pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation of Patterned Arrays of Polystyrene Colloidal Crystal Structures on Flexible Functional Substrates Niall R. Thomson,† Martyn A. McLachlan,† Chris L. Bower,‡ and David W. McComb*,† †
Department of Materials and London Centre for Nanotechnology, Imperial College London, London, SW7 2AZ, U.K., and ‡Kodak European Research, 332 Science Park Cambridge, Cambridge, CB4 0WN, U.K. Received March 17, 2009. Revised Manuscript Received August 3, 2009 This contribution presents the first preparation of patterned arrays of highly ordered polystyrene colloidal crystal structures on flexible functional substrates. The formation of patterned arrays of colloidal crystals over large areas (5 1 cm) with periodic line patterns ranging in pitch from 25 to 450 μm is demonstrated. The protocol developed to achieve this is applicable to a wide-range of substrates and is inherently scalable. Interestingly, directed colloidal deposition was found to be more susceptible to fluctuations in the deposition conditions than bulk deposition. The conditions required for directed deposition were systematically investigated, and the success of the optimized protocol was illustrated by the deposition of ordered structures on a range of functionalized rigid and flexible substrates. These advances-low-cost production on flexible functional substrates and the fabrication of structures of controlled geometry-address two of the major challenges in developing devices using colloidal crystal structures.
1. Introduction Colloidal crystals have generated intense scientific interest because of their structural properties: porosity, interconnectivity, and periodicity.1-4 Applications proposed to utilize these properties include structural color for brand differentiation, security and smart textiles. Further use of the colloidal crystal structure can be gained by using it as a template and infiltrating with another material. Fabrication of such composites from polystyrene colloidal crystals enables further applications, for example, chemical sensing, thermal barriers, photonic devices, displays, and low-cost photovoltaic devices.2,5-12 Although many routes for the fabrication of infilled colloidal crystals have been reported, further work is required to make this process scalable.1-4,9,11,12 In order to develop devices that utilize colloidal crystals, it is highly desirable to be able to form thin films over large areas and to deposit the crystals in specific geometries on the substrate.7,13 The formation of colloidal crystal structures has previously been demonstrated on a range of functional substrates and in a *Corresponding author. E-mail:
[email protected].
(1) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Langmuir 2004, 20, 9114– 9123. (2) Lee, Y. J.; Braun, P. V. Adv. Mater. 2003, 15, 563–566. (3) Lee, Y. J.; Kim, S. H.; Huh, J.; Kim, G. H.; Lee, Y. H. Appl. Phys. Lett. 2003, 82, 3779–3781. (4) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 696–713. (5) McLachlan, M. A.; Johnson, N. P.; De La Rue, R. M.; McComb, D. W. J. Mater. Chem. 2005, 15, 369–371. (6) Yablonovitch, E. Sci. Am. 2001, 285, 46–53. (7) Yang, S. M.; Miguez, H.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 425–431. (8) Arsenault, A.; Fournier-Bidoz, S.; Hatton, B.; Miguez, H.; Tetreault, N.; Vekris, E.; Wong, S.; Yang, S. M.; Kitaev, V.; Ozin, G. A. J. Mater. Chem. 2004, 14, 781–794. (9) McLachlan, M. A.; McComb, D. W.; Berhanu, S.; Jones, T. S. J. Mater. Chem. 2007, 17, 3773–3776. (10) Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Russell, A. E. Faraday Discuss. 2006, 132, 191–199. (11) Arsenault, A. C.; Puzzo, D. P.; Ghoussoub, A.; Manners, I.; Ozin, G. A. J. Soc. Inf. Disp. 2007, 15, 1095–1098. (12) Arsenault, A. C.; Miguez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Macromol. Symp. 2003, 196, 63–69. (13) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95–104. (14) Ferrand, P.; Egen, M.; Griesebock, B.; Ahopelto, J.; Muller, M.; Zentel, R.; Romanov, S. G.; Torres, C. M. S. Appl. Phys. Lett. 2002, 81, 2689–2691.
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variety of controlled geometries; to date, the substrates used have typically been rigid and brittle.14-18 The deposition of colloidal crystal structures on flexible substrates is an attractive prospect, as it would be advantageous in many of the applications proposed for colloidal crystals such as large-area, low-energy reflective displays, chemical sensing and low-cost photovoltaic devices,6 with flexible substrates providing reduced device weight and expanding the available range of colloidal deposition methods. For example, by using flexible substrates, the commonly used laboratory scale technique of evaporative vertical deposition (EVD) may be scaled-up by continuous mechanical withdrawal of substrates, i.e., roll-to-roll dip coating.19,20 The introduction of such deposition routes could further reduce the cost of colloidal crystal fabrication, currently a barrier to the adoption of periodic submicrometer structures in low-cost applications. Previous work on flexible substrates has not resulted in the formation of highly ordered colloidal crystal structures.21,22 Here we demonstrate the formation of highly ordered bulk polystyrene colloidal crystals on two different flexible substrates: silica coated polyethylene terephthalate (PET) and on electrically conducting indium tin oxide (ITO)-coated PET. While such substrates are flexible in comparison to those currently in use (e.g., glass and silica), excessive bending of the substrate, particularly toward the functionalized coating, can cause cracking, which would affect the properties of the coating (e.g., the electrical conductance of ITO). The ability to control the shape of the colloidal crystal structures deposited over large areas is also a key requirement in their adoption for application.7,13 For the first time, we (15) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Adv. Mater. 2003, 15, 1025–1028. (16) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132–2140. (17) McLachlan, M. A.; Johnson, N. P.; De La Rue, R. M.; McComb, D. W. J. Mater. Chem. 2004, 14, 144–150. (18) Mihi, A.; Ocana, M.; Miguez, H. Adv. Mater. 2006, 18, 2244–2249. (19) Blake, T. D.; Ruschak, K. J. Nature 1979, 282, 489–491. (20) Cohu, O.; Benkreira, H. Chem. Eng. Sci. 1998, 53, 533–540. (21) Maenosono, S.; Okubo, T.; Yamaguchi, Y. J. Nanopart. Res. 2003, 5, 5–15. (22) Okubo, T.; Chujo, S.; Maenosono, S.; Yamaguchi, Y. J. Nanopart. Res. 2003, 5, 111–117.
Published on Web 09/08/2009
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demonstrate the formation of arrays of colloidal crystals on a flexible substrate through directed colloidal deposition on ITOcoated PET. Many methods have been used to direct colloidal deposition on rigid substrates, for example, template directed growth, electrostatic interactions, and wettability patterned substrates.1,7,13-15,23,24 The latter technique relies on the spatial variation of the contact angles and is one of the most promising scalable approaches. There are numerous methods that can be used to deposit wettability patterns: one exciting and inherently scalable route, which has been demonstrated as a roll-to-roll process uses flexographic printing,25 another offering even higher resolution uses microcontact printing (μ-CP),26 and it is this latter method that is exploited in this contribution to demonstrate directed colloidal deposition over large areas on both flexible and rigid substrates.
2. Experimental Section The experimental protocol for the preparation of a patterned array of colloidal crystal structures is illustrated schematically in Figure 1. A range of rigid and flexible substrates (70 15 mm2) were used in this study. Glass substrates were cleaned by sequential sonication in opticlear, acetone, methanol, and deionized water. Gold-coated slides were fabricated by sputtering 40 nm of gold onto the clean glass slides. ITO-coated glass substrates (Psiotech) were cleaned by sequential sonication in isopropanol and deionized water. Two types of functionalized flexible substrates were investigated: ITO- and SiO2-coated PET. ITO-coated PET with an ITO thickness of 75 μm and sheet resistance of 50 ohms per square was used (CP Films, OC50).27 Prior to colloidal deposition, the substrates were cut to 7 1.5 cm2 and cleaned by sequential sonication in isopropanol and deionized water. SiO2-coated PET substrates were prepared by vapor deposition of ∼5 nm of SiO2, using a rapid, atmospheric atomic layer deposition process.28 Polystyrene spheres (diameter 192-456 nm, polydispersity 250 μm in width are of a similar thickness and quality to bulk colloidal crystals, with a decrease in thickness and increase in defects observed in narrower stripes (Table 2). The decrease in stripe thickness with width has been related to the decrease in meniscus length for narrow stripes.9 The similarity between continuous films and 250 μm colloidal crystal stripes suggests that mechanisms dominating their formation are comparable and, as previously demonstrated, stick-slip interactions are not observed during bulk colloidal deposition under these conditions. By increasing the width of the stripes as well as stabilizing the environmental conditions, it was possible to form high quality arrays of colloidal crystal stripes 80 and 100 μm in width with minimal evidence of stick slip effects. (Figure 6) The conditions required to minimize stick-slip effects also significantly reduced the effect of Rayleigh instabilities. This instability can alter the width and thickness of the lines and ultimately lead to the formation of periodic bulges in the colloidal arrays. In addition to the factors already addressed, the volume fraction of polystyrene spheres has been found to affect the presence of the Rayleigh instability, with colloidal crystals deposited at low volume fractions (0.01 vol%) found to be more susceptible to instability than those deposited at higher concentrations (0.03-0.05 vol%). The stabilizing effect of an increase in volume fraction is demonstrated for 25 μm arrays in Figure 7. This stabilization at increased volume fraction was predicted by Cates et al.,34 who calculated that the need for surface energy minimization inducing the instability would be overwhelmed by the energy of self-assembly of the colloidal particles at sufficiently high volume fractions. Although it is desirable to carry out colloidal deposition at higher colloidal volume fractions to utilize this effect, too high a volume fraction (>0.1 vol%) has been found to result in nonselective deposition with bulk colloidal crystals formed (Figure 7d). Colloidal concentrations in the range of 0.03-0.05 vol % were found to be optimal for the reduction of the Rayleigh instability for deposited feature sizes (25-450 μm). 3.2.2. Colloidal Crystal Arrays on Patterned Rigid Substrates. Investigation of the factors influencing mechanisms competing with self-assembly have allowed the effects of stickslip and Rayleigh instabilities to be almost eliminated, resulting in the formation of well-defined highly ordered colloidal crystal (34) Cates, M. E.; Adhikari, R.; Stratford, K. J. Phys.: Condens. Matter 2005, 17, S2771–S2778.
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Figure 7. The effect of volume fraction on the presence of Rayleigh instability during directed colloidal deposition: 25 μm colloidal arrays with 75 μm gaps between deposited at (a) 0.01 vol %, (b) 0.03 vol %, (c) 0.05 vol %, and (d) 0.2 vol %. 258 nm polystyrene spheres.
Figure 9. The effect of stripe width on reflectance spectra. Spectra have been normalized and offset to aid visualization. All colloidal crystal stripes were fabricated from 248 nm polystyrene spheres on glass substrates. (a) 25 μm colloidal crystal stripe; (b) 50 μm colloidal crystal stripe; (c) 250 μm colloidal crystal stripe; (d) bulk colloidal crystal (width >400 μm).
Figure 8. (a) Optical micrograph demonstrating the long-range order of the colloidal arrays created by the deposition of 192 nm polystyrene spheres on a MUA-patterned gold substrate; 80 μm colloidal crystal stripes with 40 μm gaps between. (b-d) SEM images of the sample shown in panel a. Panel b indicates the presence of well-defined colloidal crystal stripes, c demonstrates the selectivity of the technique, and d shows the formation of multilayer colloidal crystals in the MUA-functionalized areas.
stripes, such as those shown in Figure 8. Preferential adsorption to areas functionalized with MUA is clearly achieved with colloidal arrays formed on the 80 μm hydrophilic MUA stripes and not on the 40 μm gaps of hydrophobic gold. Figure 8c demonstrates that the technique is highly selective with little adhesion of colloidal particles in the hydrophobic gold region. Maxima in the reflectance spectra gathered from individual colloidal crystal stripes further demonstrate the presence of threedimensional periodicity in the structure (Figure 9). The dependence of the full width at half-maximum (fwhm) of the reflectance peak is associated with the decrease in thickness discussed earlier (Table 2). When stripe thickness is taken into account, the variation in fwhm is in good agreement with theory.8,35 (35) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300–303.
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Figure 10. 100 μm colloidal crystal arrays deposited by EVD of 456 nm polystyrene spheres at 0.1 vol % on ODPA patterned ITO-coated PET. (a) Optical micrograph. (b) SEM micrograph showing the edge of the colloidal crystal stripe. (c) SEM micrograph from the center of the colloidal crystal stripe, inlay shows an FFT from the area shown in image c. (d) A photograph of the bent structure.
3.2.3. Colloidal Deposition on Patterned Flexible Substrates. The progress made in thin film colloidal deposition on flexible substrates and in directing colloidal deposition on rigid substrates has enabled a further significant advance: directed colloidal deposition on flexible substrates. Optical and SEM images in Figure 10 demonstrate that highly ordered colloidal crystal arrays have been formed on flexible ITO-coated PET substrate. While there is some variation in the thickness of the colloidal crystal stripes as a result of competing mechanisms, the DOI: 10.1021/la9009306
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stripes are continuous over large areas of the substrates. Future work will attempt to minimize the effect of competing mechanisms by tailoring the difference in wettability between patterned and unpatterned areas of the substrate. The electron micrograph shown in Figure 10b illustrates the formation of multilayer colloidal structures, and once again the impressive selectivity of the μ-CP patterning is apparent. Figure 10c, an electron micrograph from the center of the colloidal crystal stripe, illustrates the degree of order in the colloidal crystals. This is confirmed by an FFT of the area in Figure 10c, which illustrates that a high degree of structural periodicity is achieved at the center of the colloidal crystal stripe (Figure 10d).
4. Conclusions For the first time we have demonstrated the formation of colloidal crystal structures of controlled geometry on both unpatterned and patterned flexible substrates. This significant advance has been achieved by establishing and combining two reproducible protocols, one for directed colloidal deposition over large areas, and the other for thin film deposition on flexible substrates. The formation of highly ordered colloidal crystals on
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patterned flexible substrates is a significant advance toward the incorporation of colloidal crystals in a range of technologically relevant applications, including smart textiles, large-area, lowenergy reflective displays, chemical sensing, and low-cost photovoltaic devices. This advance creates the opportunity to exploit new deposition techniques for the large-area fabrication of highly ordered colloidal crystals in both the bulk and area-specific form. Acknowledgment. The authors are grateful to Dr. N. Johnson (University of Glasgow) for access to optical microspectroscopy equipment. N.R.T. would like to thank John Fyson and Zsuzsanna Nagy of Kodak European Research for the fabrication of the silicacoated PET. N.R.T. also wishes to thank Kodak European Research and EPSRC for provision of a research studentship. Supporting Information Available: Formation of highly ordered colloidal crystal structures on flexible silica-coated PET deposited using 456 nm polystyrene spheres at 0.1 vol %. Optical micrographs demonstrating the effect of a hydrophobic substrate on EVD. This material is available free of charge via the Internet at http://pubs.acs.org.
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