Inward-Growing Self-Assembly of Colloidal Crystal Films on Horizontal

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Inward-Growing Self-Assembly of Colloidal Crystal Films on Horizontal Substrates Qingfeng Yan, Zuocheng Zhou, and X. S. Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received October 29, 2004. In Final Form: December 17, 2004 Colloidal crystal films have been fabricated on solid substrates with a horizontal deposition method. Scanning electron microscope images showed that the colloidal crystal films exhibit ordered face-centered cubic structures in large domains. Optical measurements demonstrated the presence of photonic band gap along the crystallographic [111] direction. The fabrication method described in this paper allows one to rapidly fabricate colloidal crystal films of different thicknesses, which can be controlled by varying colloidal suspension concentration or volume. In addition, the method also works well for growing colloidal crystal films on a hydrophilic solid substrate with a rough surface. Furthermore, the fabrication of colloidal crystal heterostructures has been demonstrated. An inward-growing mechanism responsible for self-assembly of colloidal spheres on horizontal substrates has been proposed to interpret the observed experimental results.

1. Introduction Since the pioneering works of Yablonovitch1 and Johns,2 there has been a rapidly growing interest in the fabrication of photonic crystals (PCs) because of their technological applications in photonics,3 opotoelectronics,4data storage,5 chemical and biochemical sensors,6 and so forth. Traditional methods such as lithography for fabrication of PCs are expensive and tedious, particularly for three-dimensional (3D) PCs. In contrast, self-assembly of monodisperse spherical colloids has been exploited and demonstrated as a feasible approach to 3D PCs.7 Self-assembled colloidal crystals hardly possess a complete photonic band gap (PBG) because of the constraints of small refractive index contrast of readily available spherical dielectrics such as polystyrene (PS) and silica. However, they are ideal templates for constructing 3D PCs with a complete PBG.8 Over the past few years, many methods have been demonstrated for growing colloidal crystals such as gravitational sedimentation,9,10 physical confinement,11-13 electrophoretic deposition,14 centrifugation,15 withdrawal,16 * To whom correspondence should be addressed. Fax: 6567791936; e-mail: [email protected]. (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059-2062. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486-2489. (3) Chutinan, A.; John, S.; Toader, O. Phys. Rev. Lett. 2003, 90, 1239011-1239014. (4) Painter, O.; Lee, R. K.; Scherer, A.; Yariv, A.; O’Brien, J. D.; Dapkus, P. D.; Kim, I. Science 1999, 284, 1819-1821. (5) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.; McCord-Maughon, D.; Qin, J. Q.; Ro¨ckel, H.; Rumi, M.; Wu, X.-L.; Marder, S. R.; Perry, J. W. Nature 1999, 398, 51-54. (6) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534-9537. (7) Xia, Y. N. Adv. Mater. 2001, 13, 369. (8) Busch, K.; John, S. Phys. Rev. E 1998, 58, 3896-3908. (9) Miguez, H.; Meseguer, F.; Lopez, C.; Blanco, A.; Moya, J.; Requena, J.; Mifsud, A.; Fornes, V. Adv. Mater. 1998, 10, 480-483. (10) Zhu, J. X.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R. H.; Russell, W. B.; Chaikin, P. M. Nature 1997, 387, 883-885. (11) Park, S. H.; Qin, D.; Xia, Y. N. Adv. Mater. 1998, 10, 10281032. (12) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266-273. (13) Gates, B.; Qin, D.; Xia, Y. N. Adv. Mater. 1999, 11, 466-469. (14) Holgado, M.; Garcı´a-Santamarı´a, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Mı´guez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lo´pez, C. Langmuir 1999, 15, 4701-4704. (15) Schroden, R. C.; Al-Daous, M.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305-3315. (16) Dimitrov A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311.

flow-controlled vertical deposition,17 and solvent evaporation.18-27 Among these, the solvent evaporation method is the simplest one. There are two modes allowing growth of colloidal crystal films, that is, horizontal deposition and vertical deposition. Earlier studies on solvent-evaporationinduced self-assembly focused on horizontal deposition of two-dimensional (2D) colloidal arrays.18,19 Later, Kopnov and co-workers20 fabricated 3D colloidal crystals with this method. Subramania et al.21-23 described the fabrication of inverse titania film photonic crystals on horizontal substrates by using a colloidal solution containing both PS microspheres and titania nanoparticles. The resultant films exhibited both ordered and disordered structures because of the agglomeration of titania nanoparticles. Zentel and co-workers24-26 reported the horizontal deposition of colloidal crystal films on solid substrates. The procedure is time-consuming (several days) because of the high humidity required. Very recently, Velev et al.27 developed a controllable, rapid deposition method for fabricating colloidal crystal films on horizontal solid substrates. Monodisperse colloidal particles were confined in a thin aqueous film between two plates and crystallization of the colloidal particles was achieved when the upper plate was pulled away by a drive shaft with a proper speed. Several processes for deposition of colloidal crystals on a vertical solid substrate using the solvent evaporation method have also been described.28-31 (17) Zhou Z.; Zhao X. S. Langmuir 2004, 20, 1524-15263. (18) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190. (19) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26-26. (20) Kopnov, F.; Lirtsman V.; Davidov, D. Synth. Met. 2003, 137, 993-995. (21) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho, K.-M. Synth. Met. 2001, 116, 445-448. (22) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho, K.-M. Appl. Phys. Lett. 1999, 74, 3933-3935. (23) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho, K.-M. Adv. Mater. 2001, 13, 443-446. (24) Mu¨ller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Torres, C. M. S. Chem. Mater. 2000, 12, 2508-2512. (25) Mu¨ller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Torres, C. M. S. Adv. Mater. 2000, 12, 1499-1503. (26) Egen, M.; Zentel, R. Chem. Mater. 2002, 14, 2176-2183. (27) Prevo, B. G.; Velev, O. D. Langmuir 2004, 20, 2099-2107. (28) Meng, Q.-B.; Gu, Z.-Z.; Sato, O. Appl. Phys. Lett. 2000, 77, 43134315.

10.1021/la047337p CCC: $30.25 © 2005 American Chemical Society Published on Web 02/11/2005

Self-Assembly of Colloidal Crystal Films

The technological applications that PCs promise require a simple, rapid, cost-effective, and controllable fabrication method. However, the methods mentioned above are either complex, because of special facilities needed,11-13,16,17,27 or time-consuming9,10,24-26 or material-consuming28-31 or difficult to control the crystalline orientation and film thickness.9,10,15 Following Zental’s work,24-26 here we describe the fabrication of colloidal crystal films on horizontal solid substrates with a preliminary aim of shortening crystal growth time. Large-scale (several centimeters in area) colloidal crystal films of controllable thickness have been fabricated within 2 h without the use of any special facilities. Our experimental data have also shown that the fabrication process described in this paper is feasible for both organic (e.g., PS) and inorganic (e.g., silica) spherical colloidal particles grown on both glass and silicon substrates. In addition, deposition of colloidal crystal films on a substrate with a rough surface has been demonstrated. Moreover, it has been shown that colloidal crystal heterostructures can be fabricated by multiple depositions. The self-assembly mechanism of colloidal microspheres on a horizontal substrate has been interpreted. 2. Experimental Section Colloidal crystal films were fabricated from monodisperse PS colloids with diameters ranging from 0.26 to 1.1 µm and from silica colloids with an average diameter of 0.39 µm. The PS spheres were prepared by using an emulsifier-free emulsion polymerization technique.32 The silica spheres were synthesized following the Sto¨ber-Fink-Bohn method.33 Both colloids had a standard size deviation of less than 5%. Glass substrates (microscope glass slides, 22 × 22 × 0.3 mm3, Marienfeld, Germany) and silicon substrates (20 × 20 × 0.5 mm3, HNS, Singapore) were pretreated with conventional semiconductor cleaning process. First, the substrates were bathed in a mixture containing concentrated sulfuric acid (96%, Ashland Chemical Inc.) and hydrogen peroxide (30%, Ashland Chemical Inc.) (H2SO4: H2O2 ) 3:1, volume ratio) for 10 min (WARNING: the above piranha solution reacts violently with organic materials. Handle with caution.) Then, they were treated in an ultrasonic bath containing ammonia solution (30%, J. T. Baker Inc.), hydrogen peroxide, and deionized water (ELGA system, Chemoscience Pte Ltd.) with a volume ratio of NH3:H2O2:H2O ) 1:2:5 for 5 min. Finally, the substrates were bathed in a mixture containing hydrochloric acid (36.5%, J. T. Baker Inc.), hydrogen peroxide, and deionized water (HCl:H2O2:H2O ) 1:2:7, volume ratio) for another 5 min. After that, the substrates were washed with copious deionized water and dried in nitrogen gas flow before use. A few drops of colloidal suspension of a given concentration were placed on a substrate and carefully spread to fully cover the substrate surface (the details of dropping and spreading procedure were illustrated in Figure S1 in Supporting Information). The drop volume of the suspension was measured using a finnpipet (Labsystems, J36207, 10∼100 µL). The spread suspension was exposed to ambient conditions (temperature 20 °C and relative humidity 28%). The optical photographs of the resultant samples were acquired using a digital camera (Nikon Coolpix3700). The microstructures of the colloidal crystal films were imaged with a scanning electron microscope (SEM) (JEOL JSM-5600LV) and field emission SEM (FESEM) (JEOL JSM-6700F). The surface morphology of the silicon substrates on which colloidal crystal films were deposited was imagined using a Nanoscope III atom (29) Gu, Z.-Z.; Kubo, S.; Qian, W.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. Langmuir 2001, 17, 6751-6753. (30) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630-11637. (31) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132-2140. (32) Shim, S. E.; Cha, Y. J.; Byun, J. M.; Choe, S. J. Appl. Polym. Sci. 1999, 71, 2259-2269. (33) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69.

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Figure 1. Optical photograph of a colloidal crystal film selfassembled from PS spheres of 0.26 µm in diameter on a horizontal glass substrate (the unit of the ruler below the sample is cm). A void is seen on the bottom-left corner of the colloidal crystal film because of the applying of an air flow indicated by the arrow during colloidal crystallization. Insets are SEM images of the void area; scale bars in a and b are 500 µm and 5 µm, respectively. force microscope (AFM) under the tapping mode. Optical reflection spectra were measured on a UV-vis-near-infrared spectrophotometer (Shimadzu UV-3101PC) with a 5-mm aperture.

3. Results and Discussion 3.1. Deposition of Colloidal Crystal Films. Upon dropping a colloidal suspension on a horizontal substrate, followed by careful spreading to fully cover the substrate surface, it was observed that the edge of the substrate began to exhibit iridescent color after a few minutes and the iridescent color gradually moved from the periphery toward the center of the substrate. Figure 1 shows the optical photograph of a representative colloidal crystal film of PS spheres with a diameter of 0.26 µm. A single color, namely, red iridescence, is seen, showing the uniformity of the crystal film. When a square-shaped glass substrate (2.2 × 2.2 cm) was used, it took about 2 h to complete crystallization under ambient conditions (temperature 20 °C and humidity 28%). Interestingly, a macroscopic circular void can be seen as highlighted in Figure 1. During the growth of the colloidal crystal film, an air flow was applied as indicated by the arrow. Thus, the void was formed on the left-bottom corner of the substrate. Without applying such an air flow, the void was observed to be located in the center of the film or the substrate, see Figure S2 in Supporting Information. Thus, the void can be positioned or even removed by using a gas flow. To the best of our knowledge, it is the first time to describe the formation of such a void in a colloidal crystal film. The presence of the void is a strong indication of the mechanism of colloidal crystal growth, which will be discussed in the following sections. The insets in Figure 1 are the SEM images around and inside the void. It can be clearly seen that only a few isolated monolayers or double layers formed on the substrate of the void area. The observations from Figure 1 indicate that the colloidal crystals grew inward from the periphery toward the center of the spread suspensions on the horizontal substrate. 3.2. Microstructure of the Colloidal Crystal Films. Figure 2 shows the SEM images of a colloidal crystal film deposited on a glass substrate from a PS colloidal suspension of 8 wt % concentration, viewed at different magnifications and along different directions. When viewed at a low magnification (Figure 2a), it is seen that the film is composed of many crystalline domains sepa-

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Figure 2. SEM images of colloidal crystal films grown on glass substrates from PS sphere of 0.40 µm in diameter and 8.0 wt % in concentration (relative humidity: 28%, temperature: 20 ( 0.5 °C). (a) and (b) surface images, (c) a side view, and (d) a cross section view.

rated by cracks. The domain size can be as large as 50 × 100 µm (see Figure 2b). When viewed at a larger magnification and from different directions (Figure 2c and d), it is seen that the spheres were organized as ordered close-packed face-centered cubic (fcc) structure with the (111) planes parallel to the substrate surface. The singlecrystalline close-packed array extended over the entire substrate (2.2 × 2.2 cm) except the center where the macroscopic void formed and the film periphery where the colloidal crystallization began to take place. A certain degree of disorder, probably because of particle polydispersity, site randomness, and dislocation, is also seen. The formation of the cracks is most likely due to the shrinkage of the self-assembled spherical particles during the drying process because the crystallographic orientations are the same on both sides of the cracks. SEM images of colloidal crystal films deposited on glass substrates from colloidal spheres of different diameters or compositions can be found in Figure S3 in the Supporting Information. It can be seen that colloidal spheres as large as 1.1 µm in diameter can be fabricated as ordered array by the horizontal deposition method. Besides PS colloidal spheres, silica spheres have also been packed into colloidal crystal films using this method as demonstrated by the SEM image shown in Figure S3f. All samples exhibit an fcc crystalline structure with the (111) planes parallel to the substrate surface. To investigate the thickness uniformity of the resultant colloidal crystal films, the morphology along the growth direction (from the edge to the center of the films) has been investigated. A similar layering transition phenomenon (from monolayer to multilayer) as described previously18,27 has been observed at the edge of the colloidal crystal film (see Figure S4 in Supporting Information). Nevertheless, a relatively uniform and large-scale colloidal crystal film forms several thousands of micrometers away from the film edge. The variation of the film thickness was less than five layers over a scale of 1 cm. 3.3. Optical Properties of the Colloidal Crystal Films. Figure 3 shows the optical transmission and reflection spectra at normal incidence to the sample surface (along the crystallographic [111] direction) taken from a colloidal crystal film of 17 monolayers fabricated using PS spheres with an average diameter of 0.4 µm. The dip in the transmission and the peak in the reflection spectra match very well at the wavelength of about 950 nm, indicating that the optical response is directly induced by

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Figure 3. Optical transmission and reflection spectra for colloidal crystal film with 17 layers of 0.4-µm PS spheres measured with a light incident along the surface normal ([111] direction).

Figure 4. Reflection peak wavelength versus PS spheres diameter for normal incidence.

the photonic band gap along the [111] direction.22,23 The Fabry-Perot fringes34 around the photonic band gap can be clearly seen and are mainly caused by the interference of the light reflected by the opposite surfaces of the film (the abnormal flutter in the spectra around 832 nm is a systematic error due to the detector change of the spectrophotometer). These fringes also indicate the uniformity of the thickness and the similarity of orientation of the colloidal crystal films,34 which can be confirmed by the SEM data shown in Figure 2d. In the conventional vertical deposition method, the concentration of a colloidal suspension is increasing during solvent evaporation, resulting in a variable thickness of the resultant colloidal crystal film.31 However, for the horizontal deposition method described in this paper, the concentration of the colloidal suspension spread on the substrate surface is hardly affected by water evaporation. As will be discussed later on, the meniscus area for the present horizontal deposition method is relatively very large, resulting in a fast colloidal crystallization. Water evaporated from the bulk suspension will be complementary with the colloidal particles assembled via the meniscus. Thus, the colloidal suspension concentration remains approximately a constant during the crystallization (except for the end stage). This is why the horizontal deposition method can afford uniform colloidal crystal films in a large area. Figure 4 shows the dependence of the reflection peak position upon the diameter of PS spheres in the case of normal incidence to the colloidal crystal surface. The positions of the observed reflection peaks agree very well with the diameters of PS spheres, indicating that it is an (34) Wostyn, K.; Zhao, Y.; Schaetzen, G. de; Hellemans, L.; Matsuda, N.; Clays, K.; Persoons, A. Langmuir 2003, 19, 4465-4468.

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Figure 5. Number of layers for colloidal crystal films versus (a) suspension concentration (PS spheres of 0.40 µm in diameter, drop volume of 70 µL) and (b) drop volume (PS spheres of 0.26 µm in diameter and 1.5 wt % in concentration).

intrinsic feature of the photonic band gap. The solid straight line in Figure 4 is a linear fit to the peak wavelengths, which can be predicted by the Bragg’s law for the (111) family of planes:35

λc ) 2x2/3Dneff

(1)

where λc is the wavelength at the peak position, neff is the effective refractive index of the structure, and D is the diameter of the PS spheres. From the slope of the fitted straight line, it can be obtained neff ) 1.43. This value is approximately equal to the theoretically estimated value, 1.44, which was calculated by using neff ) fnPS + (1 f)nair, where f is the dielectric filling ratio (for an ideal fcc structure f ) 0.74) and nPS and nair are the refractive indexes of PS and air (here nPS ) 1.6 and nair ) 136). 3.4. Control over the Thickness of the Colloidal Crystal Films. It is important to be able to control the thickness of a colloidal crystal film because it is related to its optical properties.31,37-39 To investigate how the deposition parameters affect the thickness of the resultant colloidal crystal films, a series of PS colloidal suspensions with different concentrations or volumes were spread on a square glass substrate with an area of 22 × 22 mm2. Shown in Figure 5a and b is the dependence of the thickness of the resultant colloidal crystal films on the colloidal suspension concentration and drop volume, respectively. Figure 6 shows the reflection spectra of the resultant samples from different concentrations and drop volumes. It is seen that the thickness of the colloidal crystal films increases with the increase in suspension concentration and drop volume. In addition, the films exhibit (35) Goldenberg L. M.; Wagner, J.; Stumpe, J.; Paulke B.-R.; Go¨rnitz E. Langmuir 2002, 18, 3319-3323. (36) Lu, Y.; Yin Y.; Li Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722-7727. (37) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300-303. (38) Galistero-Lo´pez, J. F.; Palacios-Lido´n, E.; Castillo-Martı´nez, E.; Lo´pez, C. Phys. Rev. B 2003, 68, 1151091. (39) Reynolds, A.; Lopez-Tejeira, F.; Cassagne, D.; Garcia-Vidal, F. J.; Jouanin, C.; Sanchez-Dehesa, J. Phys. Rev. B 1999, 60, 1142211426.

Figure 6. Optical reflection spectra of colloidal crystal films from (a) PS spheres (diameter 0.40 µm and drop volume 70 µL) with different suspension concentrations and (b) PS spheres (diameter 0.26 µm and concentration 1.5 wt %) with different drop volumes. The light was incident along the surface normal ([111] direction).

stronger and stronger reflectance at the corresponding Bragg response wavelength location, as has been predicted and verified by other research groups.31,37-39 Thus, one can precisely control the deposition process to obtain colloidal crystal films with a desired thickness. However, it is impossible to fabricate colloidal crystal films with a thickness of several tens of layers by excessively increasing the suspension concentration or drop volume, especially for large colloidal particles. This is because a highconcentration suspension leads to a relatively fast solvent evaporation rate, leaving behind a large amount of colloidal spheres packed in a disordered manner since they do not have enough time to organize themselves into an ordered array. In using a large drop volume, the deposition process requires a relatively longer period of time and sedimentation of colloidal particles perhaps will affect the efficiency of the self-assembly process. For example, it has been observed that a sample fabricated on a square glass substrate (22 × 22 mm2) with 200 µL PS colloidal suspension of PS spheres of 0.4 µm in diameter and 8 wt % in concentration exhibited only several ordered layers on the top of the film while disordered structures appeared at the bottom region (see Figure S5 in Supporting Information). As will be noted, thick colloidal crystal films can be obtained by repeating the deposition process. 3.5. Deposition of Colloidal Crystal Films on a Solid Substrate with a Rough Surface. In most cases, hydrophilic solid substrates with a flat surface are used for colloidal crystallization from aqueous suspensions.16-31 However, the roughness of a substrate surface is of importance from both fundamental and technological points of view.40 On one hand, the roughness is related to the crystallization kinetics, which are determined by many (40) Liu Z.-J.; Shen Y. G. Appl. Phys. Lett. 2004, 84, 5121-5123.

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Figure 8. Optical reflection spectra of the colloidal crystal films described in Figure 7. The Fabry-Perot fringes disappeared in the colloidal crystal film deposited on the silicon substrate with a rough surface.

Figure 7. SEM images of the cross sections of colloidal crystal films (PS spheres of 0.26 µm in diameter) deposited on (a) a silicon substrate with a smooth surface and (b) a silicon substrate with a rough surface.

parameters such as adhesion force between the particles and the substrate surface.41 On the other hand, the surface roughness may also affect the performance of the resultant films. To investigate the effect of the substrate surface roughness on the array fashions and the optical properties of the resultant colloidal crystal films, a few drops of a 0.26-µm PS colloidal suspension were applied on two horizontal silicon substrates, of which one had a smooth surface and the other one had a rough surface (without polishing). The AFM images of the surfaces can be found in Figure S6 in Supporting Information. Further measurements from AFM showed that the former has a rootmean-square (RMS) roughness of about 0.356 nm and the latter has a RMS roughness of about 392 nm. Surprisingly, the roughness of the substrates does not seem to severely affect the array fashions of the colloidal crystal films, as can be seen from the SEM images in Figure 7, which shows the cross sections of the colloidal crystal film deposited on the smooth and rough silicon substrates, respectively. For the colloidal crystal film grown on the rough substrate, the (111) planes of the fcc structure are still approximately parallel to the substrate surface, similar to that on the smooth substrate. From Figure 7b, it can be seen that the peak-to-valley difference in height of the substrate surface is as large as 1.0 µm, thus forming many V-shaped pits. Because the colloidal particle size (0.26 µm) is much smaller than the height of these pits, self-assembly of the colloidal particles on the rough surface is believed to be similar to that on two inclined flat substrates dipped into (41) Go¨tzinger M.; Peukert W. Langmuir 2004, 20, 5298-5303.

colloidal suspensions, during which convective self-assembly is operative.42,43 Figure 8 shows the reflection spectra of the colloidal crystal films deposited on the smooth and rough silicon substrates in the case of normal incidence to the sample surface. The reflectance peaks at the corresponding Bragg response wavelength can be clearly seen for both cases and the peaks remain a good overlap. However, obvious differences of the reflection spectra can be seen. The intensity of the reflection peak of the colloidal crystal film grown on the rough silicon substrate is much lower than that of the film grown on the smooth one. In addition, the Fabry-Perot fringes around the photonic band gap, which is generally a consequence of the thickness uniformity of the colloidal crystal film,34 disappeared in the colloidal crystal film deposited on the rough substrate because of the nonuniform thickness resulting from the rough substrate surface. The fact that colloidal crystal film can also deposit on a rough substrate suggests that, at least in small colloidal spheres, the surface roughness is likely to play a less important role than that of surface properties of the substrate. 3.6. Fabrication of Colloidal Crystal Heterostructures. The fabrication of colloidal crystal heterostructures such as two-layer opaline films,44-46 sandwichlike colloidal crystals,34,47 and colloidal photonic superlattices48 based on the vertical deposition method has been described. Our horizontal deposition method can be employed to fabricate colloidal crystal heterostructures as well. By using a multiple deposition process, we have fabricated colloidal crystal heterostructures composed of either different kinds of colloids or the same colloid with different particle sizes. For example, Figure 9a shows a two-layer colloidal crystal heterostructure formed with 0.39-µm silica spheres (the bottom layer) and 0.26-µm PS spheres (the top layer). The crystalline order of each layer was well maintained after multiple depositions. In addition, as observed by other groups,34,44-48 the interface between the compositional colloidal crystals is smooth and the top-layer colloidal crystal can grow on the surface of the bottom layer in a (42) Cong, H.; Cao, W. Langmuir 2003, 19, 8177-8181. (43) Im, S. H.; Kim, M. H.; Park, O. O. Chem. Mater. 2003, 15, 17971802. (44) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589-15598. (45) Jiang, P.; Ostojic, G. N.; Narat, R.; Mitleman, D. M.; Colvin, V. L. Adv. Mater. 2001, 13, 389-393. (46) Egen, M.; Voss, R.; Griesebock, B.; Zentel, R. Chem. Mater. 2003, 15, 3786-3792. (47) Zhao, Y. X.; Wostyn, K.; Schaetzen, G. de; Clays, K.; Hellemans, L.; Persoons, A.; Szekeres, M.; Schoonheydt, R. A. Appl. Phys. Lett. 2003, 82, 3764-3766. (48) Rengarajan, R.; Jiang, P.; Larrabee, D. C.; Colvin, V. L.; Mittleman, D. M. Phys. Rev. B 2001, 64, 2051031-2051034.

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Figure 9. SEM images of the cross-sectional views of (a) asgrown and (b) after thermal treatment at 110 °C for 30 min of a colloidal crystal heterostructure composed of 8 layers of silica spheres (0.39 µm) on bottom and 15 layers of PS spheres (0.26 µm) on top.

Figure 10. Optical reflection spectra of the colloidal crystal heterostructure composed of 8 layers of silica spheres (0.39 µm) on bottom and 15 layers of PS spheres (0.26 µm) on top. The abnormal flutter in the spectra at around 832 nm is a systematical error due to the detector change of the spectrophotometer.

way similar to that growing on a flat solid substrate, which implies that one can obtain thick colloidal crystal films by multiple deposition technique. The solid line in Figure 10 shows the normal-incidence reflection spectra of the colloidal crystal heterostructures shown in Figure 9a (solid line), with the bottom phase consisting of eight layers of silica colloids of 0.39 µm in diameter and the phase of 15 layers of PS colloids of 0.26 µm in diameter. The abnormal flutter in the spectra at

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around 832 nm is a systematical error due to the detector change of the spectrophotometer. It can be seen that the colloidal crystal heterostructure exhibits two stop bands at about 638 and 820 nm, which are exactly the stop bands of the compositional PS and silica colloidal crystals. To further elaborate, the above colloidal crystal heterostructure was treated at 110 °C for 30 min. It can be seen from Figure 9b that the crystalline structure of the PS colloidal crystal (top layer) was destroyed because PS has a lower glass-transition temperature than silica. As a consequence, the stop band corresponding to the top-layer PS colloidal crystal disappeared as can be seen from Figure 10 (the dashed line). It is thus concluded that the band structure of the colloidal crystal heterostructure along the film stacking direction is a simple superposition of that of the individual colloidal crystal films. In short, our experimental data have shown that it is possible to fabricate high-quality colloidal crystal heterostructures, as well as thick colloidal crystals, by multiple deposition processes using the horizontal deposition method described in this work. 3.7. Self-Assembly Mechanism of Colloidal Crystal Films on Horizontal Solid Substrates. The possible self-assembly mechanisms of colloidal particles have been investigated previously.18,19,31,49,50 Dencove and co-workers18 described a self-assembly mechanism of colloidal particles confined in a concave aqueous layer. The growth of the colloidal crystal film was observed to originate from the middle of the aqueous layer. In the present study, the colloidal particles were confined in a convex layer and the growth front lay along the periphery of the substrate. Although some groups have found that colloidal crystallization can be achieved at the air-water interface via electrostatic interaction and lateral capillary forces,51,52 we believe that the colloidal crystallization on a horizontal solid substrate as observed in this work is mainly driven by a convective self-assembly mechanism.49,50 On the basis of the experimental results observed in this study, especially the appearance of a circular void in the center of the resultant colloidal crystal films, we propose an inward growing self-assembly mechanism as schematically illustrated in Figure 11. In a convex aqueous layer, the edge where a meniscus exists is much thinner than the middle region. With solvent evaporation, the thickness of the aqueous layer becomes thinner and thinner. When the thickness of the aqueous layer on the substrate periphery is approximately equal to the diameter of the colloidal particles, the particles in the aqueous layer periphery are forced to stay and nucleate because of immersion capillary forces.49,50 Since the evaporation rate at the periphery is much higher than that at the center,53 there is a convective flow from the center to the periphery. More colloidal particles from the bulk suspension are driven toward the meniscus by a convective transport to eventually self-organize around the nucleus to form a crystalline structure because of the attractive capillary forces. In our experiment, the convective flow was radial in distribution. As the colloidal crystallization proceeds from the periphery to the center of the square substrate because of water evaporation and meniscus shift, the concentration of the colloidal particles in the center of the (49) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383-401. (50) Nagayama, K. Colloids Surf., A 1996, 109, 363-374. (51) Zeng, F.; Sun Z.; Wang, C.; Ren B.; Liu, X.; Tong Z. Langmuir 2002, 18, 9116-9120. (52) Im, S. H.; Lim, Y. T.; Suh, D. J.; Park, O. O. Adv. Mater. 2002, 14, 1367-1369. (53) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829.

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Figure 11. A scheme showing the inward self-assembly mechanism for colloidal crystal films deposited on a horizontal solid substrate.

aqueous layer (i.e., the center of the square substrate) decreases gradually. As a result, when the colloidal crystallization front approaches the center of the substrate, the concentration of the colloids in the central region is very low, resulting in the formation of the circular void containing only some isolated colloid clusters. The appearance of the circular void in the film center strongly supports the inward growing self-assembly nature of such a horizontal deposition process. When an air flow was applied during the colloidal crystallization, the meniscus moved faster toward the center of the square substrate along the air flow direction, leading to a position shift of the void, as can be seen from Figure 1. Thus, one can control the position of the void or even eliminate it simply by applying an extra force to obtain a colloidal crystal film free of a void. In comparison with the conventional dip-coating method or convective self-assembly on a vertical substrate,28-31,42,43 the horizontal deposition method described in this paper exhibits three important advantages. First, it enables rapid fabrication of colloidal crystal films in a large area because of the formation of a large-area meniscus, which provides the front where colloidal crystallization takes place. For a square substrate, in either the dip-coating method or convective self-assembly on a vertical substrate,28-31,42,43 the meniscus exists only on one side of the substrate and the colloidal crystallization takes place along a single direction. For the horizontal deposition method, the meniscus exists along the periphery and the colloidal crystallization takes place from the periphery to the center of the substrate; thus, the meniscus area increases largely and colloidal crystallization becomes rapid. Second, the effect of particle sedimentation on colloidal suspension concentration change can be minimized because of the

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rapid deposition process. It is well-known that conventional vertical deposition methods require a relatively long period of time and the sedimentation of colloidal particles is unavoidable, especially for large particles, resulting in a dramatic change in colloidal suspension concentration. Although introduction of temperature gradient can minimize particle sedimentation,44,54 this involves many complex facilities, leading the process to be practically infeasible. In the inward growing self-assembly process, the effect of particle sedimentation on colloidal suspension concentration can be largely minimized as crystallization is very fast (within 2 h depending on colloidal suspension concentration and volume). In addition, the effect of solvent evaporation on the colloidal suspension concentration can also be neglected since the water evaporated from the bulk suspension might be complementary with the colloidal particles assembled via the large-area meniscus under a rapid colloidal crystallization process. Third, a huge saving of colloidal suspension is possible for the present horizontal deposition method whereas conventional self-assembly methods consume a relatively large volume of colloidal suspensions.28-31,42,43 Thus, the inward growing self-assembly process described in this paper is a fast, efficient, and cost-effective method for fabrication of large-scale colloidal crystal films. Conclusions In summary, the fabrication of high-quality colloidal crystal films on horizontal substrates with different surface roughness has been demonstrated with a simple and efficient inward growing self-assembly method. For a substrate of a given area, the thickness of the resultant colloidal films grown on the substrate can be controlled by either suspension concentration of colloidal spheres or the volume of the suspension. Colloidal crystal films can be grown on solid substrate with a rough surface, suggesting that surface roughness of the substrate plays a less important role than its surface properties (hydrophilicity/hydrophobicity). In addition, the self-assembly method is feasible for fabricating colloidal crystal heterostructures. The mechanism of colloidal crystallization on horizontal solid substrates is interpreted in terms of an inward growing self-assembly. Acknowledgment. We are grateful to A*STAR (Agency for Science, Technology And Research) of Singapore for financial support. Supporting Information Available: A schematic diagram showing how to drop and spread the colloidal suspension on horizontal substrate, an optical photo showing a colloidal crystal fabricated in absence of an air flow, SEM images of colloidal crystals from PS and silica spheres with different sizes, an SEM image showing the layering transition phenomena in the periphery of the colloidal crystal film, an SEM image showing the colloidal crystal fabricated with excessive drop volumes, and AFM images showing the surface morphologies of a smooth and rough silicon substrate.These materials are available free of charge via the Internet at http://pubs.acs.org. LA047337P (54) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289-293.