Article pubs.acs.org/Langmuir
Encapsulation of Polymer Colloids in a Sol−Gel Matrix. Direct-Writing of Coassembling Organic−Inorganic Hybrid Photonic Crystals Annabel Mikosch and Alexander J. C. Kuehne* DWI − Leibniz Institute for Interactive Materials, RWTH Aachen University, Forckenbeckstrasse 50, 52062 Aachen, Germany S Supporting Information *
ABSTRACT: The spontaneous self-assembly of polymer colloids into ordered arrangements provides a facile strategy for the creation of photonic crystals. However, these structures often suffer from defects and insufficient cohesion, which result in flaking and delamination from the substrate. A coassembly process has been developed for convective assembly, resulting in large-area encapsulated colloidal crystals. However, to generate patterns or discrete deposits in designated places, convective assembly is not suitable. Here we experimentally develop conditions for direct-writing of coassembling monodisperse dye-doped polystyrene particles with a sol−gel precursor to form solid encapsulated photonic crystals. In a simple procedure the colloids are formulated in a sol−gel precursor solution, drop-cast on a flat substrate, and dried. We here establish the optimal parameters to form reproducible highly ordered photonic crystals with good optical performance. The obtained photonic crystals interact with light in the visible spectrum with a narrow optical stop-gap.
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INTRODUCTION Photonic crystals consist of periodically arranged building blocks with characteristic dimensions on the order of the wavelengths of visible light. The ordered arrangement of colloids produces a periodic refractive index alternation of the particles and a surrounding medium, which is often air.1 For large enough refractive index contrasts (neff) a photonic stop gap emerges, which leads to the reflection of light of certain wavelengths (λ) at defined angles (θ), as described by the 2 − sin 2 θ ) with d modified Bragg equation:2 mλ = 8/3d 2(neff as the building block dimension, or here the particle diameter, and m as the Bragg order, an integer number. To create photonic crystals, elaborate top-down manufacturing techniques have been developed such as nanolithography3,4 and deposition methods.5,6 However, these techniques are difficult to scale up7 and often too coarse, and it is difficult to reach resolution below 600 nm in all three dimensions of the resulting periodic structures. These drawbacks can be overcome by using bottom-up methods and exploiting the self-assembly of monodisperse colloids into colloidal crystals.7 Full threedimensional ordering is feasible, while the photonic stop gap is only limited toward the minimum achievable colloid size during particle synthesis.8 Such photonic crystals have potential applications as resonators for colloidal lasers,9 for nonbleaching, structural colors,10 and for anticounterfeiting labels.11 Crystallization of colloids into synthetic opals by sedimentation can take several days or months.12 Therefore, other selfassembly methods have been developed to facilitate faster structure formation. During convective assembly, the particles assemble into large-area defect-free photonic crystals by © XXXX American Chemical Society
exploiting capillary forces for assembly at the meniscus of an evaporating dispersion.8 Evaporation-driven self-assembly is also used in inkjet-printed photonic crystals of monodisperse silica13 or polymer particle dispersions.14,15 The particles arrange due to the evaporation of the confining dispersant. This process works particularly well when water is used as a dispersant, due to its high surface energy.13,14 Colloidal deposits produced by these two techniques show iridescence and a shift in the reflection color with increasing particle sizes as described by the modified Bragg equation. Whereas convective assembly leads to large films of colloidal crystals, direct-writing of selfassembling colloidal inks enables the creation of defined patterns without the need of any prepatterned surfaces or subsequent etching procedures.14,15 However, colloidal crystals produced by these techniques exhibit problems in their cohesion, as they are not mechanically stable and quickly delaminate from the substrate or exhibit flaking and disintegration into dust.13 Recently, a coassembly method has been demonstrated, where colloidal crystals are produced by convective assembly, while at the same time a sol−gel process takes place, forming glass in the interstitial spaces of the emerging colloidal crystals. This encapsulation of the colloidal crystals supplies mechanical integrity.16 To generate useful patterned structures, lithographic patterning of such large and defect-free coassemblies has been demonstrated.17 However, drawbacks remain with the convective coassembly method. Received: January 12, 2016 Revised: March 1, 2016
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DOI: 10.1021/acs.langmuir.6b00098 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Constituents of the formulation for coassembly. (a) Scanning electron microscopic image of synthesized polystyrene particles displaying monodispersity. The scale bar measures 5 μm. (b) Confocal scanning laser microscopic image of the first monolayer of self-assembled fluorescent particles. The scale bar represents 5 μm. (c) Artistic representation of the drop-casting and coassembly process. The colloids assemble while the dispersant evaporates, and the sol−gel precursor hydrolyzes to form a glass around the colloids. The inset shows the reaction scheme for the hydrolysis of BTES.
ambient conditions. During this time, the hydrolyzed BTES sol−gel precursor condenses into a three-dimensional glassy network, which encapsulates the particles in a solid and transparent matrix. A schematic representation of the dropcasting process and the reaction scheme of the sol−gel hydrolysis reaction to form an inorganic glass matrix are displayed in Figure 1c. Influence of pH. To study the influence of pH on the coassembly of the PS particles and BTES, we vary the pH across the positive scale from 0.5 to 13.5. Hydrochloric acid or potassium hydroxide are used to adjust the pH of the prepared dispersions. Drying colloidal dispersions often suffer from nonuniform drying phenomena occurring during evaporation of the dispersant. One drying artifact is generated by pinning of the three-point (air/solvent/substrate) contact line and results in a ring-shaped deposit. The artifact is widely known as the coffee-ring effect, where particles are dragged to the pinned contact-line of the droplet by a strong radial flow created by higher evaporation rates at the droplet periphery.14,20 Particle accumulation further supports pinning and enhances the deposition of colloids in form of a ring shape. All of our coassembled deposits are of light green color because of the added Coumarin 6. However, we observe the coffee-ring effect for coassemblies produced between 2.5 and neutral pH, while at lower and higher pH we obtain perfectly oblate-spheroid deposits and no coffee-ring. However, while the overall shape of the deposit is intact at high pH, they appear brittle and show only weak if any coloration by iridescence, which indicates a low degree of colloidal crystallinity and insufficient or incompatible coassembly of the sol−gel around the PS particles. By contrast, the coffee-ring deposits produced at lower pH exhibit smooth surfaces and strong iridescence in the red spectrum between pH 2.5 and 3.5; see Figure S1 in the Supporting Information. Iridescence is a direct indicator for ordered colloidal arrangements in the deposits. We conclude that the hydrolysis reaction of BTES, which is catalyzed both by acids and bases, occurs fast outside of the range of pH 2.5− 7.0.21 Here hydrolysis is so fast that assembly of the colloids into ordered structures cannot occur, leading to assemblies without iridescence.22 If the BTES gelation is faster than the drying rate of the droplet, radial flows are prevented and particle crystallization is inhibited. The hydrolysis of BTES then quickly rigidifies the dome-like shape of the droplet, circumventing formation of a ring deposit.23
Convective coassembly requires several hours to days to produce large-area opals, and patterning requires lithography and etching steps, further complicating the otherwise simplistic self-assembly process. Optimized conditions to obtain coassembled droplet deposits via direct-writing remain unexplored to date. The realization of such a process would allow for inkjet printing and precise alignment of mechanically stable photonic crystals with tunable photonic bandgaps. Here we explore the conditions for the coassembly of colloids and a sol−gel matrix to take place in drop-cast deposits on a substrate. We apply fluorescently labeled polystyrene particles and 1,2-bis(triethoxysilyl)ethane (BTES) as the sol− gel precursor. In other studies18 tetraethyl orthosilicate (TEOS) is used as a sol−gel precursor, which leads to a SiO2 matrix. Here we use the organosilica precursor BTES with an ethyl unit bridging the two trifunctional alkoxysilane groups. The resulting matrix is less brittle and exhibits less shrinkage during condensation. We examine the encapsulation procedure and analyze the optical performance using reflectance spectroscopy. This new approach leads to a one-step process toward printable, mechanically stable, and high-fidelity photonic crystals. We will examine and optimize the influence of the following parameters on the coassembly process and the resulting quality of the colloidal crystals: pH; monolayer substrate modification; drying control agents; volume ratios of particles and sol−gel precursor; and hydrolysis activation time.
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RESULTS AND DISCUSSION In this study, we use Coumarin 6 labeled polytyrene (PS) particles with a diameter of 260 nm synthesized via surfactantfree emulsion polymerization.19 The particles are highly monodisperse with a polydispersity of less than 3%. This allows them to readily self-assemble into hexagonally ordered arrays, as shown by scanning electron microscopy (SEM) in Figure 1a. The fluorescent dye Coumarin 6 allows analysis of the particles also by confocal scanning laser microscopy (CSLM) and adds further functionality for prospective photonic applications of the coassemblies, for example as laser gain material (see Figure 1b). The conditions for deposition of the matrix-encapsulated PS particle deposition are optimized first by individually tuning the above-mentioned parameters. To do so, we apply dispersions of particles in ethanol−water mixtures (2:1 (v/v)), which are drop-cast as 5 μL droplets onto cleaned microscope cover glass slides using a micropipette. The deposited droplets are then left to dry at B
DOI: 10.1021/acs.langmuir.6b00098 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. Performance of the coassembly and encapsulation process. (a−d) Scanning electron microscopic images of cross-sections of deposits dropcast from colloidal sol−gel precursor solutions with different PS:BTES volume ratios from (a) 63:37 vol %, (b) 50:50 vol %, (c) 40:60 vol % to (d) 33:67 vol %. (e) A photograph of a coassembled deposit showing iridescent red reflection and the green coloration of the coumarin dye. (f−h) SEM images are shown of argon-ion polished cross-sections of deposits with the following PS/BTES ratios: (f) 50:50 vol %, (g) 40:60 vol % (the red circles indicate defects); (h) 33:67 vol %. (i and j) SEM images of cross-sections of deposits with PS/BTES ratio of 40:60 vol % and an activation time of the sol−gel precursor solution of (i) 8 h and (j) 24 h. All scale bars represent 5 μm except for part e, 1 mm.
Drying Control Agents. To avert ring formation, pinning of the three-point contact line needs to be prevented. A widely used method to achieve this is substrate modification with selfassembled alkylsilane monolayers, to increase the hydrophobicity of the substrate and hence raise the contact angle of aqueous suspensions.24 Here we use trimethoxyoctylsilane monolayers on glass. We observe that the deposit diameter decreases uniformily during drying, indicating that no pinning occurs. However, several hours after completion of the coassembly process we observe catastrophic delamination, where the deposits almost completely detach from the substrates. We reject substrate modification and concentrate on the particle deposition mechanism, which is the second cause for pinning and the coffee-ring effect. Radial flows of the particles in the drying droplet seem to assist particle assembly; however, they also support the formation of ring-shaped assemblies, which are undesirable for applications. Drying control agents are low molecular weight compounds with a high boiling point. These agents effectively reduce the evaporation rate of the dispersant, resulting in a more uniform evaporation across the entire liquid/air interface. The reduced evaporation rates entail radial Marangoni flows, where the colloids are not transported toward the three-point contact line but toward the substrate, leading to particle assembly as smooth layers of particles.25 We apply diethylene glycol (DEG) as a drying control agent, because of its good solubility in water and ethanol. We obtain smooth oblate-spheroid deposits at pH 2.5 and with addition of 10 wt % of DEG. This amount of DEG is optimized and has previously been employed in colloidal self-assembly.25 The added DEG does not seem to interfere with the BTES sol−gel precursor. Particles as well as matrix are carried to the center of the deposit by the Marangoni flow, as shown by thick overlayers of amorphous glass with residual nonordered particles encapsulating the colloidal crystal (see Figure S2 in the Supporting Information). Particle to Sol−Gel Precursor Volume Ratio. Hard spheres, such as PS particles assemble into close-packed fcc or hcp structures with packing factors of 74%.26 The remaining interstitial 26% are occupied by the BTES in order to prevent crack formation. However, when we prepare such a volume ratio of 74:26 vol % of PS:BTES and investigate the crosssection by SEM, we observe incomplete encapsulation with free
interstitial space. This is because polysilsesquioxanes exhibit considerable shrinking during condensation.22 Additionally, some of the BTES remains on the surface of the assembled colloidal crystal, forming a protective layer. This material is thus missing for filling of the interstitial space. We hence increase the BTES percentage successively between 37 vol % to 67 vol %. Compositions with 37 and 42 vol % BTES produce opaque, light-green deposits with an intense red reflection; however, these deposits are very brittle. Coassemblies with 50 and 60 vol % exhibit transparent edges, while they are iridescent toward the center (see Figure 2e). Deposits with 67 vol % of BTES are completely transparent. We again examine the deposits with respect to their cross-sections using SEM to analyze the quality of the encapsulation and the coassembly process. All deposits have a flat hemispherical profile due to the addition of DEG as a drying control agent. The structure of the deposit with 37 vol % of BTES consists of well-ordered but partially nonencapsulated colloids as observed for 26 vol % of BTES. This is in agreement with the findings in other studies.16,27 Close to the air/assembly interface the order is reduced as shown in Figure 2a. The brittleness observed for deposits with 37 and 42 vol % of BTES is a result of incomplete encapsulation of the particles. Nonuniform shrinkage of the colloidal crystal structure occurs at low volume fractions of BTES, leading to cracks and areas with air in the interstitial spaces.16,27 Essentially, only volume fractions above 50 vol % of BTES are suitable for complete encapsulation, as the interstitial space are filled with BTES as well as formation of an overlayer occurs, encapsulating the entire coassembly. The deposits with 50 vol %, 60 vol %, and 67 vol % show large areas of close-packed, encapsulated particles with the typical honeycomb structure exposed at the facets of the crystal lattice resulting from the BTES glass matrix (see cross-section images in Figures 2, parts b, c, and d, respectively). The encapsulated, ordered structures extend over the entire deposit with amorphous areas occurring only at the edges of the deposit. These amorphous areas consist of unordered particles (50 vol %) and the isolated glass matrix with dispersed single particles (60−67 vol %). Deposits with high loading of BTES (50−67 vol %) exhibit complete encapsulation of particles and are more stable toward mechanical deformation. To examine the goodness and the size of the crystallites in more detail, we polish the cross-sections using a focused argonC
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Figure 3. Optical performance of the coassembled photonic crystals. (a) Absolute reflectivity spectra of deposits with BTES content of 50 vol %, (b) 60 vol %, and (c) 67 vol % at different activation times. Black squares correspond to an activation time of 0.5 h, blue circles to 1 h, green triangles to 3 h, orange upside down triangles to 8 h, and red diamonds to 24 h. (d) Micrographs (2×) of deposits with a BTES volume ratio of 60% and 1 h activation time at wavelengths (d) 620 nm, (e) 630 nm, and (f) 640 nm. The scale bar represents 20 μm. Highest intensities of the reflected light are yellow and lowest are black as illustrated by the calibration bar in each image. (g) An angle-resolved reflectance spectrum with blue as the lowest and orange the highest intensity. The corresponding deposit has a BTES volume ratio of 50% and 1 h activation time. The green line represents the Bragg fit of the acquired reflection contour graph.
glass matrix. Loss of residual solvent, which is entrapped in the glass matrix, as well as delayed hydrolysis of residual reactive silane groups in the glass matrix, are responsible for the slow shrinking process, which shows its effect after about 1 week.22,27 These stresses can be reduced by performing a prehydrolysis step, where precondensed silane oligomers are produced, reducing the amount of shrinkage in the dried state. This effect is taken advantage of in polyalkyloxysilane chemistry.29 We now optimize the precondensation time to achieve coassembled colloidal crystals, which are not prone to aging. Activation Time. Acid-catalyzed gelation of bridged polysilsesquioxanes is well understood for a variety of organic bridging groups.22,30−32 A mechanism has been proposed, where hydrolyzed monomers condense not only to form linear and branched oligomers but also to form cyclic species with reduced activity toward further condensation.31,32 This leads to slow and well controllable precondensation.30−32 Although the cyclic intermediates are less reactive, they can still form oligomers after longer reaction times.30,31 Thus, we choose BTES precondensation conditions in an acidic ethanol/water mixture. We vary the duration of precondensation or activation time33 from 30 min to 24 h, before adding the colloids in ratios to precondensed BTES of 40:60 vol % and 33:67 vol %. The precondensation is performed on a tumbling wheel to ensure mixing and avoid concentration gradients. The particles are completely dispersed in the medium and then immediately deposited onto glass substrates. With increasing activation time, the number and lengths of the cracks on the surface of the deposits decrease. However, photonic performance which we have established earlier as a measure for the colloidal crystallinity does not linearly improve with longer activation times. Iridescence is best seen for activation times between 1 and 8 h. In the sample with 60 vol % of BTES, aging is much reduced for activation times between 1 and 3 h. Longer activation times result in large silica gel particles disturbing the colloidal crystal or disordered amorphous colloids, as shown in Figure 2i,j. Reflectance Measurements. Good colloidal order is reflected by the photonic properties of the colloidal crystals.
ion beam. Smooth surfaces are generated, exposing the grain boundaries of the crystallites, which can now be precisely imaged by SEM. The SEM images of coassemblies with BTES contents of 50, 60, and 67 vol % show dark circular spots representing the PS particles in a lighter colored matrix of the BTES sol−gel (see Figure 2, parts f, g, and h, respectively). The polished cross-sections demonstrate that the coassemblies consist of large crystallites of different orientations toward each other, which are separated by thin amorphous grain boundaries. In coassemblies with 50 vol % of BTES, crystallites of different sizes above and below 10 μm and different orientations can be identified (see Figure 2f). Line defects and point defects such as vacancies occur in the crystallites and disturb crystal order (see Figure 2f). Such defects are also observed in the 60 vol % BTES composites, while the crystallites are larger in diameter in comparison to the 50 vol % coassemblies, as displayed in Figure 2g. Large crystallites of a few tens of micrometers next to very small crystallites are found in the deposit with a BTES content of 67 vol %, as shown in Figure 2h. The size of the crystallites and their disordered orientation can be explained by nucleation and growth of crystallites throughout the drying droplet. While Marangoni flow leads to accumulation and ordered self-assembly of particles at the substrates (see Figure 1b), self-assembly also occurs in the liquid droplet. While the substrate confines the (1 1 1) direction of the colloidal crystal to be parallel to the interface, growing crystallites in the volume of the droplet are free in their orientation. The curved surface of the drying droplet leads to contorted orientation of the crystallites.28 The size of the crystallites is governed by the evaporation rate, where lower rates facilitate larger crystallites. Similarly, faster evaporation and hydrolysis rates entail fast self-assembly, promoting defects by prevention of colloidal healing. While the coassemblies with 50, 60, and 67 vol % of BTES are smooth and without cracks after drying and remain like this for several days, giving us sufficient time for analysis, we realize aging effects take place after 1 week or more. Networks of small cracks emerge, followed by delamination of the coassemblies from the substrate. This is again the result of shrinkage of the D
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value. We overlay the contour plot reflection data with the modified Bragg equation and literature values for the refractive indices of PS and BTES (nPS = 1.58 and a nBTES = 1.3)24 and the determined particles sizes from SEM d = 260 nm and a packing factor f = 0.74. We see very good agreement of the calculated Bragg line with the experimental data. However, for increasing reflection angles, we also observe slight weakening and broadening of the reflection band, which will be caused by the observed tilted crystallites.
Defects such as vacancies, stacking faults, and amorphous regions cause multiple scattering effects, resulting in weakening and broadening of the reflection profile.34 We thus investigate the optimized conditions for colloidal arrangement with reflectance spectroscopy, to examine the potential for application as printable photonic crystals. We examine BTES volume percentages of 50, 60 and 67% at a pH of 2.5 with 5 wt % of DEG and again examine activation times between 0.5 and 24 h. All samples exhibit reflection maxima at 0° (i.e., perpendicular/normal to the substrate) in the red spectrum. However, we can also see that longer activation times lead to broadened and shifted reflection bands (see Figure 3a−c). The modified Bragg equation expresses that the wavelength of reflection depends on the angle θ between the incident light beam and the (1 1 1) crystal plane. The presence of differently orientated crystallites in the deposits, as confirmed by the argon-ion milled cross-sections, leads to the appearance of reflections at different wavelengths and angles. This leads to broadening of the reflection peak. Shifted and broadened reflection spectra are mainly observed for BTES ratios of 60 and 66 vol % (see Figure 3b,c). To analyze the reflection spectra, we determine the full width at half-maximum (fwhm) of the main reflection peaks in the spectra. The fwhm values for BTES ratios of 67 vol % are generally larger than for 60 and 50 vol % (see Figure S3 in the Supporting Information). The particle to BTES ratio of 50:50 exhibits the lowest fwhm between 25 and 30 nm. The smallest fwhm in the other two samples with 60 and 67 vol % of BTES is observed for activation times of 1 h. The small fwhm represents narrow reflection bands, indicating well-ordered particles with few defects. These results indicate, together with the previously conducted studies and microscopic analyses, that an activation time of 1 h is optimal. A PS particles to BTES volume ratio of 50:50 can be easily controlled, and small changes in the activation time have almost no effect. Furthermore, the small fwhm values indicate good colloidal order at least for several layers at the substrate interface, where we optically probe the samples. The reflection is not completely uniform across the entire coassembled deposit as we can observe in wavelength resolved reflectance microscopy images. When we look at wavelengths close to the reflection maximum, we observe the highest reflection intensities close to the center of the deposits (see Figure 3d,e). This can be explained by better crystal quality at the center of the substrate droplet interface, where the Marangoni flow allows for improved ordering. Moving outward from the center toward the edges of the deposit, we can observe other areas of the same crystalline orientation, which also exhibit reflection. The dark and therefore nonreflecting area will host crystallites of a different lattice orientation, as observed in the cross-section images in Figure 2f−h. We can investigate the influence of these areas by performing angle-dependent reflection spectroscopy. Here the tilted crystallites will appear by broadening the reflection profile at higher reflection angles. We record a spectrum for the 50 vol % BTES sample at an activation time of 1 h. We can record the angle dependence between −18° to 27° with respect to the incident beam normal to the substrate. We plot the data as a contour plot, where the intensity is color coded (blue for low and orange for high intensities; see Figure 3g). The contour plot shows high intensities for θ = 0° at 640 nm. In accordance with the modified Bragg law, the wavelength reflected by the (1 1 1) crystal plane in fcc packed crystals shifts to shorter wavelengths for reflection angles increasing in their absolute
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CONCLUSION In summary, we have established experimentally optimized conditions of colloid and sol−gel coassembly for directly printed colloidal crystals. Mixtures of 50 vol % of colloids to 50 vol % of BTES precursor lead to the best results, with fully encapulated colloidal crystals with BTES occupying the interstitial spaces as well as forming a protective layer over the entire deposit. 10 wt % of DEG inhibits coffee-ring formation, leading to the desired dome-shaped coassembly deposits. Precondensation at pH 2.5 with an activation time of 1 h produces high quality photonic crystal assemblies, with long term stability, where no aging is observed for months. The method is easily scalable and could be adopted for inkjet printing of colloids, as well as for convective assembly.35 The optimized conditions for coassembly will facilitate the production of inkjet-printable photonic crystals that will ultimately lead to self-assembled laser resonators and filters, which can be precisely positioned and patterned into desired structures.
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EXPERIMENTAL SECTION
Chemicals and Materials. All chemicals such as 1,2-bis(triethoxysilyl)ethane (≥99.95%) purchased from Gelest Inc., and Coumarin 6, diethylene glycol (≥99%), hydrochloric acid (4 M), potassiom peroxodisulfate (≥99%), and styrene (≥99%) purchased from Aldrich, were used without further purification. Ethanol (95− 96%) was purchased from VWR International. Demineralized water was used. Cover glasses were purchased from VWR International. Synthesis of Polystyrene Colloids. Coumarin 6 (5.3 mg, 0.020 mmol) was dissolved in styrene (4.2 mL, 39 mmol). The mixture of the flurecent dye and the monomer was then mixed with 150 mL of deionized water. The reaction mixture was stirred, heated to 80 °C, and bubbled with argon for 30 min. Potassium persulfate (165 mg, 0.610 mmol) was dissolved in 4 mL of deionized water, bubbled with argon, and added to the reaction mixture. The reaction was run for 4 h. Afterward agglomerates and residue were filtered off over glass wool and paper filter. The colloidal suspensions were purified by centrifugation at 10000−12000 rpm for 30 min followed by decantation and redispersion. This procedure was repeated three times. Colloids with diameter of 260 nm and polydispersity of less than 3% were obtained by this synthesis. Sizes and dispersity of colloids were determined by light scattering methods or scanning electron microscopy. Preparation of Colloidal Sol−Gel Precursor Solution. At first, 1 mL of the colloidal suspension was centrifuged by 13 400 rpm for 15−20 min to separate the particles from supernatant water which was then discarded. The amount of particles was then determined using TGA. The total weight fraction of the encapsulated colloidal crystal was chosen to be 40 wt %. The sol−gel solution was prepared by mixing HCl (4 M) and ethanol (95−96%) (1:2) and DEG in a weight fraction of 10 wt %. BTES was added in a weight fraction varying from 15, 20, 24, to 27 wt %, according to the volume ratio of BTES to PS colloids which was chosen to be 1:1.7, 1:1, 1.5:1, and 2:1, respectively. The volume fractions of BTES then were 11, 16, 19, and 22 vol % in the same order as above. The volumes were measured with a E
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Langmuir micropipet, and the solution was activated for 0.5, 1, 3, 8, and 24 h on a rotator at room temperature prior to redispersing the colloids. Drop-Casting of Dispersions. Cover-glasses purchased from VWR International were cleaned by ultrasonication in acetone, 2propanol, and deionized water for 10 min each. After each step, the cover-glasses were dried in a stream of nitrogen. The colloidal sol−gel solutions were applied with an Eppendorf pipet on the cleaned coverglasses as 1, 5, and 10 μL droplets and left in the fume hood to dry in a covered Petri dish at room temperature.
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(10) Zulian, L.; Emilitri, E.; Scavia, G.; Botta, C.; Colombo, M.; Destri, S. Structural iridescent tuned colors from self-assembled polymer opal surfaces. ACS Appl. Mater. Interfaces 2012, 4, 6071− 6079. (11) Xuan, R.; Ge, J. Photonic printing through the orientational tuning of photonic structures and its application to anticounterfeiting labels. Langmuir 2011, 27, 5694−5699. (12) Pieranski, P. Colloidal crystals. Contemp. Phys. 1983, 24, 25−73. (13) Ko, H.-y.; Park, J.; Shin, H.; Moon, J. Rapid self-assembly of monodisperse colloidal spheres in an ink-jet printed droplet. The ability of monodisperse colloidal spheres to self- assemble into crystalline arrays makes them interesting and versatile building blocks for advanced materials. Chem. Mater. 2004, 16, 4212−4215. (14) Park, J.; Moon, J.; Shin, H.; Wang, D.; Park, M. Direct-write fabrication of colloidal photonic crystal microarrays by ink-jet printing. J. Colloid Interface Sci. 2006, 298, 713−719. (15) Cui, L.; Li, Y.; Wang, J.; Tian, E.; Zhang, X.; Zhang, Y.; Song, Y.; Jiang, L. Fabrication of large-area patterned photonic crystals by ink-jet printing J. Mater. Chem. 2009, 19, 5499 10.1039/b907472d. (16) Hatton, B.; Mishchenko, L.; Davis, S.; Sandhage, K. H.; Aizenberg, J. Assembly of large-area, highly ordered, crack-free inverse opal films. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10354−9. (17) Schaffner, M.; England, G.; Kolle, M.; Aizenberg, J.; Vogel, N. Combining bottom-up self-assembly with top-down microfabrication to create hierarchical inverse opals with high structural order. Small 2015, 11, 4334. (18) Hatton, B.; Mishchenko, L.; Davis, S.; Sandhage, K. H.; Aizenberg, J. Assembly of large-area, highly ordered, crack-free inverse opal films. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10354−10359. (19) Müller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Sotomayor Torres, C. M. Dye-containing polymer beads as photonic crystals. Chem. Mater. 2000, 12, 2508−2512. (20) Bhardwaj, R.; Fang, X.; Somasundaran, P.; Attinger, D. Selfassembly of colloidal particles from evaporating droplets: Role of DLVO interactions and proposition of a phase diagram. Langmuir 2010, 26, 7833−7842. (21) Osterholtz, F.; Pohl, E. Kinetics of the hydrolysis and condensation of organofunctional alkoxysilanes: A review. J. Adhes. Sci. Technol. 1992, 6, 127−149. (22) Loy, D. A.; Shea, K. J. Bridged polysilsesquioxanes. Highly porous hybrid organic-inorganic materials. Chem. Rev. 1995, 95, 1431−1442. (23) Talbot, E. L.; Yang, L.; Berson, A.; Bain, C. D. Control of the particle distribution in inkjet printing through an evaporation-driven sol-gel transition. ACS Appl. Mater. Interfaces 2014, 6, 9572−83. (24) Park, J.; Moon, J.; Shin, H.; Wang, D.; Park, M. Direct-write fabrication of colloidal photonic crystal microarrays by ink-jet printing. J. Colloid Interface Sci. 2006, 298, 713−9. (25) Park, J.; Moon, J. Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing. Langmuir 2006, 22, 3506−13. (26) Norris, D. J.; Arlinghaus, E. G.; Meng, L.; Heiny, R.; Scriven, L. E. Opaline photonic crystals: How does self-assembly work? Adv. Mater. 2004, 16, 1393−1399. (27) Wang, L.; Zhao, X. Fabrication of crack-free colloidal crystals using a modified vertical deposition method. J. Phys. Chem. C 2007, 111, 8538−8542. (28) Velev, O. D. A class of microstructured particles through colloidal crystallization. Science (Washington, DC, U. S.) 2000, 287, 2240−2243. (29) Zhu, X.; Jaumann, M.; Peter, K.; Möller, M.; Melian, C.; AdamsBuda, A.; Demco, D. E.; Blü mich, B. One-pot synthesis of hyperbranched polyethoxysiloxanes. Macromolecules 2006, 39, 1701− 1708. (30) Loy, D. A.; Jamison, G. M.; Baugher, B. M.; Myers, S. A.; Assink, R. A.; Shea, K. J. Sol−gel synthesis of hybrid organic−inorganic materials. Hexylene- and phenylene-bridged polysiloxanes. Chem. Mater. 1996, 8, 656−663.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00098. Experimental prodecures, methods, and materials. Additional FESEM images of amorphous top layer (Figure S1), samples at pH 2.5 and 11 and corresponding photographs (Figure S2). Full width at half-maximum (fwhm) of reflectivity measurements normal to the substrate against activation time (Figure S3) for different PS:BTES volume ratios (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Sabrina Mallmann for help with crosssectioning by argon-ion beam polishing and SEM measurements. The authors thank the German Ministry of Education and Research for financial support in form of the “AktiPhotoPol” grant (BMBF grant no. 13N13522). This work was performed in part at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the federal state of North Rhine-Westphalia (grant EFRE 30 00 883 02).
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