pubs.acs.org/Langmuir © 2010 American Chemical Society
Colloidal Crystal Thin Films Grown into Corrugated Surface Templates F. Ramiro-Manzano,†,‡ E. Bonet,† I. Rodriguez,† and F. Meseguer*,†,‡ †
Centro de Tecnologı´as Fı´sicas, Unidad Asociada ICMM/CSIC-UPV, Universidad Polit ecnica de Valencia, Av. Los Naranjos s/n, 46022 Valencia, Spain, and ‡Instituto de Ciencia de Materiales de Madrid CSIC, 28049 Madrid, Spain Received November 20, 2009. Revised Manuscript Received January 27, 2010
The influence of patterned surfaces on the formation of one- and two-dimensional colloidal crystals is analyzed. We have used the corrugated surface of a digital versatile disc (DVD) for template surface processing. When the sphere diameter is on the order of the groove width of patterned substrates, a rich variety of particle decorations appear. However, if particle size is much larger than template patterns, large domains of particle ordering are formed.
Introduction Colloidal particle self-organization has raised great interest in science. First, it provides direct information on natural ordering processes of mineral and biological systems.1,2 Second, it allows the fabrication of crystals composed of different geometries for new research areas, such as photonic crystals.3 Finally, spherically shaped micrometric-sized colloidal particles present a well-defined optical fingerprint4 that strongly depends on the particle size. Therefore, colloidal particles either isolated or as a colloidal ensemble can be thought of as information systems. Confined colloidal crystals show a manifold variety of closed packed (faced centered cubic (FCC) and hexagonal closed packed (HCP)) structures, strongly dependent on the confinement length.5-11 Also, the influence of substrates with topographically patterned surfaces has been demonstrated on the formation of three-dimensional (3D) closed and non-close-packed colloidal crystals.12-16 Templated growing surfaces have also been used to achieve zerodimensional (0D) and one-dimensional (1D) colloidal nanostructures.17 The method used so far involves growth through sedimentation of colloidal crystals thick cells, whose substrate has a patterned surface,12,18 or through the deposition mediated by
capillary forces on the free surface of the template.13,15 In addition, chemical patterning of substrates has been shown to be another way to direct colloid crystallization onto planar surfaces.19 Harreis et al.20 showed theoretically that the presence of charged strips in the substrates would induce the formation of different arrangements. However, to the best of our knowledge, no one has studied experimentally the influence of patterned surface on the growth of thin film colloidal crystals, although a theoretical work21 has been published. The fabrication of patterned substrates usually involves expensive and sophisticated micro and nanofabrication techniques based on either electron or photon assisted lithography.22 However, here we make use of inexpensive grating substrates obtained from commercially available DVDs to process large-sized patterned surfaces. Many groups have also shown that colloidal order into confined cells is strongly dependent on the cell thickness value.5-11 Here, we report on the growth of 1D and 2D colloidal crystals when one of the confining walls has a patterned surface. We will show that both the corrugated profile of the substrate as well as the thickness of the confining cell are key parameters for the particle ordering. Specific arrangements controlled by the ratio between the cell thickness and the particle diameter have been obtained.
Experimental Section
*Corresponding author. E-mail:
[email protected]. (1) Sanders, J. V. Nature 1964, 204, 1151. (2) Stryer, L. Biochemistry; Freeman, 1998. (3) Arsenault, A.; Fleischhaker, F.; von Freymann, G.; Kitaev, V.; Miguez, H.; Mihi, A.; Tetreault, N.; Vekris, E.; Manners, I.; Aitchison, S.; Perovic, D.; Ozin, G. A. Adv. Mater. 2006, 18, 2779. Lopez, C. Adv. Mater. 2003, 15, 1679. Tarhan, I. I.; Watson, G. H. Phys. Rev. Lett. 1996, 76, 315. (4) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons, Inc.: New York, 1983 (5) Pieranski, P.; Strzelecki, L.; Pansu, B. Phys. Rev. Lett. 1983, 50, 900. (6) Pansu, B.; Pieranski, P.; Pieranski, P. J. Phys. 1984, 45, 331. (7) Neser, S.; Bechinger, C.; Leiderer, P.; Palberg, T. Phys. Rev. Lett. 1997, 79, 2348. (8) Schmidt, M.; L€owen, H. Phys. Rev. Lett. 1996, 76, 4552. (9) Ramiro-Manzano, F.; Meseguer, F.; Bonet, E.; Rodriguez, I. Phys. Rev. Lett. 2006, 97, 028304. (10) Ramiro-Manzano, F.; Bonet, E.; Rodriguez, I.; Meseguer, F. Phys. Rev. E 2007, 76, 4. (11) Fontecha, A. B.; Palberg, T.; Schope, H. J. Phys. Rev. E 2007, 76, 050402/1. (12) vanBlaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (13) Hoogenboom, J. P.; Retif, C.; de Bres, E.; de Boer, M. V.; van LangenSuurling, A. K.; Romijn, J.; van Blaaderen, A. Nano Lett. 2004, 4, 205. Yang, S. M.; Miguez, H.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 425. (14) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (15) Yang, S. M.; Ozin, G. A. Chem. Commun. 2000, 2507. (16) Dziomkina, N. V.; Vancso, G. J. Soft Matter 2005, 1, 265. (17) Yin, Y. D.; Lu, Y.; Gates, B.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 8718. (18) Yin, Y. D.; Xia, Y. N. Adv. Mater. 2002, 14, 605.
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The experimental procedure to achieve the wedge cell is very similar to the experimental method previously reported.9 In our case, one of the confining plates that forms the wedge cell is obtained from the DVD, the second one being a standard glass plate. The two plates are separated by a Mylar film (6 μm) attached along only one rim of the cell, and several binder clips on the three other sides tighten the cell. Therefore, a very small edge angle is obtained (≈10-4 rad), and the cell thickness changes continuously from zero to an arbitrary thickness value larger than the sphere diameter. We use recordable DVDs composed of two polycarbonate plates, the bottom one has a spiral distribution of rectangulartype grooves coated with a metallic and reflecting layer filled with (19) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phys. Rev. Lett. 2000, 84, 2997. Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. Allard, M.; Sargent, E. H.; Lewis, P. C.; Kumacheva, E. Adv. Mater. 2004, 16, 1360. (20) Harreis, H. M.; Schmidt, M.; L€owen, H. Phys. Rev. E 2002, 65, 041602. (21) Heni, M.; L€owen, H. Phys. Rev. Lett. 2000, 85, 3668. (22) Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press: New York, 2001.
Published on Web 02/25/2010
DOI: 10.1021/la904396m
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Letter a photosensible material, in where the information is recorded. In addition, the top plate of the DVD is patterned with a
Figure 1. (a) SEM image of a DVD-type template employed in the experiments. (b) AFM image of the DVD corrugated surface and the corresponding profile parameters: h = 105 nm, b = 500 nm, d = 240 nm.
Figure 2. SEM images: (a) a parallel collection of particles rows (“LATRIX”), (b) a row of spheres arranged linearly, and (c) a strip composed of two rows. In all cases, the particle size is 770 nm and placed over the flat substrate.
Ramiro-Manzano et al. complementary geometry of the bottom plate, i.e., it is decorated by a spiral distribution of plateau type structures that fit into the grooves of the top layer. Therefore, these DVDs can provide two different types of corrugated templates with narrow and wide grooves. Figure 1a shows a scanning electron microscope (SEM) image of one of the patterned plates. The corresponding atomic force microscope (AFM) analysis can be seen in Figure 1b with an inset showing the DVD topographic profile as well as the characteristic parameters like depth (h), width (d) of the grooves, and the length of the plateau (b). The distance (bþd) corresponds to the well-known period value of the DVD spiral distribution (740 nm). The process of mechanical separation of the two plates that form the DVD is extremely simple. The photosensible resin is removed with a mixture of water and ethanol (1:1), and the metallic film is peeled off. Then, the surface is rinsed, so one can obtain very easily patterned plates (25 25 mm2), which form one plate of the confining cell. Aqueous dispersions of sulfate-stabilized latex (polystyrene) spheres (Ikerlat Polymers) of 380 or 770 nm were washed with deionized water and ethanol through several cycles of sonication/ centrifugation. The surface charge of the particles employed was around 0.98 mC/m2. The size of the particles corresponds to values on the order of or much larger than the groove width (d). A 1% w/w aqueous suspension of particles was introduced by capillary forces into the cell through a small hole drilled on the glass plate and connected to a 2-cm-high glass tube. As the water evaporated, polystyrene spheres condensed into several configurations and colloidal crystal was formed when the sample was dried. Then, the flat confining plate was detached, and in most cases, the colloidal crystal is bound to the patterned substrate. However, in other cases, some particles are also attached to the flat surface showing nice decorations. The resulting samples are analyzed by means of SEM.
Figure 3. SEM images (bottom row) of different particle orderings, the cell thickness increasing from the left to the right side of the panel. Side (second row) and top (third row) views of the ordering model showing the sphere distribution in the grooves. The first row shows how the zigzag angle of particle strips changes from 180° (a) to 60° (e). (f) A triangular arrangement of close-packed spheres. The particle size is 380 nm. 4560 DOI: 10.1021/la904396m
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Letter
Results and Discussion Figure 2 shows particles attached to the flat substrate, and at the thinnest part of the wedge sample, forming several decorations as parallel rows, one row and a strip of two close-packed rows, respectively. We call such colloidal decoration of Figure 2a LATRIX, as it reminds us of the credit pictures of the MATRIX movie, and because such a colloidal arrangement can be thought of as a sequence of information units represented by the particles themselves. From the results of Figure 2, it is important to stress that patterned substrates can also be useful for templating colloidal arrangements on flat substrates. However, we will concentrate on the results obtained on the corrugated surface of the cell. We will also restrict ourselves on quasi-2D colloidal orderings. We will discuss two types of colloidal ordering when particle size is (A) smaller and (B) larger than the groove width. A. Particles Whose Size Is on the Order of the Groove Width (Particle Size = 380 nm). Figure 3 shows both SEM images (bottom row of images) of particles ordered in the thinnest part of the cell, as well as models (first, second, and third row of images) of the particle ordering, where particles are ordered on parallel strings with different arrangements. The bottom row panel (a4 to f4) shows how particles modify their order as the height of the cell increases, the left SEM image corresponding to the case of the thinnest cell value. We also have modeled the particle distribution as shown in the second (side view) and the third (top view) row of images. For the thinnest value (left column in Figure 3), spheres sit in the groove and they are allowed to be ordered in linear strands parallel to each other. However, as the cell thickness slightly increases, particles are not sitting in the groove, but a little bit out of the groove, allowing more particles to be ordered. As a consequence, strips form a zigzag particle distribution (b column of Figure 3) arranged around the groove direction. As the thickness is further increased, the system tries to maximize the filling fraction value, so the zigzag angle (δ) gradually changes from δ = 180° (Figure 3a) to δ = 60° (Figure 3e) corresponding to parallel strips composed of two rows of close-packed spheres. We also have modeled the influence of the cell thickness value on the zigzag particle ordering. Figure 1S of the Supporting Information shows the relationship between the cell thickness value and the zigzag angle (δ). We should stress that particles are sitting near the groove rim (see Figure 3e2). As the distance between strips is smaller than a single sphere row width, the system is composed of a periodic distribution of strips separated by empty rows. These empty rows are not able to accommodate further spheres as can be seen from the model of Figure 3e3 and the SEM image of Figure 3e4. However, when the cell thickness becomes a bit thicker (see Figure 3f), strip rows are not anchored to the grooves anymore. Then, they can shift, coming closer to each other to achieve close-packed ordering. It is important to stress that, at variance to previous methods,23 the confining flat plate and the cell height play a key role in the particle ordering. When the height of the cell increases even more, the system tries to maximize the filling fraction by pushing some spheres out of the triangular packing into the grooves of the patterned substrate (see Figure 4a5 and b6). The remaining particles rearrange into a zigzag strand distribution. Figure 4 also shows a proposal for the rearrangement mechanism, where particles adsorbed at the grooves and those placed at the flat confining plate are depicted as blue and green spheres, respectively. Figure 4a3,b3 shows (23) Mayers, B. T.; Gates, B.; Xia, Y. Adv. Mater. 2000, 12, 1629.
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Figure 4. SEM images (bottom row) of different particle orderings, as the cell thickness increases (from the left to the right side of the panel). The (a5) picture corresponds to an image taken from the colloidal distribution facing to the flat side of the confining cell. The SEM picture of (b6) corresponds to the model of (b5) (see text). The particle size is 380 nm. The third and fourth rows are the side and top views of the ordering model, respectively. The first and second rows show ordering models of the green and blue particles, respectively, with the lines connecting particles being a guide for the eye.
the side view, Figure 4a4,b4, the top view (seen from the upper flat confining plate), and Figure 4b5 shows the bottom view (seen from the bottom corrugated confining plate) of the ordering model. Then, Figure 4b4,b5 corresponds to the same particle ordering seen from different complementary sides of the confining plates. The first row of images (Figure 4a1,b1) shows schemes of the particle ordering of the green spheres (zigzag type order) in the top layer (see Figure 4a3,b3) lying on the flat confining plate. The second row of images (Figure 4a2,b2) shows schemes of the particle ordering of the blue spheres (non-close-packed linear stringlike order) in the bottom layer (see Figure 4a3 and b3) sitting on the grooved confining plate. The lines connecting particles are only a guide for the eye. As thickness in the cell increases, the particles of the linear string adsorbed at the grooves (blue particles) become closer; i.e., the s value in Figure 4a2 and b2 decreases. The remaining particles (green spheres) fit on the plateau region of the patterned substrate as shown in the SEM image of Figure 4a5 and in the model of Figure 4a4. If the height of the cell further increases, both the blue rows and the green zigzag-type strands (see Figure 4b4 and b5) rearrange themselves to accommodate more particles. Then, the particles from the blue rows approach each other, and simultaneously, the zigzag angle δ DOI: 10.1021/la904396m
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Figure 5. SEM images of particles whose size is larger than the groove width (particle size = 770 nm) attached to the corrugated plate: (a) buckling phase, (b) triangular phase, (c) HCP (100) phase; (b) being a zoom image of (d).
decreases (see Figure 4b1). Finally, all particles in both the blue rows and the green strips tend to arrange in a quasi close-packed manner as shown in the SEM images of Figure 4a5 and b6. B. Particles Whose Size Is Larger than the Groove Width (Particle Size = 770 nm). When the particle size is larger than the groove width, one should expect a more minor influence of the patterned surface. However, we will show that patterned substrates still influence particle ordering. Indeed, Figure 5 shows different particles orderings of 770-nm-diameter-sized spheres in the wedge cell. Here, the ordering sequence is very similar to that observed in nonpatterned wedge-type cells:24 1B f 2Δ f 2HCP, being B, and Δ, the buckling, and the triangular phases, respectively. Those phases can be seen in Figure 5a (buckling), b (2Δ), (24) Ramiro-Manzano, F.; Bonet, E.; Rodriguez, I.; Meseguer, F. Soft Matter 2009, 5, 4279.
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and c (2HCP). The patterned surface seems to act both as nucleation center and as a crystal growth vector, allowing high order quality in large areas of the samples. It can be seen, particularly in Figure 5d, that despite localized defects due to particle size dispersion, particle order is preserved over large sample extension leading to high-quality crystals with large domains. To conclude, we have built a wedge cell with a new confinement condition. We use a flat and a corrugated plate. The geometrical characteristics of the designed cell lead to the formation of new decoration lattices. The new variables in the wedge confinement, introduced by the corrugated surface, groove size, pitch of the substrates, as well as the cell thickness value, dictate the particle decoration deposited on both the patterned and nonpatterned confining plates. In particular, we have shown the influence of patterned substrates on the particle arrangement of very few monolayers when the modulation period of the template is on the order of the particle size. It constitutes an easy and inexpensive method to create a large variety of colloidal crystal arrangements. Commercial inexpensive CD-Rs (recordable compact disk, large groove width) or BR-R (recordable bluray, small groove width) could be used to extend these results to different sphere sizes. The results shown here could open a wide range of potential applications in the fields of colloidal, soft,25 and nanoimprint lithographies, as well as reversal nanoimprint processes.26 Acknowledgment. The authors would like to thank A. Moreno for providing wedge type cells, Dr. M. Garı´ n for helpful discussions, and the Electronic Microscopy Service of the UPV for technical support. This work has been partially supported by the Spanish CICyT projects, FIS2009-07812, and Consolider CSD2007-046. Supporting Information Available: Additional figure as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. (25) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (26) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114. Li, Z.; Gu, Y.; Ge, H.; Wu, W.; Xia, Q.; Yuan, C.; Chen, Y.; Cui, B.; Williams, R. S. Nano Lett. 2009, 9, 2306.
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