Langmuir 2007, 23, 8695-8698
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Binary Colloidal Crystals with a Wide Range of Size Ratios via Template-Assisted Electric-Field-Induced Assembly Xueguang Huang, Ji Zhou,* Ming Fu, Bo Li, Yuehui Wang, Qian Zhao, Zhengwen Yang, Qin Xie, and Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed February 21, 2007. In Final Form: June 27, 2007 Here we present a simple but general method for constructing complex binary colloidal crystals with an almost full range of size ratios, which is referred to as “template-assisted electric-field-induced assembly (TAEFIA)”. The dependence of the structures of binary colloidal crystals on size ratio (γ) and volume fraction (φ) of the colloidal suspension was investigated. Binary colloidal crystals with γ ranges from 0.10 to 0.91 were fabricated, and an attempt to fabricate a triple colloidal crystal via TAEFIA was presented. We suggest that TAEFIA is a versatile way to grow colloidal crystals with binary or more complex structures.
1. Introduction Colloidal crystals consisting of submicron particles with twodimensional (2D) or three-dimensional (3D) ordered arrangements have been intensively studied over the past decades because of their potential applications, such as photonic band gap (PBG) materials, optoelectronic devices, and biochip sensors.1 Much research has focused on the assembly of colloidal crystals with uniformly sized particles. However, because the “crystals” are constructed with only one kind of colloid “atom ”, the structure of the “crystals” was limited to a few simple types, such as face-centered cubic (FCC), hexagonal close-packed (HCP), or body-centered cubic (BCC). The colloidal crystals with many complex structures, which may produce novel physical properties1a,2 and practical application potential,3 are hard to grow with single uniformly sized particles. Binary colloidal crystals, which are composed of both large (L) and small (S) particles that alternately self-organize into ordered structures, possess rich crystal structures depending on the volume fraction (φ) and the size ratio of the small to large particles (γ ) RS/RL).4-9 This new class of colloidal crystals (colloidal alloys) not only act as ideal models for the assembly of the nanoparticle system and for the phase transition10,11 but also have many potential applications, especially in fabricating photonic crystals with full photonic bandgaps (PBGs).3 * Corresponding author. E-mail:
[email protected]. (1) (a) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (b) Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 1329. (c) Yang, S. M.; Ozin, G. A. Chem. Commun. 2000, 2057. (d) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. AdV. Mater. 2001, 13, 396. (e) Deutsch, M.; Vlasov, Y. A.; Norris, D. J. AdV. Mater. 2000, 12, 1176. (f) Hattori, H. AdV. Mater. 2001, 13, 51. (g) Andersson, H.; Van der Wijngaart, W.; Stemme, G. Electrophoresis 2001, 22, 249. (2) (a) van Blaaderen, A. MRS Bull. 1998, 23, 39. (b) Dinsmore, A. D.; Crocker, J. C.; Yodh, A. G. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (c) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383. (3) (a) Biswas, R.; Sigalas, M. M.; Subramaina, G.; Ho, K.-H. Phys. ReV. B 1998, 57, 3701. (b) Busch, K.; John, S. Phys. ReV. E 1998, 58, 3896. (c) Moroz, A.; Sommers, C. J. Phys. Condens. Matter 1999, 11, 997. (4) (a) Sanders, J. V. Philos. Mag. A 1980, 42, 705. (b) Sanders, J. V. Philos. Mag. A 1980, 42, 721. (5) (a) Hachisu, S.; Yoshimura, S. Nature 1980, 283, 188. (b) Barlett, P.; Ottewill, R. H.; Pusey, P. N. Phys. ReV. Lett. 1992, 68, 3801. (c) Dinsmore, A. D.; Yodh, A. G.; Pine, D. J. Phys. ReV. E 1995, 52, 4045. (d) Eldridge, M. D.; Madden, P. A.; Frenkel, D. Nature 1993, 365, 35. (6) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; van Blaaderen, A. Science 2002, 196, 106. (7) Wang, D.; Mo¨hwald, H. AdV. Mater 2004, 15, 244. (8) Kim, M. H.; Im, S. H.; Park, O. O. AdV. Mater 2005, 17, 2501. (9) Kitaev, V.; Ozin, G. A. AdV. Mater 2003, 15, 75.
A few methods have been developed to grow binary colloidal crystals, which can be divided into two categories: a layer-bylayer (LbL) approach and a one-stage approach. However, none of the current methods can be applied to fabricate binary colloidal crystals with a full range of size ratios from 0 to 1. In a typical LbL process such as controlled drying,6 stepwise spin-coating,7 and confined convective assembly,8 small colloidal particles were directed to assemble into a new layer on a previously formed colloidal layer under capillary interactions. Binary colloidal crystals with a large size ratio (generally γ > 0.25), which exhibit limited kinds of structures with simple stoichiometry, such as LS, LS2, LS3 LS4, and LS5, can be grown by these methods. However, there is no report on the fabrication of binary colloidal crystals with γ < 0.25 by current LbL methods. One can expect that when the size ratio is overly small, electrostatic and entropy forces will predominate in the assembly of the small particles, and the diameter of small particles will be comparable with the size of the interstitial sites within the template layer; therefore, the growth of small particles will not be directed effectively by the template, which will result in disordered binary structures. On the other hand, as a one-stage method, accelerated evaporation induced co-assembly developed by Kitaev and Ozin had been employed to fabricate varieties of surface patterns of binary colloidal crystals with γ e 0.30.9 However, this method is incapable for the growth of binary colloidal crystals with γ > 0.30 for energetically instability. Here we present a simple but general method for constructing complex binary colloidal crystals with an almost full range of size ratios (0.10 < γ < 0.91 in experiments), which is referred to as the template-assisted electric-field-induced assembly (TAEFIA). Instead of capillary forces in the usual LbL methods, an external DC electric field is introduced in TAEFIA. Under appropriate field strength, the small particles in a colloidal suspension can overcome the influences of electrostatic and entropy forces that disturb their assembling process and are (10) (a) Ackerson, B. J.; Schatzel, K. Phys. ReV. E 1995, 52, 6448. (b) Harland, J. L.; Henderson, S. I.; Underwood, S. M.; van Megen, W. Phys. ReV. Lett. 1995, 75, 3572. (c) Larsen, A. E.; Grier, D. G. Nature 1997, 385, 230. (d) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (11) (a) Hachisu, S.; Kobayashi, Y.; Kose, A. J. Colloid Interf. Sci. 1973, 42, 342. (b) Pusey, P. N.; van Megen, W. Nature 1986, 320, 340. (c) Yethiraj, A.; van Blaaderen, A. Nature 2003, 421, 513. (d) Kegel, W. K.; van Blaaderen, A. Science 2000, 287, 290. (e) Pham, K. N.; Puertas, A. M.; Bergenholtz, J.; Egelhaaf, S. U.; Moussaı¨d, A.; Pusey, P. N.; Schofield, A. B.; Cates, M. E.; Fuchs, M.; Poon, W. C. K. Science 2002, 296, 104.
10.1021/la700512j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
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Scheme 1. Illustration of the Procedure Used to Fabricate Binary Colloidal Crystals by TAEFIA
Figure 1. SEM images of binary colloidal crystals with different size ratios. (a) γ ) 180 nm/1800 nm ) 0.10, 2 V; (b) γ ) 400 nm/1800 nm ) 0.22, 2.2 V; (c) γ ) 400 nm/700 nm ) 0.57, 2.2 V; (d) γ ) 640 nm/700 nm ) 0.91, 2.2 V. Indices A and B indicate the lattice and displacement sites of 640 nm spheres, respectively.
directed by electric field force to self-assemble onto an ordered layer of large particles, resulting in two-dimensional binary colloidal crystals. In this paper, the dependence of the structures of binary colloidal crystals on the size ratio (γ) and volume fraction (φ) of the colloidal suspension was investigated, and an attempt to fabricate triple colloidal crystal via TAEFIA was presented. We suggest that TAEFIA is a versatile way to grow colloidal crystals with binary or more complex structures. 2. Experimental Section The aqueous suspensions of monodispersed polystyrene (PS) particles were purchased from Bangs Laboratories, Inc., and were diluted to certain volume fractions (2‰ without specialization) with deionized water before use. Indium tin oxide (ITO) coated glass was cut into slides (10 mm × 50 mm) to serve as substrates, which were cleaned in ethanol (98%), acetone, and deionized water successively under sonication. Scheme 1 illustrates the procedure of using TAEFIA to fabricate 2D binary colloidal crystals. An ordered monolayer of 700 or 1800 nm colloidal particles was first formed on the substrate under capillary forces at a certain temperature and humidity. The template was lightly sintered at 90 °C for several minutes to make the particles stick each other and affixed to the substrate, so as to prevent the monolayer from peeling off in the following deposition. Subsequently, the template was settled onto an EPD cell to act as the anode, and a plain sheet of platinum served as the cathode. The electrodes were separated by a Teflon spacer of 1 mm in thickness. The gap between the electrodes was filled by about 100 µL aqueous suspension of small colloidal particles. Then an electric field (about 2 ∼ 3 V/mm) was applied across the deposition cell through a DC power supply and maintained for 10 ∼ 20 min. Due to the difference of the dielectric constants of PS particles (r ∼ 2.15-2.65) and water (r ∼ 80), the electric field adjacent to the ITO substrate in the suspension was slightly distorted; therefore, the local electric filed strength at an interstitial site was greater than the field strength around. Moreover, the interstitial sites among the hexagonally close-packed large particles on the substrate and the recesses between these interstices served as traps for localizing the small particles. Because of the distortion of the electric field and the modulation of the 2D colloidal crystal template, small particles preferred to settle into the cavities in the colloidal crystal monolayer of large particles to reach a minimum of electrostatic potential energy and thus formed a 2D binary colloidal crystal. By adjusting parameters such as size ratio and volume fraction of the suspension, binary colloidal crystals with various complex structures can be synthesized.
When the deposition finished, a strong voltage was applied to the deposit cell to “freeze” the particles to the template. Scanning electron microscopy (SEM) was carried out on an FEI QUANTA 200F instrument operated at 15.0 kV. SEM samples were placed on copper surfaces and then sputter-coated with Au (about 5 nm thick).
3. Results and Discussion Figure 1 shows scanning electron microscopy (SEM) images of 2D binary colloidal crystals with different size ratios (γ) ranging from 0.10 to 0.91 fabricated by TAEFIA. As shown in Figure 1a, perfect structural order was not obtained in the TAEFIA result of 180 nm particles on 1800 nm particles due to overmuch small size ratio. In this case, the small particles filled the gaps among the template layer with local ordered structure, leading to a colloidal crystal with a complex stoichiometry. In Figure 1b, 400 nm particles were found to settle within the interstitial sites between the hexagonally close-packed 1800 nm particles in the template, and some sites in the ordered monolayer were not complete filled by 400 nm particles and, therefore, formed structure defects in the 2D binary colloidal crystal. These defects reveal that a two-step process was undergone to generate this kind of structure. At the beginning stage of TAEFIA, one 400 nm particle was attracted to move toward the template due to the electric field. Because its diameter was larger than the size of the interstitial sites in the template, the small particle lay in one original interstitial site within the template and thus formed three secondary interstitial sites. At the same time, the distribution of the local electric field was also influenced. As the electricfield-induced assembly went on, these secondary sites were occupied by other particles. As a result, each interstitial site trapped four small particles in an inverse pyramid arrangement, and each large particle was surrounded by 24 small particles. This type of structure can be denoted as stoichiometry LS8, which has not been reported previously in the literature. A similar structure was also found in the case of 150 nm particles on the 700 nm template, with γ ) 0.21, which is not shown here. Binary colloidal crystals with stoichiometry LS2 were found in the case of 400 and 700 nm particles, as demonstrated in Figure 1c, in which the 400 nm particles nearly touched each other due to a high size ratio (γ ) 0.57, whereas the critical size ratio of LS2 is γ ) 0.577). For a size ratio higher than 0.577, for example 640 and 700 nm particles (see Figure 1d), the small particles
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Figure 2. SEM images of binary colloidal crystals consisted of 240 nm colloidal suspension with different volume fractions on 700 nm particles. (a) φ ) 5‰; (b) φ ) 1%.
tended to occupy three separated interstitial sites among the ordered template layer, which resulted in a colloidal crystal with stoichiometry LS. From the result of TAEFIA on 640 nm/700 nm microspheres, it seemed that the 640 nm microspheres formed an incomplete monolayer on the template layer of 700 nm spheres, which may be caused by two factors. When a small sphere was about to settle onto the template layer, the ideal arrangement was to occupy the “lattice point”, for example site A (see Figure 1d), to form a perfect structure of LS. However, in case the small sphere settled in site B rather than site A for some reason (e.g., hydrodynamic fluctuation or friction between small and large spheres), a packing displacement was induced into the crystal lattice, which may lead to more displacements of other small spheres, such as the vicinity of site B as shown in Figure 1d. These intrinsic structural defects made the monolayer of small spheres incomplete. Another reason for the incompletion was the structural defects in the template layer, which directly influenced the arrangement of the small spheres above. These kinds of extrinsic defects can be eliminated by improving the quality of the template layer. It is necessary to point out that the LS8 binary colloidal crystal described above appears similar to some results (260 and 1280
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Figure 3. (a) High and (b) low magnification SEM images of triple colloidal crystal consisted of 150, 240, and 700 nm particles.
nm silica particles, γ ) 0.20) reported previously in the literature,9 but they are formed with a different mechanism. By accelerated evaporation, Kitaev and Ozin induced coassembly of large and small colloidal particles to form surface patterns of binary colloidal crystals. During this process, the small particles occupied not only the interstitial sites within the surface of the ordered-packed large particles but also the ones inside. On the other hand, TAEFIA was a layer-by-layer approach. Because the diameter of the small particles was larger than the size of the interstitial sites in the template of large particles, the 400 nm particles of LS8 discussed above were only settled in the interstitial sites within the surface of the 1800 nm particle template, and the interstitial sites inside the template remained not occupied. However, when the diameter of the small particles was less than the size of the interstitial sites, for example 180 nm particles on 1800 nm particles (γ ) 0.10) as illustrated in Figure 1a, the small particles moved through the gaps among the large particles and filled the cavities both inside and outside the template. The as-prepared complex structure was similar to some results (145 and 1280 nm silica particles, γ ) 0.11) of accelerated evaporation induced coassembly9 but less ordered and more porous. To demonstrate the influence of volume fraction (φ) on the structures of binary colloidal crystals generated by TAEFIA, 5‰ and 1% suspensions of 240 nm PS particles were employed
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to assemble on a 2D colloidal monolayer of 700 nm PS particles. As shown in Figure 2a, a low volume fraction of a small particle suspension led to a binary structure with a low particle number ratio (NS/NL) such as LS2, whereas a high volume fraction produced a complex structure, which can be denoted as LS4 (see Figure 2b). When a colloidal suspension with a higher volume fraction was employed, binary structures with more small particles but nonstoichiometry were generated, and ordered layers of small particles on the template monolayer were also obtained in an extreme case. In addition to binary colloidal crystals, we also apply our method to fabricate triple colloidal crystals, by depositing a third layer of 150 nm colloidal particles (S) on a colloidal crystal consisted of 240 nm (M) and 700 nm (L) particles, as demonstrated in Figure 3. Due to the fact that the LM2 crystal had not been treated further to affix the particles, some 240 nm particles were peeled off from the interstitial sites within the 700 nm colloidal layer during the second depositing. Although these structural defects affected the local arrangement of 150 nm particles, some triple ordered structures can be formed. Under high magnification as shown in Figure 3b, it can be seen that each secondary interstitial site between two 240 nm particles was further filled by five 150 nm particles, with one located at the middle site between two 240 nm particles and the other four particles settled above the one underneath. This kind of structure can be denoted as LM2S15.
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By fine adjusting the electric field strength and depositing time, we expect that triple or more complex colloidal crystals can be synthesized by template-assisted electric-field-induced assembly in a layer-by-layer manner.
4. Conclusions In summary, 2D binary colloidal crystals of polystyrene particles with size ratios from 0.10 to 0.91 have been fabricated by template-assisted electric-field-induced assembly. By adjusting the size ratio of the small to large particles and the volume fraction of colloidal suspension, we produced binary colloidal crystals with various stoichiometries, including previously reported LS, LS2, and LS4, and new LS8. Moreover, a triple colloidal crystal with stoichiometry LM2S15 was fabricated, which suggested that template-assisted electric-field-induced assembly might be a promising way to fabricate triple or more complex colloidal crystals. The dependence of the crystal structure on the applied electric field is under further investigation. Acknowledgment. This work was supported by the Ministry of Sciences and Technology of China through 973-Project under Grants of 2002CB61306, and National Science Foundation of China under Grants of 50425204, 50572043, 60608016, and 90401012. LA700512J
