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Parameters Influencing the Templated Growth of Colloidal Crystals on Chemically Patterned Surfaces Charles-Andre´ Fustin,§ Gunnar Glasser, Hans W. Spiess, and Ulrich Jonas* Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Received April 28, 2004. In Final Form: August 2, 2004 The influence of various experimental parameters on the vertical deposition and structure formation of colloidal crystals on chemically patterned surfaces, with hydrophilic and hydrophobic areas, was investigated. The pattern dimensions range from about 4 to 400 µm, which is much larger than the individual particle size (255 nm), to control the microscopic crystal shape rather than influencing the crystal lattice geometry (as achieved in colloidal epitaxy). The deposition resolution and selectivity were tested by varying the particle concentration in the suspension, the substrate withdrawing speed, pattern size and orientation, and wetting contrast between the hydrophilic and hydrophobic regions. The evolution of colloidal crystal thickness with respect to the pattern dimensions and deposition parameters was further studied. Our results show that the pattern size has a rather strong influence on the deposited number of colloid layers and on the crystal quality. Better results are obtained when the lines of a stripe pattern are oriented parallel to the withdrawing direction rather than perpendicular. The deposition resolution (defined as the minimum feature size on which particles can be deposited) depends on the wetting contrast and increases with lower average hydrophobicity of the substrate.
Introduction Ordered arrays of colloidal particles attract substantial attention due to their broad range of potential applications, such as templates for porous materials (that can act as catalysts in chemical and biological processes), masks for nonlithographic patterning, sensor arrays, and photonic crystal devices.1 For many of the above applications, it is of great interest to selectively position the colloidal particles on surfaces to obtain superstructures with specific geometries. Colloidal particles can be assembled into close-packed layers by many different techniques. The most common ones are self-assembly under the influence of external fieldssgravitational sedimentation2,3 or electrophoretic deposition,4,5 vertical deposition by evaporation6-8 or by lifting the substrate,9 and the fluidic * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: +49 6131 379 100. § Present address: Unite ´ CMAT, Universite´ Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium. (1) For reviews on colloidal crystals and assemblies, see for example: (a) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (b) Lopez, C. Adv. Mater. 2003, 15, 1679. (c) del Campo, A.; Duwez, A. S.; Fustin, C. A.; Jonas, U. Colloidal Micro- and Nanostructures Assembled at Patterned Surfaces. In Dekker Encyclopedia of Nanoscience and Nanotechnology; Schwarz J. A., Contescu C., Putyera, K., Eds.; Marcel Dekker: New York, 2004; p 725. (b) Dutta, J.; Hofmann, H. SelfOrganization of Colloidal Nanoparticles. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publisher: Stevenson Ranch, 2004, Vol. 9, p 617. (2) Davis, K. E.; Russel, W. B.; Glantschnig, W. J. Science 1989, 245, 507. (3) (a) Salvarezza, R. C.; Vazquez, L.; Miguez, H.; Mayoral, R.; Lopez, C.; Meseguer, F. Phys. Rev. Lett. 1996, 77, 4572. (b) Mayoral, R.; Requena, J.; Moya, J. S.; Lopez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Vazquez, L.; Holgado, M.; Blanco, A. Adv. Mater. 1997, 9, 257. (4) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (5) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lopez, C. Langmuir 1999, 15, 4701. (6) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (7) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (8) (a) Ye, Y. H.; LeBlanc, F.; Hache, A.; Truong, V. V. Appl. Phys. Lett. 2001, 78, 52. (b) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Go¨rnitz, E. Langmuir 2002, 18, 3319. (9) Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760.
cell method demonstrated by Xia et al.10 On the other hand, only a few approaches to control the specific placement of colloids on surfaces by self-assembly have been reported in the literature. They can be divided into three main categories: assembly of colloids onto chemically patterned surfaces on the basis of (1) electrostatic forces or (2) wettability differences and (3) assembly of colloids under spatial confinement using substrates topographically structured with holes, grooves, or microchannels. If the feature size of the templating surface pattern is in the dimension of individual particles, the crystal symmetry can be controlled by the so-called colloidal epitaxy (as introduced by van Blaaderen et al.11). If much larger pattern features are used, the overall crystal shape can be influenced, which is, for example, attempted by the technique discussed in this paper. Intermediate cases are also known in which the templating structure dimensions are a small integer multiple of the lattice spacing to induce a preferred crystal packing, sometimes referred to as graphoepitaxy.12 The first methodology (1) mentioned above employs surfaces with positive/negative or charged/ neutral monolayer patterns onto which charged colloidal particles are adsorbed.13-16 High selectivity of the colloid deposition is achieved with this approach, but only disordered particle monolayers are obtained. In the second method (2), chemically patterned surfaces with wettability contrast have been used to (i) regionally control (disordered) particle deposition on 5-50 µm areas17 (ii) to study (10) Park, S. H.; Xia, Y. N. Langmuir 1999, 15, 266. (11) van Blaaderen, A.; Ruel, R.; Wiltzius. P. Nature 1997, 385, 321. (12) (a) Geis, M. W.; Flanders, D. C.; Smith, H. I. Appl. Phys. Lett. 1979, 35, 71. (b) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95. (c) Kumacheva, E.; Golding, R. K.; Allard, M.; Sargent, E. H. Adv. Mater. 2002, 14, 221. (d) Yin, Y. D.; Xia, Y. N. Adv. Mater. 2002, 14, 605. (13) Aizemberg, J.; Braun, P. V.; Wiltzius, P. Appl. Phys. Lett. 2000, 84, 2997. (14) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (15) Himmelhaus, M.; Takei, H. Phys. Chem. Chem. Phys. 2002, 4, 496. (16) Jonas, U.; del Campo, A.; Kru¨ger, C.; Glasser, G.; Boos, D. Proc. Natl. Acad. Sci., U.S.A. 2002, 99, 5034.
10.1021/la0489413 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/11/2004
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Langmuir, Vol. 20, No. 21, 2004 9115 Scheme 1a
a (a) Combination of the photolithography technique with the silanization process for the preparation of micropatterned silane layers (1, resist coating; 2, exposure; 3, resist developing; 4, silanization; 5, resist removal; 6, colloid deposition), (b) schematics of the dipping device for the controlled withdrawal of the substrate during vertical colloid deposition, (c) substrate with patterned silane layer and colloidal crystal during withdrawal, and (d) colloid crystallization process at the drying zone driven by attractive capillary forces between particles.
particle packing in individual hydrophilic stripes with dimensions close to the particle diameter,18 (iii) to insert vacancy defects in colloidal crystals19 and (iv) to generate colloidal crystals patterned at the millimeter scale.20 The third strategy (3) produces ordered colloidal layers with good control but requires a sequence of complex manufacturing processes to fabricate the substrates.11,12b,21,22 In a previous paper, we have reported on an alternative approach to obtain ordered colloidal layers on chemically patterned surfaces at the micron scale.23 This simple method is based on the selective wetting of a colloidal suspension on the hydrophilic regions of a surface patterned with hydrophobic and hydrophilic patches. These patterns can be obtained easily by combining photolithography with silane surface chemistry using the developed photoresist structure on the substrate for regiospecific silanization (see Scheme 1). In practice, the selectively silanized substrate is vertically lifted out of a colloidal suspension at a controlled slow speed. The suspending liquid specifically wets the hydrophilic regions and induces the local deposition of particles onto these areas as the meniscus is sweeping across the surface. Colloidal crystallization is induced by attractive capillary forces present between particles at the solid-liquid-gas contact line. It has been previously shown that a concave (wetting) meniscus perpendicular to the water surface (vertical cross section ⊥ in Scheme 2) is needed to self(17) a) Kru¨ger, C.; Jonas, U. J. Colloid Interface Sci. 2002, 252, 331. (b) Jonas, U.; Kru¨ger, C. J. Supramol. Chem. 2002, 2, 255. (c) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062. (18) (a) Masuda, Y.; Tomimoto, K.; Koumoto, K. Langmuir 2003, 19, 5179. (b) Kru¨ger, C.; Barrena, E.; Jonas, U. In Organosilicon Chemistry V; Norbert, A., Weis, J., Eds; Wiley-VCH: Weinheim, Germany, 2003; p 772. (19) Lu, G.; Chen, X.; Yao, J. M.; Li, W.; Zhang, G.; Zhao, D. Y.; Yang, B.; Shen, J. C. Adv. Mater. 2002, 14, 1799. (20) Gu, Z. Z.; Fujishima, A.; Sato, S. Angew. Chem., Int. Ed. 2002, 41, 2068. (21) Yang, S. M.; Miguez, H.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 425. (22) 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. (23) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Adv. Mater. 2003, 15, 1025.
assemble particles onto the substrate surface by capillary forces (specifically immersion capillary forces). Here, we report on a detailed study of the different parameters that influence the colloid crystallization on chemically patterned surfaces to gain a detailed understanding of the underlying factors as a prerequisite for the rational design and fabrication of microstructured colloid crystals by vertical deposition. The resolution of the method, i.e., the smallest usable pattern size, and its selectivity, i.e., the precision with which the colloid deposition follows the pattern, have been tested for a stripe pattern by varying the width of the hydrophilic features between 2 and 400 µm and the hydrophobic regions between 4 and 100 µm. The influence of the main experimental parameterssthe suspension concentration and withdrawing speedson the colloid assembly process has been investigated systematically. Furthermore, the orientation dependence of the pattern with respect to the withdrawing direction has been assessed. The wetting contrast between the hydrophilic and hydrophobic regions was varied to study its impact on the colloid deposition. Finally, the method has been tested for the crystallization of colloids on discontinuous patterns, i.e., hydrophilic circular regions in a hydrophobic matrix. The complex response of the colloid crystallization process to systematic variations of structural and processing parameters led to the insight that this crystallization process is driven by different mechanisms which are acting in parallel with their individual kinetics (and which are hard to deconvolute for nonpatterned substrates). The identification and fundamental understanding of these various contributions in dependence of the structural features of the substrate pattern is essential and might be specifically exploited in fabrication processes for microstructured colloidal crystals and potential devices on the basis of the presented method. Experimental section Substrate Preparation. Silicon or glass slides (of about 26 × 12 mm2) were cleaned by immersion into a piranha solution (H2SO4/H2O2 30%, 70:30 v/v, Caution: piranha solution reacts violently with organic compounds and must be handled with
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Scheme 2. Model of the Meniscus Shape at the Drying Zone over a Silane Layer Patterna
a (||) Horizontal meniscus parallel to the water surface, (⊥) vertical meniscus perpendicular to the water surface. The meniscus length (L) is defined as the distance between the apex of the contact line in the center of a hydrophilic stripe and the base line representing the planar water surface.
extreme care) for 90 min followed by copious rinsing with ultrapure water (Milli-Q system from Millipore GmbH, Eschborn, Germany). A 1:1 mixture of a positive-tone photoresist (Microposit S1805 from Shipley, Marlborough, MA) and a thinner (Microposit EC-solvent from Shipley, Marlborough, MA) was spin coated on the clean substrates at 4000 rpm for one minute, baked at 95 °C for 1 h, exposed to UV light (350 nm, 4 s) through a chromium mask (IMM GmbH Institut fu¨r Mikrotechnik, Mainz, Germany), and finally developed (ma-D 330 from Microresit Technology, Berlin, Germany). The resulting bare substrate areas were then functionalized with different silanes. Silanization with 1H,1H,2H,2H-perfluorodecyltrichlorosilane, FAS, (from ABCR GmbH, Karlsruhe, Germany) was performed in the vapor phase (room temp (RT), 10-2 mbar, 1h) and the samples were rinsed with a solvent sequence (acetone/dichloromethane/methanol/ water), using sonication each time to remove residual photoresist and regain the bare SiOH surface. Silanization with hexamethyldisilazane, HMDS, (from ABCR GmbH, Karlsruhe, Germany) was also performed in the vapor phase (RT, atm pressure, 1h), and the samples were rinsed as described above. On some samples, a second silanization step with (aminopropyl)triethoxysilane, APTES, (from ABCR GmbH, Karlsruhe, Germany) was conducted to further functionalize the SiOH regions initially covered by the photoresist pattern. This silanization was achieved via the vapor phase (RT, 10-2 mbar, 3h), and the samples were rinsed with toluene/methanol/water in sequence, using sonication each time. Materials and Equipment. Nonfunctionalized polystyrene latex particles of 255 nm and low polydispersity (typically 2-3%) were purchased from Duke Scientific, Palo Alto, CA. The dipping device is a homemade apparatus based on a step motor (LM 60 two phases SM 440 from OWIS, Staufen, Germany) with a half step of 0.5 µm enabling withdrawing speeds in the range of 0.02 µm s-1 to several cm per minute Particle Deposition. The particle suspensions were diluted with ultrapure water to the desired concentration (typically 0.52% w/w) and filled into a 10 mL beaker that was placed inside a larger beaker. The bottom of this larger beaker was covered with water to ensure high relative humidity. The whole apparatus was placed in a closed box to protect the experiment from air drafts. The temperature for the experiments was about 21 °C. The patterned substrates were then immersed vertically into the colloidal suspension and withdrawn with the dipping device at a controlled speed. Characterization. Low voltage scanning electron microscope (LVSEM) images on native samples (nonsputtered) were taken with a LEO Gemini 1530 at acceleration voltages of 0.2-1 kV. Optical properties of the colloidal crystals were evaluated by measuring their transmission spectra at normal incidence, using a UV-vis spectrometer coupled to a microscope (Leitz).
Results and Discussion Feature Resolution of the Crystallization Process. The first investigated substrate carried a line pattern with a series of hydrophobic stripes composed of a fluoroalkylsilane monolayer (FAS, static water contact angle ) 110°), which were separated by regions of bare silica surface with its hydrophilic SiOH groups (static water contact angle < 5°). The width of the hydrophobic stripes ranged from 4 to 100 µm and the hydrophilic lines from 2 to 400 µm. During particle deposition, the lines were oriented parallel to the withdrawing direction. As shown in Figure 1, the resolution and selectivity of the colloidal deposition are excellent, having crystalline particle structures down to a hydrophilic stripe width of 10 µm (Figure 1a). Narrower lines are not covered by the latex particles. On the other hand, the selectivity is still good for a hydrophobic spacing of 4 µm between two broader hydrophilic lines (Figure 1b and c). From these experiments, it becomes evident that a minimum width of about 10 µm is required for the hydrophilic SiOH regions to allow their efficient wetting by the water meniscus during particle deposition. On narrower stripes, no appropriate wetting meniscus is formed and the whole region behaves as if entirely hydrophobic. On the other hand, the hydrophobic FAS surface is efficient for dewetting and prevention of particle deposition down to a stripe width of 4 µm. It can be concluded that for small feature sizes the wetting film at the drying zone (defined here as the region enclosing the water meniscus and the wet particle layer) is more sensitive to increases of the contact angle (dewetting) than to contact angle decreases (wetting). Parameters Influencing Particle Deposition. The number of colloidal layers is found to increase in a stepwise fashion from the edge to the center of the stripe (see Figure 2), as mentioned in a previous paper.23 This effect is explained with the specific shape of the water film that wets the hydrophilic stripes which are isolated by hydrophobic regions. In Scheme 2, the qualitative shape of the meniscus over a hydrophilic stripe is outlined. The hydrophilic surface (SiOH) is wetted by the suspension, and thus, the liquid in contact with the substrate rises above the water level to a certain height, defined in the side view as the meniscus length (L). On the hydrophobic regions, the opposite situation is the case. The liquid
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Figure 2. LVSEM images showing the stepwise increase of the thickness from the edge to the center of the stripe. The arrows indicate the withdrawing direction from the latex suspension. (a) Top view of the stripe edge, (b) cross section through a stripe (after breaking the substrate), and (c) edge at the beginning of a stripe (top view).
Figure 1. LVSEM images showing vertical stripes of colloidal crystals of different width grown on a FAS/SiOH pattern. The arrows indicate the withdrawing direction from the latex suspension. The dark areas correspond to the silica surface (SiOH) covered by latex particles, while the bright regions correspond to the hydrophobic FAS monolayer. (a) Variation of the colloid line width down to 10 µm, variation of the line separation for (b) broad colloid lines (100 µm width), and (c) narrow lines (25 µm).
dewets from the FAS monolayer and the contact line (between air, liquid, and substrate) sinks below the water level. At an edge where the hydrophilic and the hydrophobic regions meet, the abrupt change in the contact angle leads to a strong deformation of the meniscus, resulting in an effective thinning of the water film on the hydrophilic surface close to the edge. The deformation is symmetrical at both edges and creates a quasi-hemispherical shape of the meniscus parallel to the water surface. This is shown for an arbitrary position in the horizontal cross section (||) of Scheme 2. In the vertical cross section (⊥), the meniscus has a concave shape (as a prerequisite for the particle deposition). When the stripe width falls below a critical value, the deformations at both ends start to overlap and influence each other, which will result in a reduction of the meniscus length (L) and height compared to an unstructured hydrophilic substrate. The meniscus shape variation with changes in the stripe width is responsible for the different crystallization behavior and the corresponding number of layers in the crystal
film. The convex shape of the wetting film parallel to the water surface (see horizontal cross section || in Scheme 2) forms an envelope for the growing colloidal crystal whose thickness follows the height of the meniscus in discrete steps (see the insert in Figure 2). Figure 2c shows the terrace-shaped beginning of a stripe and clearly illustrates that the wetting film acts as a liquid mold. In the transition zone between two layers of different thickness, small areas with a square (tetragonal) lattice symmetry are present (compare also Figures 4 and 6 below). This phenomenon, occurring to compensate for the gradually changing film thickness due to the liquid meniscus slope and favoring the fcc (100) surface orientation, is known for extended systems and has been explained by Dushkin et al.24 The influence of the deposition conditions on the number of deposited particle layers was studied by independent variation of the suspension concentration, withdrawing speed, and hydrophilic stripe width. The concentration of the colloidal suspension was varied in the range of 0.52.0% w/w and the withdrawing speed from 0.1 to 0.4 µm s-1. The number of deposited colloid layers was measured by SEM in the center of the cross sections from cleaved samples for different stripe widths of the hydrophilic SiOH lines. The results are summarized in Table 1, which reveals that, on average, the number of layers decreases with decreasing stripe width to reach the limit of one monolayer for 10 µm. This is due to a decrease of the meniscus length (L in Scheme 2) when the stripe width is reduced. Indeed, at the edge between a hydrophilic and a hydrophobic stripe, the apparent contact angle is close to that of the (24) Dushkin, C. D.; Lazarov, G. S.; Kostev, S. N.; Yoshimura, H.; Nagayama, K. Colloid. Polym. Sci. 1999, 277, 914.
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Table 1. Film Thickness (in Number of Layers at the Stripe Center) as Measured by SEM on Cross Sections of Cleaved Samples with Different Stripe Widtha
stripe width (µm)
withdrawing speed (µm s-1)
400
0.4 0.2 0.1 0.4 0.2 0.1 0.4 0.2 0.1 0.4 0.2 0.1 0.4 0.2 0.1
100 50 25 10
a
no. of deposited layers at varying suspension concentrations (w/w) 0.5% 1.0% 1.5% 2.0% 7 16 31 6 14 31 16 20 24 11 7 4 0 1 1
16 48 62 19 28 53 22 32 21 12 7 4 1 2 1
25 51 94 33 40 66 20 31 22 12 6 4 2 2 1
42 78 141 44 51 79 20 26 23 13 6 4 2 1 2
The error on the number of layers is about 10%.
hydrophobic surface but the contact line is perpendicular (vertical, like the edge) to the water surface.25,26 The apparent contact angle then decreases progressively to reach that of the nonpatterned hydrophilic substrate at a certain distance from the stripe edges. If the stripe is not broad enough, the contact angle only reaches an intermediate value. The limiting case is reached for very narrow stripes (like the 4 and 2 µm stripes in our study) where the water does not effectively wet the stripe and the average contact angle stays about the value of a nonpatterned hydrophobic surface. The meniscus length (L), which is inversely proportional to the contact angle, is thus smaller on narrow stripes, and fewer colloid layers are deposited. The number of particle layers is indeed proportional to the meniscus length, as shown by Nagayama et al.27 Using a similar setup, Nagayama et al. have deposited monolayers of colloids on nonpatterned surfaces. From these results, they derived an equation describing the growth rate of a 2D crystal array and extrapolated it for the growth of colloid multilayers:
k)
βLjeφ 0.605vd(1 - φ)
(1)
where k is the number of particle layers, β is the ratio between the velocity of a particle in solution and the fluid velocity (and is taken to be one), L is the meniscus length, je is the solvent evaporation rate, φ is the particle volume fraction in the suspension, v is the array growth rate determined by the withdrawing speed, and d is the particle diameter. However, eq 1 has to be considered only as a first approximation for the growth of multilayers by withdrawing the substrate from a suspension since deviations from the predicted behavior have been observed (see below and ref 9). A closer inspection of Table 1 reveals several interesting features. For broad stripes (100 and 400 µm), the number of layers increases linearly with concentration (which is directly proportional to the particle volume fraction, φ) for all withdrawing speeds (Figure 3a), as can be expected from eq 1. When the number of layers is plotted against the inverse of the withdrawing speed, the evolution is linear only for low concentrations (Figure 3b), i.e., the (25) Brinkmann, M.; Lipowsky, R. J. Appl. Phys. 2002, 92, 4296. (26) Schneemilch, M.; Quirke, N. Mol. Simul. 2003, 29, 685. (27) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303.
Figure 3. Dependence of the film thickness on the suspension concentration and withdrawing speed (here for a stripe width of 100 µm). (a) Linear relationship between film thickness and concentration for different withdrawing speeds. The lines are the results of fits using a linear regression function. (b) Relationship between film thickness and the inverse of the withdrawing speed. In this case, the lines connecting the points are just guides for the eye.
thickness increases with decreasing withdrawing speed. This behavior has also been observed for colloids deposited by the same technique on nonpatterned substrates and has been linked to a change of the water evaporation rate at the drying zone in dependence of the withdrawing speed.9 From Table 1 we see also that the slope for the dependence of the particle layer thickness with respect to the suspension concentration is steeper for 400 µm stripes than for 100 µm at withdrawing speeds e 0.2 µm s-1. According to eq 1, a steeper slope for broad stripes implies a larger meniscus length, L, and/or a higher evaporation rate je, assuming the other parameters are constant. A larger meniscus length on wider stripes is most probably responsible for the steeper slope since it is unlikely that the evaporation rate is significantly influenced by variation of the stripe width or small changes of the suspension concentration. Going further into detail, it becomes apparent that the number of layers on stripes of 100 and 400 µm width is roughly equal at low concentration for all withdrawing speeds and is equal at high withdrawing speed for all concentrations but differs for the other deposition conditions (in particular, at high concentration and low withdrawing speed). Since the meniscus length is smaller for 100 µm stripes than for 400 µm, a smaller number of layers was expected for all deposition conditions but this only occurs at high concentration and low withdrawing speed. The origin of this deviation is unclear at the moment but could be linked to a different influence of the meniscus size and geometry (which vary with the
Colloidal Crystals on Chemically Patterned Surfaces Table 2. Film Thickness (in number of layers) for Different Deposition Conditions as Measured by SEM on Cross Section of Cleaved Samples from Non-patterned Substratesa
withdrawing speed (µm s-1) 0.4 0.1 a
no. of deposited layers at varying suspension concentrations 0.5% 2.0% 4 14
17 135
The error on the number of layers is about 10%.
Scheme 3. Model of the Influence of the Surface Patterning on the Local Effective Colloid Concentration at the Drying Zonea
(A) Non-patterned substrate, (B) patterned substrate with lateral particle flux.
stripe width) on the dependence between the deposition conditions and the colloid layer thickness. Comparison with Nonpatterned Substrates. It is also interesting to compare the number of layers obtained on 400 µm stripes of a patterned sample and on nonpatterned surfaces. This stripe is indeed broad enough to induce only small perturbations in the meniscus geometry, which can thus be considered equivalent to that of the nonpatterned surface. Table 2 gives such numbers for different deposition conditions. The number of layers on a nonpatterned sample is equal to the one on a 400 µm stripe only for high particle concentration at low withdrawing speed. For all other conditions, the thickness is always larger, roughly doubled, for the 400 µm stripe pattern. This could be explained by a higher local effective concentration of the colloids for patterned samples due to a lateral particle flux. For nonpatterned surfaces, the colloids are evenly distributed over the whole air-watersubstrate contact line and will be deposited everywhere since the substrate is entirely hydrophilic. For hydrophilic-hydrophobic patterned surfaces, only the colloidal particles that are over the hydrophilic stripes can be deposited, inducing a concentration variation along the contact line. Due to this discontinuity in particle concentration, a lateral particle flux from the hydrophobic to the hydrophilic areas can occur in addition to the general flux of colloids from the suspension bulk to the drying zone, leading to a higher effective local particle concentration over the hydrophilic stripes (see Scheme 3). At high concentration and low withdrawing speed, the particle flux from the supsension bulk is apparently sufficient to reach equilibrium and match the array growth rate. The effect of the lateral particle flux is thus screened, and the same number of colloid layers for patterned and nonpatterned surfaces is obtained. Using a higher withdrawing speed and/or a lower particle concentration generally results in a reduction of the number of transferred layers and a notable influence of the effective particle concentration on the patterned surface, which leads to about twice the crystal thickness compared with the nonpatterned substrate.
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Narrow stripes with a width below 100 µm (10, 25, and 50 µm) respond differently to the deposition conditions than broad ones. At a stripe width of 50 µm, the number of particle layers is roughly constant independent of the various deposition conditions. When the stripe width is further reduced to 25 µm, the number of layers is independent of the concentration but it increases with higher withdrawing speed. This intriguing behavior is in contrast to broad stripes and nonpatterned surfaces (where the thickness decreases with the withdrawing speed) and hints to a specific change in the deposition mechanism at narrower stripe widths. Probably, the meniscus length depends on the withdrawing speed, which becomes most relevant at this particular stripe width. Yet, at the lower limit of 10 µm width for the hydrophilic stripes, the particle film thickness is almost constantly 1-2 layers, whatever the conditions used. This could be due to the very small length of the meniscus on such a stripe (see above), enabling the deposition of only one or two layers and thus not being influenced by the diffusion of the particles to the interface. All these results show that, for narrow stripes (below 100 µm), the dependence of the number of layers on the experimental parameters is not as simple as for nonpatterned samples and that eq 1 is not valid in this regime. Additional parameters need to be taken into account, but on the basis of the available results, it is difficult to be more precise beyond speculation. However, the complex shape and size of the meniscus on such narrow stripes and their influence on the evaporation rate of water at the drying zone certainly play an important role. Deconvolution of Processes Important for Narrow-Stripe Deposition. To gain more insight into this complex crystallization behavior with varying stripe width, it might be helpful to reconsider the mechanism of colloid crystallization in the drying zone. As Nagayama et al. have already discussed in their seminal paper,28 colloid crystallization is initiated in a particle layer on a substrate by the strong attractive capillary forces mediated by a bridging meniscus (Scheme 1D), which tends to minimize its free surface area due to the surface tension of the liquid. Such a meniscus is formed between the particles at the drying front when the liquid film reaches a thickness equal to the particle diameter. This effect drives particle aggregation in the top layer and creates a mesoporous structure with a high surface area. The large surface area facilitates solvent evaporation and induces as a secondary process a solvent flux from the suspension bulk through the growing crystal front to the drying particle layer.29 Particles from the suspension bulk are dragged to the growing crystal front by this solvent flux and settle epitaxially there. The first process of capillary-force-driven particle aggregation requires simultaneous contact of the particles with the suspending liquid and air for the meniscus formation, so it is only active in the top layer. Since highly ordered colloidal crystals composed of up to 150 layers are instantaneously generated by the vertical deposition method (at high particle concentration and slow withdrawing speed), the second process of liquid and particle flux must prevail in such cases. A more detailed view of this process is outlined in Scheme 4, which we refer to as the “funnel and filter” mechanism. At the drying zone of the colloidal crystal film, the concave suspension meniscus makes contact with the crystalline particle (28) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (29) (a) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (b) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756.
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Scheme 4. Schematic Presentation of the “Funnel and Filter” Process Leading to the Instantaneous Formation of a Thick Colloidal Crystal Film by a Liquid Flux from the Suspension Bulk to the Drying Crystal
multilayer at the air-liquid-colloidal crystal contact line. The liquid flows from the suspension bulk through the mesoporous crystal front into the drying crystal layer with large surface area, where it efficiently evaporates. By this, liquid flux particles are dragged from the suspension bulk to the crystal front, which acts as a filter for the suspended particles. Due to its concave shape, the suspension meniscus resembles a funnel in which a given number of particles flow from further in the bulk through a large cross section (A0 in Scheme 4) and pass a much smaller cross section (A1) close to the crystal front. By this funnel effect, the particles essentially run through a strong concentration gradient, which likely induces a preordering (or precrystallization) process at higher concentration close to the crystal front (as it is known for increased particle concentrations in sedimentation and centrifugation experiments). The preordered particles are then pushed by the solvent drag against the crystal front where they are retained and crystallize epitaxially to further extend the colloidal crystal. The solvent evaporates in the crystal film and, at a later stage, forms capillary necks between the particles. These capillary necks exert very strong attractions between the particles and may be responsible for the crack formation after complete drying and desolvation (shrinkage) of the particles. Both mechanisms, capillary-force-driven crystallization and the funnel and filter mechanism, contribute to the overall colloidal crystal formation, yet with different kinetics. The balance between both contributions depends on the ratio of the top particle layer to the total crystal thickness, and the crystal thickness in turn depends on the meniscus length in the drying zone. Since the meniscus length is influenced by the width of a hydrophilic substrate pattern, it will be low on average for narrow patterns (width < 50 µm), leading to thin crystal layers and the prevalence of the capillary force mechanism. For wider patterns (>50 µm) the average meniscus length will be large and particle multilayer formation will be favored, mainly driven by the funnel and filter mechanism. At intermediate pattern width (∼50 µm), both mechanisms may equally contribute to the crystal formation and the different crystallization kinetics will combine to result in a parameter-independent crystallization behavior. Further insight might be gained by modeling the meniscus structure with, e.g., finite element analysis. Crystal Quality for Different Stripe Widths. Another important issue is related to the evolution of the crystal quality with respect to the stripe width. Figure 4
Figure 4. LVSEM images at high magnification showing vertical stripes of colloidal crystals of different width (a, 100 µm; b, 50 µm; c, 25 µm; d, 10 µm) grown on a FAS/SiOH pattern. The arrows indicate the withdrawing direction from the latex suspension. A packing of (at least) the top layer with square symmetry is particularly prominent for the 50 µm (b) and 25 µm (c) stripes.
shows several close-ups obtained on stripes of varying width. It is of course difficult to judge the bulk crystal quality on the basis of SEM images since only the top colloidal layer is visible. Nevertheless, inspection of the top layer can yield useful information about the packing symmetry. Typically, the quality of the 400 µm stripe is very similar to nonpatterned samples, but it decreases progressively with declining stripe width. The higher defect density (cracks, domain boundaries, dislocations, etc.) in narrow stripes is probably linked to the presence of more constraints due to the forced confinement of the meniscus by the hydrophobic-hydrophilic pattern. In particular, inspection of all our SEM data reveals that the overall area with square (tetragonal) lattice symmetry is larger for stripe widths below 100 µm (Figure 4b and c). Such a square packing symmetry has also been observed on nonpatterned samples and its occurrence increases when the mobility of the particles decreases, for example, at low temperature or with viscous suspensions.24,30 This lattice orientation (corresponding to the (100) plane of the fcc lattice oriented parallel to the surface) is energetically slightly less favorable, but the energy difference with a (111) fcc lattice plane orientation parallel to the surface is very small. Here again, the larger constraints in the wetting film on narrow stripes could be responsible for the more frequent occurrence of such defects. A complementary way of probing the crystal quality is to record UV-vis spectra. The relative stop bandwidth, ∆λ/λ0, where ∆λ is the half-width at half-maximum and λ0 is the wavelength at the center of the peak, is indeed inversely proportional to the quality of the packing. Thus, with this technique, we have access to information about the bulk crystal quality. A UV-vis spectrometer coupled to a microscope has been used to record transmission spectra in an area of about 50 × 50 µm2 positioned in the center of the stripes. Figure 5 presents UV-vis spectra recorded on stripes of different width (50, 100, and 400 µm) for samples prepared with a concentration of 1.0% (w/w) and a withdrawing speed of 0.2 µm s-1. For comparison, a spectrum recorded on a nonpatterned sample is also shown. The ∆λ/λ0 values extracted from the different (30) Cong, H.; Cao, W. Langmuir 2003, 19, 8177.
Colloidal Crystals on Chemically Patterned Surfaces
Figure 5. Normal incidence transmission spectra recorded on stripes of different width and on a nonpatterned sample. The spectra have been scaled to the same intensity and vertically offset for clarity. The stop band quality is expressed as ∆λ/λ0 for the half width at half-maximum, ∆λ, and maximum wavelength, λ0.
spectra confirm the SEM observations. For a stripe width of 400 µm and for the nonpatterned sample, the stop band is narrow (5.6%), in good agreement with theoretical predictions31 and results obtained by other groups on nonpatterned surfaces.8,9,22,32,33 The ∆λ/λ0 value increases a bit (6.6%) for a 100 µm stripe and more markedly for 50 µm (7.9%), signaling a decrease of the crystal quality. In the latter case, the broad stop bandwidth could be influenced by the size of the probing window, which is equal to the stripe width (50 µm). The stripe edges, characterized by a rather high defect density, are thus also contributing to the spectrum, which is not the case for broader lines. In comparison, the stop bandwidth of a spectrum (not shown) recorded on the very edge of a broad stripe is 9.6%. No stop band has been observed on the spectra recorded on 25 µm stripes. For comparison, Zentel’s group measured the stop bandwidth of colloidal crystals grown in the confined space of narrow rectangular microchannels and obtained 8.5%.22 Influence of Withdrawing Direction. As an additional important process parameter, the influence of the orientation of the hydrophobic and hydrophilic lines with respect to the withdrawing direction on the colloid crystallization process was studied. The pattern orientation relative to the water surface determines the relative width of a hydrophilic region that is covered by the drying zone during particle crystallization. The same FAS/SiOH patterns as discussed before have been used, but during these particle deposition experiments, the line axes were oriented perpendicular to the withdrawing direction (parallel to the water surface). Colloids have been deposited on those horizontal lines using different suspension concentrations and withdrawing speeds. Figure 6 shows SEM images taken on a sample prepared with a concentration of 1.5% and a withdrawing speed of 0.4 µm s-1. As opposed to the parallel orientation, most of the stripes are only partially covered by particles. The colloids are deposited in an ordered fashion only in the upper part of the stripes, the rest being covered by a submonolayer. Such inhomogeneous features could be explained by a spontaneous slippage of the meniscus from one hydrophilic line during substrate withdrawal, which is induced by the destabilization of the meniscus by the hydrophobic spacer situated just below it, followed by re-pinning on (31) Tarhan, I. I.; Watson, G. H. Phys. Rev. B 1996, 54, 7593.
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Figure 6. LVSEM images showing horizontal stripes of colloidal crystals of different width grown on a FAS/SiOH pattern at perpendicular line orientation with respect to the withdrawing direction (which is indicated by the arrows). (a) Overview with several lines partially filled by colloidal particles in the top region, (b) particle submonolayer in thin lines (2, 4, and 10 µm), (c) close-up into one line at the border between the filled and unfilled region, and (d) close-up into the top region of one line showing the stepwise increase in the number of layers.
the next hydrophilic line. When the water film is pinned on a hydrophilic stripe, colloids start to be deposited and the number of layers increases in a stepwise fashion (see close-up in Figure 6d), as discussed before for vertical lines. As the substrate is further withdrawn, the pinning region is becoming smaller and the influence of the strongly dewetting hydrophobic regions increases. At some point, the hydrophobic effect is too strong and the water film ruptures and dewets. The water film wets the substrate again when the next hydrophilic line emerges from the suspension and repetition of the whole process is accompanied by a periodic flip of the contact angle of the suspension at the substrate surface. A similar phenomenon, involving the meniscus slippage followed by its repinning, is the origin of the thickness variation sometimes observed on samples prepared by vertical deposition.34 Varying the suspension concentration and withdrawing speed did not improve the filling of the lines. The only stripes which are completely covered (but only by a submonolayer) are the very narrow ones: 10 µm and even 4 and 2 µm (Figure 6b). In contrast, stripes as narrow as the latter were not covered on samples oriented parallel to the withdrawing direction. This could be rationalized by the difference in the meniscus contact line in the two orientations. When the 4 and 2 µm stripes are oriented vertically, the pinning (wetting) region is too narrow for the water film to wet it efficiently. The dewetting forces originating from the fluoroalkyl regions on each side are dominating, and the whole region behaves as if entirely hydrophobic, with no possibility to deposit colloids. When the 4 and 2 µm stripes are oriented horizontally, the water film can wet it much more easily since the contact line runs across the whole width of the sample (∼1.2 cm), allowing colloid deposition at the whole drying zone. (32) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300. (33) Miguez, H.; Lopez-Meseguer, F. C.; Blanco, A.; Vazquez, L.; Mayoral, R.; Ocana, M.; Fornes, V.; Mifsud, A. Appl. Phys. Lett. 1997, 71, 1148. (34) Im, S. H.; Kim, M. H.; Park, O. O. Chem. Mater. 2003, 15, 1797.
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Figure 7. LVSEM image showing vertical stripes of colloidal crystals of different width grown on a HMDS/SiOH pattern (lower wetting contrast and overall hydrophobicity) down to a stripe width of 4 µm. The arrow indicates the withdrawing direction from the latex suspension.
Influence of Hydrophobic-Hydrophilic Contrast. In a further experiment, the wetting contrast between hydrophobic and hydrophilic regions has been varied to understand the role of the hydrophobic effects in the selectivity and resolution of the colloid deposition. While all previously described experiments were conducted with FAS/SiOH patterns presenting a very high wetting contrast (static water contact angle on FAS ) 110°, static water contact angle on SiOH < 5°), here, two additional silane layer systems have been studied. The first one carried a pattern of vertical lines with trimethylsilyl groups made from hexamethyldisilazane, HMDS (static water contact angle ) 77°) for the hydrophobic region on the bare SiOH surface for the hydrophilic stripes. The second system was composed of vertical lines made from FAS for the hydrophobic stripes and from (aminopropyl)triethoxysilane, APTES (static water contact angle ) 45°) with primary amino groups on the “hydrophilic” regions. In the case of HMDS/SiOH patterns, the selectivity of the colloid deposition is as good as on FAS/SiOH patterns but its resolution is better since particles are deposited down to a stripe width of 4 µm (Figure 7) instead of only 10 µm for FAS/SiOH patterns. With a lower wetting contrast between the stripes, it is apparently easier for the meniscus to follow the changes of hydrophilicity on a short scale. The dewetting forces are weaker, and the water film is thus able to efficiently wet the 4 µm stripes. The selectivity of the colloid deposition on FAS/NH2 patterns is still good, but its resolution is substantially lower than on FAS/SiOH or HMDS/SiOH patterns since the stripes are only covered down to a width of 50 µm. Despite the lower wetting contrast on FAS/NH2 patterns compared to FAS/SiOH, the average hydrophobicity of the sample is higher and the FAS regions still dewet strongly. The balance between these two effects results in a lower resolution for the colloid deposition process on FAS/NH2 patterns. Deposition on Discontinuous Patterns. In all preceding experiments, the stripe patterns presented a one-dimensional template modulation for the wetting meniscus either perpendicular or parallel to the substrate withdrawing direction. In the following section, we report on experiments performed on discontinuous patterns to study colloid crystallization under the influence of a twodimensional meniscus confinement by simultaneous wetting contrast modulation parallel and perpendicular to the withdrawing direction. Such discontinuous patterns consisted of arrays of hydrophilic circles (SiOH) surrounded by a hydrophobic matrix (FAS). Colloidal particles were attempted to be deposited on circular regions of 50, 25, and 10 µm in diameter using different deposition
Figure 8. LVSEM images showing circular arrays of colloidal crystals with a diameter of 50 µm grown on a FAS/SiOH pattern with (a) a 0.5% w/w suspension and a withdrawing speed of 0.4 µm s-1 and (b) a 1.5% w/w suspension and a withdrawing speed of 0.1 µm s-1. (c) Close-up on one of the circular array of (b). The arrows indicate the withdrawing direction from the latex suspension.
conditions. The filling of the circular patches of 50 µm increased progressively as the suspension concentration was increased and the withdrawing speed was reduced, ranging from a partial coverage to a thickness of four layers, as shown in Figure 8a and b. On completely covered circles, the thickness increased in a stepwise fashion from the edges of the circles to their center (Figure 8c), as already discussed for straight lines. On smaller circles (25 µm), a small patch of an ordered colloid monolayer was always deposited in the center, whatever the experimental parameters used (an example is given in Figure 9a). This behavior is different from the 50 µm circles, which get filled from the upper edge (Figures 8a and 9b). On 10 µm circles, no particles were deposited, even at high colloid concentration and low withdrawing speed. This once again shows that a minimal hydrophilic area is needed to allow efficient wetting by the suspension. For 25 µm circles, a wetting meniscus might be formed only when about half a circle protrudes from the water surface, i.e., when the hydrophilic area is large enough to allow the suspension
Colloidal Crystals on Chemically Patterned Surfaces
Figure 9. (a) LVSEM image showing circular arrays of small colloidal crystal monolayers assembled in the center of hydrophilic patches with a diameter of 25 µm (FAS/SiOH pattern). (b) Zoom of the 50 µm circles from Figure 8a, which are partially filled from the upper rim. The arrow indicates the withdrawing direction from the latex suspension.
to sufficiently wet it. For 10 µm circles, the hydrophilic area is always too small to allow the formation of an appropriate meniscus and no colloids can be deposited. Conclusions Colloidal particles have been selectively crystallized by vertical deposition on the hydrophilic regions of chemically patterned substrates with hydrophobic and hydrophilic lines of different width and orientation. The resulting microstructured colloidal crystals are characterized by (1) the number of deposited colloid layers, (2) the minimum feature size of the crystalline structures (as a consequence of the resolution of the deposition process), and (3) the crystal quality (given by the packing symmetry, domain sizes, defects/cracks, etc.). These factors are determined on one hand by the intrinsic parameters built into a given system, like the type of latex, type of substrate surface (functional groups and polarity), and the feature size and geometry of the silane patterns. On the other hand, the colloidal crystal quality depends on process parameters that can be controlled during the deposition experiment, like the colloid concentration in the suspension, the substrate withdrawing speed, and the overall orientation
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of the silane pattern with respect to the water surface (specifying the effective width of a hydrophilic region). Detailed knowledge about the contribution of each parameter in the crystallization process and their mutual influence is of fundamental importance for the rational design of microstructured colloidal crystals by vertical deposition, which was investigated in our experiments. In contrast to nonpatterned substrates, a nonlinear dependence of the colloid concentration and substrate withdrawal speed was identified for line patterns with dimensions below about 100 µm (line width for a perpendicular orientation of the lines with respect to the water surface). With decreasing line width, the number of deposited layers and the crystal quality generally decrease down to a lower limit of about 10 µm for hydrophilic lines and 4 µm for hydrophobic stripes separating the hydrophilic regions. The deposition is also affected by the wetting contrast between the hydrophilic and hydrophobic regions of the substrate. A lower wetting contrast and average hydrophobicity improve the resolution, while a lower wetting contrast and higher average hydrophobicity is detrimental. The overall quality of the structured colloid crystals is sufficient to show an optical stop band down to a hydrophilic stripe width of 50 µm, which is appropriate for, e.g., sensor applications in which changes in the stop band can be conveniently monitored online by spectroscopic methods. The particular deposition behavior in dependence of the various parameters (pattern dimensions, withdrawal speed, etc.) is attributed to the specific meniscus shape and size at the drying zone. This specific meniscus geometry seems to have a direct influence on at least three individual processes which operate simultaneously during colloid crystallization: (1) the capillary-force-driven assembly of the top colloid layer, (2) the funnel and filter mechanism leading to colloid multilayer crystals, and (3) a local concentration effect due to the selective deposition on the stripe patterns. The contribution of each process with its specific kinetics varies with the stripe width and apparently reaches a balance for the particular pattern size of 50 µm, which renders colloid crystallization insensitive to the process parameter (like withdrawal speed and colloid concentration). The balancing of the different contributions might be exploited in a robust fabrication process for colloidal crystal devices in this specific size regime which will be insensitive to random fluctuations of the process parameters. To gain further insight into the mechanism of the deposition technique on patterned substrates, an exact or in situ determination of the microscopic meniscus shape, and possibly the direct observation of the colloid organization process in the drying region, would be the next step. Modeling of the meniscus shape (e.g., by finite element models) and of the corresponding colloid assembly process (by mesoscopic dynamics modeling based on continuum/mean field theories and classical hydrodynamics) could further complement such studies. Acknowledgment. This work has been supported by the European Community, Marie Curie Fellowship Nο. MCFI-2002-00179 (C.A.F.). The authors would like to thank Prof. W. Hofmeister and Dr. Tobias Ha¨ger for their help with the UV-vis microscopy. LA0489413
