Large-Scale Self-Organized Growth of (001) Surface-Oriented

Dec 9, 2008 - Such (100) oriented regions of the colloidal film are apt to concentrate near the edge of the substrate, and changing the width of the s...
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Langmuir 2009, 25, 820-824

Large-Scale Self-Organized Growth of (001) Surface-Oriented Colloidal Crystals by Edge Meniscus Effect Ting Zhang,†,‡ Xinlin Tuo,§ and Jun Yuan*,†,| Beijing National Center for Electron Microscopy, Tsinghua UniVersity, Beijing 100084, China, Laboratory of AdVanced Materials, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing, 100084, China, Laboratory for AdVanced Materials, Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, China, and Department of Physics, UniVersity of York, Heslington, York YO10 5DD, U.K. ReceiVed September 19, 2008. ReVised Manuscript ReceiVed NoVember 2, 2008 Millimeter-sized films of (100) facing face-centered crystals (fcc) of colloidal nanoparticles have been fabricated by self-assembly on silicon substrates and studied by both optical and scanning electron microscopy. The top surface layer sometimes also reconstructs to form a 1 × n (n varies from 2-7) superstructure. Such (100) oriented regions of the colloidal film are apt to concentrate near the edge of the substrate, and changing the width of the substrate could control the areas of the (100)-oriented domains. The formation of the square packing surface pattern is related to the shape of the meniscus at the edges of the substrate, where the particles suffer additional shear force and higher evaporation, which can be used to control the location and size of the square arrangement of nanoparticles.

1. Introduction Thin films of colloidal crystals with different surface orientation are of growing importance, not only because of their interesting physical properties such as orientation-dependent optical responses,1-3 but also because they expose non-close-packing periodic surface structures. They can be used as optical filters as well as useful templates for biologically active substrates,4,5 chemical and biosensors,6-9 and high-density magnetic and optical data storage.10,11 Monodispersed colloidal nanoparticles can easily self-assemble to form long-range two- (2D) or threedimensional (3D) lattice structures on other substrates when suitably dried of its solvent.12 However, such self-assembly alone usually only produces (111) facing crystals with close packing ordering on the surface. This is restrictive and has prompted research into methods that can produce colloidal crystals with other orientations To date, a popular approach for fabrication of other forms of arrangement of nanoparticles is colloidal epitaxial, i.e., directed colloidal crystal growth under spatial confinement on patterned * Corresponding author. E-mail address: [email protected]. † Beijing National Center for Electron Microscopy, Tsinghua University. ‡ Department of Materials Science and Engineering, Tsinghua University. § School of Materials Science and Engineering, Tsinghua University. | University of York.

(1) Tarhan, I. I.; Zinkin, M. P.; Watson, G. H. Opt. Lett. 1995, 20, 1571–1573. (2) Tarhan, I. I.; Watson, G. H. Phys. ReV. Lett. 1996, 76, 315–318. (3) Vos, W. L.; Sprik, R.; Vanblaaderen, A.; Imhof, A.; Lagendijk, A.; Wegdam, G. H. Phys. ReV. B 1996, 53, 16231–16235. (4) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959–960. (5) Kamenetzky, E. A.; Magliocco, L. G.; Panzer, H. P. Science 1994, 263, 207–210. (6) Holtz, H. J.; Asher, S. A. Nature 1997, 389, 829–832. (7) Asher, S. A.; Alexeev, V. L.; Goponenko, A. V.; Sharma, A. C.; Lednev, I. K.; Wilcox, C. S.; Finegold, D. N. J. Am. Chem. Soc. 2003, 125, 3322–3329. (8) Velev, O. D.; Kaler, E. W. Langmuir 1999, 15, 3693–3698. (9) Sharma, A. C.; Jana, T.; Kesavamoorthy, R.; Shi, L. J.; Virji, M. A.; Finegold, D. N.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 2971–2977. (10) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (11) Siwick, B. J.; Kalinina, O.; Kumacheva, E.; Miller, R. J. D.; Noolandi, J. J. Appl. Phys. 2001, 90, 5328–5334. (12) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9, 3695–3701.

substrates, such as imprinted relief surface,13 square pyramidal pit in substrate14 or V-shaped grooves substrate.15 Stretching of substrate in lift-up lithography is another alternative to produce non-close-packing ordering on a surface.16,17 However, those methods are usually too complicated and expensive for largescale applications. In addition, the strategy utilizing the chemically patterned surfaces with wettability contrast has also been used to regionally control particle deposition on substrates,18-22 but the growth process of colloidal crystals still remains unclear, and the mechanisms that influence the size and orientation of colloid crystallization need further detailed study. Relatively large-scale, template-free growth of colloidal crystals with square surface patterns has been reported by Herna´n Mı´guez23 using a spincoating method, but their dependences on the experimental factors such as the dispersion media, particle size, and liquid concentration are not fully understood. In this work, we demonstrate the template-free formation of millimeter-scale colloidal crystals with their (100) planes parallel to the substrate surface, by the simple method of flowcontrolled vertical deposition (FCVD).24 We find that the shape of the meniscus has great influence on the formation of the square domains. More importantly, we can show that they are related to the deformation of the meniscus at the edge of the substrate. We have used this understanding to control the size and location of the square arrangement of nanospheres. This opens a new (13) Jin, C. J.; McLachlan, M. A.; McComb, D. W.; De La Rue, R. M.; Johnson, N. P. Nano Lett. 2005, 5, 2646–2650. (14) Yin, Y.; Li, Z. Y.; Xia, Y. Langmuir 2003, 19, 622–631. (15) Ozin, G. A.; Yang, S. M. AdV. Funct. Mater. 2001, 11, 95–104. (16) Yan, X.; Yao, J. M.; Lu, G.; Li, X.; Zhang, J. H.; Han, K.; Yang, B. J. Am. Chem. Soc. 2005, 127, 7688–7689. (17) Tan, B. J. Y.; Sow, C. H.; Lim, K. Y.; Cheong, F. C.; Chong, G. L.; Wee, A. T. S.; Ong, C. K. J. Phys. Chem. B 2004, 108, 18575–18579. (18) Li, T.; Xing, R. B.; Huang, W. H.; Han, Y. C. Colloids Surf., A: Physicochem. Eng. Aspects 2005, 269, 22–27. (19) Fustin, C. A.; Glasser, G.; Spiess, H. W.; Jonas, U. Langmuir 2004, 20, 9114–9123. (20) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062–3067. (21) Masuda, Y.; Tomimoto, K.; Koumoto, K. Langmuir 2003, 19, 5179– 5183. (22) Lu, G.; Chen, X.; Yao, J. M.; Li, W.; Zhang, G.; Zhao, D. Y.; Yang, B.; Shen, J. C. AdV. Mater. 2002, 14, 1799–1802. (23) Mihi, A.; Ocana, M.; Miguez, H. AdV. Mater. 2006, 18, 2244–2249. (24) Zhou, Z. C.; Zhao, X. S. Langmuir 2004, 20, 1524–1526.

10.1021/la8030717 CCC: $40.75  2009 American Chemical Society Published on Web 12/09/2008

Meniscus Effect on Self-Assembly of (001) Crystals

Figure 1. (a) An SEM image of the regular square arrangement of 290 nm colloidal spheres (the inset shows the corresponding Fourier transform). (b) An SEM image of a square arrangement of 290 nm colloidal spheres with 1 × 5 periodicity (the inset shows the corresponding Fourier transform). (c) An SEM image of the boundary between a regular square region and a square region with a 1 × 4 periodicity. (d) An optical microscopy image of the (green) areas with square arrangement of colloidal spheres on the surface.

approach for low-cost fabrication of nanostructured surfaces of a non-close-packing pattern over a large scale.

2. Experimental Section The colloidal crystals were fabricated using a solution of monodispersed polystyrene spheres with a nominal diameter of 290 nm (purchased from the Institute of Physics and Chemistry of the Chinese Academy of Sciences). The method used to grow the crystals was FCVD.24 Briefly, silicon wafers, used as substrate, were first treated in a mixture of H2SO4/H2O2 (98% H2SO4:30% H2O2 ) 3:2 (v/v)) for 8 h to make them hydrophilic. They were then ultrasonically cleaned in acetone, ethanol, and deionized water, respectively, three times. The treated silicon wafers were immersed in the colloidal suspension with a tilt angle of 10° from the liquid surface normal. The dropping speed of the suspension level was set as 7.5 × 10-4 cm/s, and the temperature of the container was maintained at 20 ( 2 (°C). Scanning electron microscopy (SEM; JEOL JSM6301F), atomic force microscopy (AFM; Nanoscope IIIa AFM) and optical microscopy (NEOPHOT32) were used to characterize the morphology of the colloidal samples on the silicon substrate. In order to perform the SEM inspection, the samples were coated with a thin gold film as customary.

3. Results and Discussion The non-close-packing nature of the surface nanostructure of the colloidal crystal films can be directly observed using both electron and optical microscopy. In Figure 1a, a simple square arrangement of colloidal spheres is observed. Such packing pattern is not always necessary without any distortion. For example, the angle between the unit cell vectors in Figure 1a is about 91°, a little larger than that of normal square lattice. In Figure 1b, a square domain with a 1 × 5 periodicity is observed. The angle between the unit cell vectors is about 92°, which also varies from place to place. But nanospheres in these 1 × 5 domains are more closely packed at least in one direction of this plane, and this is just one example of many 1 × n square arrangements observed, where n can vary from 2 to 7. Insets in the top right corners are the corresponding Fourier transform images, confirming the longrange square ordering of the spheres. In addition, two such square domains can match very well without transition zones existing between them, as shown in Figure 1c. The colloidal films with dominant square domain can be easily identified in the reflected

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Figure 2. The distribution of the closest spacing between the centers of neighboring spheres for different surface arrangement of colloidal spheres (a, b, and c are hexagonal, regular, and 1 × 5 square arrangements, respectively) and the corresponding subsurface spacing in the 1 × 5 square domains (d).

light under the optical microscope as green patches, compared with the yellowish patches for the colloidal crystal surface with close-packed hexagonal structure. An example is shown in Figure 1d, where the greenish area, corresponding to colloidal crystals of dominantly square domain, can be as large as 0.06 mm2. The different sphere packings observed showed different closest spacing between spheres. As shown in Figure 2, the average spacing between the centers of two closest spheres in a hexagonal packing pattern is about 320 nm, a value slightly larger than the size of the starting nanospheres. The regular square structure has an average spacing of 332 nm, with a broad distribution extending up to 360 nm. An examination of Figure 1a suggests that the larger spacing in the square domain corresponds to a noncontact packing of nanospheres. By contrast, the average spacing between centers of two neighboring spheres in the top layer of the 1 × 5 square domains is 295 nm, which is a representative of the top layers of other 1 × n domains. This value is the closest to the nominal size of the starting nanosphere dispersion in liquid. The SEM picture in Figure 1b also shows that the spheres are touching each other. Interestingly, the averaging spacing between the neighboring spheres in the trench between the 1 × 5 square domains is 312 nm, closer to the size of the starting nanospheres too. We will return to this slight spacing increase later. An important observation of the square arranged domains is that they are mostly multilayer stacking of nanospheres. This raised the interest about their 3D structure. The defects such as vacancy found in the top surface layer allow us to gain some information about the structure of the subsurface layer (Figure 3a). The position of the spheres in the subsurface layer indicates that upper spheres rest on the interstitial hollow sites formed by four neighboring spheres also with a square arrangement. Another more useful source of information comes from the side view of a broken colloidal film (Figure 3b). It reveals a hexagonal stacking pattern right to the bottom layer. Consistent with this observation, we have observed that the polystyrene spheres in the bottom layer can also have a square arrangement. To do that, we have ultrasonically washed away the colloidal films using toluene. However, a trace of the polystyrene spheres is still observed on the silicon wafer, as shown in Figure 3c. It is clear that the polystyrene spheres in the bottom layer have formed a regular square array. The above observation is consistent with a colloidal structure that is an approximate of the 3D ordered face-centered-

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Figure 3. (a) Top view of the surface of a colloidal film with square pattern and some vacancy defects. (b) A cross-section of a colloidal film with square arrangement on the top surface. (c) An SEM image of remnants of the PS spheres on the silicon substrate after ultrasonic treatment. (d) A schematic picture of 3D packing of nanospheres that is consistent with the observed surface pattern (some spheres in different layers are shaded to aid visualization).

Figure 5. (a) The spatial distribution of square domains on two substrates of different width. (b) An SEM image of the boundary between a regular square region and a hexagonal region. (c) A cross-section image of the boundary between a regular square region and a hexagonal region. (d) The percentage of square domains as a function of the substrate width.

Figure 4. (a) An SEM image of a 1 × 3 periodic surface structure with some vacancies. (b) An AFM image of a 1 × 5 periodic surface structure. (c) AFM line traces of the surface height profile of the colloidal surfaces along the marked lines in panel b. (d) Schematic side view of the proposed 3D nanosphere packing pattern with a periodic 1 × 3 surface structure (top) and a regular square surface structure (bottom).

crystal (fcc) with a (001) facing surface. Figure 3d is a 3D schematic view, with the top view corresponding to the (100) plane. Due to distortion, the local structure may not be exactly square and close packing, i.e., it could be face-centered tetragonal (fct) structure. From now on, we use the term “fcc structure” to mean a generalization of these structures. We can show that the 1 × 5-type superstructure observed in Figure 1b is probably a surface reconstruction of the (001)facing fcc structure. Figure 4a shows an SEM image of another domain with a 1 × 3 periodic structure. The point defects within the structure reveal that the surface nanoparticles are also sitting on the hollow sites of the subsurface square-arranged layer (Figure 4a). However, despite the obvious differences existing between the close-packing top layer and the non-close packing of the subsurface layer, the average areal density of particles has been found to be comparable. This suggests that the 1 × n domain is a manifestation of surface reconstruction of a (001) facing of nonclose packing colloidal structure. Figure 4d gives the projected image of such a structured packing pattern. An unexpected result

is the small height difference between spheres in the trench and those on the top surface. Figure 4c shows an AFM tracing over the lines shown in Figure 4b. The height difference is only 70 ( 10 nm, which is much less than the predicted height difference of 212 nm between nanoparticles in the first and second surface layers of an ideal fcc structure. It is also much smaller than that expected even if we allow for larger particle-to-particle spacing in a non-close packing of nanospheres. We believe that it is a sign for large distortion of the shape of the nanoparticles. We have discovered that these large-scale (100)-oriented regions of the colloidal films appear to concentrate near the edge of the wafer (Figure 5a), while the hexagonal arrangement is mostly observed in the middle of the wafer (which appears yellowish in Figure 5a). Support for an edge-related formation mechanism for the large square facing colloidal crystal formation comes from the results of varying the width of the silicon substrate. To get a statistical analysis of the proportion of the square domains verses the hexagonal domains, we have taken SEM images of the colloidal crystal film with 5000× magnification every 0.08 mm, and count the percentage of square array observed in every photo. As shown in Figure 5a, a large percentage of the square domains actually occurs near the edge of the wafer for the two representative substrates. What should be mentioned is that the square domains can match very well with hexagonal domain without transition zones existing between them (Figure 5b), and the adjacent regions with two different lattices possess the same

Meniscus Effect on Self-Assembly of (001) Crystals

Figure 6. (a-c) Optical microscopy images of the colloidal films on the substrates. (d-f) Photos of the convex and concave suspension meniscus observed on the corresponding silicon substrates, respectively.

thickness (Figure 5c), which indicates that square domains could form independently and not be as transition regions for hexagonal multilayer from n to n+1 layer. Using this method, we also have plotted the typical size of square domain width as a function of the substrate width (Figure 5d). When the substrate is 1.7 mm wide, the region with square domains can be about 1.3 mm wide, almost covering the whole substrate. However, as the substrate width increases, the width of the square domain narrows rapidly. If the substrate is wide enough, a large area of ordered hexagonal arrays was observed, and large square domains were rare. Only small patches of square arrangement are found in the initial stage of the film growth or in the transition layer where colloidal crystal transits from n to n+1 layer in the central areas dominated by hexagonal arrangement of spheres. They are consistent with the reports in the literature,25 and are therefore not the subject of focus in this paper. It is also noteworthy that the relationship appears to be independent of the substrate position at which the measurement is made, thus demonstrating the uniformity of the self-assembled films covering a length scale of millimeter magnitude. However, for the sample with the 3 mm wide substrate (Figure 6a), the green regions occur only near the edge of the wafer or in the transition layer and present narrowly.

4. Discussion 4.1. Optical Response and fcc-like Structural Model. Further evidence supporting the fcc-like structure of the nanosphere colloidal films is the optical response observation. Reflectivity of fcc crystals as a function of colloidal crystal orientation has been investigated both theoretically and experimentally.26-31 The greenish reflection can be associated with the Van Hover-like high density of photonic states in the (001) projections of fcc structure made up of 290 nm spheres. The corresponding yellow peak is associated with the high density of photonic states of the fcc crystal in its (111) projection. The small variation within the color bands may be attributed to films with the same crystallophic projection but different film thicknesses. (25) Ciampi, E.; Goerke, U.; Keddie, J. L.; McDonald, P. J. Langmuir 2000, 16, 1057–1065. (26) Garcia, P. D.; Galisteo-Lopez, J. F.; Lopez, C. Appl. Phys. Lett. 2005, 87, 201109. (27) Johnson, S. G.; Joannopoulos, J. D. Opt. Express 2001, 8, 173–190. (28) Miguez, H.; Kitaev, V.; Ozin, G. A. Appl. Phys. Lett. 2004, 84, 1239– 1241. (29) Galisteo-Lo´pez, J. F.; Lo´pez, C. Phys. ReV. B 2004, 70, 035108. (30) Garcia-Santamaria, F.; Galisteo-Lo´pez, J. F.; Braun, P. V.; Lopez, C. Phys. ReV. B 2005, 71, 195112. (31) Blanco, A.; Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Lo´pez-Tejeira, F.; Sa´nchez-Dehesa, J. Appl. Phys. Lett. 2001, 78, 3181–3183.

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4.2. Edge Effects. The size and location of the large square domains follow curved contact lines suggesting that the edge effect may be related to the changing shape of the contact lines at the edge of the wafer. For this reason, we have examined the shape of the meniscus at the vertically inclined Si wafer as it emerges out of the colloidal solutions, as shown in Figure 6d-f. This shows that the meniscus is found to be pinned downward at the edge of the silicon wafer. This can be understood because we have used a piece of aluminum block as a support to a narrow silicon wafer strip. The solution is nonwetting on the oxidized surface of the aluminum block comparing with the wetting behavior on a chemically treated silicon wafer. At the edge of the substrate, an abrupt change of the contact angle happened, which results in a strong deformation of the meniscus. From a comparison of the picture of the meniscus (Figure 6d) and that of the distribution of the green patches representing the square domain facing crystals (Figure 6a), we can see clearly that the (001) facing crystals are mostly concentrated near the edge of the wafer where the meniscus is heavily deformed. At the center of the wafer, green patches are found only as a line separating regions of different shades of yellow, as expected. However, the bending contour may be related to the instabilities of the bowing contact line as it sweeps through the wafer. This suggests that the contact line may be pinned at the regions where square domain crystals are formed, further deforming the meniscus locally. In addition, with the decreases of the wafer width (Figure 6e) the square domains at the edge merge to promote a large patch of square domains in the middle (Figure 6b). Interestingly, now the green regions dominate with the yellow lines decorating the edge of the green patches, i.e., the hexagonally arranged colloidal surface is now a transitory phenomenon. We also artificially make the meniscus have a concave downward bow shape as shown in Figure 6f, and we can see that the green region now appears at the interior (Figure 6c). With the change of the meniscus shape from convex gradually to concave, not only does the size of the green patch decrease, but their location also moves. The role of meniscus deformation could also explain the size of the square domain observed. This is related to the characteristic length of the capillary wave, which is given by33-35

L)

 ∆Fgσ

(1)

and is about 5 mm for the water/silica/air interface.32 Our result suggests that millimeter-sized square domain patches are possible and that our approach is both fundamentally and technologically different from chemically patterned approaches where the length of the patterned area is often much smaller than the characteristic capillary length.33-35 4.3. The Microscopic Origin of Crystal Surface Control. The details of the deformed meniscus on the orientation selectivity of the colloidal crystals are not fully understood and require further investigation, but a few key factors can be identified by considering the existing knowledge of structural formation of colloidal crystals and the features of deformed meniscus with respect to flat contact lines. The self-assembly of monodispersed colloidal particles into a monolayer of close-packed lattice at a straight liquid-air-solid contact line has been well understood (32) Dushkin, C. D.; Kralchevsky, P. A.; Yoshimura, H.; Nagayama, K. Phys. ReV. Lett. 1995, 75, 3454–3457. (33) Schaak, R. E.; Cable, R. E.; Leonard, B. M.; Norris, B. C. Langmuir 2004, 20, 7293–7297. (34) Peyrade, D.; Gordon, M.; Hyvert, G.; Berton, K.; Tallal, J. Microelectron. Eng. 2006, 83, 1521–1525. (35) Cui, Y.; Bjork, M. T.; Liddle, J. A.; Sonnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093–1098.

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as a convective assisted process.36,37 In such a convective assembly, colloidal crystallization is initiated by strong attractive immersion capillary force due to the curved meniscus between the particles as soon as the thickness of the liquid film is less than the diameter of the colloidal particles. The evaporation in the aggregated colloidal film and the resistance to water thinning due to the meniscus at the top of the nanoparticles induce a convective flow between liquid and nanoparticles from the surrounding solution. When suitably balanced, they could result in monolayer growth. As the strong attractive immersion capillary force is nondirectional, close-packing order prevails. Small nonclose-packing arrangement can be found experimentally, which may be understood as defects due to a finite space confinement effect when bounded by pre-existing close-packing domains.37 More relevant to our new approach is the multilayer colloidal film formation using the vertical deposition method. This occurs when the convective flow brings more nanoparticles than necessary for the growth of a 2D colloidal crystal. This can be either the result of high colloidal concentration in the dispersion or changes in the meniscus slope at the air-liquid interface,36,38 for example, by tilting the substrate.37 The self-assembly of multilayer colloidal films under the convective flux condition is complex and still a subject of research with many unanswered questions;38,39 however, clues may be obtained from earlier extensive and industrially important research into drying of thick colloidal films on a flat substrate.38 It is now well established that the colloidal particles first form a loose surface skin layer even before coming into contact at the dispersion air interface. Eventually the attractive immersion capillary force brings them into close contact just as in the monolayer case. In the second stage, the drying front moves downward from the top of the multilayer and laterally. This means that the structure is probably directed from the top layers. In our case, we also have to consider additional factors such as Laplace pressure due to a deformed meniscus and possibly a strong convective flow from the lateral growth front owing to the concentration variety along the contact line. A curved meniscus exerts a strong Laplace pressure. The large spacing between the self-assembled particles, much more than expected from a hard sphere model of the colloidal crystal films (Figure 2), and the much reduced spacing between the topmost and the underlying layers of nanospheres are testimony of such downward pressure. This will produce an additional shear force acting on the structure consolidation of the colloidal films, and the magnitude along with the direction is a function of the local curvature of the liquid surface.40-42 The importance of the shear is self-evident from the distribution of angles in the pseudo-square particle arrangement. This is consistent with the experimental observation that the interface between the square domain and the hexagonal facing domain often appears at an orientation that requires only shear flow moving all nanoparticles from the three particle cavity site in the hexagonal close-packed structure into the four particle (36) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303–1311. (37) Dushkin, C. D.; Lazarov, G. S.; Kotsev, S. N.; Yoshimura, H.; Nagayama, K. Colloid Polym. Sci. 1999, 277, 914–930. (38) Malaquin, L.; Kraus, T.; Schmid, H.; Delamarche, E.; Wolf, H. Langmuir 2007, 23, 11513–11521. (39) Norris, D. J.; Arlinghaus, E. G.; Meng, L. L.; Heiny, R.; Scriven, L. E. AdV. Mater. 2004, 16, 1393–1399. (40) Park, S. H.; Xia, Y. N. Langmuir 1999, 15, 266–273. (41) Shklover, V.; Braginsky, L.; Hofmann, H. Mater. Sci. Eng. C 2006, 26, 142–148. (42) Sun, Z. Q.; Chen, X.; Zhang, J. H.; Chen, Z. M.; Zhang, K.; Yan, X.; Wang, Y. F.; Yu, W. Z.; Yang, B. Langmuir 2005, 21, 8987–8991.

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contacting sites in the fcc structure. Additionally, it is known that the evaporation in the distorted meniscus is higher than that at the middle of the wafer.43,44 Meanwhile, there exists an effective water thinning and a higher local concentration of colloids at the edges of the substrate because of the wettability contrast between the substrate with aluminum support. This probably suggests that the convective flow is stronger at these areas where meniscus is deformed heavily and may modify the structure stability of the surface “seed” layer.25 Compared with the close-packing structure, the large pore size among the particles in square arrangement may allow high evaporation through the surface seed layer and could offer a broader channel for liquid flow.44,45 Therefore, the surface “seed” layer now is more in favor of non-close-packing arrangement, i.e., square arrangement. In addition, such flow also directs the subsurface layers to fill the void sites in the top layer, and an easy rule forms to follow for (001) facing colloidal growth. In this way, we can visualize that the order can propagate right to the bottom of the colloidal film. We may also understand the formation of 1 × n structure as the result of surface relaxation driven by the capillary immersion force associated with a receding drying front in a colloidal crystal whose formation is dominated by hydrodynamic and hydrostatic forces favoring a non-close-packing structure. However, until now, the controlled growth of 1 × n structure has not been tried, and the parameters influencing the number n are also not completely clear. Further systematic work is required to understand their formation in details, so that the morphology and placement of the 1 × n structure could be controlled, and the improved colloidal crystal surface quality could meet the need of potential applications.

5. Conclusions In summary, we have demonstrated a convenient and simple method for large-scale growth of (100)-orientated fcc colloidal crystalline films on nonpatterned silicon substrate by the FCVD method utilizing nanosphere self-assembly. The largest (100)orientated domains could be square millimeter magnitude, and the areas of such stacking pattern could be controlled by changing the width of the substrate. In addition, we also find that the square regions are apt to concentrate near the edge of the substrate, where the particles suffer additional shear force and higher evaporation. We have attributed these effects to the influence of meniscus. The dependence of the array formation on the meniscus opens a way for selective formation of square-facing colloidal films with different optical properties just by controlling the wettability of the substrate. The method is of much lower cost and easier manipulation than other template technologies. Colloidal crystals with large square domains exhibit different properties that are particularly valuable for application in optoelectronic devices, magnetic and optical data storage, microfiltration, and batteries. Acknowledgment. We thank the Ministry of Science and Technology of China (Basic Science Research Grant No. 2002CB613501) for financial support. LA8030717 (43) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Langmuir 1999, 15, 957–965. (44) Ko, H.-Y.; Park, J.; Shin, H.; Moon, J. Chem. Mater. 2004, 16, 4212– 4215. (45) Teh, L. K.; Tan, N. K.; Wong, C. C.; Li, S. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 1399–1404.