pubs.acs.org/Langmuir © 2010 American Chemical Society
Controlled Directionality of Ellipsoids in Monolayer and Multilayer Colloidal Crystals Tao Ding,†,§ Kai Song,*,† Koen Clays,*,‡ and Chen-Ho Tung† †
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, ‡Department of Chemistry, University of Leuven, Celestijnenlaan 200D, Leuven, BE-3001, Belgium, and §Graduate University of the Chinese Academy of Sciences, Beijing 100190, China Received January 24, 2010 We present a self-assembly approach for monolayer and multilayer deposition of ellipsoids with a controllable direction. The direction of the ellipsoids in the assembly can be conveniently tuned by external applied magnetic field. This level of control on positional and directional order suggests a way to build monolayer templates for lithography with two-dimensional patterns and three-dimensional anisotropic photonic crystals, which may open the way toward the complete photonic band gap in the visible.
1. Introduction Colloidal crystals are increasingly considered a viable platform for applications in the fields of information, catalysis, biosensing, electrics, and optoelectronics.1 To these specific ends, colloidal particles that vary in chemical nature, size, surface potential, and dielectric properties have been used as building blocks for these crystals with widely varying architecture.2 The colloidal self-assembly of spherical particles results in the energetically most favorable (random) hexagonal close-packed (rhcp) or face-centered-cubic (fcc) crystal structure with good positional order. Ordered arrays of submicrometer spheres play an important role in such diverse yet growing research fields as nanochemistry,3 nanophotonics,4 and nanotechnology. Twodimensional (2D) arrays of microspheres have been widely used as a mask for colloidal lithography, resulting in well-controlled etching or deposition.5 Three-dimensional (3D) arrays of submicrometer spheres interact with the UV-Visible-IR part of the electromagnetic spectrum, resulting in optical effects such as opalescence. Such 3D arrays form an important subclass of the colloidal photonic crystals. The emission of photons and the propagation of light can be manipulated by photonic band gap or band edge engineering, based on the reduction or the enhancement, respectively, of the local density of states in the gap or the edge. *To whom correspondence should be addressed. E-mail: kai.song@iccas. ac.cn;
[email protected]. (1) (a) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (b) Levene, M. J.; Korlach, J.; Turner, S. W.; Foquet, M.; Craighead, H. G.; Webb, W. W. Science 2003, 299, 682. (c) Yu, J.-S.; Kang, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2004, 124, 9382. (d) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O'Brien, S.; Murray, C. B. Nature 2006, 439, 55. (e) Xu, M.; Goponenko, A. V.; Asher, S. A. J. Am. Chem. Soc. 2008, 130, 313. (2) (a) Xia, Y. N.; Gates, B.; Yin, Y; Lu, Y. Adv. Mater. 2000, 12, 693. (b) Whitesides, G. M.; Grzybowshi, B. Science 2002, 295, 2418. (c) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557. (d) Grzybowski, B. A.; Wilmer, C. E.; Kim, J.; Browne, K. P.; Bishop, K. J. M. Soft Matter 2009, 5, 1110. (e) Ozin, G. A.; Hou, K.; Lotsch, B. V.; Cademartiri, L.; Puzzo, D. P.; Scotognella, F.; Ghadimi, A.; Thomson, J. Mater. Today 2009, 12, 12. (f) Velev, O. D.; Cupta, S. Adv. Mater. 2009, 21, 1897. (g) Aparicio, F. J.; Lozano, G.; Blaszczyk-Lezak, I.; Barranco, A.; Míguez, H. Chem. Mater. 2010, 22, 379. (3) Ozin, G. A.; Arsenault, A. C.; Cademartiri, L. Nanochemistry: A Chemical Approach to Nanomaterials, 2nd ed.; Royal Society of Chemistry: Cambridge, 2009. (4) Prasad, P. N. Nanophotonics; Wiley: New York, 2004. (5) Zhang, G.; Wang, D. Chem. Asian J. 2009, 4, 236.
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Scheme 1. Schematic Illustration of the Magnetically Induced Close-Packing of Ellipsoids at Air/Water (A/W) Interfacea
a The ellipsoidal particles are mainly subjected to the magnetic forces (Fm), the gravitation forces (G), flotage (Ff), and the capillary forces (Fc) during the assembly process.
To broaden the range of applications, replacing the spherical building blocks with nonspherical particles provides a variety of anisotropic structures and concomitant properties. For example, top-down lithography with a mask of a 2D array of nonspherical colloids may result in novel patterns other than the traditional triangular arrays. 3D colloidal crystals of nonspherical units may break the symmetry-induced degeneracy at the W- and U-points of the photonic band structure and may open a gate to a complete photonic band gap.6 However, the self-assembly of nonspherical particles with a lower symmetry rarely results in a close-packed pattern with directional order also. Xia’s group tried to assemble submicrometer latex ellipsoids. However, due to the lack of directional order, only random packing was obtained.7 Liddell’s group fabricated 2D and 3D arrays of asymmetric spherical dimers and mushroom caps by convective self-assembly. These nonspherical colloidal particles were close-packed into hexagonal arrays with positional order but without directional order.8 As can be expected, the absence of directional order results in the difficulty of fabricating long-range perfect order and in the lack of the corresponding properties.
(6) Haus, J. W.; Sozuer, H. S.; Inguva, R. J. Mod. Opt. 1992, 39, 1991. (7) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722. (8) (a) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 8810. (b) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 10479.
Published on Web 06/09/2010
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Figure 1. SEM images of assembled structures with magnet distance of (a) 2 mm, (b) 5 mm, and (c) 10 mm from the sample; ellipsoid suspension, 20 mg/mL; and (d) their corresponding reflectance spectra, all the scale bars are 1 μm. (e) Relationship of the number of layers of the assembled structure and the distance of the magnet from the sample. The solid line just serves as a guide to the eye.
Relatively few examples exist of applying an external force to direct those anisotropic building blocks. These include electrical force,9 in-plane compression and streching,10 capillary force,11 and magnetic force.12 Minimizing the total energy, including the energy of interaction with the electric, capillary, or magnetic field, results in the directional order. Recently, Furst’s group created films of anisotropic titania particles by combining field- and flowdirecting techniques, The directional order of the particles results in unique optical and mechanical properties of the films.13 Our group has used an externally applied magnetic field to fabricate 3D photonic colloidal crystals of magnetic submicrometer ellipsoids with excellent positional order and directional order at the same time, but highly limited to the main axis of the prolate ellipsoids only parallel to the substrate.14 (9) (a) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399. (b) Ryan, K. M.; Mastroianni, A.; Stancil, K. A.; Liu, H.; Alivisatos, A. P. Nano Lett. 2006, 6, 1479. (10) (a) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360. (b) Yu, G.; Cao, A.; Lieber, C. M. Nat. Nanotechnol. 2007, 2, 372. (11) Deng, T.; Cournoyer, J. R.; Schermerhorn, J. H.; Balch, J.; Du, Y.; Bolhm, M. L. J. Am. Chem. Soc. 2008, 130, 14396. (12) Love, J. C.; urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (13) Mittal, M.; Furst, R. M. Adv. Funct. Mater. 2009, 19, 3271. (14) Ding, T.; Song, K.; Clays, K.; Tung, C. -H. Adv. Mater. 2009, 21, 1936.
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In this paper, we report the fabrication of 3D colloidal crystals of submicrometer ellipsoids with the same long-range positional order but now with well-controlled and completely variable (0-90°) directional order. The orientation of the long axis of the particles can be adjusted by changing the relative orientation of the magnetic field. The resulting variation in the photonic band gap properties has been studied by reflectance spectroscopy.
2. Experimental Section 2.1. Magnetic-Field-Directed Assembly of Ellipsoidal Crystals. The magnetic ellipsoids made of γ-Fe2O3@SiO2 core-
shell structure (length 305 ( 15 nm, diameter 200 ( 5 nm, aspect ratio 1.5 ( 0.1) were synthesized as previously reported14 and dispersed in ethanol. The experimental procedure is schematically summarized in Scheme 1. To fabricate the monolayer and multilayer films of ellipsoids with their long axes perpendicular to the substrate (tilt angle 0°), 5 μL of ethanol dispersion of the ellipsoids (5 to 20 mg/mL) was dropped onto a glass slide (20 10 1 mm3) pretreated with piranha solution (H2SO4/H2O2 = 2:1, v/v). A drop of water was immediately dropped next to the suspension. When water and colloid suspension came into contact, ellipsoids were spread on the surface of water due to the surface tension. Then, a magnet with a surface magnetic strength of 1.2 T was placed above the water with its polar axis perpendicular to the DOI: 10.1021/la101622d
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Figure 2. SEM images of assembled ellipsoids with concentration of (a) 5, (b) 10, and (c) 15 mg/mL, magnet distance 2 mm; all scale bars are 1 μm. (d) Relationship between the number of layers and the concentration of the ellipsoids suspension. water surface at a distance of 0.2 to 1 cm. After the water was completely evaporated while the magnetic field was maintained, monolayer or multilayer films of ellipsoid arrays were formed under certain conditions. Ellipsoidal crystals made of tilted ellipsoids were obtained by tilting the polar axis of the magnet from the normal direction. 2.2. Characterization. The core-shell structure of the synthesized ellipsoids was characterized by transmission electronic microscopy (TEM, 1011, Hitachi) at an accelerating voltage of 100 kV. The microstructure characterization of the assembled ellipsoids was carried out with field emission scanning electronic microscopy (FE-SEM, S4300, Hitachi) at an accelerating voltage of 15 kV. The optical images were captured with an Olympus MX40 microscope at a magnification of 500 with a charge coupled device (CCD) detector connected to the computer. The reflectance spectra and multiangular characterization were recorded with a fiber spectrometer (AvaSpec-128, Avantes) and multiangular reflectance probe holder system (AFH-15, Avantes), respectively. The hysteresis loop was obtained with a squid at room temperature (300 K).
3. Results and Discussion Monodisperse ellipsoids with an aspect ratio of 1.5 were prepared as the anisotropic building blocks.15 These ellipsoids were made magnetically active for the convenient directional control by the externally imposed magnetic field (see Supporting Information Figure S1). The polydispersity for both the length and the width of the prolate particles was smaller than 5% because of a repetitive silica coating procedure.15 The spreading of the ethanol suspension of ellipsoids on the glass substrate only resulted in coffee-ring like structures without order. When the ellipsoids were spread on the surface of water, a closely packed monolayer was formed by the action of the surface tension.16 However, the orientational order was absent without any other (15) Ding, T.; Liu, Z.-F.; Song, K.; Tung, C.-H. Colloids Surf. A 2009, 336, 29. (16) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383.
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Figure 3. Normalized multiangular reflectance spectra of the assembled ellipsoids.
external fields applied (see Supporting Information Figure S2). After the magnetic field was applied, the magnetic forces not only aligned the orientation of the ellipsoids to the magnetic field, but also attracted ellipsoids to form a continuously packed monolayer. After the water was evaporated while the magnetic field was being imposed, the ellipsoids at the air-liquid-solid interface were pulled by the strong capillary forces to form a closely packed structure with both positional and orientational order. During the assembly process, the magnetic forces (Fm), lateral capillary forces (Fc), flotage (Ff), and gravitational forces (G) should all act in a balanced way on the magnetic colloidal particles as shown in Scheme 1. Therefore, as discussed below, the final structure of the assembled ellipsoids strongly depends on the magnetic field strength and on the concentration of the ellipsoidal suspension. 3.1. Influence of the Magnetic Field Strength on the Assembled Structures. We examined the effect of magnetic field strength on the assembled structure by tuning the distance between the magnet and the sample from 2 to 10 mm while fixing the concentration of the ellipsoids suspension at 20 mg/mL. The SEM images of the resulting structures are shown in Figure 1. When the magnet was 2 mm from the sample, the assembled structure was well-aligned with the long Langmuir 2010, 26(13), 11544–11549
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Figure 4. SEM images of self-assembled monolayers of ellipsoidal particles after water has evaporated; imposed tilt angle θ between the magnetic axis and the normal to the substrate surface of (a) θ = 0°, (b) θ = 30°, (c) θ = 60°, (d) θ = 90°. All scale bars are 1 μm.
axes of the colloids perpendicular to the substrate (Figure 1a). With the magnet further away from the sample, weaker magnetic forces work toward the alignment of the ellipsoids. When the distance between the magnet and the sample increased to 5 and 10 mm, the alignment of the orientation of the ellipsoids deteriorated, which resulted in a weaker reflectance peak or even the absence of any (Figure 1d). Notice that the reflectance peak also blue-shifted ∼30 nm when the magnet was 5 mm from the sample, which is mainly because the long axes were not totally perpendicular to the substrate (Figure 1b). It is reasonable to consider that the magnetic torque may not be strong enough to balance the gravitation force to erect the ellipsoids perpendicular to the substrate. It is also noteworthy that the number of layers of the assembled structure increases with increasing the strength of the magnetic field or decreasing the distance between the magnet and the sample (Figure 1e). A possible mechanism could be that part of the ellipsoids sank into the water during the spreading process because of the high density of the magnetic colloids. During the self-assembly process, these suspended ellipsoids are attracted and oriented by the magnetic force under the surface of the water to form multilayer structures layer by layer with order kept by templating from the top layer on the surface of the water. As a result, the number of layers increased, since more ellipsoids are attracted by stronger magnetic forces. 3.2. Influence of the Concentration of Ellipsoid Suspension on the Assembled Structures. Concentration of the ellipsoid suspension is another important factor to determine the morphology of assembled structures, especially the number of layers. Here, we fixed the magnet at a distance of 2 mm from the sample while increasing the concentration of the ellipsoid suspension from 5 to 20 mg/mL. We found that the ellipsoids were well-aligned with their long axes perpendicular to the substrate and the number of layers were increased from monolayer to multilayer (Figure 2). Obviously, increasing concentration provides more ellipsoids to sink in the water and be attracted into the self-assembled structure. Langmuir 2010, 26(13), 11544–11549
3.3. Multiangular Optical Characterization of the Assembled Structures. We measured the reflectance spectra of the assembled ellipsoid sample in Figure 1a from normal and multiangular directions (Figure 3). The reflectance peak gradually blue-shifted from 670 nm to 600, 548, and 491 nm with increasing incidence angle from 0° to 30°, 45°, and 60°, respectively, similar to the case of colloidal crystals with spherical units. This is mainly because of decreasing lattice constant between two neighbor crystalline layers along the incidence direction when the incidence light angle was increased as illustrated in the inserted scheme in Figure 3. As illustrated in the figure, there is no complete band gap covering the range of all angles, because of the low refractive index contrast in the system. 3.4. Tunability of the Orientation of the Ellipsoidal Building Blocks in the Assembled Structures. Upon changing the tilt angle between the normal to the substrate and the magnetic axis, there is a concomitant change in the direction of the deposited prolate particles. It is noteworthy that the measured tilt angle for the assembled ellipsoids from the SEM images in Figure 4 deviates from the imposed tilt angle set by the magnet (0, 30, 60 and 90°). This deviation is mainly caused by the direction of the crystal cleavage determining the SEM observation angle and by the varying effective tilt angle for large substrates close to the magnet. Furthermore, the directional deviation may also be partially attributed to the particle agitation during the evaporation of the dispersion medium. The finite distribution of particle sizes and shapes leaves some free space and allows particle agitation. This deviation is preserved after the self-assembly. The number of identically oriented layers in the multilayer stack differs a lot in the cases of vertically and obliquely applied magnetic field (a few layers for 0° tilt angle as shown in Figure 5a, to tens of layers for larger angles in Figure 5b). In a vertically applied magnetic field, ellipsoids were aggregated right below the center of magnet, while in an obliquely applied field, they were driven to float to the border of the water and the substrate. In this case, the local concentration near the border increases DOI: 10.1021/la101622d
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Figure 5. SEM images of ellipsoidal crystals with imposed magnetic tilt angle of (a) 0°, (b) 60°. (c) Magnification of the framed region in (b). (d) Top view of (b).
Figure 6. Different 3D packing structures of ellipsoids.
substantially, resulting in a thicker colloidal crystal film. Furthermore, in the region near the border of water and the substrate, positional and orientational ordering also benefit from the contact between colloids and the substrate, because the position and orientation of ellipsoids can be more easily fixed in magnetic field, with less particle agitation than in the case of a vertically applied magnetic field. In Figure 5c, a magnification of the framed region in Figure 5b, good periodicity can be clearly observed. In Figure 5d, a top-view of the colloidal crystal shown in Figure 5b, the hexagonal 2D lattice of ellipsoids can also be observed experimentally. In crystalline planes parallel to substrate, the ellipsoids are still hexagonally close-packed even with different directions. Ellipsoids from a higher layer find low-energy positions in the voids between three ellipsoids from the lower layer, as shown in Figure 6. In Figure 6b, there is a random angle (possible values varying between 0° and 90°) between the main axes of ellipsoids and the normal direction to substrate. The packing behavior shown in Figure 6a and c are two special cases (tilt angle equals 0° and 90°, respectively). The 2D packing of the ellipsoids within a single crystalline plane can also be different depending on the directional status of ellipsoidal units, which provides numerous possible crystal lattices. This is quite different from the energetically more favorable 11548 DOI: 10.1021/la101622d
Figure 7. Reflectance spectra at normal direction to colloidal crystal films, with different tilt angles between the normal and the magnetic axis. Insets: optical microscope images of the different colloidal crystal films.
2D packing behavior, as we have reported previously (tilt angle 90°).14 Figure 7 shows the normalized reflectance spectra of our colloidal arrays with different orientations. With increasing θ, the wavelength of the Bragg peaks blue-shifted from 690 nm (θ = 0°) to 612 nm (θ = 30°), 564 nm (θ = 60°), and 480 nm (θ = 90°), respectively, because the distance between successive layers is decreased. Correspondingly, the structural color of colloidal crystals changed from red to blue. The sharp selective reflectivity also illustrates the good periodicity of the photonic crystal samples.
4. Conclusions In conclusion, we present a self-assembly approach for monolayer and multilayer deposition of ellipsoids with a controllable direction. A magnet exerts the force to direct the prolate particles. Therefore, the direction of the ellipsoids in the assembly can be conveniently tuned. The reflectance peak for the photonic band gap blue-shifted with increasing tilt angle between normal and magnet axis. This level of control on positional and directional order opens the way to monolayer templates for lithography with Langmuir 2010, 26(13), 11544–11549
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2-D patterns17 and to 3D anisotropic photonic crystals which may break symmetry-induced degeneracy of photonic bands, leading to complete photonic band gap in fcc lattice.18
boratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS. Special thanks are owed to Li Chen for the optical microscopy characterization.
Acknowledgment. We acknowledge the financial support provided by the NSFC (No. 60877032) and the 973 program (Nos. 2007CB808004, 2009CB930802). We also thank Key La-
Supporting Information Available: TEM image of synthesized γ-Fe2O3@SiO2 ellipsoids with aspect ratio of 1.5, and the hysteresis loop of the synthesized ellipsoids at temperature of 300 K, and the SEM image of the monolayer of randomly packed ellipsoids without applied magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org.
(17) Love, J. C.; Wolfe, D. B.; Jacobs, H. O.; Whitesides, G. M. Langmuir 2001, 17, 6005. (18) Xia, Y.; Gates, B.; Li, Z.-Y. Adv. Mater. 2001, 13, 409.
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