© Copyright 2006 American Chemical Society
AUGUST 15, 2006 VOLUME 22, NUMBER 17
Letters Rapid Fabrication of Large-Area Colloidal Crystal Monolayers by a Vortical Surface Method Feng Pan, Junying Zhang,* Chao Cai, and Tianmin Wang Center of Materials Physics & Chemistry, Beijing UniVersity of Aeronautics and Astronautics, Beijing, 100083, P. R. China ReceiVed December 8, 2005. In Final Form: May 7, 2006 We have developed a simple approach for rapidly fabricating large-scale colloidal crystal arrays on substrates. Latex particles were first assembled into a close-packed monolayer on a vortical water surface; the monolayer was then transferred onto substrates by a withdrawer. Such an assembly method is conceptually similar to the LangmuirBlodgett (L-B) methods for film deposition but does not require an L-B trough. The samples exhibit a large-scale periodic feature based on optical microscopy and scanning electron microscopy observations and diffract the laser beam, acting as crystals. This newly developed technique is timesaving, widely accessible, and applicable to large particles (up to 2 µm). It promises to be useful in nanofabrication.
Well-ordered colloidal particles monolayers, such as polystyrene (PS), silica, and protein, are interesting objects for applications as lithography masks,1 antireflection surfaces,2 multilens arrays,3 and data storage media.4 Numerous attempts have been proposed to achieve such arrays. However, many currently available methods, for example, gravity sedimentation,5 vertical lifting deposition,6 spin-coating,7 electrophoretic deposition,8 the evaporation suspending liquid method,9 and the flow cell method,10 have a common inevitable drawback in the * Corresponding author. E-mail:
[email protected]. Address: Center of Materials Physics & Chemistry, Beijing University of Aeronautics and Astronautics, Beijing, 100083, P. R. China. Tel: +86-10-82317941. Fax: +86-10-82315351. (1) (a) Deckman, H. W.; Dunsmuir, J. H. Appl. Phys. Lett. 1982, 41, 377. (b) Frey, W.; Woods, C. K.; Chilkoti, A. AdV. Mater. 2000, 20, 1515. (c) Bullen, H. A.; Garrett, S. J. Nano Lett. 2002, 2, 739. (2) Yoldas, B. E.; Partlow, D. P. Appl. Opt. 1984, 23, 1418. (3) Hirai, T.; Hayashi, S. Colloids Surf., A 1999, 153, 503. (4) Micheletto, R.; Fuckuda, H.; Ohtsu, M. Langmuir 1995, 11, 3333. (5) Miguez, H.; Meseguer, F.; Lo´pez, C.; Blanco, A.; Moya, J.; Requena, J.; Mifsud, A.; Forne´s, V. AdV. Mater. 1998, 10, 480. (6) (a) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (b) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (7) (a) Wang, D.; Mo¨hwald, H. AdV. Mater. 2004, 16, 244. (b) Kuo, C. W.; Shiu, J. Y.; Cho, Y. H.; Chen, P. AdV. Mater. 2003, 15, 1065. (8) Bo¨hmer, M. Langmuir 1996, 12, 5747. (9) Im, S. H.; Park, O. O. Langmuir 2002, 18, 9642.
difficulty to control monolayer formation despite their evident advantages in three-dimensional (3D) array fabrication. The thin liquid film method established by Nagayama11 and the airliquid interface method including the Langmuir-Blodgett (LB) technique12 are considered to be effective methods for monolayer fabrication. In the thin liquid film method, colloidal spheres are organized into a hexagonal two-dimensional (2D) array in a thin film of liquid by the attractive capillary forces among the latex particles. Such a method usually consists of a slow evaporation process of solvent under critical controlled conditions and necessitates the flat and horizontal surface of the substrate for obtaining highly ordered monoparticle films.11 More sophisticated modifications have been performed, but a special apparatus is required.13 In the air-liquid interface method, colloid particles are spread onto a liquid surface through a spreading agent and spontaneously form a 2D aggregate at the interface (10) (a) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (b) Lu, Y.; Yin, Y.; Gates, B.; Xia, Y. Langmuir 2001, 17, 6344. (11) Nagayama, K. Colloids Surf., A 1996, 109, 363. (12) (a) Lenzmann, F.; Li, K.; Kitai, A. H.; Stover, H. D. H. Chem. Mater. 1994, 6, 156. (b) Kondo, M.; Shinozaki, K.; Bergstro¨m, L.; Mizutani, N. Langmuir 1995, 11, 394. (c) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969. (d) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B.-R.; Go¨rnitz, E. Langmuir 2002, 18, 5627.
10.1021/la053323n CCC: $33.50 © 2006 American Chemical Society Published on Web 05/23/2006
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Figure 1. Schematic description of the process of fabricating a large-scale periodic monolayer of PS spheres by the novel vortical surface method. (a) A beaker was filled with water, and a hollow Teflon ring was allowed to float on the water surface. A vortex was produced by a magnetic stirrer at a constant speed. A PS colloidal suspension dispersed by ethanol was dropped into the vortex, and a well-ordered particle array was formed on the water surface by means of the water flow. (b) The opaque water under the latex film was piped out from a penstock setting at the bottom of the beaker, and the ring sank down with the water level. (c) Clear water was injected into the beaker, and the ordered latex film was raised with the ring again. (d) A substrate was inserted into the water across the latex film and lifted up vertically at a steady speed. Large-scale colloidal arrays were formed on the double surface sides of the substrate.
by the attractive interactions among the particles, and this array can be subsequently transferred onto the surface of a solid substrate.12 Recently, relatively large domain sizes of 2D arrays (several square centimeters) were obtained by this method,14 and it has been possible to form ordered 2D arrays of colloidal spheres on various types of substrates by a similar technique.15 However, the morphology of the aggregate usually exhibits disorder and fractal characteristics. We consider that the reason may be the sluggish motions of the particles in water. In the L-B case, for example, the small particles are partially immersed into the stagnant water surface and cannot move freely to assemble largearea close-packed structures, although the compression of the surface is processed. It is believed that if the torpor particles are wakened by a certain force, the quality of the arrays will be better. In this communication, we introduce a new approach that enables the rapid fabrication of colloid crystalline monolayers on flat surfaces. It is based on the idea mentioned above and involves a two-stage process: assembly of ordered close-packed monolayer arrays of latex particles on a vortical water surface by the water flow, and the structure transfer onto the surface of the substrate. The detailed procedure is illustrated schematically in Figure 1. A 600 mL portion of Milli-Q water was poured in a 12-cmcaliber glass beaker, with a cleansed hollow Teflon ring (10 cm diameter) floating on the surface. By stirring the water with a (13) (a) Picard, G.; Nevernov, I.; Alliata, D.; Pazdernik, L. Langmuir 1997, 13, 264. (b) Picard, G. Langmuir 1997, 13, 3226. (14) (a) Kempa, K.; Kimball, B.; Rybczynski, J.; Huang, Z. P.; Wu, P. F.; Steeves, D.; Sennett, M.; Giersig, M.; Rao, D. V. G. L. N.; Carnahan, D. L.; Wang, D. Z.; Lao, J. Y.; Li, W. Z.; Ren, Z. F. Nano Lett. 2003, 3, 13. (b) Fulda, K. U.; Tieke, B. AdV. Mater. 1994, 6, 288. (15) Burmeister, F.; Scha¨fle, C.; Matthes, T.; Bo¨hmisch, M. Langmuir 1997, 13, 2983.
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Figure 2. (a) Sample of a monolayer, covering a glass slide, built from 1.38 µm PS spheres. (b) Photograph of the same sample illuminated with white light at different angles.
magnetic stirrer (Model 85-2) at 120 rpm, a stable volution was obtained in the vessel. Monodispersed latex spheres with various diameters of 0.8, 1.38, and 2.0 µm were synthesized by the dispersion polymerization reaction according to Cao et al.,16 and the standard deviation of the particle diameters was determined by scanning electron microscopy (SEM; Hitachi S4200) to be less than 4%. The newly made spheres, washed by ethanol three times and diluted to a volume fraction of 2.5%, were dropped at the rolling surface in the ring using a microsyringe (see Figure 1a). Each drop was controlled within 8 µL to reduce turbulence in the water. It could be observed that the spheres spread around quickly, and a thin film rolling with the water surface was formed. With the continual dropping, the film became more and more compact. When the whole water surface in the ring was covered and the rolling of the film became sluggish, the dropping was stopped, and the stirrer was then slowed. A flat surface with a typical iridescent color was then obtained, which indicated that an ordered microstructure had been formed. After replacing the turbid water under the colloid film with clear deionized water (see Figure 1b,c), a flat glass substrate (25.7 × 75.4 × 1.1 mm), which was pretreated by hot piranha solution (3:1 concentrated H2SO4/30% H2O2) and rinsed by Milli-Q water three times, was vertically inserted and lifted up by a custom-built withdrawer at a speed of 2 mm/min (see Figure 1d). (Caution: Piranha solution reacts Wiolently with organic matter and should be handled with extreme care!) The close-packed crystal films were then created on the double sides of the substrate. Figure 2a shows a glass-slide sample covered with a monolayer of 1.38 µm PSlatex beads. The uniform iridescent colors on the surface were observed from a certain angle of view (Figure 2b). The fabrication mechanism can be explained as follows. PS has a similar but slightly larger density (1.04 g/cm3) than water. The surface tension force and the flotage of water sustain the particles completely on the water surface because of the hydrophobic property of PS. This is also widely adopted by many other approaches.12 However, different from other approaches, a motorial liquid surface is employed in our method, which provides motion for the float particles with the water and benefits from two aspects. On one hand, the motion makes the particles move in a velocity gradient, which provides a runningin period for the particles to reduce the holes and voids caused during the spreading process and to reduce stacks by driving the (16) Tongyu, C.; Bing, D.; Junyan, D.; Yanjun, W.; Caideng, Y. Acta Polym. Sin. 1997, 2, 158.
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Figure 3. Optical microscopy graph of the monolayer surface (1.38 µm PS spheres).
Figure 4. SEM images for the close-packed arrays of 1.38 µm particles: the top view (a) and profile view (b) of the monolayer; the top view (c) and profile view (d) of the double-layer.
immethodical particles into water. On the other hand, the rotation of the stirrer paddle makes the water at the beaker bottom move away from the center, working as a centrifugal pump, and produces a water flow as shown in Figure 1a. The water flow carries the float particles to aggregate at the vortex center. Because the particles are only partly immersed in water, they cannot be drawn into the water by the flow. If the rotation speed is large enough, the aggregated particles will be compacted to a multilayer, and the under layer particles will be washed into the water by the flow. Thus, the water becomes turbid. An appropriate speed can both compress the particles into close-packed structures and reduce the turbidity of the water. The appropriate speed is found to be 120 rpm in the present case, depending on the water quantity and the stirrer paddle size. We point out here that the area of the funnel surface in the vortex must be larger than that of the flat surface, which is important in the present approach. When the stirring is slowed and the curved face becomes flat, the shrinking of the surface area will compress the particles much closer, facilitating the close-packed structure formation. Since the turbid solution containing particles inside will induce plane stacking faults in the final samples, water-replacing is performed in our approach (Figure 1b,c). During the waterreplacing process, the Teflon ring moves up and down with the water level, with the film structure in the ring unchanged. This
Figure 5. (a) The hexagonal 2D Bravais lattice of the close-packed latex array, (b) the top view of the 3D reciprocal lattice built by the Bravais lattice, (c) the side view of the 3D reciprocal lattice built by the Bravais lattice (upper panel), and the corresponding projected discrete symmetric diffraction spots (lower panel). The diameter and wavelength of the laser beam is 3 mm and 650 nm, respectively, and the diameter of the particle is 2.0 µm.
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is the main reason that the ring is used in this process. In addition, to obtain a good structure quality, the withdrawing environment must be dry (relative humidity < 50%), clean, and without disturbance, but other factors such as temperature and humidity are not strictly required. The monolayer microstructure of an optical microscopy (Model XJB-1, China) image is shown in Figure 3. A large-scale highly ordered hexagon structure can be easily observed at a random field of vision. SEM has also been used to characterize the microstructure with a higher resolution. Figure 4a,b shows the top view and the profile view of the monolayer, respectively. These images clearly indicate that the colloidal particles are stacked in a single-layered and close-packed arrangement, and such an arrangement is perfectly maintained all over the film area. Double-layered samples have also been obtained by immersing the dry, once-covered substrates into water and withdrawing a second layer. SEM images (Figure 4c,d) show that the closepacked particles in the second layer are overlaid on the interstices of the particles in the first layer, with the ordered structures unchanged. Repeating the same operation, 3D multilayered structures with a controlled thickness may be obtained. To see whether such good-quality arrays can only be formed under a specific condition, we employed different fabrication conditions using the same method. These include colloidal spheres with different diameters (0.8 µm and 2.0 µm), substrates with different materials (glass, Si, and PS), and different shapes (spherical face, glass tube, and glass fiber). Similar results were obtained, showing the generality of the present method. Ordered particle array is, in fact, a kind of 2D hexagonal grating. When the periodic structure is irradiated by a monochromatic beam with a suitable wavelength, an interesting optical diffraction phenomenon will occur. Figure 5c shows a diffraction pattern obtained from an array of PS spheres with a diameter of 2.0 µm by shining a 3-mm-diameter red (650 nm) laser beam perpendicular to the array surface. Discrete and symmetric diffraction spots with different orders were observed, which is very similar to that observed with low-energy electron diffraction (LEED).17 The general theory of light diffraction was used to explain the formation and location of the diffraction spots. Figure 5a shows schematically the periodic structure of PS particles in the colloidal films. It is a hexagonal 2D Bravais lattice. The distance between the two adjacent points is found to be equal to the diameter of the PS sphere D, and the max interplanar distance d is equal to D sin 60°. The reciprocal lattice is defined by the transformation relationship of ai‚bj ) 2πδij, where ai and bj are the primitive translation lattice vectors of the real and reciprocal space, (17) Gasser, R. P. H. An Introduction to Chemisorption and Catalysis by Metals; Clarendon Press: Oxford, 1985.
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respectively. The reciprocal lattice exhibits the same hexagonal structure (Figure 5b), where s ) 2π/d ) 2π/(D sin 60°). The reciprocal lattice is an array of parallel lines along a given direction (Figure 5c, upper panel), so-called reciprocal rods, which pass through each reciprocal lattice point. When a laser beam irradiates the Bravais lattice in a direction vertical to the sample surface, the reciprocal rods will cross through a sphere (Ewald sphere with its center on the sample) formed by the wave vector of the incident laser beam, and the radius of the sphere equals the wave vector (k ) 2π/λ). It implies that the directions from the sphere center to the intersection point of the parallel rods and the spherical surface satisfy the Bragg diffraction conditions of k - ki ) G, where G is the reciprocal lattice vector. The coherent scattering spot can be observed along these directions. All the intersection points on the sphere are projected onto a flat screen, and thus the diffraction pattern appears (Figure 5c, lower panel). Because the projections are divergent, the diffraction pattern obviously distorts and is not as regular as the reciprocal lattice. The diffraction spots are of a limited number, since, for a given Ewald sphere and given parameters of the reciprocal lattice, only those lattice points that are encircled by the circumference of the sphere can be projected into the diffraction spots. In our experiment, the radius of the Ewald sphere is |k| ) 9.24 µm-1, and the horizontal components of G of different sets of diffraction spots are G1st ) s ) 3.63 µm-1, G2nd ) x3s ) 6.83 µm-1, G3rd ) 2s ) 7.26 µm-1, and G4th ) x7s ) 9.60 µm-1. Obviously, only the zero (k0), first (k1), second (k2), and third (k3) sets of diffraction will occur and be observed. The diffraction results agreed very well with the theoretical analysis. It has been confirmed that, if the wavelength of the laser is shorter or the diameter of the PS sphere is much larger, the other set of “forbidden” spots may appear. The highly rotationally symmetric and sharp diffraction pattern is a unique characteristic of crystalline structure. The diffraction results indicate the high monocrystallinity of the present samples. In conclusion, we have developed a novel method by which a 2D large-area periodic colloidal array can be fabricated. The method is simple, low-cost, and widely accessible. Many important controlling factors in other methods, such as temperature, humidity, sphere size, and so forth, have little effect on the array quality in the present method. Furthermore, the method is steady and timesaving. Preparation of a centimetersized sample needs only a few minutes, compared with several hours, days, or even months in other methods. Such a method can be greatly expected to be applied in nanostructure fabrication. Acknowledgment. This work was supported by the Beijing Nova Program (H020821250190) and the National Natural Science Foundation of China under Grant No. 50302001. LA053323N
