In Situ Optical Microspectroscopy Monitoring of Binary Colloidal

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In Situ Optical Microspectroscopy Monitoring of Binary Colloidal Crystal Growth Dynamics via Evaporation-Induced Cooperative SelfAssembly Lei Yang, Jinze Wang, Yiduo Zhang, Yanhong Luo, Dongmei Li, and Qingbo Meng* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Post Office Box 603, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: Real-time monitoring of the binary colloidal crystal (bCC) growth via evaporation-induced cooperative self-assembly (EICSA) was studied by an in situ optical microspectroscopy technique. Evolution of the recorded reflectance spectra reveals that the whole growth process of bCCs via EICSA could be separated into three different stages corresponding to that of unary colloidal crystals because of the same evaporation model. We show the detailed cooperative self-assembly information, including the evolution of the number of layers and filling factors of different components of the growing bCCs using the scalar wave approximation method. Furthermore, when the size ratio and number ratio of the two colloids were varied, the real-time optical properties of the bCCs with various stoichiometric configurations were investigated systematically. This study would be valuable in furthering the current understanding of the bCC growth dynamics via EICSA and tailoring optical properties of hierarchical materials for applications in many fields.

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INTRODUCTION Three-dimensional binary colloidal crystals (bCCs) play a pivotal role in the fabrication of highly ordered porous materials with multiscale structures, which have been shown to be potentially useful in a wide range of fields, including photonics,1−3 catalysis,4,5 sensors,6,7 and photoelectrochemical solar cells.8−10 Various techniques aiming at realizing bCCs in a controllable fashion have been investigated over the past decade.11−26 Among all of these techniques, evaporationinduced cooperative self-assembly (EICSA), hierarchical organization of colloidal particles of two sizes in one step, is now invoking intense interest for its versatility and facility.18−26 The key idea in EICSA, by either vertical18−23 or horizontal deposition,24−26 is that the large and small spheres are spontaneously transported to and concentrated at the growth front of an evaporating binary colloidal suspension by the convective fluid flow. Large spheres are finally assembled into ordered, predominant face-centered cubic (fcc) packings, while small spheres are settled into the voids of the close-packed hexagonal large sphere arrays. Such a one-step process tends to proceed more conveniently, reject defects, and provide periodic structures with higher quality than can be reached by a step-bystep approach. Remarkably and in contrast to the great attention that has been focused on developing the EICSA technique, less effort has been devoted to understanding the dynamic process, which is crucial to unravel the EICSA mechanisms and make the rational design of more perfect structures. This lack of investigation is partially a consequence of the complicated © 2012 American Chemical Society

multibody system with different dimensions and the unavailability of direct observations of small nanoparticles. There have been a few attempts by in situ observations through optical microscopes to study the self-assembly process, in which the dimensions of the observed particles for self-assembly are all above 700 nm.27−31 However, it is very difficult to apply this approach to the cooperative self-assembly of bCCs, because the size of small particles is usually below 200 nm, which is the highest resolution of optical microscopes.32 Another critical issue for the in situ monitoring technique applications is the need for the whole three-dimensional growth information. In this regard, several authors have applied other monitoring techniques to investigate the crystallization and dry process of a unary colloidal sessile drop.33−35 Recently, a real-time investigation of the whole growth process of colloidal photonic crystals from nucleation to drying in a bulk suspension was achieved by our group.36 By coupling an in situ reflectance monitoring with microscopic capabilities, the disadvantage of traditional methods of averaging information over large sample volumes was overcome. Also with a greatly enhanced signal-tonoise ratio, this in situ characterization technique has proved to be an efficient way for understanding the direct relation between structural and optical properties of colloidal photonic crystals during self-assembly. Received: December 27, 2011 Revised: February 3, 2012 Published: February 9, 2012 4160

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In this work, we present a full account on the in situ optical microspectroscopy investigation of the bCC growth dynamics via EICSA. The real-time evolution of the optical properties of the bCCs was achieved when the film was being deposited vertically in an in situ cuvette. To the best of our knowledge, this is the first real-time study of the cooperative self-assembly system of multiscale colloids, which serves as an excellent model system for understanding the mechanism of the widely occurred self-organization phenomenon in nature. To relate the real-time reflectance spectra obtained to specific characteristics of the growing bCCs, the Bragg diffraction profiles were fitted by the scalar wave approximation (SWA) method.37−39 Moreover, to further study the optical properties of the bCCs, a comparison was made to the results from systematical experiments performed by manipulating the size ratio and concentration of the two monodispersed spheres.

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EXPERIMENTAL SECTION

Polystyrene Colloidal Suspension and Substrates. Monodisperse polystyrene (PS) colloidal spheres of 100 nm [size distribution of 5.1% coefficient of variation (CV)], 140 nm (6% CV), 170 nm (5% CV), and 820 nm (3% CV) were bought from Duke Scientific Corporation. Binary PS colloids were prepared by mixing small spheres (100, 140, or 170 nm) and large spheres (820 nm) with Figure 1. (a) Schematic of the in situ cuvette under optical microspectroscopy monitoring with a glass substrate stuck vertically to the rear wall. Geometry of a (b) tetrahedral site and (c) octahedral site formed by four and six mutually adjacent spheres, respectively. (d) Real-time evolution of the reflectivity for bCC B4 during EICSA. The black dots indicate the peak position of each reflectance spectrum.

Table 1. Parameters of Binary Colloidal Samples Used in This Study large spheres

sample A0 B2 B4 B8 C2 C4 C8 D2 D4 D8

small spheres

DL (nm)

VF (%)

DS (nm)

820

1.5

NA

820

1.5

170

820

1.5

140

820

1.5

100

VF (%) NA 0.0267 0.0535 0.1069 0.0149 0.0299 0.0597 0.0054 0.0109 0.0218

diameter ratio (DS/L) NA 0.207

0.171

0.122

volume fraction ratio (VFS/L)

particle number ratio (NS/L)

NA 0.0178 0.0356 0.0713 0.0100 0.0199 0.0398 0.0036 0.0073 0.0145

NA 2 4 8 2 4 8 2 4 8

stream, which was on the top of and far away from the meniscus region. To be located far from the meniscus was deemed crucial because the environment of the film deposition on both the front wall and the substrate should be as nearly the same as possible and not disturbed. The real-time data acquisition was started as soon as a Bragg diffraction signal from the growing bCC on the front wall became detectable. Moreover, the postgrowth structural properties of the bCCs on the substrate were characterized by scanning electron microscopy (SEM; FEI, NovaNano 430).

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RESULTS AND DISCUSSION

Growth Dynamics of bCCs Analyzed by the Reflectance Evolution. Shown in Figure 1d are representative reflectance spectra corresponding to the real-time data collected for sample B4 (Table 1). In the initial period, the Bragg diffraction peak was absent because most of the light was scattered by the homogeneous colloidal suspension. We defined as zero time for the growth process the instant at which we can detect the weakest Bragg diffraction peak generated by the photonic band gap (PBG) structure nucleating at the cuvette wall.36 As the growth proceeds, the blue shift of the peak position goes hand in hand with the increase of the stop-band attenuation (also see Figure S2a of the Supporting Information). The black dots in Figure 1d highlight the peak position variations, as discerned from data in Figure 2a. As witnessed repeatedly by many experiments and simulations, monodispersed spherical particles are more inclined to self-assemble into the fcc structure with the (111) plane parallel to the substrate interface during vertical deposition.40,41 When the light is incident on the fcc colloidal

different volume fraction ratios (see Table 1) and diluted with ultrapure water (Milli-Q Synthesis System, Millipore S.A., Molsheim, France). The glass observation cuvette (Yixing Jingke Optical Instrument Co., Ltd.) and glass substrates were first immerged in chromosulfuric acid for more than 3 h, followed by washing with copious amounts of ultrapure water, and then dried in a nitrogen gas flow. The substrate was stuck vertically to the inner wall of the cuvette before use, as shown in Figure 1a. Experimental Setup and Investigated Procedure. The in situ optical microspectroscopy monitoring setup, which was previously developed by our group,36 basically consists of two parts and was built on a nonvibration optical table (shown schematically in Figure S1 of the Supporting Information). The first part was a reflected light microscopy attached to a monochromator and a PbS detector, and the second part was the vertical deposition system, where the glass cuvette was used as the suspension container and its front wall was acting as the substrate. After the cuvette filled with suspension was fixed to a three-dimensional motorized precision stage, the objective should be positioned on the center of the front wall and focused on a particular spot, where it remained for the entire period of data collection. The evaporation of the solvent was controlled by introducing a hot air 4161

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Figure 2. (a) Evolution of the peak position with elapsed time corresponding to the reflectance spectra for bCC B4 recorded in the real-time monitoring experiment (show in Figure 1). Schematic diagrams in the plot represent the EICSA growth process of the three stages mentioned in the text. The peak position extracted from the SWA-calculated reflectance as a function of the (b) number of layers for bCC B4 with its interstitial voids full of water and (c) effective dielectric constant of the background medium (Em) for a single 11-layer colloidal crystal, respectively. Normalincidence reflectance spectra for the growing bCC B4 when the elapsed time is (d) 14 min, (e) 35 min, (f) 63 min, (g) 91 min, and (h) 182 min. The red solid curves are calculated using the SWA with different values of Em of (d) 1.810, (e) 1.540, (f) 1.378, (g) 1.253, and (h) 1.155. The dashed lines highlight the peak position variations.

colloidal particles), neff is the effective refractive index of the crystal, and θ is the incident angle. In our case, the incidence angle is 0 under normal incidence, and the effective refractive index is given by a volume-weighted average. neff = [(0.74 + φS)nPS2 + φw n water 2 1/2

+ (0.26 − φS − φw )nair 2]

Here, φw and φS are the volume fractions occupied by water and small spheres, respectively, and 0.74 is the volume fraction of big spheres for a close-packed structure. Additionally, a simple geometric analysis showed that, in these close-packed sphere arrays, two types of interstitial sites exist: the tetrahedral site and the octahedral site (depicted in panels b and c of Figure 1). One cubic unit cell contains four large spheres, eight tetrahedral voids (T), and four octahedral voids (O). As for sample B4, only one small sphere (170 nm) can be placed at a random position within the T void for 0.172 < γS/L < 0.225, while the larger octahedral site, formed by six large particles, may host at least eight for γS/L < 0.229.22 Hence, all small spheres for sample B4 can be fitted in the interstitial voids among the large spheres during cooperative self-assembly, and φS should be 0.74VFS/L in theory. In terms of eqs 1 and 2, the growing bCCs, therefore, exhibit a blue shift in the peak position because of not only the increasingly compact lattice but also the decreased effective refractive index. Besides, the Bragg peak position extracted from the SWA-calculated reflectance also appears largely dependent upon the number of layers in the [111] direction of the fcc structure, as explained later in this section.

Figure 3. Variations of volume fractions of different components in the interstitial voids of bCC B4 with the elapsed time. The total volume fraction of the three components (PS, water, and air) is 0.26. The open circles indicating the volume fraction variation of water are extracted from the SWA, while the red solid curve represents the best fit from the evaporation model mentioned in the text. Two dotted lines divide the purple area into three regions corresponding to the three stages for bCC growth.

crystal (CC), the diffracted wavelength (λ) from the crystal can be given by the modified Bragg’s law42 mλ = 2d111(neff 2 − sin 2 θ)1/2

(2)

(1)

where m is the diffraction order, d111 is the interplanar spacing of (111) planes, which equals (2/3)1/2D (D is the diameter of 4162

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Figure 4. (a) Photograph of the growing bCC B2 in the in situ cuvette illuminated by the light coming from the objective. (b) Low-magnification SEM top view onto bCC B2. (c) Close-up image of bCC B2. Red circles indicate the small spheres in the middle slit of two big spheres. (d) SEM cross-section view of bCC B2. (e) SEM top view onto bCC B4 (inset: a higher magnification view) and (f) SEM top view onto bCC B8. All scale bars represent 1 μm, unless otherwise specified.

In our previous study,36 the whole growth process of unary colloidal crystals was separated into three stages, which could also be used to elucidate the evolution of optical properties of the bCC during EICSA because of the same evaporation model. At the very beginning, the large and small particles are entrained toward the growth front at the same velocity, and the large spheres nucleate first in a close-packed network because of their higher volume fraction and larger size. With the evaporation and convective flow of the liquid medium through the pore space of the network, the small spheres can easily move and fill the voids of the fcc structure. The data in Figure 2a suggest that the drying front in the early stage of the bCC growth (T1) has not entered the monitored region. After the growth front has receded far enough from the top edge of the monitored region that the wicking action of the interstitials can no longer sustain the flow of the solvent, water in the upper region of the bCC starts to evaporate, as schematically shown in Figure 2a (T2). During this stage, two main factors make the blue shift of the Bragg peak position. One is the evaporation of water in the interstitials, and the other is the minor size change during drying. The colloidal particles are suspended in an aqueous medium and, thus, must have a hydrophilic and most likely charged surface layer, leading to a hydration shell of the particle in the wet crystal lattice. The hydration shell leads to an effective larger sphere radius compared to the dry particles in the dry and air-filled crystal. In the third stage, corresponding to the region (T3) in Figure 2a, the growth front drops out of the monitored region and the bCC completely undergoes a drying

process. The time dependence of the Bragg peak position is manifested in good agreement with the separation of the bCC growth process, which further confirms the three-stage growth mechanism. Panels d−h of Figure 2 show a typical result for the fit of the experimental reflectance spectra with that calculated from SWA, full details of which have been described elsewhere.36,37 When the number of layers (N) and the effective dielectric constant of the background medium (Em) were varied, the calculated Bragg diffractive profiles were fitted to be as identical as possible with the experimental reflectance. A remarkable and unexpected difference between the calculated and experimentally measured bandwidth of the photonic gap, however, can be observed. Considering the complicated dynamic process of EICSA, many factors influence the optical properties of the growing film, including the thickness polydispersity, medium in the interstices of the crystal, the hydration shell of the particle, the degree of order in the bCC, etc. Especially, the bCC fabricated by this self-assembly approach is far from a perfect photonic crystal, whereas the bandwidth is found to be very sensitive to the long-range disorder in the polycrystalline domain structure.43 It should also be noted that the stop bandwidth predicted by the SWA overestimates the exact result of the plane wave expansion method, as indicated in ref 44. We hereby fit the experimental spectra according to the peak position. The peak position extracted from the calculated reflectance by SWA as a function of the number of layers at a fixed Em and effective dielectric constant of the medium at a 4163

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Figure 5. SEM images of (a) bCC D2 (inset: a higher magnification view), (b) bCC D4 from a cross-section view, (c) bCC D4, (d) bCC D8, (e) bCC C2, and (f) bCC C4 (inset: a higher magnification view). The red circles in panel b indicate the congregated particles within the voids of the fcc lattice between the large spheres. All scale bars represent 1 μm.

fixed N is shown in panels b and c of Figure 2, respectively. Spectroscopically, the evolution of the Bragg peak position as the growth proceeds would be affected by a competition between the number of layers and effective dielectric constant of the medium. The reflectance spectrum at the critical time (14 min) separating stage one and stage two is simulated by fixing the parameters N = 8 and Em = 1.810 (Em is theoretically calculated to be 1.846 and 1.155, when the interstices of the bCC in the monitored region is completely filled with water and air, respectively). After this critical time, it is the evaporation of water in the crystal that makes the blue shift of the peak position. When the number of layers comes to 11 (91 min), the thickness of the crystal no longer changes and final drying occurs, corresponding to the varying Em from 1.253 to 1.155, as shown in panels g and h of Figure 2. Moreover, the volume fractions of water, air, and small spheres in the bCC were obtained according to the varying Em by SWA, as shown in Figure 3. The aforementioned volume fraction of small spheres equals 0.74VFS/L under the ideal condition that all small spheres can be filled in the interstitial voids. In accordance with the same evaporation model that the growth rate of the film equals the evaporation rate of water,36 φw (open circles) extracted from SWA can be fitted very well and the fitted distance between the growth front and dry front is about 140 μm. The detailed real-time information of the EICSA process obtained here may have potential important usages for optimizing the growth condition of bCCs. Spectra Analysis of bCCs with Different Stoichiometric Configurations. Because both the size ratio (DS/L) and the relative concentration (NS/L) of the two monodispersed

spheres have distinct influence on the crystallization and structure formation of bCCs,22,24 in situ monitoring and realtime studies of EICSA with different DS/L and NS/L have been systematically investigated. To better understand the optical spectroscopy comparison of the samples in the following discussion, structural characterization of them by SEM is first presented. Figure 4a shows the photograph of the growing bCC B2 in the in situ cuvette illuminated by the light coming from the objective. Films (1 × 1 cm) deposited both on the front wall and the substrate exhibit iridescent colors under illumination, indicating the highly ordered structure of photonic crystals. The SEM images shown in panels b−f of Figure 4 illustrate the structures of bCCs (B2\B4\B8) containing 820 and 170 nm spheres with DS/L = 0.207 and NS/L values ranging from 2 to 8 (see Table 1). The SEM images in panels b and c of Figure 4 demonstrate that the 3-fold hollow sites on the surface are occupied by a cluster of three small particles, while sporadically, a few small spheres settle in the middle slit of two big spheres (as indicated by the red circles). When the volume fraction of the small spheres was increased to NS/L = 4, the above contingent phenomenon becomes common; thus, the number of small spheres surrounding each large sphere increases from 12 to approximately 15 (see Figure 4e). It is noteworthy, however, that the bCCs deposited here do not have the configuration that each 3-fold hollow site between three large particles is filled with only one small colloid, which have been observed by authors in refs 24 and 25. It is understood that the distribution of colloids in our case under the thermodynamically non-equilibrium environment is very inhomogeneous because of the hot air stream on the top of the 4164

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surface (see panels c and d of Figure 5). It can also be seen from the cross-sectional SEM image of sample D4 in Figure 5b that numbers of small particles congregate and are embedded within the voids of the fcc lattice between the large spheres (as exemplified in the red-circled areas). From the above structure characterization of the samples, it can be summarized that (a) the volume fraction occupied by the small spheres in the bCCs increases with the increasing size ratio and number ratio and (b) in most cases, the distribution of the small particles in the interstitial space of the fcc structure is uneven, which may have a certain influence on the optical properties of the bCCs. In the following, let us first consider the real-time optical properties of the growing bCCs with different size ratios. The Bragg peak position of the bCCs with the number ratio NS/L = 4 varying from the elapsed time is plotted in Figure 6a. Overall, considering the same evaporation condition, the peak position of unary CC and different bCC samples have the similar varying trend. As for the postgrowth optical properties of the samples, it is expected that the peak position of the bCCs should have certain red shifts compared to that of unary CCs because of the larger effective refractive index. For instance, the Bragg peak position is observed to red shift from 2022 to 2031 nm with an estimated neff from 1.460 to 1.473, corresponding to samples B4 and A0, respectively. The peak position change of bCCs with different size ratios, however, is barely noticeable, especially for samples C4 and D4. It is easily explicable that the very slight difference of neff between C4 and D4 (1.467 and 1.463, respectively) can only result in a 2−3 nm shift of the peak position, while the wavelength resolution of the optical monitoring in our case is 2 nm. The inset in Figure 6a shows the normalized reflectance of the postgrowth unary CCs and bCCs (B4/C4/D4). Although there is no appreciable distinction between the Bragg peak position of the samples, the band edge on the left side of the Bragg peak exhibits distinct red shifts with the increasing size ratio, indicating the bandwidth variations of these samples. Because the bandwidth is a sensitive function of the dielectric contrast,38 the time dependence of full width at half maximum (FWHM) for the growing unary CCs and bCCs (B4/C4/D4) is plotted in Figure 6b. It is found that, in stage one when the film grows thick, FWHM of the Bragg peak decreases rapidly; during stage two, water in the interstitial space of the growth film starts to evaporate and, spectroscopically, FWHM would be affected by a competition between the increasing number of layers and the decreasing effective dielectric constant of the background medium (Em), because FWHM is inversely proportional to Em (when the value of Em is between 1 and 2; see the inset of Figure 6b); and in the final drying process, FWHM increases slowly as expected. Additionally, an increase of the refractive index contrast in the fcc photonic lattices (less free volume occupied by small spheres) leads to an increase of the bandwidth, and it is also notable that the difference of FWHM between different bCCs is more distinct. For instance, also, as for samples C4 and D4, the FWHM gap between them calculated by SWA is approximately 6 nm. More detailed information on how the peak position and FWHM of reflectance spectra vary as the number ratio is increased can be seen in Figure S3 of the Supporting Information. It should also be stressed in this context that, although the bandwidth is a sensitive function of the dielectric contrast, it also depends upon the number of layers38 and long-range irregularity within the fcc structure;43 therefore, it is difficult for the above SWA analysis based on the assumption that all of the bCC samples

Figure 6. (a) Peak position and (b) FWHM as functions of the elapsed time for unary CC A0, bCC B4, bCC C4, and bCC D4. The inset in panel a shows the normalized reflectance of the postgrowth samples, and the inset in panel b shows FWHM for a single 11-layer CC as a function of Em.

cuvette, and more small particles congregate on the surface of the large spheres, owing to the strong evaporation-induced convective flow. The cross-sectional SEM image of sample B2 shown in Figure 4d reveals that the distribution of the small spheres in the interstitial voids is not as ordered as expected. The relatively minor NS/L and particle displacement during the SEM sample preparation are the two main reasons. When the number ratio approaches 8, a disorder structure was obtained because large amounts of small spheres hampered the large spheres to self-assemble into a close-packed array, as shown in Figure 4f.45 The top surface morphologies of bCC C series (140 nm) are pretty similar to those of sample B series (170 nm), as shown in panels e and f of Figure 5. However, because the small sphere for bCC C series cannot stand in the slit of two big spheres as stable as that for B series without the restriction of the two neighboring clusters of three small particles, the number of small spheres surrounding large spheres on the top surface of samples C2 and C4 with different number ratios varies slightly. For sample D series (100 nm), when the size ratio (DS/L = 0.122) is much smaller than the constriction pore size (DS/L = 0.155), the small spheres can be easily transported and redistributed in the interstitial space of the fcc lattice. As a result, for bCC D2 with a lower number ratio (NS/L = 2), the small particles stay in the middle slit of two neighbor large spheres because the 3-fold hollow site on the surface cannot hold them (see Figure 5a). When the number ratio was increased to 4 and 8, more and more small spheres would occupy the free volume between the large colloids on the 4165

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(4) Ren, M. M.; Ravikrishna, R.; Valsaraj, K. T. Environ. Sci. Technol. 2006, 40, 7029−7033. (5) Wang, Z. Y.; Kiesel, E. R.; Stein, A. J. Mater. Chem. 2008, 18, 2194−2200. (6) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534−9537. (7) Lee, Y. J.; Pruzinsky, S. A.; Braun, P. V. Langmuir 2004, 20, 3096−3106. (8) Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306−6310. (9) Mihi, A.; Calvo, M. E.; Anta, J. A.; Miguez, H. J. Phys. Chem. C 2008, 112, 13−17. (10) Lee, S. H. A.; Abrams, N. M.; Hoertz, P. G.; Barber, G. D.; Halaoui, L. I.; Mallouk, T. E. J. Phys. Chem. B 2008, 112, 14415− 14421. (11) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A; van Blaaderen, A. Science 2002, 296, 106−109. (12) Wang, D. Y.; Möhwald, H. Adv. Mater. 2004, 16, 244−247. (13) Kim, M. H.; Im, S. H.; Park, O. O. Adv. Mater. 2005, 17, 2501− 2505. (14) Zhou, Z. C.; Yan, Q. F.; Li, Q.; Zhao, X. S. Langmuir 2007, 23, 1473−1477. (15) Burkert, K.; Neumann, T.; Wang, J. J.; Jonas, U.; Knoll, W.; Ottleben, H. Langmuir 2007, 23, 3478−3484. (16) Meng, Q. B.; Gu, Z. Z.; Sato, O.; Fujishima, A. Appl. Phys. Lett. 2000, 77, 4313−4315. (17) Meng, Q. B.; Fu, C. H.; Einaga, Y.; Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 83−88. (18) Kitaev, V.; Ozin, G. A. Adv. Mater. 2003, 15, 75−78. (19) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894−897. (20) Cong, H. L.; Cao, W. X. J. Phys. Chem. B 2005, 109, 1695−1698. (21) Wang, J. J.; Li, Q.; Knoll, W.; Jonas, U. J. Am. Chem. Soc. 2006, 128, 15606−15607. (22) Wang, J.; Ahl, S.; Li, Q.; Kreiter, M.; Neumann, T.; Burkert, K.; Knoll, W.; Jonas, U. J. Mater. Chem. 2008, 18, 981−988. (23) Zheng, Z. Y.; Gao, K. Y.; Luo, Y. H.; Li, D. M.; Meng, Q. B.; Wang, Y. R.; Zhangt, D. Z. J. Am. Chem. Soc. 2008, 130, 9785−9789. (24) Wang, L. K.; Wan, Y.; Li, Y. Q.; Cai, Z. Y.; Li, H. L.; Zhao, X. S.; Li, Q. Langmuir 2009, 25, 6753−6759. (25) Yu, J.; Yan, Q. F.; Shen, D. Z. ACS Appl. Mat. Interfaces 2010, 2, 1922−1926. (26) Cai, Z. Y.; Teng, J. H.; Xiong, Z. G.; Li, Y. Q.; Li, Q.; Lu, X. M.; Zhao, X. S. Langmuir 2011, 27, 5157−5164. (27) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303−1311. (28) Meng, L. L.; Wei, H.; Nagel, A.; Wiley, B. J.; Scriven, L. E.; Norris, D. J. Nano Lett. 2006, 6, 2249−2253. (29) Ishii, M.; Harada, M.; Nakamura, H. Soft Matter 2007, 3, 872− 876. (30) Yan, Q.; Gao, L.; Sharma, V.; Chiang, Y. M.; Wong, C. C. Langmuir 2008, 24, 11518−11522. (31) Yang, L.; Gao, K. Y.; Luo, Y. H.; Luo, J. H.; Li, D. M.; Meng, Q. B. Langmuir 2011, 27, 1700−1706. (32) van Putten, E. G.; Akbulut, D.; Bertolotti, J.; Vos, W. L.; Lagendijk, A.; Mosk, A. P. Phys. Rev. Lett. 2011, 106, 193905. (33) Koh, Y. K.; Wong, C. C. Langmuir 2006, 22, 897−900. (34) Huber, P.; Blattler, T.; Textor, M.; Leitenberger, W.; Pietsch, U.; Geue, T. Colloids Surf., A 2008, 321, 113−116. (35) Bohn, J. J.; Tikhonov, A.; Asher, S. A. J. Colloid Interface Sci. 2010, 350, 381−386. (36) Yang, L.; Zhang, Y. D.; Luo, J. H.; Luo, Y. H.; Gao, K. Y.; Li, D. M.; Meng, Q. B. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2011, 84, 031605. (37) Mittleman, D. M.; Bertone, J. F.; Jiang, P.; Hwang, K. S.; Colvin, V. L. J. Chem. Phys. 1999, 111, 345−354. (38) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300−303. (39) Satpathy, S.; Zhang, Z.; Salehpour, M. R. Phys. Rev. Lett. 1990, 64, 1239−1242.

have a perfect fcc crystal lattice with the same number of layers to fully explain the experimental results. Therefore, further studies including a more precisely controlled deposition method and comprehensive simulations of the Bragg diffraction peak for imperfect colloidal crystals are still needed.

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CONCLUSION An in situ microspectroscopic investigation of EICSA allows for valuable insights to be obtained into the bCC growth dynamics. The evolution of optical properties and data treatment in the context of the SWA method have enabled a real-time determination of the number of layers associated with each of the bCC growth stages elucidated. Detailed information in the support of the three-stage growth mechanism, including the filling factor of each component in the growing bCCs, can also be extracted from the comparison between experimental and calculated reflectance spectra. Furthermore, when the size ratio and number ratio of the two colloids were varied, the real-time reflectance spectra as well as the postgrowth configurations of the bCCs were investigated systematically. It was found that an increase of the refractive index contrast (a decrease of Em) leads to a blue shift of the peak position and an increase of the bandwidth. Also noteworthy is that the bandwidth exhibits more sensitivity in charactering the optical properties of the bCCs with different stoichiometries. This in situ optical microspectroscopy monitoring appears as a powerful approach for studying the growth dynamics of colloidal crystals and tailoring optical properties of hierarchical materials for applications in photonics,1−3 sensing,6,7 etc.

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ASSOCIATED CONTENT

* Supporting Information S

Schematic diagram of the experimental setup for reflectance measurement (Figure S1), real-time evolution of the reflectivity for bCC B4 and B8 during EICSA (Figure S2), and peak position and FWHM as functions of the elapsed time for unary CC A0, bCC D2, and bCC D4 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-82649242. Fax: +86-10-82649242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from the Natural Science Foundation of China (20725311, 20873178, and 51072221) and the Foundation of the Chinese Academy of Sciences (KJCX2-YW-W27).

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REFERENCES

(1) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437−440. (2) Arsenault, A. C.; Clark, T. J.; Von Freymann, G.; Cademartiri, L.; Sapienza, R.; Bertolotti, J.; Vekris, E.; Wong, S.; Kitaev, V.; Manners, I.; Wang, R. Z.; John, S.; Wiersma, D.; Ozin, G. A. Nat. Mater. 2006, 5, 179−184. (3) Rinne, S. A.; Garcia-Santamaria, F.; Braun, P. V. Nat. Photonics 2008, 2, 52−56. 4166

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(40) Pusey, P. N.; Vanmegen, W.; Bartlett, P.; Ackerson, B. J.; Rarity, J. G.; Underwood, S. M. Phys. Rev. Lett. 1989, 63, 2753−2756. (41) Woodcock, L. V. Nature 1997, 385, 141−143. (42) Lopez, C.; Vazquez, L.; Meseguer, F.; Mayoral, R.; Ocana, M.; Miguez, H. Superlattices Microstruct. 1997, 22, 399−404. (43) Braginsky, L.; Shklover, V. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 085107. (44) Galisteo-López, J. F.; Palacios-Lidón, E.; Castillo-Martínez, E.; López, C. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 115109. (45) The absence of a distinct Bragg diffractive peak through the whole real-time monitoring experiment for sample B8 also indicates the disorder structure, as shown in Figure S2b of the Supporting Infromation.

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