Nonspherical Colloidal Crystals Fabricated by the Thermal Pressing of

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Nonspherical Colloidal Crystals Fabricated by the Thermal Pressing of Colloidal Crystal Chips Z. Q. Sun, X. Chen,† J. H. Zhang, Z. M. Chen, K. Zhang, X. Yan, Y. F. Wang, W. Z. Yu, and B. Yang* Key Laboratory for Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China Received May 3, 2005. In Final Form: July 12, 2005 Nonspherical colloids and their ordered arrays may be more attractive in applications such as photonic crystals than their spherical counterparts because of their lower symmetries, although such structures are difficult to achieve. In this letter, we describe the fabrication and characterization of colloidal crystals constructed from nonspherical polyhedrons. We fabricated such nonspherical colloidal crystals by pressing spherical polymer colloidal crystal chips at a temperature slightly lower than the glass-transition temperature (Tg) of these polymer colloids. During this process, the polymer microspheres were distinctively transformed into polyhedrons according to their crystal structures, whereas the long-range order of the 3D lattice was essentially preserved. Because a working temperature lower than Tg effectively prevented the colloidal crystals from fusing into films, the spherical colloidal crystals were transformed greatly under pressure, which lead to obvious change in the optical properties of colloidal crystals. Besides their special symmetry and optical properties, these nonspherical colloidal crystals can be used as templates for 2D or 3D structures of special symmetry, such as 2D nano-networks. We anticipate that this fabrication technique for nonspherical colloidal crystals can also be extended to nonspherical porous materials.

1. Introduction Ordered structures, such as photonic crystals (PCs) and colloidal crystals (CCs), have been studied intensely from the viewpoint of their unique ability to control the propagation and spontaneous emission of electromagnetic waves.1-3 Recently, there has been considerable success with the preparation of 3D complete band gap materials and PCs in the range from visible to infrared. Applications of these CCs or PCs range from optical filters and switches to microwaveguides, low-threshold lasers, and even biosensors. PCs created by self-assembly colloid-template (CCs or artificial opals) 4,5 strategies have provided a simple and flexible method for these photonic band gap materials. As we know, the distinctive properties of CCs are based on functions of the self-assembled crystal structures, which greatly depend on the size, shape, and dielectronic nature of building blocks. Unfortunately, many reports have demonstrated that most existing self-assembled CCs made of dielectronic spheres could not possess 3D complete band gaps resulting from the symmetry-induced degeneracy of the polarization modes.6-8 Some solutions for this degeneracy may be obtained by using shape anisotropic or dielectrically anisotropic objects as building blocks in these band gap structures and materials.8,9 However, in comparison to conventional spherical particles, additional difficulties must be overcome in the preparation of nonspherical colloid particles and their ability to selfassembly into 3D CCs. So far, many different methods * Corresponding author. E-mail: [email protected]. † Present address: Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle/Saale, Germany. (1) Bykov, V. P. Sov. J. Quantum Electron. 1975, 4, 861. (2) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (3) John, S. Phys. Rev. Lett. 1987, 58, 2486. (4) van Blaaderen, A. MRS Bull. 1998, 23, 39. (5) Colvin, V. L. MRS Bull. 2001, 26, 637. (6) Leung, K. M.; Liu, Y. F. Phys. Rev. Lett. 1990, 65, 2646. (7) Sozuer, H. S.; Haus, J. W.; Inguva, R. Phys. Rev. B 1992, 45, 13962. (8) Haus, J. W. J. Mod. Opt. 1994, 41, 195. (9) Li, Z. Y.; Wang, J.; Gu, B. Y. Phys. Rev. B 1998, 58, 3721.

have been proposed to prepare nonspherical particles,10-15 but most existing elegant and available procedures and methods for preparing CCs, such as controlled drying16 and sedimentation,17 will not provide effective control over the self-assembly process or might not be suitable for these nonspherical building block systems. In this case, a variety of elegant and available procedures have been proposed to prepare CCs of nonspherical particles by treating spherical CCs. CCs constructed from polygons have been prepared by annealing arrays of polystyrene beads at a temperature higher than their Tg.18 As we know, polymer latexes will obviously deform at temperatures higher than Tg for a long time, inducing fusing between spheres. This results in the widening of the spectroscopic peak and even the disappearance. By ion irradiation, thin 3D spherical CCs have been deformed into spheroidal oblates,19 but this method may be not suitable for thick CCs and the costs are relatively high. Other approaches are based on inducing some materials into the voids of colloidal crystals as the framework to deform colloids and then produce a nonspherical system,20-23 whereas the indraft and removal (10) Matjevic, E. Chem. Mater. 1993, 5, 412. (11) Matjevic, E. Langmuir 1994, 10, 8. (12) Sugimoto, T.; Itoh, H.; Mochida, T. J. Colloid Interface Sci. 1998, 205, 42. (13) Ocana, M.; Morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1995, 171, 85. (14) Snoeks, E.; van Blaaderen, A.; van Dillen, T.; van Kats, C. M.; Brongersma, M. L.; Polman, A. Adv. Mater. 2000, 12, 1511. (15) Lu, Y.; Yin, Y.; Xia, Y. N. Adv. Mater. 2001, 13, 271. (16) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (17) Mayoral, R.; Requena, J.; Moya, J. S.; Lopez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Vazquez, L.; Holgado, M.; Blanco, A. Adv. Mater. 1997, 9, 257. (18) Gates, B.; Park. S. H.; Xia, Y. N. Adv. Mater. 2000, 12, 653. (19) Velikov, K. P.; Dillen, T.; van Polman, A.; van Blaaderen, A. Appl. Phys. Lett. 2002, 81, 838. (20) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (21) Lu, Y.; Yin, Y.; Li, Z. Y.; Xia, Y. N. Langmuir 2002, 18, 7722. (22) Ji, L. J.; Rong, J. H.; Yang, Z. Z. Chem. Commun. 2003, 1080. (23) Zeng, F.; Wu, S. Z.; Tang, T.; Sun, Z. W.; Wang, C. Y.; Liu, X. X.; Tong, Z. Colloid Polym. Sci. 2004, 282, 651.

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Figure 1. Schematic illustration of the procedure that we used to prepare NSCCs.

of the assistant structures would increase the complexity of these methods. We previously demonstrated that CCs confined between two planar plates fabricated by the two-substrate vertical deposition method, which we called colloidal crystal chips, possess high quality and stability.24,25 In this letter, we develop an alternative approach toward nonspherical colloidal crystals (NSCCs) based on the mechanical stability of these colloidal crystal chips. First, colloidal crystal chips constructed from low-cross-linked polystyrene beads were prepared by two-substrate vertical deposition. Then we pressed these colloidal crystal chips directly at a temperature slightly below Tg of the polymer colloids. After a short time of thermal pressing, polystyrene beads were transformed into polyhedrons, and NSCCs were obtained. In our experiments, the heating procedure made polymer spheres tend to transform, whereas the high pressure would extrude air in the interstices and dominated the deformation process smoothly and swiftly. Moreover, using a temperature lower than Tg of the polymer microspheres prevented CCs from fusing into the membrane. The confinement from two substrates would also help to retain the ordered structure of the original CCs and better control the deformation of microspheres. By characterizing the morphology of transformed particles in NSCCs in a detailed way, we found that our method would lead to NSCCs transformed more distinctly than other existing methods. Also, the spectra changes in the process were relatively large. (The position of the diffraction peak can be shifted by 93 nm for the sample of 265 nm microspheres.) We anticipate that this fabrication technique for NSCCs can be extended to the fabrication of nonspherical porous materials. 2. Experimental Section 2.1. Materials. All carboxyl-modified polystyrene microspheres of low-cross-linking densities used in this work were prepared using surfactant-free emulsion polymerization as mentioned in our previous report26 (polydispersity index lower than 4%). All CCs shown in Figures 2-6 were constructed from microspheres of 265 nm diameter. Silicon, silica, quartz, and gold wafers were cut into about 10 × 10 mm2 pieces, were soaked in a 7:3 volumetric mixture of 98% H2SO4/30% H2O2 for 20 min under boiling (caution: strong oxide), and then were rinsed with deionized water several times. Silver enhancer solutions A and (24) Chen, X.; Chen, Z. M.; Fu, N.; Lu, G.; Yang, B. Adv. Mater. 2003, 15, 11413. (25) Chen, X.; Sun, Z. Q.; Zheng, L. L.; Chen, Z. M.; Wang, Y. F.; Fu, N.; Zhang, K.; Yan, X.; Liu, H.; Jiang, L.; Yang, B. Adv. Mater. 2004, 16, 1632. (26) Chen, X.; Cui, Z. C.; Chen, Z. M.; Zhang, K.; Lu, G.; Zhang, G.; Yang, B. Polymer 2002, 43, 4147.

Figure 2. (a) Morphology of the CCs fabricated by twosubstrate vertical deposition. (b-f) Typical SEM images of the NSCCs.

Figure 3. AFM images of the inner layers in the NSCCs. (a, c) Section analysis and (b) phase item. B were obtained from Sigma. Deionized water was used for all experimental processes. 2.2. Preparation of Spherical and Nonspherical Colloidal Crystals Chips. First, high-quality, stable CCs were formed between two desired substrates via two-substrate vertical deposition. We placed two close pretreated substrates vertically in the dispersion of polymer microspheres. The dispersion was sucked between the two substrates through capillary force. Then we dried this system at room temperature or higher temperature up to 70 °C. Finally, we achieved stable colloidal crystals between two substrates once the dispersed water evaporated completely. Then the sandwichlike colloidal crystal chips were pressed under a pressure of about 5 MPa while they were heated to 110 °C (Tg of the colloids is 116 °C) for less than 10 s. After completely cooling, NSCCs were achieved. 2.3. Fabrication of Silver Nano-Networks on Gold Wafers. To fabricate 2D silver nano-networks on gold substrates, we prepared NSCCs with a gold wafer, and then we put them

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Figure 4. (a) SEM images of CCs fabricated by two-substrate vertical deposition at 50 °C. The sample is shown to be a mixture of fcc lattices with (111) and (100) orientations. The inset presents an enlarged image of the fcc lattice with (100) facets parallel to substrates. (b) (100)-oriented NSCCs formed in thermal-pressed CCs with a mixture of fcc structure. The inset is an enlarged image.

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Figure 6. (a) UV-visible transmission spectra of a colloidal crystal chip before (red line) and after (green line) it had been thermally pressed. These two spectra were obtained with the light propagating along the normal to the (111) plane. The Bragg diffraction peak blue shifted more than 90 nm whereas the PS spheres were transformed. (b, c) Optical images of the colloidal crystals (taken by optical microscope in reflection mode, spanning an area of 0.26 × 0.21 mm2) corresponding to the spectra in part a. the normal to the (111) plane by a Shimadzu UV-3100 spectrophotometer. The photographs of the CCs were taken with an Olympus BX51microscope.

3. Results and Discussion

Figure 5. (a) Morphology of (111)-oriented NSCCs’ surface contact with the substrates. (b) Two-dimensional hexagonal silver nano-networks on a gold substrate fabricated by using the NSCCs shown in part a as templates. (c) Morphology of (100)-oriented NSCCs’ surface contact with the substrates. (d) Two-dimensional square nano-networks according to the NSCCs shown in part c. into the solution of silver enhancer (1:1 A/B). The solution was sucked into the interstices in the NSCCs. After reacting for 30 min at room temperature, the NSCCs were picked out and dipped into toluene for 3 h to remove the polymer templates. The gold substrates were finally rinsed with deionized water and alcohol three times in sequence. 2.4. Instrumentation. SEM micrographs were taken with a JEOL FESEM 6700F electron microscope with primary electron energy of 3 kV. For the samples of the silver nano-networks on gold substrates, they were not sputtered with a thin layer of Au or Pt prior to imaging. AFM images were recorded in contact mode with a Nanoscope IIIa scanning probe microscope from Digital Instruments under ambient conditions. The transmission spectra of 3D CCs were taken with the light propagating along

3.1. Nonspherical Colloidal Crystals. Figure 1 outlines the schematic procedure that we used to fabricate the NSCCs. First, high-quality, stable CCs were formed between two desired substrates via two-substrate vertical deposition (a and b). When we placed two close pretreated substrates vertically in the dispersion of polymer microspheres, the dispersion was sucked between the two substrates through capillary force. The microspheres crystallized between the two close substrates from the top to the bottom, resulting from the evaporation of dispersed water when we dried this system. A convective transportation of microspheres toward the upper crystallized microspheres was then maintained by the continuous flow of the dispersion due to the evaporation of dispersed water and the capillary force between two desired substrates. Finally, we achieved stable colloidal crystals between two substrates once the dispersed water evaporated completely. By pressing these colloidal crystal chips directly at a temperature slightly below Tg of the polymer colloids, polystyrene beads were transformed into polyhedrons, and NSCCs were obtained. Part d of Figure 1 illustrates the transformation of particles in the facecentered-cubic (fcc) lattice; the view is from the (111) facet. Well-controlled two-substrate vertical deposition endowed the fcc lattice with high-quality CCs. Figure 2a presents SEM image of spherical CCs before thermalpressing treatment, indicating that high-quality CCs were fabricated with the (111) plane parallel to the surfaces of the substrates. According to the (111)-oriented fcc lattice of the CCs, we speculate that the transformed polymer beads in the NSCCs would be quasi-rhombic dodecahedrons (as illustrated by the model we drew in Figure 1d). As pressure is applied in one direction, the fabricated nonspherical particles show different extents of deformation at different axes. Figure 2b-f shows typical SEM

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images of NSCCs. Figure 2b shows that the inner-side morphology of the first layer are triangular cones, whereas Figure 2c indicates that the top view of the inner layers is similar to the inner side of the first layer (I, II, and III are the 13th, 14th, and 15th layers, respectively). This implies that the inner layers were equally transformed as the outer ones because of equally operated and relatively high pressure. In fact, there are many dots on the transformed particles that can be attributed to the conglutination between every two adjacent microspheres. Figure 2d shows the SEM image of the inner layer view at a tilt of 45° to the normal of the (111) plane. From this image, we can see that the deformation caused by the compression between particles of the same layer was not as obvious as that caused by the compression between particles of different layers. This could be attributed to the high pressure applied in one direction during our experiment procedure. The section image shown in Figure 2e indicates that the transformation had obviously occurred in different types of facets. It can be seen clearly that there are distinct boundaries between the transformed building blocks, especially for the isolated polyhedron particle (Figure 2f) that was dragged out when the NSCCs were split for characterization, indicating that there are still distinct interfaces between nonspherical colloidal particles. This suggests that NSCCs could be redispersed to fabricate nonspherical colloids of special symmetry. Figure 3a-c shows AFM images of the inner particles in Figure 2c. Both the phase image (Figure 3b) and the section analysis (Figure 3a and c) show that the inner particles in CCs have been transformed into the polyhedron shape. From all of these images, we can see that the particles in the original CCs have undergone obviously deformation into quasi-rhombic dodecahedrons. We also noticed that temperature control was important in generating CCs over a large area by two-substrate vertical deposition. When polystyrene microspheres were assembled at 30 °C (samples shown in Figure 2), fcc assemblies with the (111) plane parallel to the substrates were observed over the entire substrates. When the temperature was elevated to 50 °C (samples shown in Figure 4), a mixture of fcc CCs with different orientations was obtained. These defects might be caused by the fact that the high temperature broke up the equilibrium between the evaporation of the medium and the crystallization of the microspheres in the two-substrate vertical deposition process.27 Figure 4a shows SEM images of a mixture of the fcc lattice with both (111) and (100) orientations. The inset shows an enlarged image of the (100) facets parallel to the substrates. By thermally pressing the colloidal crystal chip mixture, NSCCs with different orientations were fabricated. Figure 4b presents the NSCCs derived from spherical CCs with (100) faces parallel to the substrates. The polymer microspheres were also transformed into quasi-rhombic dodecahedrons. The transformed particles are shown to be birettas from the view along the direction of the pressure (enlarged image in inset). This also can be explained by crystal orientation and pressure tropism. 3.2. Silver Nano-Networks on Gold Wafers. For the particles of the first layer, there should be a flat surface on the side adhered to the substrate. Figure 5a and c shows the morphology of the surface adhered to the substrates of NSCCs. Figure 5a shows the lattice with the (111) plane parallel to the substrates. In an enlarged image, as shown in the inset of Figure 5a, it can be seen clearly that these particles possess hexagonal planar surfaces and there are (27) Park. S. H.; Xia, Y. N. Langmuir 1999, 15, 266.

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distinct boundaries. The dots on every particle were formed in the process of splitting the colloidal crystal chips for characterization because of the conglutination between the particles and the substrate. Figure 5c shows the condition of NSCCs with the (100) plane parallel to the substrates. (There are no dots on this image because we removed one substrate by etching with hydrofluoric acid before characterization.) In this case, square-planar surfaces were formed according to the lattice. On the basis of the template strategy, the as-prepared NSCCs can be used to construct 2D nano-networks. To realize this objective, we changed one of the two substrates in our experiment to gold wafers. After the process illustrated in Figure 1, we used these nonspherical colloidal crystal chips with one gold substrate as a template in CCACN.24 When we dipped them into a solution of silver enhancer (1:1 A/B), the solution was sucked into the interstices in the NSCCs. After reacting for 30 min at room temperature, the silver enhancer formed silver patterns on the bare surfaces of the gold substrates, which were not covered with polymer particles. By removing the polystyrene particles with toluene, silver structures were left on gold wafers. Figure 5b and d shows SEM images of the silver nano-networks on the surface of the gold wafer corresponding to NSCCs shown in Figure 5a and c. As the template effect of the transformed CCs, the silver networks are shown to be hexangonal and square in shape, respectively, and are on the nanoscale in terms of size. As for this method, the size of the resulting networks can be easily tuned by changing the size of the microspheres used to prepare CCs. This method can also be extended to other systems for preparing 2D networks of a variety materials and structures. 3.3. Optical Properties of Spherical and Nonspherical Colloidal Crystals. The transfiguration of microspheres, which had changed the repeat period of CCs, would surely change the optical properties. Figure 6a is the UV-visible transmission spectra of the CCs with spherical (265 nm in diameter, red line) and nonspherical (green line) building blocks. Figure 6b and c presents the optical microscope images corresponding to the spectra of Figure 6a. The position of the diffraction peak (λmin) has blue shifted from 640 to 547 nm, and the color of the CCs has also changed from red to green. It is well known that λmin of the spherical CCs in the spectra can be approximately identified by using Bragg equation for the first-order diffraction for the fcc structure (111) plane

λmin ) 2

(32)

1/2

R[f n2block + (1 - f)n2void - sin2 θ]1/2

where R and f are the diameter and the volume diffraction of the spherical building blocks, respectively; n is the refractive index; and θ is the angle between the incident light beam and the normal to the surface of the crystal. In this study, sin θ is 0, nblock is 1.59 (refractive index of polystyrene), and nvoid is 1.0 (refractive index of air). f should be assumed to be 0.7405. As for the NSCCs, the position shift of the reflective peak occurred because of the deformation of the microspheres and the increase in f. The increase in f would lead to the red-shift trend. The particles’ deformation, decreasing the plane space of the CCs, would lead to the blue-shift trend. The result indicates that the plane space decrease is dominant in this process. Because we could not obtain the plane space of the NSCCs directly, we had to make another assumption during the theoretical calculation. From the SEM image of Figure 5a we can see that the particles are tightly packed into each other, so we could assume that the air in the

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Table 1. Calculated and Observed λmin of CCs and NSCCs Constructed from Different-Sized Building Blocks R ) 220 nm

λmin (CCs) λmin(NSCCs) ∆λmin

R ) 246 nm

R ) 265 nm

calcd (nm)

obsd (nm)

calcd (nm)

obsd (nm)

calcd (nm)

obsd (nm)

525 423 102

518.5 455.5 63

587 473 114

585 507.5 77.5

632 510 122

640 547 93

voids of the CCs had been extruded out completely. Because the area of the (111) planes was hardly changed during the thermal-pressing process, the layer separation can be calculated by adding a coefficient of 0.7405 to the layer separation of spherical CCs, and f could be assumed to be 1.0 in the NSCCs. The calculated λmin values of CCs with 265 nm polystyrene beads and their corresponding NSCCs are 632 and 510 nm, respectively; the difference between the observed and the calculated values is probably due to the assumptions used in calculation, especially that the particles are closely packed in spherical CCs and the complete extrusion of the air in the NSCCs. From Figure 2a we can see that the CCs are not very closely packed, and Figure 2c shows that there are still voids remaining in the NSCCs. Despite this difference, we can still attribute these diffraction peaks to the first-order Bragg diffraction of the CCs. To study the block size dependence of the spectral shifts, we carried out the thermal-pressing process on colloidal crystals constructed from microspheres of 220 and 246 nm diameter. (The morphology of the NSCCs was similar to the characterizations shown above.) The calculated and observed λmin values of the CCs and NSCCs constructed from building blocks of different sizes are listed in Table 1. Both the calculated and observed diffraction peak shifts (∆λmin) show the trend to be in direct ratio to R, which can be easily explained by the Bragg equation. This implies that our thermal-pressing process carried out on CCs of larger building blocks would lead to spectral shift over a much wider range.

We have developed an alternative and facile approach toward NSCCs. In our experiments, sandwichlike colloidal crystal chips were pressed at a temperature slightly lower than Tg of the polymer building blocks. Because a working temperature lower than Tg and the cross linkage as well as surface groups effectively prevented the polymer microspheres from fusing into each other, the colloidal particles were obviously transformed into quasi-rhombic dodecahedrons. Moreover, the NSCCs may be redispersed to gain nonspherical particles because there were still distinct interfaces between the particles in NSCCs. We also found that the transformation was carried out according to the crystal structures, and CCs of different lattices would result in NSCCs of different morphology and symmetry. By using as-prepared NSCCs as templates, we constructed 2D silver nano-networks of special symmetry on gold wafers. Because the elements of our colloidal crystal chips (block size, crystal structure, and substrates) are adjustable, this method can be extended to other systems for preparing 2D networks of a variety of materials, sizes, and structures. From the viewpoint of applications in optical devices and photonic crystals, we found that the optical properties of these NSCCs can be manipulated on the basis of both the size and shape of the building bulks. We are carrying out further investigations involving the control and extension of these NSCCs and their promising band gap characteristics. We expect that this kind of NSCC would be important for advanced applications in sensors, filters, and photonic band gap materials. Acknowledgment. This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University and the National Natural Science Foundation of China (grant no. 200340062). LA051185W