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Effect of Evaporation Temperature on the Quality of Colloidal Crystals at the Water-Air Interface Sang Hyuk Im and O Ok Park* Center for Advanced Functional Polymers, Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea Received May 1, 2002. In Final Form: August 4, 2002 Three-dimensional assemblies of 230 nm polystyrene (PS) colloidal particles were prepared on the suspension surface by evaporating the water in which the particles were suspended. The assembled colloidal particles were then transferred from the water surface onto a glass substrate by simple evaporation and sedimentation. In this study, we analyzed the structures of the colloidal assemblies formed at evaporation temperatures of 30, 40, 60, and 90 °C. At 30 °C, the rate of particle sedimentation is faster than the rate of crystallization on the water surface. Consequently, the PS particles randomly stack on the glass substrate before forming nuclei on the water surface. At higher evaporation temperatures, on the other hand, the rate of crystallization on the water surface exceeds the sedimentation rate, leading to an improvement in the quality of the resulting colloidal crystal. However, crystalline quality diminishes at evaporation temperatures greater than 60 °C because the high crystal growth rate leads to the formation of defects. As a result, there exists an optimum evaporation temperature that yields the highest quality crystals. Importantly, this novel process enables the rapid (within 1 h) fabrication of large-scale three-dimensional colloidal crystals.
1. Introduction Photonic band gap crystals in the optical wavelength region have attracted attention because of their potential applications in areas such as lasers,1 waveguides,2 optical filters,3 and sensors.4 In particular, the preparation of three-dimensional colloidal crystals of colloidal particles has been intensively studied in recent decades because the fabrication of such crystals is relatively easy and inexpensive. The methods typically employed to prepare colloidal crystals are gravity sedimentation,5 vertical deposition,6,7 vertical deposition with a temperature gradient,8 and electrophoresis.9 With the exception of electrophoresis, these methods require considerable time (1-2 days) to assemble colloidal particles into multilayer structures. Although electrophoresis can rapidly assemble particles into multiple layers, it cannot assemble particles into a regular structure over a large area. In this study, we report a novel method that enables the rapid (within 1 h) assembly of colloidal particles into a regular structure over a large area. This method induced particle assembly at the surface of the colloidal suspension by evaporating the suspending liquid (water in the present experiments). Having established the feasibility of the * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Yoshino, K.; Tatsuhara, S.; Kawagishi, Y.; Ozaki, M.; Zakhidov, A. A.; Vardeny, Z. V. Appl. Phys. Lett. 1999, 74, 2590. (2) Vogelaar, L.; Nijdam, W.; van Wolferen, H. A. G. M.; de Ridder, R. M.; Segerink, F. B.; Kuipers, E. F. L.; van Hulst, N. F. Adv. Mater. 2001, 13, 1551. (3) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (4) MacPherson, W. N.; Gander, M. J.; McBride, R.; Jones, J. D. C.; Blanchard, P. M.; Burnett, J. G.; Greenaway, A. H.; Mangan, B.; Birks, T. A.; Knight, J. C.; Russell, P. St. J. Opt. Commun. 2001, 193, 97. (5) Miguez, H.; Meseguer, F.; Lopez, C.; Blanco, A.; Moya, J.; Requena, J.; Mifsud, A.; Fornes, V. Adv. Mater. 1998, 10, 480. (6) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (7) Goldenberg, L. M.; Wabner, J.; Stumpe, J.; Paulke, B.-R.; Gornitz, E. Langmuir 2002, 18, 3319. (8) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature (London) 2001, 414, 289. (9) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A. Chem. Mater. 2000, 12, 2721.
method, we examined the influence of the evaporation temperature on the quality of the final colloidal crystal. Our findings were in accord with the work of Ye et al.10 on the vertical deposition method, which showed that crystalline quality changes with the evaporation temperature such that there exists an optimum evaporation temperature that produces the highest quality crystalline structure. 2. Experimental Section Polystyrene (PS) submicron-sized particles as building blocks of colloidal crystals were synthesized by emulsifier-free emulsion polymerization.11,12 Deionized water (450 g) was poured into a reactor, and the water was kept at a temperature of 80 °C and stirred at 350 rpm. Sodium styrene sulfonate (0.03 g) as an emulsifier and sodium hydrogen carbonate (0.25 g) as a buffer were inserted into the water. After 10 min, styrene monomer (50 g) was inserted into the solution. After 1 h, potassium persulfate (0.25 g) as an initiator was introduced into the solution. Finally, polymerization was performed under a nitrogen atmosphere for 18 h. The effective diameter of the prepared PS colloidal particles measured by a Zeta Plus (Brookhaven Instrument Corp.) is 240 nm, and their polydispersity is 0.005. The values are averages of measuring five times. But the diameter of the corresponding PS colloids measured by scanning electron microscopy (SEM) is 230 nm. To assemble the PS particles three-dimensionally, a glass substrate without particular pretreatments except for cleaning with methanol was placed on the bottom of a Petri dish and a 1 wt % PS colloidal solution was poured into the Petri dish (diameter, 5 cm). The dishes were readily placed in a convection oven at 30, 40, 60, and 90 °C, respectively.
3. Results and Discussion PS particles suspended in water were assembled into a three-dimensional structure by evaporation of the water (10) Ye, Y. H.; LeBlanc, F.; Hache, A.; Truong, V. V. Appl. Phys. Lett. 2001, 78, 52. (11) Yi, G.-R.; Moon, J. H.; Yang, S.-M. Chem. Mater. 2001, 13, 2613. (12) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3187.
10.1021/la025888e CCC: $22.00 © 2002 American Chemical Society Published on Web 11/14/2002
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Figure 1. (a) Three-dimensionally assembled 230 nm PS colloidal particles formed on the water surface (photographed at a tilted angle) after 10 min of heating at 60 °C in a convection oven. (b) The colloidal crystal transferred onto a glass substrate by simple evaporation and sedimentation.
via heating the suspension in a convection oven. This threedimensional structure of PS particles appears colored because the ordered structure generates a photonic band gap that reflects light of a particular wavelength. This coloring is evident in Figure 1a, which clearly shows the azure color of 230 nm PS particles assembled on the water surface viewed at a tilted angle. The same suspension has a red color when viewed at the normal angle. The mechanism of particle assembly is as follows. Evaporation of the water in the PS colloidal suspension causes the PS particles at the water surface to protrude from the surface. Then, the lateral capillary force between the protruding particles readily assembles the particles at the surface.13,14 Over time, the particles at the surface assemble into a single domain that floats on the water surface because the effective density of the assembled PS particles is lower than the density of water. This floating effect is possible for PS particles because the density difference between PS (1.04 g/cm3) and water (1 g/cm3) is sufficiently small. Ultimately, an ordered monolayer of PS particles is generated on the water surface. Further evaporation of the water leads to the formation of a multilayer structure through repetition of the above process. This implies that large PS colloidal particles (over 400 nm) can assemble into three dimensions on the suspension surface because the assembled colloids float (13) Dimitrov, A. S.; Dushkin, C. D.; Yoshimura, H.; Nagayama, K. Langmuir 1994, 10, 432. (14) Velikov, K. D.; Durst, F.; Velev, O. D. Langmuir 1998, 14, 1148.
Figure 2. SEM cross-sectional images of colloidal crystals prepared with changing concentrations of the colloidal suspension at 60 °C: (a) 1 wt %, (b) 0.2 wt %, and (c) 0.1 wt %.
on water. This novel mechanism enables the preparation of three-dimensional ordered assemblies of PS colloidal particles, which can be transferred from the water surface to a glass substrate by simple evaporation and sedimentation. Importantly, this novel process allows the rapid assembly of three-dimensional colloidal crystals over large areas.
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Figure 3. SEM surface images of colloidal crystals prepared at (a) 30 °C, (b) 40 °C, (c) 60 °C, and (d) 90 °C. The scale bars represent 2 µm.
Unfortunately, it is difficult to control the exact numbers of layers in the vertical direction through this method. But it is possible to control the number of layers by changing the concentration of a colloidal suspension. To control the thickness of the colloidal crystal, we assembled the colloidal crystals with 1, 0.2, and 0.1 wt % PS colloidal suspensions through the previously described method at 60 °C. The SEM cross-sectional images of resulting colloidal crystals are shown in Figure 2. These figures indicate that it is possible to roughly control the number of layers of the colloidal crystal in the vertical direction by changing the concentration of the colloidal suspension. Generally, the depth of the photonic band gap in colloidal crystals depends on the number of layers stacked in the vertical direction. Thus stacked colloidal crystals (typically over 10 layers) are needed to have high reflectivity at a specific wavelength, such as optical filters.15 To test the effect of evaporation temperature on crystallization behavior, we compared the quality of the crystals produced at evaporation temperatures of 30, 40, 60, and 90 °C. Samples were prepared at each temperature by simple evaporation and sedimentation. It is important to examine the crystal states because they are related to the photonic band gap states. The crystal states can be analyzed using SEM surface images, as shown in Figure 3. A sample assembled over a 12.5 × 8.5 µm2 area at 30 °C is shown in Figure 3a. This sample contains both ordered and disordered regions, with the ordered regions (15) Bertone, J. F., et al. Phys. Rev. Lett. 1999, 83, 300.
showing a red color at the normal angle. In contrast, samples assembled over the same area at 40, 60, and 90 °C have ordered structures (Figure 3b-d, respectively). In particular, the sample prepared at 60 °C shows no planestacking faults. The higher quality of the crystal prepared at 60 °C in comparison to those prepared at 40 and 90 °C can be explained by considering the effects of increasing temperature on crystallization. First, the degree to which the face-centered cubic (fcc) structure is favored over the hexagonal close-packed (hcp) structure increases with increasing evaporation temperature.16,17 Theoretical calculations also show that the thermodynamically stable crystal phase appears to be the fcc phase.18,19 Therefore, the tendency toward the fcc phase with increasing temperature leads to a decrease in plane-stacking faults at higher temperatures.20,21 Increasing the evaporation temperature also increases the kinetic energy of the colloidal particles, allowing them to explore more conformations and therefore to find a favorable lattice. Thus, the number of point defects decreases with increasing evaporation temperature, as is evident in Figure 3b-d. However, increasing the evaporation temperature in(16) Vlasov, Y.; Astratov, V.; Baryshev, A.; Kaplyanskii, A.; Karmov, O.; Limonov, M. Phys. Rev. E 2000, 61, 5784. (17) Stoytchev, M.; Genack, A. Phys. Rev. B 1997, 55, 8617. (18) Woodcock, L. Nature (London) 1997, 388, 235. (19) Bruce, A.; Wilding, N.; Ackland, G. Phys. Rev. Lett. 1997, 79, 3002. (20) Dux, C.; Versmold, H. Phys. Rev. Lett. 1997, 78, 1811. (21) Amos, R.; Rarity, J.; Tapster, P.; Shepherd, T.; Kitson, S. Phys. Rev. E 2000, 61, 2929.
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Figure 4. SEM cross-sectional images of colloidal crystals prepared at (a) 30 °C, (b) 40 °C, (c) 60 °C, and (d) 90 °C. The scale bars represent (a,b) 10 µm and (c,d) 5 µm.
creases the rate of evaporation, thereby reducing the time scale on which particles must arrange themselves into the crystal structure. For this reason, when the evaporation temperature is increased to 90 °C the number of defects increases relative to the sample at 60 °C because the rate of crystal growth is too fast for the particles to shift to favorable lattice sites.10 Assembly of the colloidal particles required 120, 60, 30, and 20 min at 30, 40, 60, and 90 °C, respectively. Thus, the increase in evaporation rate with increasing temperature enables the rapid (within 1 h) preparation of highquality colloidal crystals. The resulting colloidal crystal fabricated at 60 °C is shown in Figure 1b. In addition to in-plane order, the cross-sectional structure of the crystals will also affect the photonic band gap states of the colloidal. To elucidate the cross-sectional structure, the samples prepared at 30, 40, 60, and 90 °C were cut with a diamond blade and SEM images were taken of the exposed cross section. These images are shown in Figure 4. In the sample prepared at 30 °C, almost all of the layers are randomly stacked with the exception of some scattered ordered layers in the upper part of the crystal (Figure 4a). In the sample prepared at 40 °C, the upper half of the layers is regularly stacked whereas the lower half is randomly stacked (Figure 4b). In contrast, the samples prepared at 60 and 90 °C show regular stacking of all layers (Figure 4c,d). These figures indicate that all colloidal particles float on the suspension surface over 60 °C in the present experimental conditions. The increase in regular stacking with increasing temperature
Figure 5. Schematic illustration of a cross section of the colloidal system showing the two principal effects influencing the colloidal particles: settling onto the glass substrate and assembling at the suspension surface.
can be explained as follows. At low temperatures, the settling rate of the colloidal particles is faster than the rate at which they assemble on the water surface. Consequently, layers of irregularly deposited particles form on the glass substrate. As the temperature is increased, however, the rate of assembly on the water surface exceeds the settling rate as illustrated schematically in Figure 5. Of course, the colloids in the suspension dispersed over 2 months. It takes a long time for colloids to sediment to the bottom. However, in our system, water is evaporated very fast and consequently the colloids have
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optical properties. The crystal prepared at 60 °C has the deepest stop band because the presence of defects in colloidal crystals affects the photonic bandtail states and thus enhances the transmission in the gap. Hence the stop band becomes shallower in the presence of defects.10 In addition, we compared the absorption spectrum of colloidal crystals prepared by our method with that of samples prepared by gravity sedimentation. We prepared two different colloidal crystals as follows. First, we prepared colloidal crystals in a manner similar to that described above except for changing the concentration of the colloidal suspension to 0.2 wt % (60 °C). Second, a 0.2 wt % colloidal suspension was placed in a vibrationless oven at 25 °C for 15 days. The resulting colloids show that ordered regions are scattered and most regions are disordered. This indicates that it is very difficult to make ordered colloidal crystals over large areas through simple gravity sedimentation. The absorption spectra of two resulting colloidal crystals are shown in Figure 6b. The spectrum of the colloidal crystal prepared by gravity sedimentation is to measure the ordered region. This figure shows that the photonic band gap of the colloidal crystal prepared by our method is deeper than that of the sample prepared by gravity sedimentation and consequently the former colloidal crystal is ordered more regularly than the latter.
Figure 6. (a) UV-vis absorbance spectra of colloidal crystals prepared at 30-90 °C; 1 wt % PS colloidal suspensions. (b) Absorbance spectra of colloidal crystals prepared by our method (60 °C) and gravity sedimentation (25 °C for 15 days); 0.2 wt % PS colloidal suspensions.
to float on the water surface or settle down because the water diminishes gradually. Even if colloids do not actually sediment, they (which are not ordered colloidal particles on the water surface) cannot help settling down in a disordered state because the water dries. Finally, we investigated the photonic band gap states of the crystals using ultraviolet-visible (UV-vis) spectroscopy. The UV-vis spectra of the samples prepared at 30, 40, 60, and 90 °C are shown in Figure 6a. These absorption spectra can represent the transmission spectra because PS colloids do not absorb in the visible region. As expected from the SEM results, the sample prepared at 30 °C has poor optical properties whereas the crystals prepared at higher temperatures have relatively good
4. Conclusions We prepared three-dimensional assemblies of PS colloidal particles by evaporating the water in which the particles were suspended. In contrast to techniques that assemble colloids on a substrate, our novel process assembles the particles on the suspension surface and then transfers the resulting structure onto a glass substrate by simple evaporation and sedimentation. The sample prepared at 60 °C showed the highest crystalline quality in terms of both in-plane and cross-sectional structure. This temperature gave crystals of higher quality because it has the optimal combination of particle assembly rate on the water surface. In addition, to assemble colloids on a suspension surface three-dimensionally, the following conditions must be satisfied. (1) The effective density of the assembled colloids is lower than that of the suspension medium. (2) The density difference between the colloids and the suspension medium is low enough to float the colloids on the suspension surface by evaporation of the medium. Therefore, if the above conditions are satisfied, any types of lattices and any sizes of microspheres can be available. Furthermore, the thickness of the colloidal crystals can be roughly controlled by changing the concentration of the colloidal suspension. Acknowledgment. The authors are grateful to the Center for Advanced Functional Polymers, and this work was also partially supported by the Brain Korea 21 Project. LA025888E
