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Mechano-Optical Octave-Tunable Elastic Colloidal Crystals Made from Core-Shell Polymer Beads with Self-Assembly Techniques Wendel Wohlleben, Frank W. Bartels, Stephan Altmann, and Reinhold J. Leyrer* BASF Aktiengesellschaft, Polymer Research, 67056 Ludwigshafen, Germany ReceiVed September 5, 2006. In Final Form: December 6, 2006 Elastic colloidal crystals, even without a full photonic band gap, hold promise for fascinating applications and for easy large-scale fabrication by self-assembly. However, high mechanical robustness is required for optical, decorative, or security applications, such as tunable optical modulators/filters or optical tension indicators. Here, we present brilliantly colored filled-pore colloidal crystals that withstand elongation by 100%, i.e., one optical octave. We employ a variety of vertical deposition techniques to self-assemble monodisperse core-shell polymer beads with a filmforming shell and flexible core. We find a good theoretical description of crystal thickness for all techniques. The crystals have centimeter-sized macroscopic order, and their orientation is fully controlled by the substrate plane and meniscus line.
Introduction Colloidal crystals fascinate both for their fabrication process by self-assembly of nano-to-microparticles and for their applications in optics, in compact sensors, or in photonic devices.1-3 One important property for outdoor and industrial applications is robustness with regard to mechanical forces and environmental conditions. A recent self-assembly approach addressed mechanical toughness by core-shell polymer dispersions.4 In that approach, an elastomeric shell enhanced adhesion between neighboring polymer spheres, such that the entire structure withstands nanometer to micrometer deformations due to mechanical impact. A more demanding desideratum, e.g., for switching of optical filters, is true elasticity of a colloidal crystal material, allowing stretching of the periodicity and hence the optical band structure. Recently, porous elastic photonic crystals, synthesized by a multistep process with template inversion, have demonstrated reversible mechano-optical switching across ∆λ ) 50 nm in wavelength.5 Shifts of ∆λ ≈ 100 nm have been reported for hydrogel stabilized crystals, requiring however good encapsulation of the entire colloidal crystal device.6 Without external mechanical forces, the switching range is much lower, e.g., ∆λ ) 5 nm has been achieved with redox-tunable defects.7 To finally increase the robustness against changes in environmental conditions, filled-pore colloidal crystals are advantageous. Their contrast in the refractive indices of the periodic structure certainly is worse (lower) than for open-pore crystals,8 *
[email protected]. (1) Meseguer F. Colloids Surf., A 2005, 270, 1-7. (2) Lourtioz, J.-M.; Benisty, H.; Berger, V.; Ge´rard, J.-M.; Maystre, D.; Tchelnokov, A. Photonic Crystals - Towards Nanoscale Photonic DeVices; Springer: Berlin, 2006. (3) Busch, K., Lo¨lkes, S., Wehrspohn, R. B., Fo¨ll, H., Eds. Photonic Crystals - AdVances in Design, Fabrication, and Characterisation; Wiley-VCH: Weinheim, 2004. (4) Wang, J.; Wen, Y.; Ge, Z.; Zheng, Y.; Song, Y.; Jiang, L. Macromol. Chem. Phys. 2006, 207, 596-604. (5) Arsenault, A. C.; Clark, T. J.; von Freymann, G.; Cademartiri, L.; Sapienza, R.; Bertolotti, J.; Vekris, E.; Wong, S.; Kitaev, V.; Manners, I.; Wang, R. Z.; Sajeev, J.; Wiersma, D.; Ozin, G. A. Nat. Mater. 2006, 5, 179-184. (6) Lawrence, J. R.; Shim, G. H.; Jiang, P.; Han, M. G.; Ying, Y.; Foulger, S. AdV. Mater. 2005, 17, 2344-2349. (7) Fleischhaker, F.; Arsenault, A. C.; Wang, Z.; Kitaev, V.; Peiris, F. C.; von Freymann, G.; Manners, I.; Zentel, R.; Ozin, G. A. AdV. Mater. 2005, 17, 24552458. (8) Kalinina, O.; Kumachcheva, E. Macromolecules 1999, 32, 4122.
but it is unaffected by environment, as the contrast cannot be changed by immersion with liquids, vapors, and gases. In this contribution, we follow a simple strategy that promises colloidal crystals that feature all the above-mentioned properties (elastic, robust, filled-pore), namely, the self-assembly of coreshell particles made by two- or multistep emulsion polymerization.4,9-12 In order to obtain brilliant color, long-range order is required, meaning that the size distribution of the core-shell particle must be monodisperse with a few percent deviation only.13 Furthermore, for good film quality (i.e., nonturbid and crackfree), the shell material must have a low minimum film formation temperature (MFT), determined by glass transition temperature (Tg), elastic modulus (resistance to particle deformation), and viscosity. Shells of adjacent particles can then coalesce and interdiffuse after self-assembly. The final product is a crystalline arrangement of cores in a continuous matrix, being responsible for elasticity and robustness. Besides self-assembly of coreshell particles, alternative techniques for filled-pore colloidal crystals include the shear-induced ordering of core-shell particles in a melt process11,12 and in situ polymerization of monomers9a,14 or binders15 around the particles making up the crystal, requiring crystallization of colloid particles in a monomer-containing phase or posttreatment filling of interstitial voids with matrix-forming materials. For self-assembly, we focus on vertical deposition with its known variantssone can raise the substrate,16 and alternatively, (9) (a) Eilingsfeld, H. J.; Siemensmeyer, K.; Schuhmacher, P.; Rupaner, R. F.; Leyrer, R. J. DE19717879 A1, 1997; Chem. Abstr. 129 (25), 332185. (b) Rupaner, R.; Leyrer, R. J.; Schuhmacher, P. DE19820302 A1, 1998; Equivalent EP 955323 A1; Chem. Abstr. 131 (24), 323065. (c) Hamers, C.; Rupaner, R.; Leyrer, R. J.; Schuhmacher, P.; Yang, Z. DE19834194 A1, 1998; Chem. Abstr. 132 (11), 138857. (10) He, X.; Thomann, Y.; Leyrer, R. J.; Rieger, J. Polym. Bull. 2006, 57, 785-796. (11) Ruhl, T.; Spahn, P.; Hellmann, G. P. Polymer 2003, 44, 7625. (12) Ruhl, T.; Spahn, P.; Winkler, H.; Hellmann, G. P. Macromol. Chem. Phys. 2004, 205, 1385. (13) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132-2140. (14) Carlson, R. J.; Asher, S. A. Appl. Spectrosc. 1984, 38, 297. Asher, S. A. U.S. Patents 4627,689 and 4,632,517, 1986. Pan, G.; Kesavamoothy, R.; Asher, S. A. Phys. ReV. Lett. 1997, 78, 3860. Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. Foulger, S. H.; Jiang, P.; Lattam, A.; Smith, D. W.; Ballato, J.; Dausch, D. E.; Grego, S.; Stoner, B. R. Langmuir 2001, 17, 6023. (15) Yoshihisa, K.; Kyoshi, S. Jpn. Kokai Tokkyo Koho JP 08,231,900 [96,231,900]; Chem. Abstr. 1996, 125, 303402y. (16) Gu, Z.-Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760-765.
10.1021/la062602n CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
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one lowers the meniscus by evaporation13 or by removal of suspension.17 Vertical deposition is the workhorse method for crystalline self-assembly of polymer colloids and can yield large crystal areas (monocrystal 200 µm × 200 µm by SEM, crackdisrupted) with 22 µm thickness17 and an axis orientation that is (2° uniform over macroscopic (4 mm) areas.13 Controllable absolute orientation has not been reported yet, and we provide evidence that even this is possible with vertical deposition. Besides vertical deposition, we implement some of the following alternative self-assembly techniques: Horizontal deposition produces polycrystalline ordering and proceeds rather slowly for polymer colloids.1 A large crystal area of approximately a square centimeter (monocrystal >70 µm × 70 µm by SEM) and thickness of ∼40 µm has been reported for flux-assisted crystallization with inclined substrate and oscillating meniscus.18 An equally large crystal area (monocrystal >60 µm × 40 µm by SEM, unspecified thickness) has been reported for horizontal deposition on liquid Ga, i.e., a substrate of vanishing friction and roughness.19 However, the surface tension of Ga prevented nondestructive pick-off of the crystals for further processing. Crystallization within two parallel plates produced crystals of area of approximately a square centimeter (monocrystal >30 µm × 30 µm by SEM) and thickness of ∼10 µm, but in its original implementation had some technical limitations for a large ratio of crystal height/bead diameter.20 Here, we report filled-pore colloidal crystals that are elastic and mechanically stable at 100% (∼cm) compression/elongation. Furthermore, we produce them with a minimal number of fabrication steps: semibatch synthesis plus self-assembly, followed automatically by film formation. We find that polymer design faces a tradeoff between elasticity and refractive index contrast and demonstrate a successful compromise. Further, we enlarge the monocrystalline area by comparing the majority of the above-mentioned self-assembly arrangements with identical monodisperse polymer material. The final product is a brillianteffect color that is tunable across the entire vis spectrum. Experimental Methods A. Synthesis of Core-Shell Dispersions. With Hard Core (recipe #1). The reactions were performed in a 2000 mL four-necked glass reactor equipped with a reflux condenser, temperature controller, nitrogen gas inlet, monomer and initiator inlets, and mechanical stirrer. Core-shell particles were synthesized using three-stage emulsion polymerization in the presence of a polystyrene seed. The reaction was carried out at 358 K and at a stirring rate of 150 rpm (with a horseshoe mixer). Prior to polymerization, the reaction mixture was purged with nitrogen, and a slight positive nitrogen pressure was maintained during the reaction. Stage 1. Synthesis of core particles: 258.6 g water, 5.1 g polystyrene seed dispersion, and 26 g sodium persulfate solution (3.6% NaPS) were precharged (initial feed) into the reactor and prereacted for 5 min. Then, 110 g sodium persulfate solution (3.6%) and monomer emulsion from 540.0 g styrene (S), 35.0 g n-butyl acrylate (n-BA), 13.3 g divinyl benzene (DVB), 10.0 g acrylic acid (AS), 1.7 g allyl methacrylate (AMA), 16.4 g dodecyl- and tetradecylethylene oxide (2.5 -EO-) sulfonic acid sodium salt (28% Texapon NSO), 27.3 g NaOH solution (25%), and 501 g water were fed dropwise over a period of 4 h. It was assumed that the reaction ran in a starve-fed mode so as to avoid secondary nucleation. After monomer feed was completed, the reaction was continued for an (17) Zhou, Z.; Zhao, X. S. Langmuir 2004, 20, 1524-1526. (18) Cademartiri, L.; Sutti, A.; Calestani, G.; Dionigi, C.; Nozar, P.; Migliori, A. Langmuir 2003, 19, 7944-7947. (19) Griesebock, B.; Egen, M.; Zentel, R. Chem. Mater. 2002, 14, 40234025. (20) Park, S. H.; Qin, D.; Xia, Y. AdV. Mater. 1998, 10, 1028-1032.
Wohlleben et al. additional hour, and then the system was cooled down to room temperature. The core-particle dispersion with a solid content of 40% was obtained. Stages 2 (interlayer) and 3 (shell) synthesis of core-shell particles: In the reactor, 325.8 g core-particle dispersion from the previous stage and 5 g sodium persulfate solution (1.6%) were precharged and prereacted for 5 min. Then, 45 g sodium persulfate solution (1.6%) was fed over a period of 3 h. During the same time, the monomer emulsion made from 11.7 g S, 9.1 g n-BA, 1.5 g AS, 1.2 g AMA, 1.9 g dodecyl- and tetradecylethylene oxide (2.5 -EO-) sulfonic acid sodium salt (28% Texapon NSO), 1.5 g NaOH solution (25%), and 55.0 g water was fed during 45 min and postreacted for 15 min. Then, another monomer emulsion made from 71.5 g n-BA, 34.6 g methyl methacrylate (MMA), 1.3 g AS, 1.2 g dodecyl- and tetradecyl-ethylene oxide (2.5 -EO-) sulfonic acid sodium salt (28% Texapon NSO), 0.8 g NaOH solution (25%), and 53.5 g water were fed over a period of 2 h. Following the monomer feed, the reaction was continued for an additional hour followed by cooling down to room temperature. The core-shell particle dispersion with a solid content of 40% was obtained (recipe #1). With Flexible Core (recipe #2). For the synthesis of the core particle analog to recipe #1, the continuous feed consists of 24% (parts per hundred monovalent monomer) styrene, 24% n-butyl acrylate, 2% diallyl phthalate (DAP), 0.35% Texapon NSO, 2% acrylic acid, initiated by 0.25% NaPS at 75 °C. In the second continuous feed, the shell (without an interlayer) is produced using 39% n-butyl acrylate, 9% methyl methacrylate, 0.5% diallyl phthalate, 2% acrylic acid, and 0.35% Texapon NSO initiated by 0.25% NaPS at 75 °C. The product is a 50/50 core/shell polymer dispersion at a solid content of 50.4% (recipe #2). This product is used for crystallization without any further purification such as filtering or centrifugation. B. AUC and HDF to Characterize Colloids. Latex size distributions were characterized in dispersion with hydrodynamic fractionation (HDF) and analytical ultracentrifugation (AUC). Both are fractionating, nonimaging, integrating techniques such thatsin contrast to light scatteringsa distribution of sizes is determined with high resolution and high statistics. We use the polydispersity index PI ) (D90 - D10)/D50 because it is more sensitive to the detrimental coarse fraction than the usual standard deviation of particle size. On non-film-forming particles (polystyrene), AUC and HDF diameters were compared with diameters from AFM (atomic force microscopy) images after drying. Systematically, the AUC (sedimentation) diameter is 3% below and the HDF (hydrodynamic size exclusion) diameter 5% above the AFM (hard core) diameter. This is the expected ordering for hard-core particles with some adsorbed surfactants. C. Various Self-Assembly Techniques. Microscope slides are pretreated in H2O2 /H2SO4 ) 30:70 overnight, making them superhydrophilic. Alternatively, for future lamination, the substrate was corona-treated PE foil. Our standard self-assembly technique was deposition from a 0.5-10 wt % diluted dispersion onto a vertical ((5°) substrate by evaporation.13 The evaporation took place in a temperature-controlled oven at 23 ( 0.5 °C, at uncontrolled humidity. The measured evaporation rate je was 15 ( 5 nm/s ) 0.054 mm/h. We compared these samples to those prepared by a lifting stage with substrate-meniscus speed Vm ) 0.4-1.2 mm/h from a typical 2 wt % dispersion.16 The lifting stage was especially constructed for smooth movement and held five substrates from individual dispersions each. Substrates were lifted from an open 100 mL glass or through a slit in the cover of the glass, in order to exclude evaporation, with the latter resulting in no deposition. A similar meniscus speed was realized with an alternative approach where 0.1 mL/min of dispersion, corresponding to Vm ) 3.9 mm/h, was removed by a pump.17 Again, a freely ventilating and a closedhead room were tested, again resulting in no deposition without evaporation. Finally, we reconstructed the agitated surface or “flux-assisted” deposition with identical geometrical arrangement as in the original publication.18 The air flow was varied from 400 to 1500 L/h, and
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Figure 1. Successful crystallization of core-shell particles with vertical deposition. Colloidal crystal from core-shell polymer dispersion (recipe #2), prepared by vertical deposition without mechanical movement from a 2% dispersion. (A) Color flop photographs, scale bar is 1 cm. (B) AFM. Bottom right: 5 µm × 5 µm scan. Middle: FFT of 50 µm × 50 µm AFM trace (top right). The cores form a monocrystal with 〈111〉 plane hexagonal symmetry in a matrix of coalesced shells. A very minor spot from a 〈100〉 oriented layer is visible from the weak rectangular feature in the FFT. The monolayer edges appear in AFM (triangles), in its FFT (triangular lines), and in the photograph (60° multilayer edge), indicating a 2 cm × 2 cm sized monocrystal. 1-2 mm altitude surface waves were visible, as originally described. All deposition experiments with lifting, pumping, and air flow were performed at 23 ( 2 °C. D. AFM, Color Flop, and Mechanics to Characterize Colloidal Crystals. Finished samples were analyzed first by photographs with angle-of-view ∼50° from normal and with white-light illumination at angles of 75° and -45° from normal. The apparent color flop is typical for effect colors and is an indication of ordered layers due to Bragg scattering. We use this data for qualitative estimates of surface quality and of macroscopic ordering. Atomic force microscopy (AFM, Asylum MFP3D) in tapping mode is used to probe the mesocrystalline structures. AFM is sensitive to the soft/hard structuring of the filled-pore colloidal crystal, and in comparison to SEM, it requires less (no) preparation, which was suspected to cause artificial cracks.13 Prior to mechano-optical measurements, 50 µm thick colloidal crystal films were laminated on a ∼200 µm thick black carrier that was cast from an acrylic emulsion polymer (Acronal 360D) containing 2 wt % pigment (Basacid schwarz). Lamination has the positive side effect that the first deposited layer is now the visible top layer, enhancing brilliance, because due to growth on the perfectly flat substrate, the sample now shows an evenly perfect surface. Mechanical properties were measured in a tensile testing machine (Zwick) by extending a 5 mm wide stripe of sample from 3 cm to 9 cm length (200%), while monitoring the acting force. The force divided by the sample cross section gives the mechanical strain in megapascals. The typical stretching speed was 2%/min, observing both extension and recovery, with the slope of the force-strain curve (giving the elastic modulus Y′) determined at both small extensions of a few percent and at large extensions of 50-200%.
Results and Discussion A. Core-Shell Particles (recipe #2) Crystallize with Vertical Deposition. The core-shell dispersion described above (recipe #2) achieves a diameter of D ) 325 nm and PI ) 0.08 (standard deviation ) 2.6%) as measured in the AUC. The dispersion from recipe #1 has 336 nm diameter and is thus a good comparison. In recipe #2, the refractive index contrast between core (n ) 1.530) and shell (n ) 1.465) is compromised by restraints for elasticity, as discussed below in paragraph D. The core is styrene/ butyl acrylate based with a glass transition temperature Tg of 17 °C, a little below room temperature, and the shell remains mainly butyl acrylate based with a very low glass transition temperature of -21 °C, far below room temperature. Hence, the MFT is also sufficiently low for good film formation after crystallization. We used the emulsion polymer dispersion without further purification, i.e., there was no need for filtration or centrifugation. An aqueous dispersion of 2% solid content was then held in a 100 mL beaker, and a pretreated microscope cover slide was immersed. After a few days, AFM images of the surface were taken, and the color flop was photographed (Figure 1). Clearly, the entire scanned range of 50 µm × 50 µm is monocrystalline with the fcc 〈111〉 plane parallel to the surface. The FFT (fast Fourier transform) confirms that there are practically no impurity peaks from 〈110〉 or 〈100〉 oriented layers and no disordered regions that would appear as circles around the origin (Figure 1b). The essential element of vertical deposition is the meniscus and its movement relative to the substrate. The meniscus of
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Figure 2. Faster preparation with lifting stage vertical deposition. Sample prepared by drawing the substrate with 0.38 mm/h from a 2% dispersion (recipe #2). (A) Color flop photographs, scale bar is 1 cm. (B) AFM. Bottom right: 5 µm × 5 µm scan. Middle: FFT of 50 µm × 50 µm AFM trace (top right). The hexagonal spots in the FFT are less sharp than in Figure 1; hence, crystal orientation is homogeneous only within a few degrees. In vertical direction, the surface is modulated in thickness.
height l is the fuzzy border between the diluted dispersion and the close-packed crystal.21-23 The evaporation rate je in the meniscus region attracts solvent convection, and particles are driven upward to the crystallization zone. By equaling the evaporation induced particle flux with the crystal growth rate, Dimitrov and Nagayama derive an expected crystal thickness h21,24
h)
βljeφ V(1 - )(1 - φ)
(1)
In practice, the meniscus speed V is the added effect of mechanical movement Vm and evaporation je: V ) Vm + je.17 Furthermore, crystals are grown from dilute dispersions with concentration φ , 1, and close-packing structure has a volume filling of 1 - ) 0.74. Hence, we can simplify eq 1 to
h)
( )
lφ Vm +1 0.74 je
-1
(2)
For pure evaporation (Vm ) 0), the thickness should be independent of evaporation speed je and only proportional to concentration and meniscus height, but independent of latex diameter.21 With additional Vm, the thickness should decrease (21) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311. (22) Salamanca, J. M.; Ciampi, E.; Faux, D. A.; Glover, P. M.; McDonald, P. J.; Routh, A. F.; Peters, A. C. I. A.; Satguru, R.; Keddie, J. L. Langmuir 2001, 17, 3202-3207. (23) Gorce, J.-P.; Bovey, D.; McDonald, P. J.; Palasz, P.; Taylor, D.; Keddie, J. L. Eur. Phys. J. E 2002, 8, 421-429. (24) Here, β is the relative speed of particles and their local fluid environment. In the following, we set β ) 1.
nonlinearly. These expectations have been confirmed by the different mechanical implementations of vertical deposition,13,16,17 but have never been tested in parallel with otherwise identical material. This test will be presented in paragraph B. Surface energy and polarity of substrate and colloids on one hand and surface tension of the solvent on the other hand determine meniscus height l and capillary forces between particles.21,25 Hence, chemical pretreatment of the substrate and surfactants in the solvent strongly influence the meniscus. Our crystals are grown in comparatively high concentration in order to reach thick layers. The particular sample has h ) 30 µm thickness, corresponding to x3/2h/D > 110 layers. With Vm ) 0 in eq 2, we deduce an effective meniscus length l ≈ 1.0 mm, which is very reasonable but a little smaller than the meniscus length observed by eye. Since Dimitrov and Nagayama21 assume that all solid material that is swept to the crystal edge incorporates into the cystal, this approach certainly can overestimate the resulting crystal thickness. The AFM height picture shows numerous monolayer edges with characteristic hexagonal 60° angles (Figure 1b). The thickness of the sample increases toward the bottom according to eq 2, because proceeding evaporation increases the concentration of the dispersion. The same characteristic 60° angles are evident in the macroscopic photographs (Figure 1a). Although not desired, these layer steps are indication of 2 × 2 cm2 monocrystalline 〈111〉 fcc order, because without macroscopic order, one would not expect the 60° layer edges to run through the entire sample. (25) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383-401.
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Table 1. Thickness of Elastic Colloidal Crystals Grown in Vertical Deposition under Various Conditions
recipe
concentration %
lifting speed Vm nm/s
pumping speed Vm nm/s
measured thickness µm
#2 #2 #2 #2 #2 #2 #2 #2 #2 #2 #2 #3 ) #2 + 0.05% Steinapol NLS #4 ) #2 + 0.05% Texapon NSO #5 ) #2 + 0.05% Lutensol AT 18 #6 ) #2 + 0.05% Lumiten ISC
1 1 2 5 10 2 5 10 2 20 2 2 2 2 2
0 105 105 105 105 320 320 320 0 0 105 105 105 105 105
0 0 0 0 0 0 0 0 1100 1100 0 0 0 0 0
14 1.5 3.5 6.5 17 0.6 1.5 2.5 0.4 3.0 4.0 2.0 2.5 1.8 3.3
Microcracks that are usually observed in vertical deposition (discussed, e.g., in ref 17) and that are believed to appear upon evaporation of water from a crystalline, but not yet closestpacking, arrangement are not observed. This is an important advantage of filled-pore colloidal crystals, and it is due to the flexibility of the film-forming shell. B. Comparison of Crystallization Techniques. In the lifting apparatus, higher concentrations were used in order to maintain sample thickness. The resulting samples have crystalline order (Figure 2b), but their thickness decreases with faster lifting speed or higher pump speed. In Table 1, we compare measured thicknesses with predictions of eq 2. To derive the “expected
expected thickness h µm
ratio of measured/expected thickness
1.7 3.4 8.4 17 1.2 3.1 6.1 0.4 3.7
0.9 1.0 0.8 1.0 0.5 0.5 0.4 1.1 0.8
thickness”, we inserted the measured evaporation speed je ) 1.3 mm/day ) 15 nm/s and the meniscus length l ) 1.0 mm as derived from the first line of the table, i.e., pure evaporation without mechanical movement of the substrate. For a wide range of experimental conditions, the agreement is astonishingly fine, especially within experimental sessions, grouped by thick lines in the table. Regarding the homogeneous deviation of the session with lifting speed 1.14 mm/h, we suspect that the evaporation rate was different, because we observed it is neither time (humidity)- nor location (ventilation)-independent. All samples from the lifting stage show a periodic modulation of crystal thickness with a period of 1.3 mm (Figure 2a). This
Figure 3. Agitated surface/flux-assisted vertical deposition. Sample prepared with flux-assisted (800 L/h air) self-assembly from a 0.5% dispersion (recipe #2). (A) Color flop photographs, scale bar is 1 cm. (B) AFM. Bottom right: 5 µm × 5 µm scan. Middle: FFT of 50 µm × 50 µm AFM trace (top right). The hexagonal FFT spots are very sharp, but impurities with rectangular FFT features are present. The surface has very fine (∼100 µm) vertical furrows but no horizontal modulation.
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Figure 4. Top and bottom layer after horizontal deposition. AFM images (2 µm × 2 µm) of (A) the first and (B) the last deposited layers after removal of the crystal from the substrate. Junk material (fine and coarse fraction) that could not be integrated into the crystal accumulates in the dispersion (recipe #2) and dries onto the top layer of the crystal. This adverse effect is circumvented in vertical deposition.
period is the same within (0.06 mm for lifting speeds Vm ) 0.38 mm/h ()105 nm/s) and 1.14 mm/h ()320 nm/s) and for concentrations φ ) 2%, 5%, and 10%. Although a similar modulation phenomenon with a less strict periodicity (0.8-1.4 mm spacing) is observed for the “pumping/flow control” setup, we attribute the modulation effect to mechanical irregularities of the lifting setup. We failed to reproduce the brilliant results from the fluxassisted technique with a constantly agitated surface.18 We found that, even for more than a 10-fold concentration of the original demonstration (now 0.5%), the resulting thickness was very low (now 7 µm, originally 40 µm). Crystalline ordering was achieved, however (Figure 3). Variation of air flux between 500 and 1200 L/h does not lead to more efficient deposition. For concentrations above 1%, the surface is very corrugated on the millimeter to centimeter scale, giving rise to a milky appearance; hence, we abandoned further development of the flux-assisted technique. Finally, we cross-checked again horizontal deposition and discovered an interesting aspect (Figure 4). While the first layer on the substrate has a crystalline arrangement (Figure 4a), the top surface of the sample is a noncrystalline collection of extremely polydisperse particle size distribution Figure 4b). We assume that the particles that are incompatible with the periodic arrangement stay in dispersion while the main fraction crystallizes. The leftover, i.e., polydisperse, material is found on top of the crystal in horizontal deposition. In contrast, in vertical deposition, this incompatible material does not interfere, as the dispersion never dries completely. Especially when working with noncentrifuged, non-raffinated particle size distributions, this is another advantage of vertical over horizontal deposition. C. Influence of Surfactants, Humidity, and Meniscus Orientation. If crystalline domains approach the centimetersize of the substrate, is there any means of controlling the macroscopic orientational axis? Whenever high crystallinity is reached with our samples, we observe that the nearest-neighbor direction is horizontal, as shown by the FFT spots along the vertical axis (Figures 1-3). Hence, the orientation of this crystal axis is not random. One can suspect that the orientation is correlated either to the edges of the substrate or to the meniscus. To test this, we deposited a sample on a substrate that was tilted by 15° against the horizon, and we observed that the nearestneighbor direction is not tilted against the horizon but is tilted by -15° relative to the substrate (Figure 5; all AFM scans are recorded relative to the substrate edges). This is clear evidence that the meniscus, being always horizontal, determines the nearestneighbor direction. Hence, vertical deposition gives us complete
Figure 5. Macroscopic alignment of the nearest-neighbor direction along the meniscus line. AFM pictures (50 µm × 50 µm) and FFT of two vertical-deposition samples (1% dispersion (recipe #2)) with different tilt angle between substrate and meniscus. The macroscopic orientation is not related to substrate edges and is not random, but it is fixed by alignment of the nearest-neighbor axis with the meniscus line.
control of crystal orientation: plane 〈111〉 parallel to the substrate, nearest-neighbor direction parallel to the meniscus. In an intuitive image, the nucleus of crystal growth in vertical deposition is a pearl necklace of particles along the meniscus line. Next, we tested the influence of humidity. If samples are grown by lifting a substrate from the dispersion in a hermetically enclosed beaker, the resulting thickness is 50 × 50 µm2 monocrystal, while the sample with Lutensol AT 18 has only ∼5 µm sized polycrystalline areas with mixed 〈100〉 and 〈111〉 planes at the surface, leading to nonuniform layer-to-layer distance and hence vanishing interference color. Obviously, the concentration and the type of anionic surfactants are critical for crystal quality, and non-ionic surfactants destroy crystalline ordering. However, the surfactants tend to reduce crystal thickness (Table 1). In the context of eq 2, this reduction of thickness can be attributed to a reduction of meniscus length l due to lowered surface tension. D. Mechanics of Elastic Colloidal Crystal. The freestanding colloidal crystal at a thickness of ∼50 µm is easily stretchable by hand, with Y′ ) 3.2 Mpa at
