Gravity-Assisted Convective Assembly of Centimeter-Sized Uniform

Jan 17, 2013 - ABSTRACT: We have developed an inexpensive, robust, and easily controlled method, a gravity-assisted convective self- assembly method, ...
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Gravity-Assisted Convective Assembly of Centimeter-Sized Uniform Two-Dimensional Colloidal Crystals Ran Ye, Yong-Hong Ye,* Zhenting Zhou, and Huanhuan Xu Department of Physics, Nanjing Normal University, Nanjing 210097, China ABSTRACT: We have developed an inexpensive, robust, and easily controlled method, a gravity-assisted convective selfassembly method, to fabricate centimeter-sized uniform twodimensional colloidal crystals. In this method, centimeter-sized two-dimensional colloidal crystals can be formed when the suspension concentration is in the range of 0.32−2.5 wt %. Once the ordering process starts, the formation of two-dimensional colloidal crystals is not affected when the environmental temperature gradually increases from 3 to 10 °C. Experimental results indicate that, in this method, gravity plays an important role in the colloidal crystal formation. The colloidal particles are transported to the edge of the suspension−glass interface, and the extra particles can be eventually moved to the edge of the slide by gravity.

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trolled.17−25 It would be useful to develop deposition procedures which are not very sensitive to the experimental parameters. In this paper, we report the fabrication of large-scale 2D colloidal crystals from aqueous solutions by a gravity-assisted confined convective self-assembly method. In this method, the colloidal particles are transported to the edge of the suspension−glass interface mainly by gravity, and the transported particles are assembled into 2D colloidal crystal by the lateral capillary forces. The colloidal crystals formed by this method are very uniform, and once the ordering process starts, the colloidal crystal formation is not affected when the environmental temperature is gradually increased from 3 to 10 °C. Moreover, the extra particles can be finally moved to the edge of the slide due to gravity, and centimeter-sized 2D colloidal crystals can be formed when the suspension concentration is between 0.32 and 2.5 wt %.

INTRODUCTION Significant attention has been paid to various methods of fabricating two-dimensional (2D) and three-dimensional (3D) colloidal crystals, as these periodic structures can provide a lowcost approach to fabricate large-scale periodic microstructures.1−7 Plasmonic crystals, metamaterials, optical devices, and 2D regular arrays have been fabricated by using 2D colloidal crystals as templates.8−13 2D colloidal crystals have been obtained by Langmuir−Blodgett deposition,14 spin-coating,15 electrophoretic deposition,16 and convective self-assembly.17,18 Among these methods, continuous convective assembly is a popular method to create 2D colloidal crystals from colloidal suspensions. Nagayama and co-workers have exploited the vertical deposition method that relies on capillary forces to assemble monolayer colloidal films. A monolayer or multilayer colloidal film is obtained by withdrawing the substrate from the microsized colloidal particles.17 Tsapatsis and co-workers have studied the formation of ordered monolayer and multilayer colloidal films from silica nanoparticles.19 To reduce material consumption, a modified convective assembling method, confined convective assembly, has been proposed, and the formation of centimeter sized 2D close-packed micro- and nanoparticle arrays has been demonstrated.20−24 For convective self-assembly, the water evaporation rate is highest at the edge of the suspension-glass interface, which causes a water flux transporting the colloidal particles to the edge of the growing crystals, and the particles assemble into ordered structures due to the lateral capillary forces.25 The thickness of the assembled colloidal crystals depends on the meniscus thinning rate and the velocity of colloidal particles moving toward the edge of the growing crystal. In order to form a monolayer colloidal film, the experimental parameters (such as the water evaporation rate, the suspension concentration) should be carefully con© 2013 American Chemical Society

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EXPERIMENTAL SECTION

Colloidal Particles and Glass Substrate. Glass slides were cut into 40 × 25 × 1 mm3 pieces and were used as substrates in our experiment. Prior to use, the glass slides were thoroughly cleaned by washing them with soap solution, subsequent immersion for more than 24 h in chromic acid, followed by washing with deionized water. The cleaned slides were kept under deionized water and vacuum-dried just before use. Polystyrene microspheres with a diameter of 970 nm (size dispersion 5−10%) were purchased from Bangs Laboratories, Inc. as an aqueous suspension of 10 wt %. The suspension was further Received: October 10, 2012 Revised: January 9, 2013 Published: January 17, 2013 1796

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diluted to 2.5, 1.7, 1.0, 0.63, 0.5, 0.32, 0.25, and 0.20 wt % with deionized water before use. Self-Assembly of Colloidal Crystals. Wedge-shaped cells were used in the experiment. The wedge-shaped cell was made up of two glass slides at a contact angle of ∼2°, and colloidal suspensions were injected into the space regions between the two slides. The cell was placed on a tilted bench. The bench was both tilted forward at an angle of ∼2° and tilted left at an angle of ∼1°. The cells were kept at a low temperature (3−10 °C) with environmental humidity ranging from 60% to 80% during the experiment. After evaporation of the solvent, 2D colloidal crystals were self-assembled on the lower slide. Sample Characterization. Sample structures were characterized by optical microscopy (Leica DM 2500M) and scanning electron microscopy (SEM, JSM-5200). The transmittance spectra of the samples were recorded with a Shimadzu spectrometer at normal incidence in the range of 0.4−5.0 μm.

at the edge of the suspension−glass interface, and the band’s left side is wider than the band’s right side. There are still colloidal particles in the suspension as the suspension looks milky. Moreover, centimeter sized uniform and transparent region appears in the front part of the slide. Sixteen hours later, as shown in Figure 1d, there is still a broomlike white band at the edge of the suspension−glass interface, but this band is lifted upward. The uniform and transparent region in the front part of the slide widens. The suspension becomes transparent, which indicates that there are little colloidal particles in the suspension. Figure 2 shows the optical images of three samples at different times of the process. The samples are fabricated with different concentrations (0.32, 1, and 2.5 wt %, respectively) while using the same environmental temperature, volume of colloidal suspension, and the same sized colloidal particle (970 nm in diameter). It is found that a centimeter-sized transparent and uniform film is created on the lower glass slide for the samples. However, the size of the formed film is affected by the suspension concentration. For the samples fabricated with a lower suspension concentration, there are areas without film, and these areas will increase when the suspension concentration is decreased further. For the sample fabricated with a suspension of a higher concentration, the size of the formed film is larger, there are more extra particles on the left edge of the substrate, and multilayer colloidal crystals are formed in some areas of the film. Moreover, our experimental results also indicate that once the film starts to form, the film formation is not sensitive to the environmental temperature. Figure 3 shows optical images of the cell at different times of the ordering process. The suspension concentration is ∼0.63 wt %. During the ordering process, the environment temperature is gradually increased from 3 to 12 °C and then decreased to 10 °C. As shown in Figure 3, continuous 2D colloidal crystals can be formed when the environmental temperature is gradually increased from 3 to 10 °C. When the temperature is around 12 °C, multilayer colloidal crystals are formed. When the temperature is decreased to 10 °C, 2D colloidal crystals are formed. In our experiments, wedge-shaped cells are used. Studies have shown that the wedge-shaped cell can improve the crystalline quality of the 2D colloidal crystals because of the straight drying front of the suspension in the cell.26 Figure 4a−c shows the optical images of the cells that are put on a horizontally placed bench, a front tilted at 2° bench, and a both front tilted at 2° and left tilted at 1° bench, respectively. The particle diameter is 970 nm and the suspension concentration is 0.63 wt % for the three samples. As shown in Figure 4, centimeter sized 2D colloidal crystals can be formed in a wedge-shaped cell in both left and front tilted or untilted samples, while there are many multilayer areas in the sample that is only front tilted. However, compared with the both left and front tilted sample, the formed 2D colloidal crystal has more voids in the untilted sample. Moreover, the morphology of the suspension is different for the tilted and untilted samples. In the tilted samples, there is a white band at the edge of the suspension-glass interface. In the untilted sample, the suspension looks milky and uniform. Sample Characterization. Figure 5a and b shows the optical images of the formed film. The particle diameter is 970 nm, and the suspension concentration is 0.5 wt %. The film is transparent and uniform when illuminated and observed perpendicular to the surface (Figure 5a). When observed at a

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RESULTS AND DISCUSSION Assembly of Colloidal Crystals. Figure 1a schematically shows the setup used in the study, while Figure 1b−d shows

Figure 1. (a) Schematic of the setup used in the study; (b) optical image of the cell right after the injection of the colloidal suspension; (c) optical image of the cell 8 h after the ordering process; (d) optical image of the cell 16 h after the ordering process. The suspension concentration is 0.5%, and the particle diameter is 970 nm. The cell is placed on a bench that is both front tilted at 2° and left tilted at 1°.

the optical images of the cell at different times of the ordering process. The concentration of the colloidal suspension is 0.5 wt %, and the particle diameter is 970 nm. As shown in Figure 1b, when the suspension is injected into the cell, as the colloidal particles are uniformly dispersed in the suspension, the suspension looks milky and uniform across the whole cell. Eight hours later, as shown in Figure 1c, a white band appears 1797

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Figure 2. Optical images of three samples at different times of the ordering process fabricated with different suspension concentrations: (a−c) 0.32 wt %; (d−f) 1.0 wt %; (g−i) 2.5 wt %. The particle diameter is 970 nm for the samples. The cells are placed on a bench that is both front tilted at 2° and left tilted at 1°.

Figure 4. Optical images of the cells during the ordering process. The cells are put on (a) a horizontally placed bench; (b) a front tilted at 2° bench; (c) a both front tilted at 2° and left tilted at 1° bench. The particle diameter is 970 nm and the suspension concentration is 0.63 wt % for the three cells.

Figure 3. Optical images of the cell at different times of the ordering process. During the ordering process, vf is about (b) 0.035 μm/s; (c) 0.054 μm/s; (d) 0.14 μm/s; (e) 0.20 μm/s; (f) 0.17 μm/s; (g) 0.14 μm/s. (a) Right after the injection of the suspension; (h) the final formed film. The suspension concentration is ∼0.63 wt %, the particle diameter is 970 nm, and the cell is placed on a bench that is both front tilted at 2° and left tilted at 1°.

some are much smaller. Figure 5d is the SEM image of a sample fabricated using the proposed method with polystyrene spheres with a lower standard deviation (∼5%). The particle diameter is 710 nm, and the concentration of the suspension is 0.5 wt %. SEM image indicates that the crystalline structure of the sample is improved. Figure 6 shows the transmission spectra of the 2D colloidal crystals at normal incidence. For the samples with particle diameter of 970 and 710 nm, there is a clear stop band at about 1184 and 866 nm, respectively. The stop band’s relative bandwidth of the 710 nm sample is narrower than that of the 970 nm sample. Self-assembled ordered layers of monodispersive particles have a 2D periodic structure and have photonic bands.28 The photonic bands are caused by

shallow angle, the film is of brilliant and rainbow color (Figure 5b). The uniform appearance of the sample indicates that the thickness of the film is uniform. Figure 5c is the SEM image of the above sample. SEM images indicate that the colloidal particles are hexagonally close-packed, and 2D colloidal arrays are formed on the substrate. However, the centimeter sized sample is of multiple domains. One reason is due to the large size deviation of the colloidal particles,27 as the polystyrene spheres obtained from Bangs Laboratories have a standard deviation of 5−10%. As shown in Figure 5c, the size of the particles is not uniform; some particles are much bigger, while 1798

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capillary forces. To illustrate the proposed mechanism of 2D colloidal crystal formation, the schematic of the ordering process at different times is shown in Figure 7. Figure 7a and b shows left side views of the experimental cell, while Figure 7c and d shows top views of the cell.

Figure 5. (a) Optical image of the formed film when illuminated and observed perpendicular to the surface; (b) optical image of the formed film when observed at a shallow angle; (c) SEM image of the sample fabricated with particles of 970 nm diameter; (d) SEM image of the sample fabricated with particles of 710 nm diameter.

Figure 7. Schematic of the ordering process at different times: (a) left side view of the cell at an earlier time t1; (b) left side view of the cell at a later time t2; (c) top view of the cell at an earlier time t1; (d) top view of the cell at a later time t2.

At an earlier time t1, as the cell is both left and forward tilted, due to gravity, colloidal particles will move toward the front and left part of the cell. As shown in Figure 7a and c, colloidal particles aggregate at the suspension−glass interface, and there are more particles at the front left part. Not all the particles aggregate at the edge of the suspension−glass interface, and the particles aggregate within an area in the front part of the cell. The concentration of particles in the front part of the cell would be higher than the concentration of particles in the original suspension. In the meantime, a meniscus region is formed at the edge of the suspension−glass interface due to wetting by the suspension. As water evaporates out of this thin meniscus, the lateral capillary forces will organize the colloidal particles (for example, the red particle) into close-packed arrays, and colloidal crystals will form in the front part of the slide. At a later time t2, as the ordering process evolves, the concentration of particles near the edge of the suspension− glass interface will decrease because some particles are selfassembled into colloidal crystals on the substrate. However, the particles at the back part of the cell will immigrate to the front part (for example, the yellow particle), and the particle concentration at the edge of the suspension−glass interface will still be kept at a high value. The above assumption is proved by experimental results shown in Figures 1 and 2. As

Figure 6. Transmission spectra of the 2D colloidal crystals with particle diameters of 970 nm (solid curve) and 710 nm (dotted curve).

intersphere light scattering in the array, and a stop band with a narrower bandwidth indicates a better crystalline quality of the 2D periodic structure. The approximate position (λ) of this stop band is given by λ = neff(2/3)1/2d, where neff is the effective refractive index of the medium and d is the sphere diameter. The calculated position is 1140 and 834 nm for the samples of 970 and 710 nm particle diameter, respectively, close to the experimental ones. Formation Mechanism of 2D Array from Colloidal Particles. Considering all of our experimental observations, we propose the following mechanism of the array formation in our method. First, a meniscus region is formed at the edge of the suspension−glass interface due to wetting by the suspension. As water evaporates out of this thin meniscus, a “nucleus” of ordered phase appears when the thickness of the water layer becomes approximately equal to the particle diameter.17 Then, the particles move toward the ordered array mainly by gravity and the particles assemble into ordered structures by the lateral 1799

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reported in this paper is different from the one that has been reported in ref 29. In our experiments, the particles will be transported to the edge of the suspension−glass interface in the tilted cell due to gravity, but most of the particles will not sediment and the particles will be pushed backward as the ordering process evolves. Moreover, in the tilted cell, the velocity associated with particle transportation due to gravity (vg) is about 0.2 μm/s. vg is almost independent of both the environmental temperature (3−12 °C) and the relative humidity (60−80%). The velocity of the 2D colloidal crystal formation (vf) depends on both the environmental temperature and the relative humidity. vf is 0.035, 0.054, 0.14, 0.20, 0.17, and 0.14 μm/s, while the environmental temperature is 3, 7, 10, 12, 11, and 10 °C, and the relative humidity is 80%, 80%, 70%, 65%, 70%, and 70% in Figure 3b−g, respectively. These velocities are estimated via experiments. Figure 3 indicates that when the environmental temperature changes gradually, if vf is close to vg, multilayer colloidal crystals are formed; otherwise, if vg becomes dominant, continuous 2D colloidal crystals are formed. The estimated velocities and the results of Figure 3 also indicate that the transportation of particles to the edge of the suspension−glass interface due to gravity plays an important role in the ordering process. It should also be pointed out that tilting of the cell is important in the formation of large-scale 2D colloidal crystals in this method. Our explanation is that when the substrate is both front and left tilted, the particles will have the tendency to move the particles to the front and left part of the cell due to gravity. As proved in Figures 1−4, the white band at the edge of the suspension−glass interface is broomlike. The formation of the white band also indicates that the suspension concentration at the edge of the suspension−glass interface is kept high and constant during the ordering process; therefore, compared with the untilted sample, the tilted sample is more continuous, as shown in Figure 4a and c. For a particle near the edge of the suspension−glass interface, if the sum of the capillary forces from the ordered array and the van der Waals forces (among the particles, and between the particle and the substrate) is bigger than gravity, the particle will be captured by the ordered array, otherwise, the particle will slide away (for example, the blue particle shown in Figure 7). By properly choosing the left titled angle, we can balance the lateral capillary forces, gravity, and the van der Waals forces, and obtain large-scale 2D colloidal crystals on the slide. Therefore, in this method, centimeter-sized 2D colloidal crystals can still be formed even when the suspension concentration is 2.5 wt %, as the extra particles can be finally moved to the left edge of the substrate due to gravity, as shown in Figure 2. If the cell is only front tilted, as the extra particles cannot be moved to the left edge of the substrate, multilayer colloidal arrays will be likely to form, as shown in Figure 4b and c. Compared to other convective assembly methods, the disadvantage of this method is the relative low process rate. For the 40 × 25 × 1 mm3 glass substrate used here, the typical growth rate is only about 2500 μm2/s. Studies are going on to increase the growth rate and improve this method.

shown in Figure 1c, a narrow milky band near the edge of the suspension−glass interface is formed. As the ordering process evolves, as shown in Figure 1d, there is still a white band at the edge of the suspension−glass interface, and the white band is lifted upward. In the conventional convective assembly, the colloidal particles are transported toward the edge by the convective water influx. In this method, the colloidal particles are transported to the edge mainly by gravity. Previous studies have shown that large particles have the tendency to sediment during the ordering process.29 To examine whether this could be the case in our experiments, we let the particles sediment for 24 h first; during this period, we keep the cell in an environment with a high relative humidity so that the water evaporation rate is low and there is almost no growth of the 2D colloidal crystals. After sedimentation, the relative humidity is decreased to ∼60%, and 2D colloidal crystals are formed. The cell is put on a bench which is both front tilted at 2° and left tilted at 1°, and it is kept at ∼5 °C. The concentration of the suspension is 0.5 wt %. Figure 8a is

Figure 8. Optical images of a cell at different times of the ordering process. (a) After sedimentation for 24 h; (b) the growth of the 2D colloidal crystals; (c) the final formed film. The cell is put on a bench which is both front tilted at 2° and left tilted at 1°, and it is kept at ∼5 °C. The suspension concentration is 0.5 wt %.

the optical image of the cell after the particles are kept for sedimentation for 24 h. As shown in Figure 8a, there is a white band at the edge of the suspension−glass interface, and there is almost no growth of the 2D colloidal crystals. Figure 8b and c shows the optical images of the cell at different times of the process after the relative humidity is decreased to ∼60%. As shown in Figure 8b and c, the white band is lifted upward, and a uniform and transparent thin film appears in the front part of the slide. This experiment shows that the ordering process

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CONCLUSIONS We have developed a gravity-assisted convective self-assembly method to fabricate centimeter sized uniform 2D colloidal crystals. Unlike the other convective self-assembly methods, gravity plays an important role in the transportation of particles to the thin meniscus region in this method. As the extra 1800

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particles can be finally moved to the edge of the substrate by gravity, centimeter-sized 2D colloidal crystals can be formed when the suspension concentration is in the region of 0.32−2.5 wt %.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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