Fabrication of Colloidal Grid Network by Two-Step Convective Self

Apr 1, 2011 - 5290 dx.doi.org/10.1021/la200515w |Langmuir 2011, 27, 5290-5295. ARTICLE pubs.acs.org/Langmuir. Fabrication of Colloidal Grid Network ...
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Fabrication of Colloidal Grid Network by Two-Step Convective Self-Assembly Yasushi Mino, Satoshi Watanabe, and Minoru T. Miyahara* Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan

bS Supporting Information ABSTRACT: We explored a “template-free” approach to arranging colloidal particles into a network pattern by a convective selfassembly technique. In this approach, which we call “two-step convective self-assembly,” a stripe pattern of colloidal particles is first prepared on a substrate by immersing it in a suspension. The substrate with the stripes is then rotated by 90° and again immersed in the suspension to produce stripes perpendicular to the first ones, resulting in a grid-pattern network of colloidal arrays. The width of the colloidal grid lines can be controlled by changing the particle concentration while maintaining an almost constant spacing between the lines. On the basis of these results, we propose a mechanism for grid pattern formation. Our method is applicable to various types of particles. In addition, the wide applicability of this method was employed to create a hybrid grid pattern.

’ INTRODUCTION The arrangement of colloidal particles into ordered structures is a fundamental technique with various potential applications, including photonic crystals,1 3 surface-enhanced Raman scattering (SERS) substrates,4,5 bio6,7 and chemical8 sensors, nanoscale reactors,9 and catalysts.10 For these applications, an effective approach is the utilization of a particulate self-organization process owing to its ability to form colloidal structures over large areas. Among the numerous selforganization methods, convective self-assembly11,12 has attracted much attention because it provides simple and economical routes to the fabrication of colloidal structures. In this process, particles are carried into the solvent substrate contact line by convective flows induced by solvent evaporation, and then the particles form a closepacked array owing to a lateral capillary force between them. However, most of the configurations produced by this technique have been limited to relatively simple patterns such as continuous films,13 15 rings,16 18 and stripe patterns.19 21 It is quite challenging and important to overcome this structural limitation. A colloidal network, which has the potential for use in various applications, has not been produced by the convective self-assembly method. Although topographical22 26 or chemical27 29 premodification of the substrate has enabled precise control of the particle assembly to produce the desired colloidal pattern, it has been difficult to produce a network pattern on large substrates. Recently, several elegant studies have successfully demonstrated the formation of colloidal network patterns by template-assisted evaporative lithography.30 33 Celio et al. produced square and hexagonal network patterns of polystyrene colloidal particles by confining a suspension between a designed template and a substrate to control the shape of the contact line.30 This work was followed by similar approaches by other researchers.31,32 Vakarelski et al. fabricated a microwire network composed of Au nanoparticles by using two-dimensional arrays of r 2011 American Chemical Society

50 100 μm latex particles as a template.33 Although evaporative lithography is a robust technique for producing network patterns, it requires additional processes for preparing templates, and it would be difficult to control the particle assembly process on a manufacturing scale. A “template-free” approach is required to fabricate colloidal network patterns on a large scale easily and economically. In our previous study,21 we investigated the convective selfassembly of colloidal particles by changing the particle concentration, withdrawal rate of the substrate, and surface tension and successfully fabricated well-defined stripe patterns of silica particles on hydrophilic substrates. Furthermore, we proposed a new mechanism for stripe pattern formation and confirmed its quantitative validity. In the present work, we extend the stripe formation technique to fabricate a grid-patterned network of colloidal arrays. A schematic of this patterning approach, which we call two-step convective self-assembly, is shown in Figure 1. In the first step, a hydrophilic substrate is immersed in a suspension to produce a stripe pattern of colloidal particles on the substrate. In the second step, the substrate with the stripe pattern is rotated by 90° and again immersed in the suspension. The particles then assemble into a stripe pattern in the same manner as in the first step, resulting in a grid network pattern. Similar strategies for fabricating grid patterns have been demonstrated by a couple of group of researchers.34,35 Huang et al. demonstrated the formation of a grid pattern composed of single colloidal particle lines by a Langmuir Blodgett technique.34 Their method is quite effective for producing single particle lines, but it is difficult to fabricate multilayered particle lines and control the line width flexibly because of the nature of the Langmuir Blodgett Received: February 8, 2011 Revised: March 18, 2011 Published: April 01, 2011 5290

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Figure 1. Schematic illustration of two-step convective self-assembly method for fabrication of grid patterns. In the first step, a hydrophilic glass substrate was immersed vertically in a suspension in a glass vessel, which was placed in an incubator. After the first assembly, the same substrate was treated with a plasma cleaner to restore its hydrophilicity. In the second step, the substrate with the stripes was rotated by 90° and again immersed vertically in the colloidal suspension.

technique. Kim et al. arranged CdSe nanoparticles into a grid pattern by a horizontal convective self-assembly method.35 Although they produced well-defined grid patterns by the periodic translation of a substrate, horizontal assembly methods are, in general, not suitable for large-scale fabrication because the amount of suspension confined between the blade and the substrate is limited. Herein, we demonstrate the formation of a colloidal grid network pattern by the two-step convective self-assembly technique. Well-defined grid patterns form on hydrophilic glass substrates. We examine the effect of particle concentration and demonstrate that the width of the colloidal lines increases with the particle concentration, whereas the spacing between the lines remains almost constant. On the basis of these results, we propose a mechanism for grid pattern formation. Furthermore, we apply our method to different types and sizes of particles to demonstrate its wide applicability. Taking advantage of this applicability, we use different suspensions in the first and second step and produce hybrid grid patterns where the vertical and horizontal stripes are constructed from different types of particles.

’ EXPERIMENTAL SECTION Materials. Suspensions of silica particles with a diameter of 45 nm (Cataloid SI-45P), 120 nm (Spherica Slurry 120), and 270 nm (Spherica Slurry 300) were purchased from Catalysts & Chemicals Ind. Co., Ltd. (Japan), and suspensions of silver particle with a diameter of 10 15 nm (Finesphere SVW001) were purchased from Nippon Paint Co., Ltd. (Japan), where the diameters are those reported by the manufactures. The suspensions were diluted to the desired particle concentration using ultrapure water with a resistivity of 18 MΩ 3 cm obtained from Direct-Q 3 UV Water Purification System (Millipore Corp., Bedford, MA). Micro coverglasses with dimensions of 18  18 mm2 (Matsunami Glass Ind., Ltd., Japan) were used as substrates. As a preliminary cleaning procedure, the glass substrates were washed with acetone (99.5%, Kishida Chemical Ind., Ltd., Japan), ethanol (99.5%, Kishida Chemical Co., Ltd., Japan), and ultrapure water in an ultrasonic bath. Then, right before use, they were dried with compressed air and treated with a plasma cleaner (Harrick Plasma Inc., NY) to increase their hydrophilicity. Particle Assembly. In the first step, as shown in Figure 1, a hydrophilic glass substrate was immersed vertically in a 16-mL suspension (particle concentration φ1 [vol/vol], where the subscript 1 indicates the first step) in a glass vessel, which was set in an incubator (Yamato Scientific Co., Ltd., Japan) at 60 °C for at least 17 h. The substrate with the stripe pattern was then treated by the plasma cleaner again to restore its hydrophilicity. In the second step, the substrate with the stripes was rotated by 90° and immersed vertically in a 16-mL suspension of φ2

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Figure 2. (a) Bright-field optical microscopy image of stripe pattern obtained using 120 nm silica particle suspension with φ1 = 1.0  10 5 in first step. (b) SEM image of particulate line. (subscript 2 indicates the second step). The evaporation rate of the suspensions in the incubator was 0.24 ( 0.01 μm/s, which was calculated by measuring the total amount of solvent that evaporated during the experiment. The evaporation condition we used in the present work was the same as that in our previous work,21 although the formation rate was slow due to the slow evaporation. Speeding up the process requires further investigation, and we are now working on the improvement of the assembly technique for higher throughput, which will be reported in the near future. After the assembly of the particles, all samples were observed with a digital microscope VHX-VK-600 (Keyence, Japan), and the microstructures of the patterns were studied with a scanning electron microscope (SEM) (JSM-6700F field emission SEM, JEOL, Japan).

’ RESULTS AND DISCUSSION Grid Pattern Formation. Figure 2a shows an optical micrograph of a typical stripe-patterned colloidal array fabricated on a flat glass substrate using a 120 nm silica suspension of φ1 = 1.0  10 5 in the first step. As shown in Figure 2b, which shows an enlarged image of a particulate line in Figure 2a, the stripes are a close-packed colloidal monolayer array. The stripe pattern exhibits a well-ordered periodicity of the order of micrometers in its width and spacing, i.e., 7.2 ( 0.6 μm and 29.7 ( 2.0 μm, respectively, based on an optical microscopic measurement. Previously, we have clarified the formation process of the stripe patterns as follows.21 When the particle concentration is fairly low, the number of particles carried into the contact line is so small that the growth rate of a particulate line is slower than the rate of liquid level descent. As evaporation progresses, the rate difference increases the distance between the growing front of the particulate line and the liquid level and gradually elongates the meniscus into a concave shape. The process has negative feedback, that is, meniscus deformation enhances the rate difference. Thus, after a certain period, the elongated and curved meniscus breaks off and the next line grows steadily. This process repeats periodically, resulting in the formation of a well-defined stripe pattern parallel to the contact line. The detailed formation mechanism with schematic images have been described in ref 21. Performing the second step produces a grid pattern as shown in Figure 3. However, the fabrication was not as simple as it may seem, because simply immersing the substrate after the first step did not yield the grid pattern. The key requirement we determined for the reproducible formation of well-defined grid patterns is plasma treatment before the second immersion in the suspension. This is because, after the first assembly step, the hydrophilicity of the substrate is weakened due to its exposure to the atmosphere. As a result, the second deposition without 5291

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Figure 3. (a) Optical micrograph and (b) SEM image of grid pattern formed using 120 nm silica particle suspension with φ2 = 1.0  10 5 on substrate with stripe pattern (Figure 2a) in second step. The vertical and horizontal stripes were formed in the first and second step, respectively. In this grid pattern, (c) most junctions are monolayers, whereas (d) only a few junctions are bilayers, where the second layer is indicated by an arrow.

plasma treatment does not produce grid patterns, but accumulates particles only around the vertical stripes and consequently gives disconnected second stripes. Figure 3a shows an optical micrograph of the grid pattern of 120 nm silica particles. The particle concentration in the second step was φ2 = 1.0  10 5, which was the same as φ1. In the figure, the stripes formed in the first step are the vertical ones, while those formed in the second step are the horizontal ones. In what follows, all of the images of grid patterns are also shown in the same orientation. The optical micrograph analyses show that, after the second step, the width and spacing of the stripes formed in the first step remained almost constant (6.7 ( 0.8 μm and 30.0 ( 2.4 μm, respectively). This indicates that the colloidal structures deposited on the substrate were not removed by the second immersion in the suspension. However, the width and spacing (shown in Figure 3b) of the stripes formed in the second step were 5.7 ( 0.8 μm and 27.3 ( 2.6 μm, which were narrower than the first ones. SEM images of the grid pattern are shown in Figure 3b d, and higher magnification SEM images of junctions of the grid are shown in Figure S1 (See the Supporting Information, SI). As shown in Figures 3b and S1, the stripes formed in the second step connect smoothly with the first ones without clear boundaries and broad the width near the junctions. Furthermore, contrary to our expectations, most of the junctions were monolayers (Figure 3c, S1a), whereas Kim et al. demonstrated that the stripes formed in the second step overlapped the first ones at the junction.35 In our grid pattern, only a few junctions were bilayer; in addition, the bilayer area was limited to the lower part of the junction (Figure S1b of the SI, bright area indicated by an arrow in Figure 3d). Effect of Particle Concentration. To further examine the grid formation process, we varied φ2 from 3.0  10 6 to 1.0  10 4 with φ1 = 1.0  10 5. The resultant structures are shown in Figure 4. Optical micrographs of the grid patterns are shown in

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the upper panels (Figure 4a e), and for comparison, the lower panels (Figure 4f j) show those of stripe patterns produced on flat substrates under the same concentration conditions as those employed for the second step, as shown in Figure 4a e. Thus, the difference between the upper and lower panels of Figure 4 simply originates from the presence of the vertical (first) stripes. At the low particle concentration of φ2 = 3.0  10 6, the condition under which very thin stripes were formed without vertical stripes (Figure 4f), particles were deposited only near the vertical stripes, resulting in a disconnected structure between the vertical stripes (Figure 4a). At the higher particle concentration of φ2 = 5.0  10 6, horizontal stripes connected with the vertical ones, although they sagged (Figure 4b), unlike the straight stripes formed without verticalstripes (Figure 4g). Increase in the particle concentration up to 1.0  10 5 produced well-defined, straight, horizontal stripes (already shown in Figure 3a). At the particle concentration of φ2 = 2.0  10 5, bilayers with a compass shape were discretely formed on the junctions (yellow area indicated by an arrow in Figure 4c), although the assembly without vertical stripes produced monolayer stripes (Figure 4h). Note that the color of the particulate lines in the bright-field image originated from the interference of light and varied depending on the number of particle layers.36 Further increases in the concentration up to φ2 = 5.0  10 5 and 1.0  10 4, at which bilayer stripes were formed without vertical stripes (Figure 4i,j), increased the bilayer region to link the discrete bilayer domains (Figure 4d) and even produced trilayers at the junctions (blue area indicated by an arrow in Figure 4e). In this manner, the structure of grid patterns varied depending on the particle concentration. A characteristic feature was that the particles preferentially accumulated near the vertical stripes. Mechanism of Grid Pattern Formation. On the basis of these results, we propose a mechanism for grid pattern formation. Because the formation mechanism of first stripes has been described in detail in our previous report,21 here we focus on the process of second stripe formation. Figure 5 shows a schematic of the second particle assembly process on a substrate that has preformed vertical stripes. When a substrate with the vertical stripes is immersed in a suspension, the meniscus is raised by the vertical stripes because they are hydrophilic and possess a porous body that draws in the solvent (Figure 5a). We confirmed the meniscus deformation by direct observation of the meniscus on the substrate with the stripes (Figure 5b). As wet particulate films (areas enclosed by orange circles in Figure 5a) increase solvent evaporation as compared to the case without films, biased convective flows toward the vertical stripes (arrows in Figure 5a) are induced. The biased flows transfer particles preferentially toward the vertical stripes, and these particles are trapped in the gaps at the side edges of the vertical stripes, allowing smooth connection of the vertical and horizontal stripes without any clear boundaries, as shown in Figure 3. As a low particle concentration carries only a small number of particles between the vertical stripes, breakage (Figure 4a) and sagging (Figure 4b) of the horizontal stripes occur. When the particle concentration φ2 increases to ∼10 5, a sufficient number of particles are carried not only near the vertical stripes, but also between them to fix the contact line stably, resulting in the formation of a well-defined grid pattern (Figure 3a). It should be noted that under these fairly low concentration conditions, most junctions are monolayers, as shown in Figures 3a and 4a,b, despite the preferential transfer of particles into the vertical stripes by the biased flow. Figure 5c,d shows the process under low and high concentration conditions, respectively. There is not enough space for particles to pass 5292

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Figure 4. (a e) Optical micrographs of grid patterns formed using 120 nm silica particles, demonstrating effects of particle concentration in second step. With a fixed concentration in the first step (φ1 = 1.0  10 5), we varied the concentration in the second step; φ2 = (a) 3.0  10 6, (b) 5.0  10 6, (c) 2.0  10 5, (d) 5.0  10 5, and (e) 1.0  10 4. (f j) Optical micrographs of stripe patterns formed without vertical stripes using 120 nm silica particle suspensions with same concentration as that in upper panels: φ = (f) 3.0  10 6, (g) 5.0  10 6, (h) 2.0  10 5, (i) 5.0  10 5, and (j) 1.0  10 4.

Figure 5. Process of particle assembly on substrate with vertical stripes. (a) 2D schematic and (b) optical micrograph of meniscus raised by vertical stripes. (c, d) 3D schematics of meniscus raised by vertical stripes, demonstrating particle deposition process under conditions of (c) low and (d) high particle concentration.

through the red-colored region in the schematic because the space is confined by the meniscus, whereas enough space exists for the green-colored region. Thus, the particles carried by the biased convective flows do not enter the red space, but instead align along the contact line to form a monolayer junction. At a low particle concentration, because the growing front of the particulate line is separate from the green region, no particles are deposited in the green region. In addition, because of the preferential transfer of particles toward the junctions, the particulate

line width around the vertical stripes grows faster, resulting in the broadened stripe shape at junctions (Figure 3a). However, a high particle concentration means the growing front is rather close to the green region, because more particles are carried into the meniscus edge to form a thicker line. As a result, the particles are carried toward and stacked on the green region to form discrete domains of bilayers on the junctions (Figure 4c). A further increase in the concentration results in more particles being carried toward the meniscus edge, resulting in a continuous bilayer with a corrugated shape (Figure 4d) and trilayer domains on the junctions (Figure 4e). At these high particle concentrations, the particles deposit on the vertical stripes as well as at their sides. As a result, the growing front of the second particle deposition is aligned, and consequently the bottom part of the stripes becomes flatter than the case of low concentrations (Figure 4c e). Versatility of Our Method. In general, convective assembly techniques can be used to arrange various types and sizes of particles. To examine the versatility of our method, we applied the above method to other particles. Figure 6a,b shows optical micrographs of grid-patterned colloidal arrays fabricated from small-sized silica particles (45 nm) and large-sized silica particles (270 nm), respectively. The grid pattern was also produced from particles of a different material: silver particles with diameters of 10 15 nm (Figure 6c). These grid patterns exhibited a high periodicity, similar to the one obtained using 120 nm silica particles, demonstrating the general applicability of our method to various types of particles. Taking advantage of this wide applicability, we used two different suspensions in the first and second step to produce a hybrid grid pattern where vertical and horizontal stripes were constructed from different types of particles. Figure 6d shows this hybrid grid pattern, which was obtained with a first step using 120 nm silica particles (vertical stripes) and a second step using 10 15 nm silver particles (horizontal stripes). As shown in Figure 6d, when the particle size used at the second step was smaller than that at the first step, the smaller particles at the second step tended to deposit on the vertical stripes and form multilayer junctions (See the Supporting Information, Figure S2, which shows SEM images). This result coincides with our proposed mechanism, because the red 5293

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’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of the junctions of silica grid patterns (Figure 3) and hybrid grid pattern (Figure 6d). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Young Scientists (B), the Global Centers of Excellence (G-COE) Program, and the Core-to-Core (CTC) Program from Japan Society for the Promotion of Science (JSPS). ’ REFERENCES Figure 6. Optical micrographs of grid patterns fabricated using suspensions of (a) 45 nm silica particles at φ1 = φ2 = 1.0  10 5, (b) 270 nm silica particles at φ1 = φ2 = 1.0  10 4, and (c) 10 15 nm silver particles at φ1 = φ2 = 5.0  10 6. (d) Optical micrograph of silica-silver hybrid grid pattern obtained in first step using 120 nm silica particles at φ1 = 1.0  10 5 and second step using 10 15 nm silver particles at φ2 = 5.0  10 6.

spaces in Figure 5c,d are accessible only for smaller particles. The anisotropic property of this kind of hybrid grid pattern, it is hoped, will provide interesting technological applications in electronics and sensors. This method can produce well-defined hybrid grid patterns, which is an advantage over other methods, particularly those that use templates.

’ CONCLUSIONS We have demonstrated the fabrication of well-defined gridpatterned colloidal arrays by the two-step convective self-assembly method and investigated the formation process of the grid pattern. Various grid patterns were fabricated by varying the particle concentration in the second step. The width of the colloidal grid lines was controlled by the particle concentration, with an almost constant spacing between the lines. The characteristics of the grid pattern were preferential assembly of the particles around junctions and smooth connections between the stripes formed in the first and second steps. On the basis of these results, we proposed a formation mechanism in which the preformed vertical stripes induce biased convective flows, thereby carrying the particles toward the stripes, and deforming the shape of the meniscus to restrict particle deposition, resulting in the characteristic morphology of grid patterns. Different sizes and types of particles also produced grid patterns successfully, thereby demonstrating the wide applicability of our method. Furthermore, by using different particles in each step, hybrid grid patterns were produced. The two-step convective self-assembly method is simple and lowcost; moreover, it is applicable to various types of particles and thus has the potential for technological applications in a variety of fields.

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