplifies a subtle change in the index, and the effect (22) McCartney, J. T.; Ergun, S. Proc. Conf. Carbon, 3rd,1959, 2, 223-231. (23) Busch, K.; John, S. Phys. Rev. Lett. 1999, 83, 967-970. (24) Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 26-29. (25) Blanford, C. F.; Yan, H.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 401-407. (26) Kang, D.; Maclennan, J. E.; Clark, N. A.; Zakhidov, A. A.; Baughman, R. H. Phys. Rev. Lett. 2001, 86, 4052-4055. (27) Schroden, R. C.; Al-Daous, M.; Stein, A. Chem. Mater. 2001, 13, 2945-2950. (28) Gu, Z.-Z.; Kubo, S.; Qian, W.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. Langmuir 2001, 17, 6751-6753. (29) Ozaki, M.; Shimoda, Y.; Kasano, M.; Yoshino, K. Adv. Mater. 2002, 14, 514-518.
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can, thus, be exploited as a sensor element that shows a remarkable shift of 13 nm over this refractive index change of only 0.011. Conclusion We have demonstrated the utility of a novel glassy carbon precursor polymer for the simple fabrication of photonic crystals. The overall excellent processability and space-filling properties of the polymer combined with a high carbon yield leads to structures that maintain fidelity to the templating structure, even on the nanoscale. The potential of this method in sensor applications has been shown in reflectance measurements of the inverse carbon opal filled both with air and with a series of solvent solutions. The combination of excellent processability and low shrinkage suggests that BODA precursors and the in situ templating pyrolysis route may be an attractive alternative for the fabrication of nanoscale glassy carbon devices such as photonic crystal sensor elements. Experimental Section General Methods. Tetraethyl orthosilicate used in the solgel colloid synthesis was distilled prior to use. All other chemicals and reagents were purchased and used without further purification. 1H NMR 500 MHz, proton decoupled 13C NMR 125 MHz, and 19F NMR 188 MHz spectra were obtained with a JEOL Eclipse+500 system. Chloroform-d was used as the solvent, and the chemical shifts reported were internally referenced to tetramethylsilane (0 ppm), CDCl3 (77 ppm), and CFCl3 (0 ppm) for 1H, 13C, and 19F nuclei, respectively. Yields refer to isolated yields of compounds estimated to be greater than 98% pure, as was determined by 1H NMR. Infrared (IR) analyses were performed on neat KBr disks using a Nicolet Magna spectrometer 550. Elemental analysis was performed by Atlantic Microlabs (Norcross, GA). SEM images were obtained with a Hitachi S-4700 field emission scanning electron microscope at an acceleration voltage of 1 keV. The samples were spray-coated with Pt for about 30 s under a vacuum. Reflectance measurements were obtained with an Ocean Optics PC2000 fiber optics spectrometer at either 100× or 200× magnification. The attenuation experiments were performed by placing a drop of solvent solution on the opal surface and measuring the reflectance while wet and then again after the solvent had evaporated, to ensure a constant geometry. Monomer Synthesis. BODA monomers 1 and 2 were prepared from bisphenols, as described previously via dibromination followed by trifluoromethane sulfonato esterification of 2,2-bishydroxyphenyl-1,1,1,3,3,3-hexafluoropropane to give the bis-bromo-bis-triflate.14 Subsequent Pd-catalyzed coupling of the corresponding terminal alkyne affords BODA monomers in good yield. The synthesis and characterization of Monomer 2 was described previously.14 Monomer 1 was prepared similarly as
Letters follows: To a dried 100-mL round-bottom flask equipped with a condenser, a nitrogen sparging tube, and an inlet septum were added 16 mL of dimethylformamide, 20 mL of triethylamine, 2.00 g (2.65 × 10-3 mol) of the bis-bromo-bis-triflate of 2,2-bishydroxyphenyl-1,1,1,3,3,3-hexafluoropropane followed by 0.1372 g (1.96 × 10-4 mol) of bis(triphenylphosphine) palladium(II) dichloride and 0.0375 g (1.96 × 10-4 mol) of copper(I) iodide at room temperature. The mixture was purged with N2 and heated to 50 °C with stirring, after which 1.29 mL (1.33 × 10-2 mol) of 1-methyl-3-butyn-1-ol was added dropwise over 15 min. The temperature was raised to 90 °C, and the mixture was allowed to react for 2 h. The crude product was washed twice with water, reduced under a vacuum, and purified by column chromatography (silica gel, 10:1 hexane/ethyl acetate) and gave yellow crystals in 81% yield, mp 90-91 °C. FTIR (KBr disk; cm-1): 544, 655, 735, 827, 913, 966, 1175, 1200, 1374, 1400, 1499, 1676, 2242, 2986, 3348. H1 NMR (300 MHz, CDCl3) δ: 1.6(s), 3.5(s), 7.21(m), 7.31(d), 7.57(m). 13C NMR (75 MHz, CDCl3) δ: 31.2, 65.4, 79.7, 79.9, 99.4, 100.3, 125.9, 126.8, 128.4, 128.5, 129.2, 131.0, 132.0, 132.1, 132.3, 132.5, 162.7. 19F NMR (188 MHz, CDCl3) δ: -64.31(t). Anal. Calcd (found) for C35H34F6O4: C, 66.38 (66.12); H, 5.43 (5.54). Fabrication of Carbon Inverse Opal. Sol-gel derived SiO2 nanospheres (213 nm in diameter by SEM) were prepared by a modified Stober-Fink-Bohn method30,31 and permitted to selfassemble by sedimentation into the close-packed opal structure.19 The silica opal was then sintered between 900 and 1000 °C in a quartz tube to create necks between the spheres. The particle necks are important because they will later be used to facilitate the complete removal of SiO2. The opal was washed with saturated aqueous NaHCO3 and dried. Monomer 1 was melted above 90 °C into the pores between the SiO2 spheres for 2 h, cured at 240 °C for 10 min, then heated to 900 °C under N2 at 10 °C/min to complete the pyrolysis and carbonize the polymer. Initial attempts with monomer 2 followed the same procedure with the exception of melting temperatures above 190 °C and curing above 210 °C. The glassy carbon impregnated opal was then treated with HF (48% aq) for 2 h to remove the SiO2 template, washed well with water, and allowed to dry.
Acknowledgment. We are grateful to the National Science Foundation (CAREER Award DMR-9985160), Army Research Office (DAAD19-00-1-0114), Defense Advanced Research Projects Agency, and South Carolina EPSCoR for financial support and to P. Jiang (Clemson University) for helpful advice and expertise. D.W.S. is a Cottrell Scholar of Research Corporation. LA034237V (30) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-68. (31) Bogush, G. H.; Tracy, M. A.; Zukovski, C. F., IV. J. Non-Cryst. Sol. 1988, 104, 95-106.
