Facile fabrication of centimeter-scale stripes with inverse-opal photonic crystals structure and analysis of formation mechanism

Abstract

Due to their excellent photonic characteristics, colloidal crystals with periodic porous structures have attracted attention in fields such as bioseparation and photonic devices. In this work, we report a facile method to fabricate ultra-long colloidal crystal stripes with opal and inversed-opal structures. The colloidal crystal stripes were grown in a self-assembly process between polymeric microspheres (∼450 nm in diameter) and silica particles (14 nm ± 5 nm), and formed during vertical evaporation of the solvent. The as-grown colloidal crystal stripes can be automatically curled and peeled from the glass substrate. The polymeric spheres were subsequently removed by sintering the composite under 500 °C, yielding porous stripes with inverse-opal structures. Polystyrene-block-poly-(methyl methacrylate)-block-poly-(acrylic acid) (P(St-MMA-AA)) composite microspheres were synthesized to be used as polymeric microspheres in this process. To successfully fabricate ICPC stripes, the continuous colloidal film must be pinned on the substrate surface, directional self-assembly of colloidal particles must occur, and there must be strong interaction among colloidal particles with sufficient magnitude of inner stress. Displaying characteristics such as centimeter-scale length, a periodic porous structure, and structural colors, these stripes have potential applications in bioanalysis, optical guides, and novel photonic devices.

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Introduction

Colloidal photonic crystals (CPCs) with periodic arrangement of monodisperse latex spheres have attracted tremendous attention in recent years. CPCs possess unique light manipulation properties, making them attractive for researchers working with novel optical devices,^1,2 biological analysis,^3–5 and catalysis.^6,7 In particular, due to a combination of their specific periodic structures and large surface area ratios, inversed-opal colloidal photonic crystals (ICPCs) are extensively used in wave guides,^8–11 chromatography separation,^12–14 and biochemical sensors.^15–17

ICPCs of certain lengths have been demonstrated to be capable of functioning as optical fibres with ability to overcome the limitations of traditional silica optical fibers.¹⁸ There are two main techniques to fabricate novel ICPCs: via the lithographic template method^19–21 or capillary supporter method.^22,23 The CPCs microfibers fabricated using lithography possess a complete photonic bandgap along both transverse and longitudinal directions, and can be tailored to lie in the optical telecommunication wavelength range. When using the capillary supporter method, colloidal particles are carried inside/onto the surface of capillary tubes by capillary force. The long ICPCs are fabricated by coating colloidal particles onto the surface of a capillary tube; these ICPCs have ordered microporous structures and can be used in photonic devices. Lithography is a complicated, low-yielding technique that produces size-limited ICPCs; ICPCs grown in capillary tube have many cracks, limiting their applications in many fields. Cracking is common in the fabrication of three-dimensional CPCs/ICPCs, and many researchers aim to overcome this issue such that large-area CPCs/ICPCs films may be obtained.^24,25 Herein, we introduce a facile technique to fabricate fibrous ICPC stripes using a simple vertical deposition method. Compared to conventional evaporation fabrication of large-area colloidal film, the fibrous shape is the key in this synthetic process. The resulting ICPC stripes can be size-controlled, and are produced at a considerable yield. The stripe's periodic and porous microstructure is promising for use as the media of bioseparation, which is low-cost.²⁶ The resultant ICPC stripes possess localization properties, and are able to reflect certain wavelengths, giving them potential in application to photonic crystals.²⁷ The main features of this technique are the use of differently-sized polymeric microspheres and silica particles, which are mixed to form a working suspension. Soft, hydrophilic polystyrene-block-poly-(styrene-methyl methacrylate)-block-poly-(acrylic acid) (P(St-MMA-AA)) composite microspheres are used as polymeric microspheres.

In this work, we investigate the facile fabrication of fibrous ICPCs stripes, and characterize their macroscopic and microscopic morphologies. The geometrical dimension of the ICPC stripes were systematically controlled by changing experimental parameters such as particle concentration, concentration ratio between silica particles and polymeric microspheres, and size of microspheres. Finally, the formation dynamics and mechanism of the stripes were explored in detail.

Experiment

Materials

All chemicals are reagent grade and were used as purchased without further purification. Styrene (St), methyl methacrylate (MMA), acrylic acid (AA), ammonium persulfate (APS), sodium dodecyl benzenesulfonate (SDBS), ammonium bicarbonate, ethanol, and concentrated sulfuric acid (98%) potassium dichromate were purchased from Shanghai Chemical Reagent Co. Ltd. Polystyrene (PS) nanospheres (280 nm in diameter, 10 wt%, polydispersity under 5%) were purchased from Polysciences, Inc. The mean diameter of the silica particles (Changshu Kam Platinum Trade Co., Ltd) were 14 nm ± 5 nm and volume fraction of the solution was 34%. Deionized (DI) water was used in all experiments. Glass slides (2.5 cm × 7.5 cm × 1.2 mm) were purchased from Forever China Chemical Technology Co. Ltd. Chromic acid solution was prepared by dissolving 16–18 g potassium dichromate into 500 ml sulfuric acid under 200 °C; it was used to clean the glass substrates and make the surfaces hydrophilic.

Synthesis of monodisperse P(St-MMA-AA) composite nanospheres

Monodisperse composite latex spheres of P(St-MMA-AA) were synthesized by emulsion polymerization using established procedures found in literature.^28–30 St (20 g), MMA (1 g), AA (1 g), DI water (100 g), SDBS (0–0.038 g), and ammonium bicarbonate (0.5 g) were sequentially added into a four-necked flask equipped with an N₂ inlet, a reflux condenser, and a mechanical stirrer. The reaction mixture, stirred at 300 rpm, was initially kept at 70 °C for approximately 30 min, and then washed with DI water 3–5 times. Completion of the evaporation process yielded peeled and curled CPC stripes, as shown in Fig. 2(a), while patterns were left on the glass substrate surface, as shown in Fig. 2(c). The entire process of evaporation was completed in approximately one day. The obtained stripes were subsequently placed in a furnace (Thermo Scientific, F30400-60) under 500 °C for 2.0 h, with a heating/cooling rate of 1.0 °C min⁻¹. ICPC stripes were obtained after the sintering temperature cooled to room temperature.

Fig. 1 Schematic illustration of the experimental setup and CPC stripes fabrication process.

Fig. 2 (a) Optical microscope image of the stripes' macroscopic morphology, (b) photograph of two single stripes, (c) the line pattern left on the glass substrate when the stripes were peeled from the substrate, with the gap between lines denoted as ‘spacing’, (d) stripes' width distribution.

Fabrication of ultra-long ICPCs stripes

The precursor solution was prepared by adding P(St-MMA-AA) (C_P) spheres and silica particles (C_S), and 20 ml water into a glass vessel (25 ml beaker), as shown in Fig. 1. The concentration ratio of P(St-MMA-AA) microspheres and silica particles is defined as ϕ_S/P = C_S/C_P. A clean glass slide was inserted into the vessel at a small incline angle (≤15°) in the vertical direction. The glass vessel and slide were placed into an oven (TZL-9050 G, Suzhou Percival Laboratory Equipment Co., Ltd) for solvent evaporation; the temperature (T) in the oven was kept at 50 °C. To remove surface contaminants and impart hydrophilicity, the glass substrates were pretreated with chromic acid solution for about 30 min, then washed with DI water for 3–5 times. Completion of the evaporation process yielded peeled and curled CPC stripes, as shown in Fig. 2(a), while patterns were left on the glass substrate surface, as shown in Fig. 2(c). The whole evaporation time was about 24 h. The obtained stripes were subsequently placed in a furnace (Thermo Scientific, F30400-60) under 500 °C for 2.0 h, with a heating/cooling rate of 1.0 °C min⁻¹. ICPC stripes were obtained after the sintering temperature cooled to room temperature.

Characterization and measurements

The structures of the colloidal spheres and the microscopic morphology of ICPCs stripes were observed using a field emission scanning electron microscope (FE-SEM) (S-4800, Hitachi Ltd., Japan). The samples were sputtered with gold film before SEM observation. A microsphere structure was further confirmed based on images taken using Transmission Electron Microscope (TEM) (JEM-2100, JAPAN). The mean size and polydispersity of colloidal spheres were measured by a laser light scattering method (Zetasizer Nano S90, MALVERN Instruments Ltd., England). Super depth of Field Three-Dimensional Microscope (SDFTDM, VHX-100/VW-6000/5000, KEYENCE CORPORATION, JAPAN) was used to obtain a full, large-scale view; it was also used alongside image analysis software (Image J 1.44p, National Institutes of Health, USA) to measure the dimensions of the stripes. Optical properties were obtained by reflective spectrometer (PG2000, Ideaoptics instrument Co. Ltd. ShangHai, China). Fourier transform infrared (FTIR) spectra were recorded in the range 200–4000 cm⁻¹, using a spectrometer (Thermo Nicolet 5700, Thermo Fisher Scientific Inc., USA).

Results and discussion

The monodisperse P(St-MMA-AA) composite colloidal spheres used in this work has a diameter 247 nm ± 5 nm, as shown in Fig. S1.† Its zeta potential is indicated in the ESI.† Furthermore, FTIR spectra demonstrated the existence of C=O and –OH bonds. Their presence at peak positions of 1730 cm⁻¹ and 3430 cm⁻¹ correspond to the chemical bonding of the composite microspheres, as shown in Fig. S1(c).†

Fig. 2(a) shows macroscopic images of the CPCs stripes, which have a straight and fibrous shape. The CPCs stripes formed using this procedure have an average length of 2.5 cm as shown in Fig. 2(b). Colloidal crystal stripes automatically curl and peel off from the glass substrate after solvent evaporation, leaving residual line patterns on the glass substrate, which correspond to the linear cracks of the colloidal crystals (Fig. 2(c)). The residual patterns were used to measure the width distribution of the stripes; the statistic widths of the stripes are shown in Fig. 2(d). The width distribution is uniform and conforms to a Gauss distribution of 96.0 ± 10.0 μm. SEM images of peeled stripes and line patterns are given in the ESI (Fig. S2†). The macroscopic comparison images of the CPC and ICPC stripes are shown in Fig. S3.† The morphology of the peeled ICPC stripes retains nearly the same structure as the CPC stripes before sintering. The damage from sintering is low.

The microscopic structures of the stripes, before and after sintering, were studied using SEM (Fig. 3). The top and cross-sectional views of the CPC stripes are shown in Fig. 3(a) and (c), respectively. CPC stripes with a width of 100 μm were separated by linear cracks, in accordance to the statistical results shown above. The thickness of the stripes under these circumstances was measured to be approximately 60 μm. The inset images clearly indicate that the P(St-MMA-AA) microspheres form an ordered hexagonal structure, with nano-sized silica particles filling the gaps between the microspheres.

Fig. 3 SEM micrographs of stripes before and after sintering. (a) and (b) Top-view of the stripes before and after sintering, (c) and (d) side-edge images of the strips before and after sintering. Insets in (a)–(d) are the magnified graphs showing detailed information. All images are taken from the same batch shown in Fig. 2(a). C_P = 0.4 × 10⁻² g ml⁻¹, total volume is 20 ml; ϕ_S/P = 0.4, D is 250 nm and its polydispersity (PDI) is 3.0%.

Meanwhile, the top and cross-sectional views of ICPC stripes are shown in Fig. 3(b) and (d), respectively. Previous CPC stripes curled and peeled off from the glass substrate as after evaporation. The obtained CPC stripes were heated under 500 °C for 2.0 h to remove the polymeric microspheres. The resulting ICPC stripes present long range order and crack-free structures, and preserve the geometrical characteristics of the CPC stripes, as shown in Fig. 3(b). The widths of both stripes are approximately 100 μm, in accordance with the spacing of the line patterns shown in Fig. 2(c). The thickness of the stripes is approximately 60 μm under such conditions. Crack formation is a common phenomenon in colloidal crystals;^31–33 presenting a crucial disadvantage for the application of colloidal crystals formed using a convective deposition method. The cracks in the system presented here are directionally elongate along the liquid level descent direction only, causing the formation of fiber-like stripes. Furthermore, the ICPC stripes possess excellent interconnectivity, as shown in the inset images in Fig. 3(b) and (d). The defect-free and ordered interconnects in the ICPC stripes provide excellent channels for the transport of particles such as light and molecules. This opens the possibility of waveguide and biological separation applications for colloidal crystal stripes.^26,27

Fig. 4(a) shows the measurements for changes in width and thickness along the vertical growth direction of the CPC stripes. The typical measurement method is shown in Fig. S4(a),† with “Top” and “Bottom” denoting the start and end points of stripe formation as shown in Fig. 1. The large fluctuation at the bottom comes from the two meniscuses appearing on the glass slide and the vessel bottom, inducing the colloidal particles redistributing. The quantity of colloidal particle on the substrate reduces. Generally, both width and thickness increase along the longitudinal direction, although the width is fluctuating. Non-continuous cracks lead to a staircase-shaped trend line for increasing width, which is in accordance with literature. E. R. Dufresene et al. observed a staircase-shaped trajectory of crack growth for aqueous suspensions mixed with monodisperse silica nanoparticles.³⁴ It was found that the film composed of monodisperse silica nanoparticles fractures due to the material solidifying with individual cracks that undergo intermittent propagating.

Fig. 4 (a) Changes in width and thickness for typical CPC stripes along the longitudinal direction, C_P = 0.4 × 10⁻² g ml⁻¹, D = 270 nm, ϕ_S/P = 0.4. The inset image shows the reason for the concave tendency in thickness line. (b) Dependence of width on the concentration of P(St-MMA-AA), D = 270 nm, ϕ_S/P = 0.4. (c) Dependence of width on the concentration ratio between P(St-MMA-AA) and silica particles, ϕ_S/P. C_P = 0.4 × 10⁻² g ml⁻¹, D = 270 nm. (d) Dependence of width on P(St-MMA-AA) sphere diameter. C_P = 0.4 × 10⁻² g ml⁻¹, ϕ_S/P = 0.4.

It was observed that the increase of width and thickness is synchronous as proposed by K. A. Shorlin et al.³⁵ Consequently, in order to simplify the measurements and reduce sample damage, the studies presented here focused on width, with the assumption that findings are also applicable to thickness.

The width dependence of the CPC stripes on growth parameters was studied. The concentration of P(St-MMA-AA) microspheres, C_P, the concentration ratio between silica particles and P(St-MMA-AA) spheres, ϕ_S/P = C_S/C_P, and the diameter of P(St-MMA-AA) spheres, D, are summarized in Fig. 4(b)–(d). These parameters, especially C_P, have a significant effect on the formation of the stripes in this study. In general, the width of the stripes increases alongside an increasing C_P, ϕ_S/P and D. In order to eliminate the effects of sampling and edges, a central area (approximately 1.7 × 1.0 cm²) of the stripe (approximately 3.5 × 2.0 cm²) is chosen as the sampling, as shown in Fig. S4(b).† The width increases with the thickness of the colloidal crystal film, as shown in Fig. 4(a). In our experiments, a colloidal suspension with a volume of 20 ml is placed into the oven at the fixed temperature (T = 50 °C). A batch of vessels (ten samples) containing suspensions of varying concentrations are used to grow the stripes.³⁶ The average width of a stripe is measured as the function of C_P, ϕ_S/P and D, as shown in Fig. 4(b)–(d). The error bars indicate a standard deviation of the measured width from at least 50 linear gaps between the residual lines on a sample substrate. The solid lines show the best fittings. The stripe width increases nonlinearly as the P(St-MMA-AA) spheres concentration, C_P, increases from 0.0 to 1.0 × 10⁻² g ml⁻¹, and ϕ_S/P increases from 0.1 to 1.0. It was observed that higher C_P and ϕ_S/P value results in thicker and wider stripes. Varying ϕ_S/P and D can change the microstructure of ICPC (Fig. S5†), reflective spectra (Fig. S6†), and spectra along the length of the stripe (Fig. S7†). According to the optical spectra, the reflective peaks shift from 560 to 585 nm, which corresponds a change of 10 nm in the lattice constant. In comparison to the size of the colloidal particles, the change in lattice constant is insignificant, and it was concluded that the structure and photonic property of the CPCs are reasonably uniform.

Dynamic mechanisms of stripe pattern formation

In order to clarify the formation mechanism of the stripes, in situ observation of the experiments were carried out under an optical microscope. The observation setup is shown in Fig. S8(a).† A culture dish (5.5 cm in diameter, 1.8 cm in height) containing 25.0 ml of suspension was placed in an oil bath with a temperature of 50 °C. A washed glass substrate was inclined on the surface of the colloidal suspension at an angle of 25°. The suspension had the following parameters: C_P = 1.5 × 10⁻² g ml⁻¹, ϕ_S/P = 0.4, D = 287 nm, PDI = 4.3%. Fig. 5 shows a snapshot of the stripe formation process with a duration time of 2 h 44 min 25 s. At 9′25′′, a stable layer of colloidal crystal film (Fig. 5(a)) was pinned to and growing on the surface of substrate (Fig. 5(b)). When the colloidal crystal film was measured per 16 s and shown in Fig. 5(e) and (f), the trajectory (normalized) is staircase-shape and in agreement with E. R. Dufresene et al.,³⁴ proving that the cracks intermittently propagate. In fact, during the crack elongation, the film between two cracks peeled from the surface of the substrate (Fig. S8(b)†) without applying any external force. The peeling was driven by force from inner stress within the film. The inner stress accumulates during the growth of the colloidal film and releases when the inner stress increases beyond the local yield strength of the film. Brittle fractures occur to quickly release the stress, forming numerous cracks. The accumulated inner stress is released intermittently, not regularly, in agreement with the theoretical simulations presented by Stefan Medina et al.^37–39 Inner stress also accumulates along the thickness of the stripe, inducing stripe peeling from the substrate surface. According to the analysis shown in Fig. 5, it is hypothesized that three factors contribute to formation of the stripes. The factors are the accumulation of inner stress, the directional release of stress, and achieving a stress magnitude that is large enough to peel the stripes from the substrate surface.

Fig. 5 Time-resolved snapshots of the CPC stripe growth process under optical microscope. The scales are consistent for all microscope images. (a) The liquid level at 9′25′′. (b) The colloidal film pinning and extension on the substrate surface at 18′33′′. (c) Crack appears at 31′12′′. The white line aims at easy recognition. (d) The number of cracks reaching at 38′57′′. (e) Cracks after 2 h 44 min 25 s. (f) Crack trajectory measured with an interval of 16 s, the duration time was 44′25′′. Normalization was applied to the data points of the measured lengths of the cracks. C_P = 1.5 × 10⁻² g ml⁻¹, ϕ_S/P = 1.0, D = 289 nm, PDI = 3.0%.

Substrates with different wettability must be treated using alternate methods. For example, the substrate in Fig. 6(a) was soaked in bichromate solutions for one hour because it was too hydrophilic for contact angle measurements. Meanwhile, the substrate in Fig. 6(b) was washed with cleaner essence and its contact angle was measured to be 44.60°. The final substrate was treated with a solution containing fluoro-alkyl silanes for one hour, and its contact angle was found to be 108.73° (Fig. 6(c)). The morphology of the colloidal films on these substrates was distinct. It is difficult for colloidal crystal films to pin on the surfaces with large contact angles; the colloidal films in Fig. 6(a) and (b) split into stripes and peeled off from the substrate surface. Y. Song and J. Wang et al. previously obtained crack-free colloidal crystal films by adjusting the hydrophobic characteristics of substrates.⁴⁰ The addition of surfactant (sodium dodecyl benzene sulfonate, at a concentration of 0.4 mg ml⁻¹) into the aqueous suspension (C_P = 0.4 × 10⁻² g ml⁻¹, ϕ_S/P = 0.4 and D = 270 nm) was found to affect the surface tension. The obtained colloidal film is shown in Fig. 6(d) and (e). The film showed an unexpected ladder-like morphology that formed parallel to the liquid level. This phenomenon, stick-slip,^41–43 is induced by the concavely curved meniscus. The capillary force in the film is a result of film curvature and local surface tension.⁴⁴ Upon the accumulation of surfactant at the air–liquid interface, the surface tension lowers, causing a flow away from such a region.⁴⁵ The film continuity is broken, and no stripes are formed.

Fig. 6 The morphology of the substrate after colloidal solution deposition. (a) The substrate was treated with bichromate solutions for one hour (too hydrophilic to measure the contact angle) and the stripes were peeled from the substrate surface. (b) When the substrate, with a contact of 44.60°, was not pretreated, the colloidal film could pin to the surface. (c) The substrate was treated with a solution of fluoro-alkyl silanes for one hour and the contact angle was measured to be 108.73°. No continuous colloidal film was pinned to its surface. (d) Colloidal film self-organizes by P(St-MMA-AA) and silica particles; the mixed suspension was added with 0.005 g SDBS. (e) The magnified images from the indicated area in (d). Experimental conditions were: C_P = 0.4 × 10⁻² g ml⁻¹, ϕ_S/P = 0.4, D = 270 nm.

The directional elongation of cracks is mainly attributed to directional drying. To understand the relationship between crack propagation and directional evaporation, videos (shown in Videos 1 and 2†) were recorded in situ of a sessile drop drying on a glass slide. The video recordings captured the instant of crack formation began; Fig. 7(a) and b show the two sessile drops during drying, and their final morphology. Propagation direction and final liquid receding direction are indicated using color-coded arrows. There are two receding and elongation modes, from inner to outer and outer to inner, respectively. In both modes, the elongation direction is consistent with the liquid receding direction. Simultaneously, the films in the two modes do not fully dry before the liquid begins to recede. Fig. 7(c) shows the final micro morphology of the dried film. It can be seen that the cracks extend along the radial direction of the sessile drop. In reality, the liquid receding direction also occurs from the outer edge of the sessile drop to its center. We consider the drying of sessile drop as a directional process in the radial direction. Consequently, the point of the film that is fully dry at the beginning of the process determines the crack position, as the crack propagates along the direction of liquid recession.⁴⁶ J. W. Hutchinson's simulations showed that the maximum tensile stress develops at the tip of a perfect crack,⁴⁷ in accordance with our experimental results here. According to this conjecture, with vertical film deposition, the full drying of the colloidal film begins from the top edge of the formed film and elongates along vertical direction, forming stripe pattern on the surface. Once cracks form, the gaps facilitate the evaporation of water, inducing further cracks that elongate from the same gap (Fig. S9†). In other word, horizontal liquid receding generates longitudinal cracks. However, it is difficult to realize horizontal receding of water liquid level.

Fig. 7 Images captured from the Videos 1 and 2,† representing two crack propagating modes, (a) from inner to outer edge of the film ring and (b) from outer to inner edge. (c) The microscopic morphology of the dried sessile drop. The parameters are as follows: D = 230 nm, ϕ_S/P = 0.4, C_P = 1.0 × 10⁻² g ml⁻¹ (a), 2.0 × 10⁻² g ml⁻¹ (b), 0.1 g ml⁻¹ (c). The volume of the sessile drop was 10 μl.

As the stripe patterns have thickness, inner stress also accumulates within. When the inner stress exceeds the adhesion interaction between the stripe pattern and substrate, the stripe pattern peels from the substrate surface. As a result, the colloidal film has sufficient yield stress to inhibit the fracture along the film thickness. Because the large yield stress of the film mainly depends on strong interaction between colloidal particles, the inner stress should be stronger than the adhesion interaction. The microsphere composition and the incorporation of silica particles play an important role in the augment of film yield stress. Therefore, colloidal films formed by the composite microsphere and PS microspheres were studied; the resulting films are shown in Fig. 8. Although the colloidal film can split into stripes and peeled from substrate surface (Fig. 8(a) and(b)), it shows poor crystalline order. Meanwhile, the films formed by PS microspheres self-organization are covered with horizontal cracks, despite the main orientation being along the liquid level descending direction (Fig. 8(c) and (d)). This study indicates issues with vertically descending liquid level generating longitude cracks. Through TEM measurements, it was discovered that the contact area between the spheres served as the main distinction between PS microspheres and composite microspheres (Fig. S10†). Large contact area corresponds with increased difficulty of separating contact spheres. Weak interaction among PS spheres induces film fracture before the inner stress is enough to peel the film from the substrate, resulting in the appearance of horizontal cracks. Similarly, silica nanoparticles fill the intervals between colloidal spheres, acting as a glue to adhere spheres together. This results in the production of long, peeled stripes. The poor crystal order is a result of film formation occurring too quickly for ordered self-assembly of polymer spheres to complete. Hydrophilic silica particles filling the intervals between spheres inhibit the speed of water evaporation. Therefore, in addition to longitudinal cracks from vertical liquid descending, problems also arise from tight interactions facilitating the stripe formation.

Fig. 8 SEM images of colloidal films prepared with three different suspensions. (a) Colloidal film organized by P(St-MMA-AA) nanospheres, C_P = 0.4 × 10⁻² g ml⁻¹, D = 250 nm. (b) The magnified images from the specific area in (a). (c) Colloidal film organized by hard spheres of PS. The concentration of PS spheres was 0.4 × 10⁻² g ml⁻¹, D = 280 nm. (d) The magnified images from the specific area in (c).

Fig. 9(a)–(g) show the morphology of stripes fabricated under different temperatures ranging from 30–90 °C. The width was measured using the same method as Fig. S4(b),† and the result of measurement is shown in Fig. 9(f). High temperature conditions produce stripes with large curvature and small average width. When the fabrication temperature is below 50 °C, it is difficult to peel the stripes from the substrate surface due to weak inner stress accumulation. The variation decreases as temperature increases. Inner stress arises from plastic deformation of the microspheres under an external force, namely, capillary force.^48,49 The composite microspheres have a hydrophilic constitute (P(MMA-AA)) that is soft in aqueous solutions. Therefore, the hydrophilic constitute undergoes plastic deformation more easily under the capillary pressure. When the fabrication temperature increases, the plastic deformation rises, augmenting inner stress. As a result of the larger plastic deformation, the tensile stress becomes stronger for the large contact area. The accumulated inner stress becomes great enough to release. The final stripes produced are longer, finer and more bent in shape.

Fig. 9 The morphology of the stripes from films fabricated under different working temperatures, 30–90 °C corresponding with pictures (a)–(g). Their width is drawn (h). All scale bars are 5 mm.

The pinning and continuous growth of colloidal film, directional deposition, strong interaction and sufficient magnitude inner stress simultaneously contribute to the creation of CPC stripes. These requirements are satisfied when fabricating stripes via synchronized self-assembly of composite microspheres and silica particles on hydrophilic substrate via vertical deposition. Therefore, adjusting the ingredient of the suspension was used to control stripe sizes.

Conclusion

Crack-free ICPCs stripes were fabricated by evaporating a suspension of composite nanospheres comprised of P(St-MMA-AA) and silica particles. Adjusting the suspension recipe, in particular, the concentration of the polymeric nanospheres, allowed for control of stripe sizes. Several conditions were identified as critical for successful fabrication of ICPC stripes. Strictly ensuring the pinning of continuous colloidal film on the substrate surface, directional self-assembly of colloidal particles, strong interaction among colloidal particles and sufficient magnitude of inner stress, leads to the production of ICPC stripes.

Acknowledgements

We gratefully acknowledge the financial support from the National Science Foundation of China under Grant 51073113, 91027039 and 51373110, the Natural Science Foundation of the Jiangsu Higher Education Institutions of China under Grant 10KJA540046. We also acknowledge support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Qing Lan Project for Excellent Scientific and Technological Innovation Team of Jiangsu Province (2012) and Project for Jiangsu Scientific and Technological Innovation Team (2013).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07314j

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