Self-Assembly of Crack-Free Silica Colloidal Crystals on Patterned

ARTICLE pubs.acs.org/JPCC

Self-Assembly of Crack-Free Silica Colloidal Crystals on Patterned Silicon Substrates Zhongyu Cai,† Jinghua Teng,‡ Deying Xia,§ and X. S. Zhao*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602 § Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡

bS Supporting Information ABSTRACT: Face-centered cubic (fcc) (111)-orientated twodimensional (2D) and three-dimensional (3D) crack-free silica colloidal crystals (CCs) were fabricated on patterned silicon substrates via a vertical deposition method. The influence of the surface properties of the substrate and the concentration of colloidal microspheres on the self-assembly were studied. The results showed that monolayer of silica microspheres selfassembled on a hydrophobic substrate surface while on a hydrophilic surface multilayer of silica microspheres formed under otherwise the same experimental conditions. The number of CC layers grown on the patterned substrate was proportional to the volume fraction of the colloidal microspheres. The optical properties of the samples were less influenced by the patterned surface with increasing the volume fraction of the colloidal suspension when the patterned substrates were fully covered by the CCs. The results presented in this paper reveal that the vertical deposition method can allow the formation of crack-free and well-ordered 3D CCs in large domains on patterned surfaces.

1. INTRODUCTION A great deal of recent research effort has been paid to the fabrication of ordered structures on patterned template for various applications, such as photonics,1 3 optoelectronic devices,4,5 and cell micropatterning.6 Many methods have been described for forming colloidal crystals (CCs) on a patterned substrate surface, such as the physical confinement method,7 12 oscillatory shear method,13 template-directed self-assembly method,14 21 colloidal epitaxial or graphoepitaxial assembly,5,22 27 and capillarydirected self-assembly (CDSA) method.28,29 However, these methods are either relatively complicated8,9 or the structures fabricated using these methods are restricted to relatively simple and small patterns, such as square periodic arrays of holes, grooves, and one- and two-dimensional (2D) gratings.16,20,29 31 It has been a challenge to assemble microspheres on patterned templates with a long-range order. Two factors may account for this challenge. One is that a patterned structure may impose constraints and influence the integration of the crystal. The other one is that stick slip motion may prevent getting large domains along the direction of evaporation (or withdrawal) of the solvent of a colloidal suspension.32 Among the available methods for self-assembly of colloidal particles, the vertical deposition method33 has been shown to be a facile approach to growing CCs on patterned surfaces.16,17,19,34,35 By using the vertical deposition method, Ye et al.16 investigated r 2011 American Chemical Society

2D colloidal arrays self-assembled on one-dimensional (1D) surface grating patterns and observed that the colloidal structures were significantly influenced by the ratio of the diameter of the microsphere over the periodicity of the grating. Yi et al.17 studied the effect of the modulation depth on three-dimensional (3D) colloidal self-assembly with symmetric and asymmetric 2D templates. The authors found that when the ratio of the surface modulation over the diameter of the bead was less than 0.28, the 3D crystallization of colloids was not observed. The crystallization of colloidal particles on a patterned surface, however, is influenced by many other factors, such as the surface properties (hydrophobic or hydrophilic surface), surface patterning, and concentration and types of colloidal particles. These factors have not been investigated apart from a recent work by Hoogenboom et al.,19 which investigated the influence of surface topography of template on the CC structure. It was found that a large squaresymmetric close-packed 2D crystal as well as a 3D non-closepacked simple cubic and body-centered-cubic crystal could be grown. However, when growing large close-packed crystals along the directions other than along the hexagonal close-packed plane parallel to the substrate, defects and large hexagonally arranged Received: February 20, 2011 Revised: April 19, 2011 Published: May 03, 2011 9970

dx.doi.org/10.1021/jp201674x | J. Phys. Chem. C 2011, 115, 9970–9976

The Journal of Physical Chemistry C

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Figure 1. SEM images of silica spheres of volume fraction of 1.00 10 3 grown on a crosslike patterned substrate coated with 5 nm of silica thin film (a c): (a) top view, (b) a higher-magnification top view, (c) multilayer of silica spheres aligned along the surface pattern; and without coating of silica film (d f): (d) top view, (e) a high-magnification top view, and (f) a monolayer of silica sphere aligned along the surface pattern.

crystal grains were observed. In these studies, ordered structures in small domains were obtained when feature size of the pattern matched well with the diameter of the colloidal microsphere.16,17,19 In addition, these previous studies all employed polystyrene spheres, and no optical properties of the structures were presented.16,17,19,20 In this study, we used silica microspheres and silicon substrates of different patterns to investigate the influence of substrate on the growth of CCs. The surface chemical properties and patterning of the substrate and the concentration of silica colloidal suspension on the self-assembly of CCs were examined. Optical responses of the fabricated samples were characterized.

2. EXPERIMENTAL SECTION Materials. Silicon wafers with a diameter of 125 mm were obtained from Rockwood Electronic Materials (France). The chemicals used in this study including AZ P4330 (Clariant Pte Ltd.), acetone (Tedia Co. Inc.), absolute ethanol (99.99%, Merck), ammonia (NH4OH, 28%, Fisher), and tetraethyl orthosilicate (TEOS, 98%, Fisher) were used as received. Methods. In this study, two silicon wafers with different surface patterns were used as the substrates for the self-assembly of colloidal crystals. One had a flowerlike pattern, and the other one had a crosslike pattern (see Figure S1). The silicon patterns were fabricated on a silicon wafer employing the conventional photolithography technique (see details in the Supporting Information). To investigate the influence of surface chemical properties (hydrophobicity and hydrophilicity), one piece of the square symmetric pillar-patterned substrate was coated with a layer of silica thin films ∼5 nm in thickness by using a plasma-enhanced chemical vapor deposition (PECVD) method, while the other one was not coated with silica film. Silica mocrospheres of 442 nm in diameter with a standard deviation of 1.4% were synthesized following a modified St€ober method.36 The silica microspheres were dispersed into deionized water with different volume fractions. Two types of patterned silicon wafer were used as substrates to assemble silica microspheres using the vertical deposition method.33 The growth of CCs was conducted as described below.

A patterned silicon substrate was vertically immersed in a colloidal suspension containing silica spheres of different volume fractions (0.68 10 3, 1.00 10 3, 1.36 10 3, and 2.72 10 3, respectively). Then, the suspension with the silicon wafer was placed in an oven of 60 °C for 2 days to obtain CCs grown on the substrates. Characterization. The contact angle data were collected with a VCA Optima contact angle measurement instrument. The measurement was carried out five times on different part of the same sample, and the average was taken as the contact angle of the substrate. All the scanning electron microscope (SEM) images, including the top view and cross-sectional images of samples, were obtained by using a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). Optical microscope (Leica Microscope) was used to take an overview of samples. Optical spectra were measured on an ultraviolet visible near-infrared (UV-vis-NIR) spectrophotometer (Shimadzu UV-3600F). Laser diffraction was measured by using laser beam (λ = 325 nm and the diameter of the laser beam is ∼1.2 mm), and a diffraction pattern was observed on a screen placed in front of the sample.

3. RESULTS AND DISCUSSION SEM images of the two types of patterned silicon wafer substrates are shown in Figure S1. One type was a flowerlike square symmetric pillar-patterned substrate, and the other one was a crosslike square symmetric pillar-patterned substrate. The periodicity was 10 μm, and both the diameter and the height of the pillars were 5 μm. Influence of Surface Properties. The influence of substrate parameters was investigated by using two pieces of crosslike pattern substrates with different surface properties with a colloidal volume fraction of 1.0 10 3. Figures 1a c show that multilayer of silica microspheres assembled on the silicon substrate coated with a layer of silica thin film. From the top view of the sample shown in Figure 1a, it can be seen that well-ordered arrays of silica microspheres grew on the whole part of patterned silicon substrate except for the areas in the vicinity of the pillars. The detailed structure is shown in Figure 1b. Highly ordered silica CCs are seen in the areas not close to the pillars. Disorders and small cracks occurred around the pillars under this volume fraction. The multilayer 9971

dx.doi.org/10.1021/jp201674x |J. Phys. Chem. C 2011, 115, 9970–9976


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Figure 2. Reflectance spectra of (a) hydrophilic silicon substrate with crosslike pattern before (i) and after growth of silica CCs (ii) and (b) hydrophobic silicon substrate with crosslike pattern before (i) and after growth of silica CCs (ii).

Figure 3. SEM images of silica spheres of volume fraction of 1.0 10 3 grown on flowerlike patterned silicon substrate coated with 5 nm of silica thin film: (a) top view, (b) a high-magnification top view, and (c) multilayer of silica spheres aligned along the surface pattern.

feature of the self-assembled microspheres can be seen clearly from Figure 1c. This is mainly due to the hydrophicility of the thin silica film with a measured contact angle of 42.9° (see Figure S2a in the Supporting Information). Figures 1d f demonstrate that monolayer of silica microspheres assembled on the patterned silicon substrate without silica coating. The silicon wafer is hydrophobic while the silica colloidal microspheres were dispersed in water. The measured contact angle on the silicon surface was 99.5° (see Figure S2b in the Supporting Information). Figure 1d clearly shows silica microspheres assembled uniformly on the surface pattern. However, it can be seen from Figure 1e that silica microspheres assembled in a disordered way near the pillars while orderly assembled far from the pillars. The silica microspheres assembled on the patterned silicon substrate with high quality except some vacancies on the patterned silicon substrate. Moreover, single layer of silica microspheres formed on the crosslike pillar patterned bare silicon surface without any coating from Figure 1f. Such 2D monolayers are of particular importance to nanoscale surface patterning, which is a key platform for a variety of applications.37 The optical properties of the obtained samples were characterized at normal incidence by using reflectance spectroscopy to assess the CCs grown on the pillar patterns. Figure 2a shows the reflectance spectra of the silicon substrate and silicon substrate with self-assembled multilayer silica microspheres. It can be seen that there is an enhanced reflection region in the wavelength range of ∼550 to ∼1200 nm. Among the peaks seen from the silicon substrate with self-assembled multilayer silica microspheres, only the sharp peak at the wavelength around 970 nm is due to the pseudo stop band of the silica CCs according to the Bragg equation. Comparing the spectra of the samples before and after assembly of silica microspheres, it was found that the reflection was greatly enhanced at the wavelength range of 550 1200 nm,

from ∼5% to ∼25%. This optical property makes it can be utilized as a solar cell back-reflector.38,39 For the crosslike silicon pattern self-assembled with monolayer silica microspheres shown in Figure 2b, the spectra of patterned silicon substrate and patterned silicon wafer self-assembled with silica microspheres are similar, except that the reflection of the latter is much higher than that of the silicon substrate, which increased from ∼15% to ∼25% at the wavelength range of 550 1200 nm. This implies that the monolayer thin film of silica CCs enhanced the reflection of the patterned silicon substrate as silica microspheres do not absorb light but to reflect light back. This is very useful because the visible and near-infrared ranges constitute of most of the solar energy. Influence of Patterned Structure. We also studied the influence of patterned structure on the CCs self-assembled on silicon surface. The CCs assembled on flowerlike patterned substrate with 5 nm silica film coating are shown in Figure 3. In Figure 3a, the brighter lines of the surface demonstrate well-ordered arrays of silica microspheres, while the darker area is not well-ordered silica microspheres around the pillars. This was further illustrated from Figure 3b, which shows that the silica spheres assembled on the patterned silicon substrate with high quality except some vacancies or minor gaps. Figure 3c confirms that CCs grown on the patterned silicon substrate are in multilayer as well. When comparing microspheres grown on different types of patterns (crosslike and flowerlike) under otherwise the same experimental conditions, as shown in Figures 1b and 3b, no distinguishable differences can be observed. This can be attributed to the large ratio of the periodicity of the patterns over the diameter of the microspheres used in this study. However, the order of the microspheres grown on the substrate depends on the distance to the pillars, implying that the pillars imposed constraints on the selfassembly of the silica microspheres. 9972



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Figure 4. Microscopy images of samples. (a) Silicon wafer with crosslike pillar patterns. (b) Silica colloids (volume fraction = 0.68 10 3) deposited on silicon substrate. (c) Silica colloids (volume fraction = 1.36 10 3) deposited on silicon substrate. (d) Silica colloids (volume fraction = 2.72 10 3) deposited on silicon substrate.

It is thus can be concluded that the hydrophobic surface resulted in the formation of CC monolayer while the hydrophilic surface led to the formation of CC multilayer under a volume fraction of 1.0 10 3. The different patterns imposed a predetermined lattice orientation in the final CCs, which may extend to the entire thickness of the crystal (see Figure S4 in the Supporting Information). This constraint resulted in the formation of highly ordered CCs at the position far from the pillars while in a less order around the pillar patterns, and the CCs show highly order from the bottom layer to the top layer. Influence of Concentration of Colloidal Suspension. We further investigated the influence of concentration of colloidal suspension on the growth of CCs on the crosslike patterns coating with silica film by using three different volume fractions of colloidal suspensions. Figure 4 shows the optical microscopy images of obtained samples. It can be seen that the morphology of the patterned silicon substrate became more and more uniform when increasing the volume fraction of the silica colloidal suspension from 0.68 10 3 to 2.72 10 3. As shown in Figure 4b, the pattern was partially covered by CCs. When increasing the volume fraction to 1.36 10 3, as can be seen in Figure 4c, the pattern was almost covered by the CCs. The CCs grown on the patterned silicon substrate with a volume fraction of 2.72 10 3 were more uniform than that of grown with a volume fraction of 0.68 10 3. From Figure 4d, it can be seen that with increasing the volume fraction of colloidal suspension the patterned silicon substrate was fully covered by CCs, and the well-ordered domain can be as large as several hundred micrometers. The detailed structures of the obtained samples were further analyzed with FESEM in order to examine the effects of concentration on the growth of CCs on the patterned silicon substrate. As shown in Figure 5, three groups of samples fabricated from silica colloidal suspensions with a volume fraction of 0.68 10 3, 1.36 10 3, and 2.72 10 3. It can be seen that face-centered cubic

(fcc) (111)-orientated 3D crack-free silica CCs were self-assembled on silicon patterns. From Figure 5a, silica microspheres aligned along crosslike patterns in high order. As observed from Figure 5b, the silica microspheres assembled with higher order at the center of any two adjacent pillars than that near the pillars. All the microspheres arranged into fcc structure except some vacancies. When increasing the volume fraction of the colloidal suspension to 1.36 10 3, the vacancies became less and the pillars were exactly covered with less ordered structure around the patterns (Figures 5c,d). This may be due to the increase in the CC thickness resulting from the increase of the volume fraction of the colloidal suspension. This conclusion can be confirmed by the samples fabricated by silica colloidal suspension with a volume fraction of 2.72 10 3 (Figures 5e,f). High-quality silica CCs grown on the patterned silicon substrate fully covered the pillars. The presence of vacancies as shown in Figure 5f was most probably due to the size deviation of the microspheres and slight difference in free energy between the fcc structure and the hexagonally close packing (hcp) structure.40 It can also be observed that the ordered domains were as large as 100 μm 100 μm, which were much larger than the CCs fabricated on a flat surface (the largest defectfree domains were on the order of 5 μm 5 μm). In addition, when comparing the resultant CCs grown on flat substrate without patterns under the same experimental condition, more vacancies and uncontrolled orientations can be observed (see Figure S3 in the Supporting Information). These results were similar to the results of the self-assembled block copolymer on 2D periodic patterned substrates by using a graphoepitaxy technique but with a higher ratio of period of substrate to diameter of microspheres and a larger scale range of surface pattern feature and particle size in this study.27 Similarly, the formation of crack-free CCs in this study can be explained by the free-energy model developed by Bita et al.27 That is, the templated CCs may either assume a strained spacing and fit inside the pattern lattice, or form local 9973



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Figure 5. Silica microspheres self-assembled on crosslike patterned silicon substrate coating with silica film. (a) Silica multilayer uniformly aligned along the surface pattern (volume fraction = 0.68 10 3). (b) High magnification of (a). (c) Silica multilayer uniformly aligned along the surface pattern, and the patterns are almost fully covered with silica CCs (volume fraction = 1.36 10 3). (d) High magnification of (c). (e) The crosslike pillar patterns are fully covered with silica CCs (volume fraction = 2.72 10 3). (f) High magnification of (e).

defects and relieve the long-range stress. In this study, the formation of crack-free CCs mainly can be attributed to the patterns relieve the long-range stress during the self-assembly and drying process. The number of CCs layers is proportional to the volume fraction of colloidal suspension used in this study from the crosssectional images of the obtained samples (see Figure S4 in the Supporting Information). It can be found 6, 12, and 24 layers of CCs grown on the patterned substrate when varying the volume fraction of colloidal suspension. Under the present conditions, the relation between the number of CCs layers and the volume fraction of colloidal suspension is shown in Figure 6. The thickness of the CCs grown on the patterned substrate increased with increasing the volume fraction of the colloidal suspension. The patterns had less effect on the microsphere arrangement of the top layer. In addition, vacancies of the resultant samples decreased with increasing the volume fraction of the colloidal suspension. However, the most striking feature of the samples fabricated in this study is that the CCs do not contain cracks though some samples may possess a few vacancies. This may indicate that the patterns on the substrate not only guided the growth of CCs along the surface of the patterned substrate but also restricted the alignment of the silica microspheres. In the present case, the silica microspheres in the bottom layer may stay at the initial position during the drying process since the displacement of the microspheres is constrained by the patterned pillars, and the subsequent layers are restricted with the same in-plane lattice constant by the microspheres beneath. This restriction led to the formation of a slightly nonclosed fcc structure without cracks but with some gaps between microspheres as are seen from Figures 5b,d,f. Laser diffraction was used to further probe the long-range order in the fabricated samples and to quantify the effect of surface patterning on the crystal quality. The laser diffraction was observed by passing a laser beam (λ = 325 nm) through a screen with a pinhole placed in front of the sample. Two different diffraction patterns can be observed from the patterned sample with self-assembled CCs when adjust the distance between the sample and the laser. One is the regular patterned silicon substrate (Figure 7a), and the other is the self-assembled silica CCs on the patterned substrate (Figures 7b d). As shown in Figure 7a, the diffraction

Figure 6. Relationship between the number of CC layers and the volume fraction of colloidal suspension.

patterns obtained from patterned silicon substrate alone featured with dot arrays indicate the highly ordered arrangement of small pillars on the silicon substrate. From Figures 7b d, for the CCs grown on the patterned silicon substrate, the first-order diffraction pattern can be seen in six bright diffracted light spots forming a regular hexagon, reflecting the perfect hexagonal lattice. These laser diffraction images indicate the long-range order on the topology of the as-fabricated silica CCs assembled on the crosslike patterned silicon substrate. The results are in agreement with the microscopy results. At lower volume fraction, the thickness of CCs is small; thus, the diffraction image is weaker. In the present paper, we also investigated the influence of colloids concentration on the optical properties of the fabricated photonic crystals assembled on the crosslike patterned silicon substrate. As shown in Figure 8, from UV to visible wavelength range, Fabry Perot resonance of the crystal film plays an important role, and the obtained samples showed relatively low reflections. While at the near-infrared wavelength range, all the samples showed higher reflections, especially at the pseudo stop band of the photonic crystals. This indicates that Bragg diffraction 9974



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the reduction in the interplanar spacing and effective refraction index.5,42

Figure 7. Laser diffraction of silica film coating silicon substrate with crosslike pattern. (a) Produced by the pattern alone (without assembling with silica microspheres). (b d) Silica CCs film coating on patterned silicon substrate assembled with different thickness of silica microspheres. (b) Volume fraction = 0.68 10 3. (c) Volume fraction = 1.36 10 3. (d) Volume fraction = 2.72 10 3.

4. CONCLUSIONS In summary, fcc (111)-orientated 2D and 3D crack-free silica CCs, to our best knowledge, were first self-assembled on silicon patterns in large domains via a facile vertical deposition method. The surface properties of the pattern significantly affect the selfassembly process. The hydrophilic surface benefits the assembly of silica microspheres onto the patterned silicon substrate. All the results indicated that the surface pattern imposes a predetermined lattice orientation in CCs, which may extend to the entire thickness of the crystal. The influence of the concentration of colloidal suspension may also play an important role on the selfassembly of colloidal suspension on the patterned silicon substrate. It is notable that no cracks appear in the fabricated CCs grown on the pillar patterned silicon substrate. It is vital that the critical defects are minimized in the practical applications since the applications of CCs are greatly restricted by practical difficulties encountered in fabricating large single crystals with adjustable orientation. Thus, this study may pave an alternative avenue for the applications of optoelectronic devices integrated in 3D photonic crystal films. ’ ASSOCIATED CONTENT

bS

Supporting Information. FESEM images of silicon wafer patterns, contact angles of hydrophilic and hydrophobic surface, FESEM images of silica CCs deposited on a silicon substrate without surface patterning, and cross-sectional views of the silica CCs assembled on the patterned silicon wafers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (65)65164727; Fax: (65)67791936; e-mail: chezxs@nus. edu.sg.

Figure 8. Reflectance spectra of silica film coating silicon crosslike substrate assembled with different volume fractions of silica sphere colloidal suspension: (i) 0.68 10 3, (ii) 1.36 10 3, and (iii) 2.72 10 3.

caused by the crystal may be the dominant effect. This can be attributed to more than 10 layers of CCs grown on the patterns from the SEM images shown in Figure 5 and Figure S4. The sharp peaks at the pseudo stop band imply a well-ordered structure formed. The enhancement of the reflection at the pseudo stop band is useful because CCs assembled on patterned silicon substrate can be used as back-reflector of thin film solar cell to improve the light absorption and consequently to enhance the light-to-electricity conversion.38,41 When compare the assembled photonic crystals on patterned silicon substrate, we may find that the reflection at the pseudo stop band of the CCs enhances with increasing the volume fraction of colloidal suspension. As the volume fraction was increased, the reflection peak was shifted slightly toward the short-wavelength region. This may be due to

’ ACKNOWLEDGMENT This work was partially supported by Ministry of Education of Singapore under Project MOE2008-T2-1-004. We thank Ms. Chew Ah Bian from Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), for her help on the fabrication of patterns. ’ REFERENCES (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (2) Yablonovitch, E. Sci. Am. 2001, 285, 46. (3) Yablonovitch, E. Nature 1999, 401, 539. (4) Lin, S.-Y.; Chow, E.; Hietala, V.; Villeneuve, P. R.; Joannopoulos, J. D. Science 1998, 282, 274. (5) Jin, C. J.; McLachlan, M. A.; McComb, D. W.; De La Rue, R. M.; Johnson, N. P. Nano Lett. 2005, 5, 2646. (6) Yap, F. L.; Zhang, Y. Biomaterials 2007, 28, 2328. (7) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (8) Yin, Y.; Xia, Y. Adv. Mater. 2002, 14, 605. (9) Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2003, 19, 622. (10) Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 267. (11) Yin, Y. D.; Lu, Y.; Gates, B.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 8718. 9975



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Self-Assembly of Crack-Free Silica Colloidal Crystals on Patterned

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