Langmuir 2005, 21, 4971-4975
4971
Template-Directed Assembly on an Ordered Microsphere Array Sonia Grego,* Thomas W. Jarvis, Brian R. Stoner, and Jay S. Lewis RTI International,† 3040 Cornwallis Road, Research Triangle Park, North Carolina 27709 Received December 16, 2004. In Final Form: March 25, 2005 We investigated the capability of an ordered array of microspheres to act as a template for deposition and ordering of a subsequent layer of microspheres. An evaporation-based technique was used to deposit monolayers of large colloidal spheres. A novel technique for selective deposition of polyelectrolyte film was used to stabilize the arrays and optimize the bead-substrate interaction. The template behavior of facecentered cubic and body-centered cubic (bcc) microsphere arrays was studied by optical and scanning electron microscopy, and the packing geometry was found to have a dramatic effect on the arrangement of the subsequent layer. A geometrical interpretation of the experimental data explains why a bcc bead array is well suited to act as a template for an additional layer of microspheres.
Introduction Colloidal self-assembly of crystalline structures has generated interest in recent years for the development of novel materials for photonic and sensing applications. Growth of colloidal crystals with a different packing than the usual (111)-oriented face-centered cubic (fcc) substrate has been obtained on a template using a variety of methods: sedimentation,1 convective assembly,2,3 hydrodynamic flow in confined spaces,4 and others. The template is either a polymer film patterned by a variety of methods (optically, e-beam, imprinting) or square pyramid pits or grooves anisotropically etched in silicon.5 It has not yet been demonstrated that an array of microspheres can be used to template the deposition of an ordered array of spheres of the same size, although such a templating function has been suggested.6,7 A potential application of the capability of using microspheres as templates for themselves would be the deposition of particles with different refractive indexes or other material properties, creating an engineered planar defect in the colloidal structures. Planar defects are of great interest for photonic device applications because they provide optical functionality such as the creation of a “pass-band” peak within the photonic stop band of a crystal, with potential use in waveguiding and highly selective filters. The development of methods to create such a defect layer in a self-assembled colloid structure has recently become an active field of research. A few methods have been used to engineer defects in photonic crystals, for example, using photopolymerization8 and infiltration by chemical vapor deposition of a self-assembled crystal to grow a defect slab of silica.9,10 * To whom correspondence should be addressed. E-mail: sgrego@ rti.org. † Formerly MCNC Research & Development Institute. (1) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (2) Zhang, J.; Alsayed, A.; Lin, K. H.; Sanyal, S.; Zhang, F.; Pao, W.-J.; Balagurusamy, V. S. K.; Heiney, P. A.; Yodh, A. G. Appl. Phys. Lett. 2002, 81, 3176. (3) Yi, D. K.; Seo, E.-M.; Kim, D. Y. Appl. Phys. Lett. 2002, 80, 225. (4) Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2003, 19, 622. (5) Yang, S. M.; Mı´guez, H.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 425. (6) Lee, I.; Zheng, H.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 572. (7) Ye, Y.-H.; Badilescu, S.; Truong, V.-V.; Rochon, P.; Natansohn, A. Appl. Phys. Lett. 2001, 79, 872.
An approach was recently suggested for depositing a single layer of microspheres as a two-dimensional defect in a three-dimensional self-assembled colloid crystal.11 This method uses the Langmuir-Blodgett technique to deposit an ordered hexagonally packed array of microspheres and allowed the authors to investigate the optical properties of the defect; however, it does not provide control of the packing geometry. The defect insertion by Langmuir-Blodgett was not a demonstration of microsphere templating, because the deposited monolayer of beads assembles in the minimal energy configuration at the airliquid interface before being transferred to the air-solid interface. In our work a combination of surface relief, chemical bonds, and evaporation forces was used to create a robust, ordered bead array in (100) orientation. The capability of the array to act as a template for subsequent bead deposition was investigated. We used a microfabricated square-patterned silicon substrate for the deposition of a first ordered monolayer of polystyrene microspheres. A second deposition of bead suspension created, in suitable conditions, a second layer that ordered itself using the first layer as a template. Experimental Section We used polystyrene beads with a mean diameter of 4.4 µm and various surface functionalizations: carboxylated (Polysciences, Bangs Laboratories) and biotin/streptavidin-coated (Bangs Laboratories), as well as plain polystyrene beads (Polysciences). Aliphatic amine beads (Interfacial Dynamics Corp.) with a mean diameter of 4.6 µm were used to exploit glutaraldehyde cross-linking. We fabricated (100) substrate patterns on silicon wafers by anisotropically dry reactive ion etching a two-dimensional 200 × 200 array pattern in photoresist (1811, Shipley). The pitch was selected to produce either close-packed fcc or bcc lattice crystal structures for 4.4 µm beads (Figure 1). To prevent the microspheres from obstructing one another, we changed the geometry (8) Lee, W. M.; Pruzinski, S. A.; Braun, P. V. Adv. Mater. 2002, 14, 271. (9) Palacios-Lido´n, E.; Galisteo-Lo´pez, J. F.; Jua´rez, B. H.; Lo´pez, C. Adv. Mater. 2004, 16, 341. (10) Te´treault, N.; Mihi, A.; Herna´n, M.; Rodrı´guez, I.; Ozin, G. A.; Meseguer, F.; Kitaev, V. Adv. Mater. 2004, 16, 346. (11) Zhao, Y.; Wostyn, K.; de Schaetzen, G.; Clays, K.; Hellemans, L.; Persoons, A.; Szekeres, M.; Schoonheydt, R. A. Appl. Phys. Lett. 2003, 82, 3764.
10.1021/la0468850 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/30/2005
4972
Langmuir, Vol. 21, No. 11, 2005
Grego et al. We deposited polyelectrolyte films according to the procedure described in the literature.12 Clean samples were dipped alternately in solutions of poly(diallyldimethylammonium chloride) (0.08 M PDDA with 0.1 M NaCl) and poly(sodium 4-styrenesulfonate) (0.04 M PSS with 0.1 M NaCl). Because we used commercially available negatively surface-charged microspheres (typically 6 µequiv/g), we coated substrates with an odd number of polyelectrolyte layers, typically between 3 and 9, producing a positive surface charge. The thickness of a PDDA/PSS/PDDA trilayer was measured to be 40 Å by ellipsometry and was negligible compared to the surface relief pattern depth. Polystyrene microspheres were coated with alternate layers of the polyelectrolytes PDDA and PSS, according to a procedure described in the literature.13 Beads were mixed in a solution of 0.1 M polyelectrolyte with 0.5 M NaCl for ionic balance and left to equilibrate for 20 min. After that, the beads were rinsed via repeated centrifugations.
Results and Discussion
Figure 1. SEM image of the 1.2 µm deep substrate arrays. The well diameter d and the pitch are indicated. (a) fcc lattice with 4.2 µm diameter cylindrical wells with pitch ) 4.65 µm. (b) bcc configuration with well diameter 4.3 µm and pitch ) 5.2 µm. Scale bar ) 5 µm.
Figure 2. Diagram of the perfusion chamber used in the experiment with lettering indicating A, silicon substrate; B, spacer; C, coverglass; D, eight arrays with template wells; and E, the apron of the substrate where the bead suspension is deposited. from the nominal fcc lattice to a slightly larger pitch so that for each experiment we had arrays with pitches of 4.5, 4.65, and 4.85 µm. We calculated the pitch of the (100) plane corresponding to a bcc lattice with a 4.5 µm unit cell as 5.2 µm and used this pitch value for the bcc patterns. In both fcc and bcc samples, multiple arrays were fabricated with different well diameters in the range 2.7-4.3 µm. We had two cylindrical well depths, shallow wells of 0.3 µm and deep wells of 1.2 µm. To deposit microspheres on a substrate we built a perfusion chamber with a volume of 15-20 µL using two parallel spacers (250-300 µm thick) and a coverslip (Figure 2). Eight arrays of 200 × 200 etched wells with different diameter and pitch values were exposed to the bead suspension in the perfusion chamber. The perfusion chamber was left in a horizontal position and unsealed on two sides. Solutions were pipetted onto the apron immediately adjacent to the covered chamber (region E in Figure 2). Capillary action then drew the liquid into the chamber itself. Solutions were pipetted into the chamber consecutively while maintaining a full, wet chamber or after previously perfused bead suspensions had evaporated (in a time span of approximately 30-60 min at a rate of 5-10 µm/s, depending on surfactant choice). We explored the effect of experimental parameters such as bead concentration and the use of surfactants such as decyltrimethylammonium bromide, SDS (sodium dodecyl sulfate), and Tween-20. All perfusion chamber experiments were conducted at room temperature.
We obtained excellent quality (100) fcc and bcc patterns in silicon using photolithography and reactive ion etching (Figure 1a,b). Because we used beads with diameters of 4.4-4.6 µm, deposition using the evaporation method with a vertical slide described in the literature was not feasible without agitation of the suspension.5 Good quality ordering of the microsphere array directed by the template substrate was achieved by letting the suspension evaporate in the chamber, exploiting capillarity effects.14 To use the bead array itself as a template, its adhesion to the substrate had to be sufficiently robust to remain intact after the subsequent wetting and drying processes. We found, however, that the polystyrene bead array was relatively fragile. As a result of the large microsphere size, the drag force of the meniscus removed a significant fraction of the adhered beads during subsequent reperfusions. Considerable bead loss was avoided by bead surface functionalization to improve adhesion to the substrate followed by bead-bead cross-linking. We explored several surface functionalizations with the goal of creating a robust array of microspheres. A biotinstreptavidin bond alone was found to produce insufficient adhesion. The biotin-streptavidin bond (with biotinylated substrate- and streptavidin-coated beads) proved able to withstand perfusion chamber rinsing but not the drag force of the liquid meniscus when the solution dried. Ultrathin polyelectrolyte films were found to produce adequate adhesion by interacting with the charged microspheres. The silicon wafers were treated by immersion in a freshly prepared piranha solution to create a hydrophilic negative surface where the cationic polyelectrolyte PDDA was dip-coated.15 An aqueous polyelectrolyte dip-coating process was used to deposit the multilayer film. We observed that highly charged negative carboxylated or aliphatic amine beads (typically 6 µequiv/g) had greater adhesion to the substrate than plain polystyrene or biotin/streptavidin-coated beads. The polyelectrolyte films exhibited resistance to many solvents, permitting a selective deposition of the polyelectrolyte coating on the bottom of the wells: a conformal film was deposited during the dipping process with the photoresist pattern still in place; by photoresist lift-off in acetone or photoresist-strip EKC802 (EKC Technologies) the deposited polyelectrolyte was removed from the substrate except from the bottom (12) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (13) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; Mo¨hwald, H. Polym. Adv. Technol. 1998, 9, 759. (14) Grego, S.; Jarvis, T.; Stoner, B. R.; Lewis, J. S. Mater. Res. Soc. Symp. Proc. 2004, 820, R5.4.1. (15) Zheng, H.; Rubner, M. F.; Hammond, P. T. Langmuir 2002, 18, 4505.
Assembly on an Ordered Microsphere Array
Langmuir, Vol. 21, No. 11, 2005 4973
Figure 4. Effect of depth of the pattern on ordering array. (a) Optical microscope image of beads on a fcc pattern with wells 0.3 µm deep; (b) same as part a but with wells 1.2 µm deep. Scale bars ) 15 µm. (c) Drawing to scale of a 4.4 µm bead in a 0.3 µm deep well and (d) in a 1.2 µm deep well.
Figure 3. Procedure to create selective deposition of polyelectrolyte films in well arrays.
and sides of the wells (Figure 3).14 The silicon surface outside the patterned well after photoresist lift-off was hydrophobic, and it did not favor microsphere adsorption which occurred mainly in the patterned wells. By varying the number of polyelectrolyte film layers deposited on the pattern we discretely tuned the surface charge force that affected bead-substrate interaction, with an increased number of layers producing increased adhesion. We also evaluated polyelectrolyte-coated spheres, to test beads with higher surface charge. This allowed us greater control over the dynamic interactions between the microspheres and the substrate. The surface charge was not quantified, but the optimal number of polyelectrolyte layers on both beads and substrates was determined empirically. The process was improved to find a point where the attraction force was not so strong as to cause disordered bead adhesion. Using optical microscopy, we observed the arrangement of microspheres deposited on the substrate after the passage of the meniscus. As the meniscus swept across the patterned substrate, the beads accumulated exclusively in the wells (a similar effect was observed by Ozin5 for a template consisting of large grooves). The order quality of the deposited layer depended on the well depth: it was good for a 1.2 µm depth (25% of the bead diameter) while 0.3 µm depths produced an imperfect arrangement (Figure 4a,b). We interpreted this result as a geometrical effect, as shown in Figure 4c,d. In the limiting case of no well, with a template pattern consisting only of polyelectrolyte thin films (height of a few nanometers, negligible compared to bead size), the deposition yielded disordered bead adhesion.14 Besides requiring an adequate well depth, highly ordered arrays were only obtained with well diameters larger than 4 µm. We interpreted this result with a similar geometrical argument: for the smaller diameter wells, the points of contact between the beads and the sidewalls of the wells are farther away from the equator of the microspheres. On the contrary, for a well diameter larger than 4 µm the beads sit lower in the wells and their arrangement is regulated by the well pattern. A dependence in coverage on the pitch of the anisotropically etched silicon pattern was not observed.
Figure 5. SEM images of self-assembled bilayers of 4.4 µm carboxylated beads formed after one deposition on (a) a bcc template (top-down view), (b) a fcc template (top-down view), and (c) a fcc template (tilted-angle view). Scale bar ) 5 µm.
A double layer (bilayer) formation, rather than a monolayer, was found to be the most energetically favored result of a single perfusion and evaporation process (Figure 5a-c). This self-assembled bilayer occurred over a wide range of particle concentration values (between 0.2 and 10%, w/v), with the caveat that for very low microsphere concentrations the patterned area was not entirely covered, although where coverage did occur a bilayer was often present. Self-assembled bilayers formed, although less consistently, in shallow 0.3 µm deep well patterns. A bilayer assembly was observed with beads of all the surface chemistries tested: carboxylated, aliphatic amine, and polyelectrolyte-coated. We believe that this phenomenon is related to the shape of meniscus that wets the patterned areas.16 It has been reported that the number of deposited bead layers depends on the height of the meniscus in discrete steps.16 We believe the geometry of the perfusion chambers and substrates consistently produces a meniscus height corresponding to a bilayer. Further investigations are necessary to explain the details of this mechanism. (16) Fustin, C.-A.; Glasser, G.; Spiess, H. W.; Jonas, U. Adv. Mater. 2003, 15, 1025.
4974
Langmuir, Vol. 21, No. 11, 2005
Grego et al.
Figure 6. Concept tested in this investigation: an ordered microsphere array can serve as a template for deposition of a subsequent layer of beads.
The object of this investigation is to test whether a microsphere array can be used as a template for subsequent ordered deposition of identical beads (Figure 6). To prove the templating capability of a self-assembled bilayer, a third layer needs to be deposited, but in our evaporation conditions this third layer has proved difficult to obtain. The adhesion of the third layer microspheres to the template bead layer needs to be increased to withstand the drag force of the meniscus. We achieved such adhesion using polyelectrolyte-coated beads. Both positively and negatively charged microspheres13 were fabricated and deposited in charge-alternating perfusions, obtaining robust multilayers but losing the template-directed ordering effect. The adhesion strength of polyelectrolytecoated beads of opposite charge precluded the mobility required for ordering. To test the concept of bead array templating, a multistep process was developed to deposit an ordered microsphere array of a selected surface chemistry on a template monolayer of microspheres. Increasing the surfactant concentration in the first bead perfusion suppressed the bilayer self-assembly and produced a single monolayer, although with an accompanying increase in vacancy defects. For SDS surfactant concentrations greater than 10-3 M SDS or Tween-20 surfactant concentrations greater than 10-4 M, we observed a monolayer and no significant self-assembled bilayers. It proved difficult to find a point in the parameter space that consistently produced a wellordered, high-density monolayer, as was obtained by Langmuir-Blodgett.11 Because the use of polyelectrolytecoated beads providing strong adhesion was detrimental to the ordering, we used a two-step deposition procedure to ensure ordered adhesion onto substrate-bound microspheres. Using patterned silicon substrates with five layers of polyelectrolyte coating, aliphatic amine microspheres with 10-3 M SDS surfactant were perfused and produced an ordered monolayer (Figure 7a,c). To prevent bead loss, the amine aliphatic groups on the bead surfaces were then cross-linked by treatment with a 1% glutaraldehyde solution for 30 s, followed by copious rinsing with deionized water. No significant bead loss was observed during this procedure. The sample was then perfused a second time with an aliphatic amine bead suspension. Bead deposition of the second layer exhibited a strong dependence on the pitch of the bead array used as a template. Parts b and d of Figure 7 show the very same region as respectively parts a and c of Figure 7. If the first layer was a close-packed square pattern with tight pitch (fcc lattice), we obtained a disordered second layer (Figure 7b). If the microsphere array was a bcc lattice with spheres separated by a submicrometer gap due to the 5.2 µm pitch, the second layer arranged along the template (Figure 7d). Such dependence on pitch was not observed in the deposition of the first layer of beads. The effect of the different pitches on the deposition of the second layer (but not the first) is interpreted in terms of the depth provided by the template pattern. We observed earlier in this manuscript that ordering along a template occurs for wells approximately as deep
Figure 7. Template effect of a monolayer of aliphatic amine beads: (a) initial monolayer on a fcc lattice; (b) disordered second layer assembled on a fcc bead monolayer; (c) initial monolayer on a bcc lattice; and (d) ordered second layer assembled on a bcc bead monolayer. Scale bars are 15 µm.
Figure 8. Schematic of the geometry of three spheres in a vertical plane. The segment h is defined as the perceived well depth for the upper sphere on a template formed by the lower two spheres.
as 25% of the bead diameter. The earlier observations were based on cylindrical wells with straight walls produced by silicon etching, while the physical template provided by an array of microspheres has concave surfaces, so the comparison of the well depth in the two cases is not straightforward. We defined well depth for microsphere templates as the distance h in Figure 8. Geometrical calculation of the well depth h for microspheres in a closepacked fcc array resulted in h ) 0.64 µm, with the angle R ) 45°. For the bcc pattern, which had a pitch of 5.2 µm, h ) 1 µm, with the angle R ) 34°. Therefore, a bead deposited on a bcc lattice sat “lower” between the four lower neighboring beads than a bead deposited on a fcc array. This depth difference was sufficient to allow templating to occur for a bcc array but not for a fcc array. Conclusions We demonstrated that it is possible to use microspheres as a template for the ordered deposition of same-sized beads with an evaporation method. Adhesion forces were required for retaining the beads, but a sensitive balance had to be found between adhesion and the bead mobility necessary to produce order. We used a selective deposition of a polyelectrolyte multilayer on a silicon substrate and optimized bead-substrate interaction to produce a robust ordered monolayer of spheres on a (100) fcc and bcc substrate. We observed that the silicon well depth had a very important effect on the achievement of order, and in our experimental condition a depth of approximately 25% of the bead diameter proved necessary. We used a multiple
Assembly on an Ordered Microsphere Array
perfusion procedure and glutaraldehyde cross-linking of microspheres to deposit a second layer of microspheres over a first ordered array. The pitch of the first bead array, now used as a template, had a significant effect on the capability of directing the pattern of the subsequent layer. When the first layer was a close-packed square pattern with tight pitch (fcc lattice), we obtained a disordered second layer. On the contrary, when the first layer was a bcc lattice with spheres separated by a larger pitch, the second layer arranged along the template. A geometrical interpretation of these results was given in terms of the different well depths provided by the fcc and bcc array to the second layer deposition. These observations are
Langmuir, Vol. 21, No. 11, 2005 4975
relevant in research areas aiming, for example, at building large area ordered materials using micrometer-sized polyhedral clusters of a few spheres as building blocks.17 Acknowledgment. We thank Dr. V. Paunov for fruitful discussions. This work was partially supported by DARPA through SPAWARSYSCEN Grant N66001-03-1-8900. LA0468850 (17) Yi, G.-R.; Manoharan, V. N.; Michel, E.; Elsesser, M.; Yang, S.-M.; Pine, D. J. Adv. Mater. 2004, 16, 1204.
