pubs.acs.org/Langmuir © 2009 American Chemical Society
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Photonic Crystals of Oblate Spheroids by Blown Film Extrusion of Prefabricated Colloidal Crystals Tao Ding,†,‡,§ Zhan-Fang Liu,†,‡ Kai Song,*,†,‡ Koen Clays,*, and Chen-Ho Tung† †
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Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, ‡Beijing National Laboratory for Molecular Sciences, Beijing 100190, China, §Graduate University of the Chinese Academy of Sciences, Beijing 100190, China, and Department of Chemistry, University of Leuven, Celestijnenlaan 200D, Leuven BE-3001, Belgium Received March 22, 2009. Revised Manuscript Received April 24, 2009 Three-dimensional photonic crystals (or photonic band gap materials) have been fabricated with oblate spheroids as the photonic building block. The nonspherical shape was realized by the blown film extrusion process of a prefabricated colloidal photonic crystal of spherical polystyrene particles, with its voids infiltrated by polyvinyl alcohol. The extrusion was applied on the composite film at a temperature above the glass transition temperature of both polymers. The uniformly applied two-dimensional stretching retains the positional order in the prefabricated colloidal crystal; transforms the spheres into oblate spheroids; and results in orientational order between the spheroids. The morphology of the particles can be predictably changed from a sphere into an oblate spheroid with a specified aspect ratio by the extent of the blown film extrusion. Therefore, the concomitant photonic band gap properties can be tuned in a convenient way.
Introduction In analogy to the forbidden electronic band gap for electrons with an energy between the valence band and the conduction band in semiconductors, the photonic band gap is a forbidden spectral range for photons in a structure with a dielectric constant spatially modulated with a periodicity in the optical wavelength range (hundreds of nanometers). Since being proposed by Yablonovich and John,1,2 intense research in this domain is directed toward achieving a complete photonic band gap in the optical regime. A complete band gap results if, within a specific range of optical frequencies, photons are not allowed to propagate in all three dimensions in the photonic crystal. Colloidal particles offer the intrinsic advantage of facile self-assembly in a threedimensional (3D) photonic crystal, that is, a structure with a periodicity of the dielectric constant in 3D. However, the nature of colloidal particles (e.g., silica and polystyrene, PS) translates into a low dielectric contrast with the air voids. Also, the closest packing of spheres (face-centered cubic, FCC, or random hexagonal close packed, RHCP) results in a large fill factor of 0.74. For these two reasons, no complete band gap in the optical spectrum has been observed from colloidal particles. It has been suggested by previous studies that, for an FCC lattice consisting of colloidal spheres, the most prevailing structure for colloidal crystals, there can only exist a pseudophotonic band gap in the photonic band structure, no matter how high the dielectric contrast, because of a symmetry-induced degeneracy at the W- or U-points of the photonic band structure.3,4 As suggested by computational studies, this degeneracy could be broken by using shape-anisotropic5 or dielectrically anisotropic6 colloidal particles as building blocks. *To whom correspondence should be addressed. E-mail: kai.song@ iccas.ac.cn (K.S.);
[email protected] (K.C.). (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Haus, J. W. J. Mod. Opt. 1994, 41, 195. (4) Biswas, R.; Sigalas, M. M.; Subramania, G.; Ho, K.-M. Phys. Rev. B 1998, 57, 3701. (5) Haus, J. W.; Sozuer, H. S.; Inguva, R. J. Mod. Opt. 1992, 39, 1991. (6) Li, Z. Y.; Wang, J.; Gu, B. Y. Phys. Rev. B 1998, 58, 3721.
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One approach is the direct self-assembly of sub-micrometer nonspherical colloids. Polystyrene ellipsoids have been used as building blocks for direct self-assembly. The lack of long-range order pointed toward the difficulty of organizing nonspherical colloids into 3D crystalline lattices.7 In recent years, Liddell and Hosein have demonstrated the fabrication of crystalline films of mushroom caps and asymmetric dimers of spheres.8,9 Similarly, it was difficult to get orientational ordering of the nonspherical particles, besides the positional ordering, and to fabricate 3D photonic crystals with a perfect superlattice through the method of direct self-assembly. One more recent approach is based on realizing that, for an ellipsoidal particle, there is the need to induce, in addition to the positional order, also the orientational order. While the positional order can still be induced by convective self-assembly, the orientational order requires an additional handle on the particles. This has been realized recently by means of a magnetic core and an externally applied magnetic field.10 Deformation strategies applied on prefabricated colloidal crystals are studied as alternative strategies. By means of ion irradiation, colloidal crystals of silica and zinc sulfide spheres can be anisotropically deformed into spheroidal oblates.11 However, this may cause nonuniform deformation in different parts of the thick colloidal crystals. Because polymers can be deformed rather easily at a temperature higher than their glass transition temperature (Tg), mechanical stretching and compressing have also been used to deform initially spherical colloidal particles into ellipsoids or polyhedrons.7,12,13 (7) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722. (8) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 8810. (9) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 10479. (10) Ding, T.; Song, K.; Clays, K.; Tung, C.-H. Adv. Mater. 2009, 21, 1936. (11) Velikov, K. P.; van Dillen, T.; Polman, A.; van Blaaderen, A. Appl. Phys. Lett. 2002, 81, 838. (12) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (13) Sun, Z. Q.; Chen, X.; Zhang, J. H.; Chen, Z. M.; Zhang, K.; Yan, X.; Wang, Y. F.; Yu, W. Z.; Yang, B. Langmuir 2005, 21, 8987.
Published on Web 05/14/2009
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The fabrication of oblate spheroidal particles, which may have different packing behaviors than spheres,14,15 has not been frequently reported. This shape cannot be prepared by simple mechanical stretching, since biaxial stretching may lead to nonuniform deformation of the particles. Recently, the blown film extrusion technique has been used by Yin et al. to prepare diskshaped polymer spheroids.16 Monodisperse PS microspheres were dispersed in an aqueous solution of polyvinyl alcohol (PVA) and embedded in a composite PS/PVA film after evaporating the solvent. By applying the blown film extrusion process at a temperature higher than the Tg’s of both PVA and PS, the softened PS colloids were deformed into oblate spheroids by this uniform 2D stretching force. In another recent report, unidirectional chemical etching was presented as a general approach toward individual spheroidal particles.17 In this work, we report the use of the blown film extrusion technique to fabricate a colloidal crystal of oblate spheroids. To this end, we first made a colloidal photonic crystal with positional order between the PS spheres. We then applied the same approach of infiltrating the air voids with PVA followed by blown film extrusion but on the PS/PVA photonic crystal composite. After dissolving the PVA, the consequences for the morphology of the PS particles and for the orientational order between the particles as well as for the spectral features of the photonic band gap were studied as a function of extent of extrusion.
Experimental Section Materials. Low molecular weight PVA (22 000-26 000) was bought from Alfa Aesar. Styrene, glycerol, glycol, and 2-propanol were all analytical pure and obtained from Beijing Chemical Works. Deionized water was purified by using a Millipore Milli-Q water purification system. Fabrication of PS Colloidal Crystal Films. Monodisperse polystyrene spheres were prepared by emulsifier-free emulsion polymerization as mentioned in our previous report.18 The PS colloidal crystal films were fabricated by dropping the PS/ethanol suspension onto the bottom of a glass dish. Prior to use, the glass dish was made hydrophobic by pretreating with anhydrous toluene containing 0.1% (v/v) octadecyltrichlorosilane (OTS, Alfa Aesar) under ambient condition for 24 h, followed by washing the physically absorbed OTS with toluene and drying under a flow of nitrogen. This ensures the facile peeling off of the film in a later stage. Capillary force drives the PS colloids to selfassemble into sedimented ordered arrays during the evaporation of the solvent.
Fabrication of the PVA Films Embedding the PS Colloidal Crystals. This opaline lattice was infiltrated with an aqueous solution of PVA. To ensure good flexibility and high mechanical strength of the PVA film, plasticizers were added. Typically, 7 g of PVA, 2.4 g of glycerol, and 0.6 g of glycol were dissolved in 40 mL of water at 90 °C. A film was cast on the glass dish containing the prefabricated PS colloidal crystals. After drying overnight at room temperature, a strong yet flexible composite PS/PVA film was obtained. A ceramic filter funnel was put onto the film, with its wide opening touching the film. To stick the funnel to this sealing film, an additional amount of concentrated PVA solution was dropped along the rim of the funnel. After solidification at room temperature, the PVA film, together with the funnel, was (14) Donev, A.; Stillinger, F. H.; Chaikin, P. M.; Torquato, S. Phys. Rev. Lett. 2004, 92, 255506. (15) Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikire, P. M. Science 2004, 303, 990. (16) Hu, Y.; Ge, J.; Zhang, T.; Yin, Y. Adv. Mater. 2008, 23, 4599. (17) Deng, T.; Coumoyer, J. R.; Schermerhom, J. H.; Balch, J.; Du, Y.; Blohm, M. L. J. Am. Chem. Soc. 2008, 130, 14396. (18) Liu, Z.-F.; Ding, T.; Zhang, G.; Song, K.; Clays, K.; Tung, C.-H. Langmuir 2008, 24, 10519.
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Scheme 1. Schematic Representation of Fabricating Colloidal Crystal of Oblate Spheroids by Blown Film Extrusion Process
carefully peeled off from the bottom of the hydrophobic glass dish.
Blown Film Extrusion of the Composite PS/PVA Film. The narrow tip of the funnel was connected with an inflator. The film was fully immersed in a silicone oil bath preheated at 120 °C. This temperature is higher than the Tg’s of both PS (95 °C) and PVA (85 °C). A determined amount of air was slowly pumped into the funnel to blow the initially planar film into a stretched bubble. This air injected in the sealed system resulted in a uniform 2D stretching force on the soft and deformable film. The whole system was then removed from the oil bath and allowed to cool in air while maintaining the stretching force. The bubble was peeled off from the funnel followed by rinsing with 2-propanol to remove the silicone oil adhered to the surfaces. Finally, the PVA matrix of the PS/PVA composite film was dissolved in plenty of water by simple dipping. After removing the PVA matrix, the photonic crystal film collapsed into millimeter square pieces. Reflection spectra were taken from these pieces which were collected from random locations of the photonic crystal films. Good reproducibility was found in the wavelength position of reflection peaks. That also means the uniformity of blown film extrusion. Characterization. The scanning electron microscope (SEM, model JEOL S4300 or S4800) images of the colloidal arrays were captured at an accelerating voltage of 15 kV. The reflection spectra were measured with a fiber spectrometer (AvaSpec2048, Avantes). These spectra were taken with light impinging perpendicular to the film and, hence, also to the (111) planes of the opaline lattice.
Results and Discussion During the blown film extrusion process at a temperature higher than the Tg’s of both PS and PVA, both the macroscopic composite PS/PVA film and the individual PS building blocks embedded by PVA in the colloidal crystal showed different stretching ratios with different volumes of injected air. The composite film was stretched and deformed into a regular crown shape such that the extent of the extrusion process can simply be represented by the ratio of the height of the crown (H) to the diameter of the wide opening of funnel (D, the original diameter of the composite film before extrusion) as illustrated in Scheme 1. Figure 1A shows the SEM image of the (111) crystalline plane of a spherical PS colloidal crystal, with a PS microsphere diameter of 270 nm, indicating the high crystallinity of the colloidal crystal before extrusion (H/D = 0). Over a relative small domain size, compared with the complete composite film, the biaxial stretching force can be considered parallel to the (111) crystalline plane or, equally, to the surface of composite film. As a consequence, the macroscopic bubble expansion effect affects not only the macroscopic film but also the PS microspheres. The stretching force results in the PS spheres expanding in two dimensions in the (111) crystalline plane, and in shrinking in the direction normal to the (111) crystalline plane. This gradual process results in a colloidal crystal of PS oblate spheroids, as shown in Figure 1B-D. DOI: 10.1021/la901004m
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Figure 2. Reflection spectrum changes of PS opal during the process of blow stretching with the H/D expansion ratio = 0.2: (1) original PS colloidal crystal; (2) composite PS/PVA film before stretching; (3) composite PS/PVA film after stretching; and (4) bare PS colloidal crystal after stretching.
side views of colloidal crystals, the minor axis of the oblate spheroids can be measured as ∼210 nm, while the major axis increases from 270 to 310 nm. Figure 2 demonstrates the spectrum changes during the extrusion process. The original self-assembled PS colloidal crystal film gives a reflection peak at a wavelength of 614 nm. After being embedded into the PVA matrix, the main changes are the red-shift to 671 nm and the large decrease of the amplitude of the reflection peak, due to the refractive index matching between PS and infiltrated PVA. As a consequence of the blown film extrusion, the peak position of the stop band blue-shifts to 549 nm, with the amplitude of the reflection remaining small. It is clear that, during the expansion in the (111) plane, the distance between the crystalline planes in the (111) direction decreases as a result from the shrinking in the direction normal to the (111) crystalline plane or, congruently, a decreasing minor axis of the oblate spheroids. After removal of the PVA, the bare colloidal crystal with oblate spheroidal PS building blocks is obtained. The reflection peak position further blue-shifts to 508 nm, while the reflection amplitude increases simultaneously, as a consequence of replacing the PVA by air again. The position of the reflection peak of a colloidal crystal can be approximately calculated by the Bragg equation for diffraction under normal incidence (eq 1).19 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mλ ¼ 2d111 fnPS 2 þ ð1 -f Þnair 2
Figure 1. SEM images of the PS opaline lattice: (A) (111) crystal plane before extrusion; (B) (111) plane after extrusion; (C, D) images of stretched PS opal captured from different angles; H/D expansion ratio = 0.2. Scale bars in (A-C) are 2 μm; scale bar in (D) is 1 μm.
Figure 1B shows the (111) plane of a colloidal crystal after blown film extrusion, with the H/D expansion ratio = 0.2. After removing the infiltrated PVA matrix, it can be clearly observed that the extrusion does not destroy the periodic structure of the colloidal crystal. Please note that the PS spherical building blocks in different parts of the colloidal crystal uniformly transformed into oblate spheroids, as shown in Figure 1C and D. From these 10220 DOI: 10.1021/la901004m
ð1Þ
where m is the order of Bragg diffraction, λ is the wavelength of the stop band maximum, d111 is the interlayer spacing of (111) crystalline planes, and f is the volume fraction occupied by PS colloids, approximately 0.74 for close-packed spheres. Here, for disk shaped ellipsoids, we approximated the distance between two (111) planes d111 as (2/3)1/2h, where h is the minor axis of the oblate spheroid. As the refractive indices of PS and PVA are nPS = 1.59 and nPVA = 1.5, respectively, the calculated Bragg wavelengths for PS bare opal, PVA embedded PS opal before and after stretching, and disk shaped PS colloidal crystal are 643, 691, 537, and 500 nm, respectively, which are in agreement with the reflection spectra in Figure 2, considering the approximation made. As we change the extent of extrusion, as described by the value for the H/D expansion ratio, oblate spheroids with different sizes (19) Jiang, P.; Berton, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132.
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Figure 4. Reflection spectra of blown film extrusion stretched PS colloidal crystal in the normal direction to the (111) plane at different extents of extrusion (H/D).
and morphologies are obtained. As shown in SEM images in Figure 3A-D, PS spheres with diameter R0 of ∼270 nm can be uniformly transformed into oblate spheroids with the major axis (Rs) of ∼290, ∼310, and ∼340 nm at H/D values of ∼0.1, ∼0.2, and ∼0.3, respectively. Simultaneously, along with the expansion in the major axis, the minor axis of spheroids decreases, because the volume of PS particles maintains a constant during the extrusion as there is no removal of material, as is the case for generating spheroidal particles by chemical etching.17 Under the conditions of the blown film extrusion, the relationship between the deformation of PS colloids, as expressed in the ratio Rs/R0, and the extent of extrusion, expressed by the expansion ratio H/D, should follow the equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ Rs =R0 ¼ 1 þ ð2H=DÞ2 However, as shown in Figure 3E, the experimental values for Rs/R0 are higher than the ones simulated with the use of eq 2. This has been observed earlier for blown film extrusion for the preparation of individual spheroidal particles, and a number of possible causes has been presented, such as difference in elasticity between the PS particles and the PVA matrix, and the morphology away from regular spherical crown.16 Spectral characterization was also applied to these ellipsoidal disk arrays with different expansion ratios. Figure 4 shows the reflection spectrum changes for the PS colloidal crystal with the expansion ratio (H/D) varied over 0, 0.1, 0.2, and 0.3. From the compression in the normal direction, the position of the reflection peak blue-shifts because of the deformation of the PS microspheres and the decrease of the minor axis of spheroids, resulting in a smaller distance between two (111) planes d111. As the bubble expansion is increased from 0.1 over 0.2 to 0.3, the reflection peak is blue-shifted from 560 over 508 nm to finally 480 nm. The minor axis of oblates, h, can be calculated as R03/Rs2. From the original diameters of PS microspheres and the major axis of the oblates, the corresponding h values of oblates at different H/D ratios of 0.1, 0.2, and 0.3 are 234, 205, and 170 nm, respectively. Thus, the reflection peak positions of colloidal crystals of PS oblates can be predicted by Bragg’s law to be 557, 489, and 405 nm. The large deviation for the H/D value of 0.3 Figure 3. SEM images of stretched PS arrays with the H/D expansion ratio of (A) 0, (B) 0.1, (C) 0.2, and (D) 0.3. All the scale bars are 1 μm. (E) Expansion ratio Rs/R0 of PS disks as a function of the degree of expansion ratio (H/D). Here, Rs is the major axis of the ellipsoidal disks, R0 is the diameter of the original PS spheres, H is the height of the spherical crown bubble, and D is the original film diameter (wide opening of filter funnel) Langmuir 2009, 25(17), 10218–10222
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is attributed to the irregular morphology of nonuniformly stretched particles at high stretching ratio (see Figure 3D) and errors of measurement.
colloidal crystal structure consisting of disk-shaped ellipsoids may break the degeneracy of the band gap at W- and U-points compared with colloidal crystals from completely spherical particles.
Conclusions
Acknowledgment. We acknowledge the financial support provided by the NSFC (No. 60877032) and the 973 program (Nos. 2007CB808004, 2009CB930802). We also thank the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS.
A novel colloidal crystal structure with oblate spheroidal building blocks was fabricated by blown film extrusion of a PS colloidal crystal embedded in a PVA matrix. As the expansion ratio (H/D) increases, the stretching ratio (Rs/R0) of the original PS spheres increases. As a result, the original PS spheres were transformed into oblate spheroids. Consequently, the crystal lattice constant was changed which resulted in the change of the reflection peak. The spectral position of the reflection can predictably be altered by variation of the expansion ratio. This new
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Supporting Information Available: Plots showing reflection spectra of different locations of stretched films with H/D = 0.1 and 0.2. This material is available free of charge via the Internet at http://pubs.acs.org.
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