Langmuir 2003, 19, 9669-9671
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Colloidal Lines and Strings G. Su, Q. Guo,* and R. E. Palmer Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom Received June 27, 2003. In Final Form: September 2, 2003 Self-assembly of polystyrene spheres guided by pattered InP substrates has been studied using atomic force microscopy (AFM). Using photoresist stripes and posts, 1-D and 2-D colloidal particle structures were found to preferentially nucleate at the step edges of the photoresist. The dangling bonds created by photocleavage of molecular bonds at the steps are identified as playing a major role in attracting the colloidal suspension.
Introduction Colloidal spheres can self-assemble into ordered twodimensional (2-D) and three-dimensional (3-D) structures under the influence of gravitational, convective, or electrohydrodynamic forces.1-12 The ordered 2-D structures have been utilized for large-area micro- and nanostructure fabrications,13,14 while the 3-D structures have been successfully adapted for photonic materials research.15 Currently there are two major issues around the formation of colloidal crystals. One is the fabrication of extended 3-D and 2-D structures with very low concentrations of defects. This is largely driven by the needs for high-quality photonic crystals and etch masks. Several techniques have been developed to improve the quality of colloidal crystals, for instance, spin coating13 and the applications of electric fields16 and intense optical fields.17 As a result, ordered 2-D monolayers over approximately centimeter distances, e.g., have been grown successfully.18 The other issue is the fabrication of complex colloidal structures at the micrometer scale based on surface templates.3,19,23 This usually involves the introduction of lateral potential energy modulations via artificially struc* To whom correspondence should be addressed. E-mail: Q.Guo@ bham.ac.uk. (1) Donselaar, L.; Philipse, A.; Suurmond, J. Langmuir 1997, 13, 6018. (2) Park, S.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (3) Ye, Y.; Badilescu, S.; Truong, V.; Rochon, P.; Natansohn, A. Appl. Phys. Lett. 2001, 79, 872. (4) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; IvanoV, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (5) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; IvanoV, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (6) Ye, Y.; LeBlanc, F.; Hache´, A.; Truong, V. Appl. Phys. Lett. 2001, 78, 52. (7) Micheletto, R.; Fukuda, H.; Ohtsu, M. Langmuir 1995, 11, 3333. (8) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lopez, C. Langmuir 1999, 15, 4701. (9) Masuda, Y.; Tomimoto, K.; Koumoto, K. Langmuir 2003, 19, 5179. (10) Gong, T. Y.; Wu, D. T.; Marr, D. W. M.; Langmuir 2003, 19, 5967. (11) Yin, Y.; Li, Z. Y.; Xia, Y. Langmuir 2003, 19, 622. (12) Gong, T. Y.; Wu, D. T.; Marr, D. W. M. Langmuir 2002, 18, 10064. (13) Deckman, H. W.; Dunsmuir, J. H. J. Vac. Sci. Technol. 1983, B1, 1109. (14) Seeger, K.; Palmer, R. E. J. Phys. D: Appl. Phys. 1999, 32, L129. (15) Tarhan, I. I.; Watson, G. H. Phys. Rev. Lett. 1996, 76, 315. (16) Trau, M.; Saville, D. A.; Sksay, I. A. Science 1996, 272, 706. (17) Burns, M. M.; Fournier, J. M.; Golovchenko, J. A. Science 1990, 249, 749. (18) Bertone, J.; Jiang, P.; Hwuang, K.; Mittleman, D.; Colvin, V. Phys. Rev. Lett. 1999, 83, 300. (19) Blaaderen, A. V.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321.
tured surfaces. Once a colloidal suspension is applied to the structured surfaces, the particles move to sites of lowest energy, giving rise to a patterned two-dimensional colloidal architecture. Sometimes it is the variation of the liquidsolid interfacial energy which causes the liquid film to break systematically at points of high interfacial energy. In such a case, the colloidal particles are carried by the liquid film to specific positions on the structured surface. Using structured surfaces, guided growth of colloidal monolayers on gold decorated mica substrates has been achieved previously in our laboratory.23 Similar techniques have been applied for the fabrication of silicon pillars at predefined locations.22 The possibility of attaching microspheres to specific sites on a surface has a wide range of implications. For instance, it provides a good opportunity to study heterogeneous nucleation and growth of colloidal monolayers. A detailed understanding of site-specific attachment of colloidal particles can also provide the basis for designing biosensitive surfaces applicable in sorting and detection of proteins and cells. We report in this paper the results from our recent work on the formation of 1-D straight lines and curved strings of polystyrene spheres on patterned InP substrates. InP has been chosen as the substrate, rather than silicon, mica, or glass, because of its interesting optical activity. Its direct band gap (1.35 eV) can easily be accessed with visible light. If fluorescence spheres or fluorescence-tagged proteins are attached to the substrate and excited by a pulse of visible light, the fluorescence decay is expected to cause localized interband transitions, leading to a localized conductivity change which could be detected with integrated microelectrodes. Experimental Section Details of the polystyrene microspheres used in the experiments were given in a previous publication.23 The spheres 500 nm in diameter were applied to the substrate by dropping a 1-2 µL of colloidal suspension onto the substrate with a micropipet. The substrates are double-side polished n-type InP wafers provided by Agilent Technologies UK Limited. Surface contamination was removed by ultrasonic cleaning in acetone for 5-15 min., which gave rise to surfaces on which the polystyrene spheres assembled into extended colloidal monolayers. By controlling the temperature and the humidity of the drying environment, (20) Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phys. Rev. Lett. 2000, 84, 2997. (21) Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 267. (22) Bale, M.; Turner, A. J.; Palmer, R. E. J. Phys. D: Appl. Phys. 2002, 35, L11. (23) Guo, Q.; Arnoux, C.; Palmer, R. E. Langmuir 2001, 17, 7150.
10.1021/la035149d CCC: $25.00 © 2003 American Chemical Society Published on Web 10/10/2003
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Figure 1. AFM images of polystyrene spheres on photoresistpatterned InP substrates: (a) Spheres filling holes within the photoresist film and (b) spheres filling the trenches between photoresist stripes. the drying time of colloidal suspension droplet on the surface of InP wafer was adjusted to about 30 min. Shorter drying times resulted in a low degree of ordering because the spheres did not have enough time to reach to equilibrium positions before the complete evaporation of the liquid. Surface patterning was achieved using photolithography with PMMA photoresist. A Dimension 3100 atomic force microscope (AFM) was used to image various forms of colloidal structures.
Results and Discussion Figure 1 shows AFM images of the microspheres assembled on top of two photoresist-patterned InP substrates. In both cases, a 2 µm thick PMMA photoresist film was spin coated onto the InP substrate. This was followed by exposure to UV light through a mask and the removal of the exposed photoresist with an acetone wash. In Figure 1a the spheres are seen to fill the diamondshaped wells, similar to that observed on photoresistcovered silicon22 and indium tin oxide (ITO) substrates.24 Apart from a small number of isolated spheres sitting on top of the photoresist film, the overall majority of the spheres are found inside the wells. Since the diameter of the spheres is 500 nm and the photoresist film is 2 µm thick, the colloidal structure formed inside the wells is up to four layers tall when measured from the InP surface. In Figure 1b the InP substrate was covered by a regular array of photoresist stripes of 4 µm wide before the application of the colloidal suspension. On this substrate, the spheres are found to fill the trenches between the (24) Kumacheva, E.; Golding, R. K.; Allard, M.; Gargent, E. H. Adv. Mater. 2002, 14, 221.
Su et al.
PMMA stripes. Again, hardly any spheres are found on top of the photoresist. On the basis of these observations, it is clear that the colloidal suspension always avoids the unexposed photoresist. It cannot be concluded yet if the bare InP surface or the sidewalls of the PMMA is responsible for the nucleation of the colloidal structures. To find out the initial nucleation sites for the colloidal structures, the colloidal suspension was diluted to 25% of its initial concentration and applied to a striped substrate. Figure 2 shows AFM images of the colloidal structures formed from the diluted suspension. In contrast to the image shown in Figure 1b, the spheres in Figure 2 are observed to accumulate at the edges of the PMMA stripes, giving rise to many straight lines of one sphere wide, along the direction of the stripes. In addition to the straight lines, localized monolayer islands are also present, as can be seen in Figure 2a. The monolayer islands are found always to grow out from the edge of the stripes, providing clear evidence that the initial nucleation occurs at the edge of the stripes. Moreover, it can be seen in parts a and b of Figure 2 that lines in the second layer start to form at the very edge of the stripes well before the completion of the first layer. One of these second-layer lines is highlighted with two arrows in Figure 2b. Therefore, it is without any doubt that the colloidal lines nucleate at the edges of the PMMA stripes and the monolayers grow out the lines. Once a sphere moves to the edge of a stripe, it is trapped by the step edge. However, spheres can still move along the steps, joining other spheres, to form short segments of colloidal chains. This occurs when the coverage at the step edge is well below that required for the formation of a full line. Several short lines (3-10 spheres in length) can be seen in Figure 2b. The formation of short lines indicates an attractive interaction among the spheres along the step edges. Without this attractive interaction, one expects to see mostly isolated spheres at low coverages due to the random arrival of individual spheres. The attractive force probably arises from capillary interactions which have been identified as an important factor in the assembly of closed packed two-dimensional colloidal layers.5 If this is the case, then the short colloidal lines are likely to be formed when the thickness of the liquid film at the step edges falls below the diameter of the spheres. The nucleation of colloidal structures at the edge of the stripes is also consistent with the structure of the multilayers shown in Figure 1b where domains are observed. Four domains are indicated by arrows in the figure. Domains grow out of opposite edges, and when two domains meet a domain boundary is created. When there is positional mismatch between two adjacent domains, the boundary becomes a line defect. Since the spheres always tend to avoid the top of the resist stripes (only isolated spheres separated by very long distances can be observed on top of the stripes), the property of the sidewalls of the stripes is thus distinctly different from that of the top surface. This change of surface property is very likely induced by the exposure to UV light. The exposure to UV light creates a large number of unsaturated chemical bonds due to the photocleavage of the PMMA molecules, making the sidewalls of the stripes polar. The formation of lines and monolayers within the trenches can be understood as follows. When the colloidal suspension is applied to the striped surface, the liquid moves quickly into the trenches, carrying the polystyrene spheres with it, because water does not wet the untreated PMMA. Within the trenches, the liquid film gets pinned by the edges of the photoresist stripes as illustrated in Figure 2c. Due to the evaporation of water,
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Figure 2. (a and b) AFM images of colloidal lines along the edges of the PMMA stripes. (c) Illustration of the pinned water film and the motion of the spheres on the surface of InP.
Figure 3. AFM image of curved colloidal strings formed around photoresist posts.
the liquid film becomes thinner with time. The pinning of the liquid film at the stripe edges, in combination with evaporation, leads to a net flux of water molecules moving toward the edges of the stripes. The spheres are carried by the water flux to the edges where they get trapped. If the concentration of the colloidal suspension is high enough, two-dimensional monolayer forms. For low concentrations, line/s form along the step edges. The water film is expected to eventually break somewhere midway in the trench, giving rise to two liquid tubules attached to each step edge in the trench. Due to variations in local concentration, the spheres along the edges of the stripes form either single or multiple lines. The strong interaction between the colloidal suspension and the edges of the photoresist stripes provides a useful route for the assembly of curved lines of spheres: colloidal
strings. Figure 3 shows several colloidal strings formed around photoresist posts. In this case the edges of the PMMA posts serves as nucleation sites, similar to that of the striped surface. Strong capillary forces, once again, are believed to play an important role for drawing the spheres close together once they are trapped by the edge of the photoresist. The lateral dimensions and the density of the posts, as well as the concentration of the spheres in the suspension, can be adjusted to control the width of the strings. Reducing the density of the posts, for example, favors the formation of wider strings and monolayers, while single sphere wide strings are expected with low concentrations of spheres in the suspension. In summary, colloidal lines and strings have been fabricated on photoresist-patterned InP substrates. The edges of the photoresist stripes are identified as the nucleation sites for the formation of lines and strings. Colloidal monolayers are formed from lines and strings, and the structure of the monolayer is strongly influenced by the initial structure of the lines/strings. The colloidal lines and strings can be applied as model systems for the investigation of diffusion and thermal fluctuation of particles in a 1-D potential well. They can also be incorporated into biosensors acting as templates for the attachment of biomolecules.25 Spheres decorated with biorecognition functionalities could be served as protein binding sites for bioassay. Acknowledgment. This work was supported by the UK EPSRC and Agilent Technologies. LA035149D (25) Gleason, N. J.; Nodes, C. J.; Higham, E. M.; Guckert, N.; Aksay, I. A.; Schwarzbauer, J. E.; Carbeck, J. D. Langmuir 2003, 19, 513.
