Spectroscopic Investigation of Opal Formation from Suspensions

Aug 2, 2017 - spectrometer than that in open air. Bragg Wavelength and Peak Width Simulation. Bragg wavelength and width of extinction peak of wet opa...
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Spectroscopic Investigation of Opal Formation from Suspensions M. Muldarisnur†,‡ and F. Marlow*,†,§ †

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany Department of Physics, Andalas University, Kampus Unand Limau Manis, 25163 Padang, Indonesia § Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Duisburg, Germany ‡

S Supporting Information *

ABSTRACT: We report an in-situ observation of wet opal formation from a dilute colloidal suspension by using time-resolved transmission spectroscopy. The formation involves rather complex partial processes that include particle migration, particle ordering (crystallization) into differently oriented domains, and continuous compaction. The initial particle ordering results in an fcc lattice with an interparticle distance larger than particle diameter. The crystallization is followed by a slow but continuous compaction until the wet opal fills the capillary cell completely. The time behavior of the background of the extinction spectra indicates that there is no disordered dense state preceding the opal growth front. Instead, it seems that the continuous compaction process heals pointlike defects but at the same time induces domain-related defects. Similar formation processes likely occur for other deposition methods like vertical and horizontal deposition methods as well.

1. INTRODUCTION The self-assembly of colloidal particles in artificial opals is one of the most investigated routes to three-dimensional photonic crystals. Many opal deposition methods, with a different degree of controllability and resulting opal quality, have been proposed.1 However, the crystallization process at the beginning of opal formation has only rarely been investigated. This fact is surprising because one would naturally agree that better controlled opal deposition is only possible with a thorough understanding of the crystallization process. A better controlled deposition is expected to result in opals with improved quality, i.e., a low concentration of internal defects. The existence of internal defects limits the applicability of opals in optical devices.2 The formation mechanism has been intensively investigated in two related research fields: latex coatings and crystallization of hard-sphere colloid particles.3 Particles used for opal deposition differ from those for these studies. Particles for opal deposition are monodisperse and slightly deformable and possess surface charges. The charges have a crucial role in stabilizing suspension against agglomeration, and at the same time, they are expected to induce interactions4−6 affecting the hydrodynamics of colloidal particles. The formation of artificial opals from a stable suspension is therefore a complex process involving particle migration toward a crystallization growth front. The complete process of opal formation can be divided into several partial processes as shown in Figure 1. The process starts with homogeneously distributed particles in a suspension and potential densification processes (1). The particles, under external influences, experience a disorder-to-order phase © 2017 American Chemical Society

Figure 1. (a) Partial processes of opal formation from the suspension state to dry opals1 discussed in the text and (b) the measurement setup. The growth front is labeled with solid black arrows whereas evaporation at the open edges is shown as dashed arrows. The brown rectangle is the approximate detection area. This setup allows an easy and fully perturbation-free observation of the formation process.

transformation at the so-called growth front (2). Further growth results in an ordered arrangement of all particles surrounded by solvent that fills in interstitial voids (3). After solvent evaporation and possible particle rearrangements (4), a dry opal is obtained which may also show evolution processes (5). Depending on deposition method, some steps might be overlap or absent. In addition, the dominant interactions in different steps may also be different. Only a few works have been published concerning explanation of particle ordering. For the sedimentation method, Received: June 7, 2017 Revised: August 1, 2017 Published: August 2, 2017 18274

DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279

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The Journal of Physical Chemistry C

Characterization. The ordering of particles during opal formation was investigated by performing time-resolved UV− vis transmission measurements at normal incidence with respect to the opal surface. The spectra were taken using a Cary 5G UV−vis−NIR spectrometer (Varian Inc.) operated in a cyclic mode. The UV and vis−NIR light are provided by Deuterium and quartz−iodine incandescence lamps, respectively. The transmitted light from the sample was detected with a photomultiplier or with a PbS photocell, respectively. The spectra were collected for wavelengths between 400 and 1500 nm. The first spectra were taken 12 h after deposition started. After that, spectra were collected automatically every 30−45 min. The CDM setup22,23 was modified to enable transmission measurements during opal deposition. In the modified setup, a capillary cell with a bent capillary tube was oriented vertically as shown in Figure 1b. The assembled cell was glued tightly to avoid suspension leakage. The setup was mounted into the sample compartment of the spectrometer. The sample position was adjusted so that the beam probed the opal at about 3 mm from one of the cell open edges. The beam position was fixed while the growth front swept the beam area as deposition progressing. The propagation of the growth front toward the center of the cell during deposition enables the detection of opal regions having different ordering states at different times. There is no noticeable difference between opals grown using the modified and the original setup. Lattice orientation, optical properties, and advancement of the growth front are very similar. The only difference was a slightly longer deposition time in the modified setup. Slower deposition is likely due to a slower solvent evaporation in the sample compartment of the spectrometer than that in open air. Bragg Wavelength and Peak Width Simulation. Bragg wavelength and width of extinction peak of wet opals were simulated using the MIT Photonic Band (MPB).25 The peak width is directly related to the width of the partial gap in the band structure. The calculations were done for wet opals with different interparticle distance corresponding to nontouching fcc, close-packed, or sintered fcc lattices. The refractive indices of polystyrene particles and water surrounding particles were assumed to be 1.59 and 1.33, respectively.

the settling kinetics of particles and its influence on the resulting opal quality have been investigated.7,8 Other works discussed the ordering of particles on a solid substrate at the vicinity of a meniscus leading to formation of opal monolayers9 or multilayers.10 The ordering is caused by attractive capillary forces between immersed particles when the thickness of the solvent is less than the particle diameter.9 The balance between particle flux and solvent evaporation was found to determine the thickness of the resulting opals.9,11−13 The drag force exerted by the suspension flow was suggested to be responsible for the strong preference of particles to form an fcc (facecentered cubic) arrangement when grown on a solid substrate.11,14,15 Opal formation has been studied for the vertical deposition method.16,17 On the basis of the shifts of the Bragg peak, the authors proposed a three-step formation process. The steps are (1) the formation of nontouching ordered particles, (2) solvent evaporation indicated by an abrupt blue-shift of the Bragg peak, and (3) a dry close-packed opal is obtained. A similar result has been reported for opals made by using the drop-casting method.18 In this paper, we introduce a distinction between different stages of wet opal formation based on time-resolved spectral changes. We interpret the changes in the measured spectra during opal formation as results of particle ordering, lattice compaction, and defect formation. Especially the defect formation has a large impact on the potential use of opals as photonic materials. The transformation of wet to dry opal displayed in Figure 1 has been reported;19,20 therefore, we will focus on the formation of the wet opal here. In our previous paper, we have shown that opal drying is not simply a solvent removal process. Instead, it involves water redistribution, particle rearrangement, and particle sintering. During wet opal formation different scenarios are possible such as the occurrence of a dense but less-ordered volume preceding the growth front or initial multidomain growth. Roughly this question was already discussed in short,21 but here we will go in detail. The opal samples were prepared by using capillary deposition method (CDM)22,23 that results in high quality opals.24 The CDM is ideal for studying opal formation because the crystallization and the drying process are inherently separated. Separated processes are advantageous because this lowers the risk of misinterpretation of the data. Besides, the thickness of CDM-made opals is constant and predetermined. These advantages are not obtained in most of available opal deposition methods.

3. RESULTS The spectra measured at sampled times are presented as a deposition map (Figure 2), a surface plot of extinction versus wavelength and time. The map shows the variations of the intensity and the shift of the Bragg peak with time. A background extinction outside of the Bragg peak region is low but visible during the whole deposition process. Here, a numerical value of the background (Ebg) is defined as the extinction at a wavelength of 1350 nm. This wavelength was chosen to differentiate the background from the vibration band of water existing between 1400 and 1500 nm. The shape of the background curve is independent of the defining wavelength. The use of a shorter wavelength only shifts the whole curve to higher values. The vibration band corresponds to a combination vibration of symmetric (ν1) and asymmetric (ν3) stretching of the water molecules.26 The water vibration band delivers quantitative information about local water content in the growing opal film. Because the water band is not visible in the color code chosen for Figure 2, a yellow dashed line was added.

2. EXPERIMENTAL SECTION Sample Preparation. In the CDM, the opal is grown inside a capillary cell made by sandwiching two glass slides (25 mm × 20 mm) separated by two thin polymer spacers. The spacers determine the resulting opal thickness and provide a small separation between glass slides generating capillary forces on the suspension. In this study, we used spacers with a thickness of 15 μm. Suspensions of polystyrene particles (diameter of 264 ± 8 nm) were purchased from Microparticles GmbH. The purchased 10 wt % suspension was used without further treatment except dilution with Milli-Q water to form 0.5 wt % suspension. The diluted suspension was ultrasonicated for 10 min to ensure mixing homogeneity. 18275

DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279

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The Journal of Physical Chemistry C

nously while the others show increasing and decreasing periods. The vertical dashed lines were added to indicate that at these times at least one of the spectral features is changing. The exact time for the changes depends strongly on solvent evaporation rate and details of the deposition process (see Supporting Information). It may differ from one sample to another up to 10 h. Nevertheless, the overall temporal trends were found to be very reproducible. Therefore, it is advantageous to describe different stages of opal formation using characteristic times ti (i = 1, 2, ..., 6). The times refer to the characteristic spectral changes indicated as dotted lines in Figure 3. Figure 3a shows that the maximum extinction increases monotonously from t1 to t4. The slope of Emax is however not constant but turns out to be larger between t3 and t4 than between t1 and t3. After reaching its maximum value, the maximum extinction decreases about 10% between t4 and t6. The water peak shows a steplike dependence on time. It only changes between t2 and t3 from 0.077 to 0.06. The background curve in Figure 3b, despite being relatively low, has a characteristic “peak” shape between t1 and t3 before it increases slowly with time. The Bragg peak (Figure 3c) that initially appears at 663 nm blue-shifts slowly after t2 when the water peak decays abruptly. The slope of the blue-shift is 0.4 nm h−1 between t2 and t5 and then increases to 0.87 nm h−1 between t5 and t6. The initial broadening of the Bragg peak (Figure 3d) is followed by monotonous narrowing until wet opals were formed. The fwhm has a relatively high uncertainty between 23 and 27 h because of low Bragg peak intensity. The fast (73%) narrowing of the Bragg peak between t2 and t4 is followed by a much slower further narrowing between t5 and t6. All spectral features reach their final values for the wet opal at t6.

Figure 2. A deposition map of an opal film (particle diameter 264 nm, thickness 15 μm). Inset: cut of the map for some selected times indicated with the same line styles as in the main figure. A yellow dashed line was added to indicate the edge of the enhanced water band region. To improve visibility, the spectra in inset are shifted by 0.05, 0.10, and 0.15, respectively.

The maximum extinction (Emax), appearing as a dark red region in the deposition map, occurs at t ≈ 60 h. No peak is visible up to 20 h after the deposition started; only the water band and a continuous background are observed. The maximum extinction does not increase continuously. Instead, it decreases slightly after reaching a maximum value. Along with the intensity change, the Bragg peak shows a blue-shift and peak narrowing. The details of the spectral changes during wet opal formation are analyzed in Figure 3. The spectral features show pronounced differences in their evolution with different characteristic times. The Emax and normalized full width at half-maximum (Δλ/λB) change significantly before 60 h of deposition time. On the contrary, background and Bragg wavelength (λB) change until the very end of wet opal formation process. The Bragg wavelength decreases monoto-

4. DISCUSSION Influence of Opal Parameter Changes on the Spectra. The optical properties of wet opals are not only determined by the lattice constant and refractive indices, but they are also influenced by the ordering quality, i.e., the form and the concentration of internal defects. The spectra of opal with wellordered particles show a high maximum extinction, a low background, and a sharp peak. Internal defects cause lowering and broadening of the Bragg peak and increase the spectral background. The defects are expected to have no influence on Bragg wavelength. Therefore, the increase of Bragg peak extinction may result from (1) the enlargement of ordered volume in the area probed by the beam and (2) the improvement of particle ordering quality. The Bragg peak is a characteristic of the ordered volume only. This means that when the beam probes ordered and suspension regions simultaneously, the suspension volume does not contribute to the Bragg peak. The suspension volume, however, does affect the background and (slightly) the fwhm. The blue-shift of the Bragg peak is caused by the shrinkage of interparticle distance during opal growth. However, the shrinkage is also accompanied by an increase of the particle filling fraction and, consequently, of the effective refractive index that should cause a red-shift in contrast to the above trend. The net influence turns out to be the blue-shift that can be proven using the Bragg equation (see Supporting Information). The shrinkage not only causes the blue-shift but also affects the width of the Bragg peak. An MPB calculation shows that the Bragg peak broadens steadily when the particles get closer;

Figure 3. Temporal changes of the Bragg peak (black), background (gold), and water peak (brown). Details: (a) maximum extinction, (b) background, (c) Bragg wavelength, and (d) normalized fwhm. The moving-average trends to guide the eyes are shown as solid lines in (c) and (d). The vertical dotted lines show characteristic times. Here, the characteristic times are t1 = 21 h, t2 = 39 h, t3 = 48 h, t4 = 61 h, t5 = 87 h, and t6 = 95 h. 18276

DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279

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sweep beam area with a width of ca. 3 mm. This is in a close agreement with t3 − t1 = 27 h. Stage d. Opal compaction takes place between t3 and t4. Compaction likely enforces particles to fill previously empty lattice sites and results in improved ordering quality. Better ordering quality of particles explains why maximum extinction continues to increase even though the beam-enclosed opal volume does not change anymore. The compaction results in a blue-shift and narrowing of the Bragg peak simultaneously as shown in Figure 3c,d. Stage e + f. Further opal compaction (t4 ≤ t ≤ t5) leads to formation of defects such as dislocations or domain boundaries. Defect states in the stop band allow partial light transmission manifested as a lowering of maximum extinction and an increase of background in the measured spectra. The normalized fwhm remains nearly constant as a result of compromise between peak narrowing during compaction process and peak broadening due to scattering by defects. The change of the spectra between t5 and t6 can be considered as a continuation of the previous stage. At the end of deposition process, the growth front becomes round and opal grows from all sides (see Figure 5). A slight increase in the slope may be ascribed to a faster deposition process.

i.e., the interparticle distance decreases. The maximum width of Bragg peak occurs at x = 1.25 (Figure 4). Further decrease of

Figure 4. Effect of the normalized interparticle distance on the Bragg wavelength (black squares) and the theoretical width of the partial gap (blue circles) for an infinite crystal. Dashed line indicates close-packed arrangement with touching particles. The calculation was performed by assuming that the fcc arrangement is preserved during the compaction process.

interparticle distance causes a little surprisingly a narrowing of the Bragg peak. Here, x is the center-to-center particle distance (dc−c) normalized to the particle diameter (D). In this notation, the close-packed arrangement corresponds to x = 1, whereas expanded and sintered fcc correspond to x > 1 and x < 1, respectively. One may explain this effect by the balance of the two materials in the photonic crystal. For many 3D systems, the partial band gap and thus the Bragg peak have the maximum width when the thin material has a little bigger volume than the thick part. For opals, both volumes are of the same size at x = 1.14. If x becomes smaller, then the thin-index regions are too small and have only a low influence on the photon propagation resulting in narrow Bragg peaks only. Stages of Wet Opal Formation. Stage a. At t < t1, the probed area is still in the suspension state. The crystal growth front reaches the probed area at t1. Before t1, there is no Bragg peak because the probed area contains only randomly distributed particles (dilute suspension state). The intensity of water peak is at its maximum because the probed area contains mostly water molecules. Stage b + c. Between t1 and t3, the growth front sweeps over the probed area. In this period, the increase of Emax is directly related to a steady growth of the ordered volume. The increase of Emax at a nearly constant Bragg wavelength (t1 < t < t2) corresponds to initial particle ordering. The background increases quickly, likely because of strong scattering by domains of the dense ordered particles. The constant fwhm is in agreement with the fact that Bragg wavelength is also constant because they have a similar dependence for x < 1.25 (see Figure 4). During t2 < t < t3, the growth front propagation is accompanied by particle compaction. The water peak decays to its minimum because the suspension volume in the probed area vanishes continuously while being replaced by ordered particles. The compaction causes a blue-shift and peak narrowing. The growth of the ordered volume in the probed area between t1 and t3 is in agreement with the visual observations. It took 95 h to fill the capillary cell completely with the opal. By considering the width of the cell, 9.5 h is needed to fill 1 mm area of the cell. It means that the growth front needs 28.5 h to

Figure 5. Scheme of particle ordering at the growth front. The black arrows indicate growth direction. The growth front becomes curved (red and orange dashed lines) after the opal film filled more than the half of the capillary cell. The magnified scheme of the growth front shows the proposed mechanism of particle ordering. The disordered dense particle system preceding the growth front undergoes a disorder-to-order transformation.

Stage g. At t > t6, the wet opal film is formed, and all spectral features reach their final constant values. The opal reaches its stable interparticle distance in the solvent. The solvent evaporation at the edge of the capillary cell is compensated by the influx of solvent from the suspension container while the particles cannot enter the cell. The wet opal remains unchanged as long as the capillary cell is connected to the suspension. Genesis of Order. The continuous blue-shift of the Bragg peak with a nearly constant slope during the opal deposition shows that compaction occurs in the whole deposition time. This finding indicates that the crystallization at the growth front results in nontouching ordered particles that are slowly pressed together. Slow compaction is possible due to the existence of the electric double layer surrounding the colloidal particles. The double-layer interaction causes the particles to feel the existence of the others at a certain distance. Compaction takes place because the drag force on particles due to solvent evaporation is much stronger than the interparticle repulsion.4,6 In our previous work,19 it was shown that the resulting wet opal is a fcc structure with nontouching particles. Particles are separated from each other by a thin solvent layer. 18277

DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279

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The fact that the Bragg peak and the background start to rise at the same moment indicates that there is no significant disordered dense layer in front of the crystal growth front. Genesis of Disorder. As we have seen, the changes of height of the Bragg peak, its width, and the position are quite well understandable. However, the background of the spectra shows an unexpected and nonmonotonous trend. The background has a “peak” shape between t1 and t3 before it then reincreases. The increase of background between t1 and t2 can be explained as a result of scattering by domains in thin volume preceding the growth front of the final one-domain region within the probed area. Disorder-to-order transformation is followed by the formation of domain boundaries that scatter light strongly. The scattered light appears as background in the spectra. The appearance of the multidomain volume within the probed area causes the steep decrease of the background between t2 and t3. The growth front leaves the probed area at t3. The compaction process goes on continuously after the growth front leaves the probed area. In this stage, ordered particles in the wet opal form more distinct or sharper domain boundaries that can scatter the incoming light stronger. This is our explanation of the reincrease of the scattering background.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the IMPRS for Surface and Interface Engineering in Advanced Materials (SurMat) and Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft for the support.

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5. CONCLUSIONS We have distinguished opal formation into different stages based on time-resolved transmission measurements. The changes of spectral features during opal formation have been interpreted as manifestation of three specific internal changes in wet opal films: crystallization, compaction, and defect formation. Particle ordering leads to an fcc lattice with interparticle distance larger than particle diameter. The crystallization step is followed by slow but continuous compaction process until wet opal fills the capillary cell completely. The compaction process likely heals point defect but, at the same time, induces domain boundary related defects. We found that the wet opal is an expanded fcc lattice with nontouching particles. A compaction process has also been proposed for opals made by using vertical16 and horizontal18 deposition methods. This shows that the formation mechanism for CDM-made opals shares some similarities with those opal deposition methods. However, it must be emphasized that growth and drying processes occur simultaneously in those deposition methods; therefore, interpretation of measurement data needs to be done with great care.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05590. (A) Temporal change of extinction spectra; (B) interpretation of the blue-shift; (C) reproducibility (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(F.M.) E-mail [email protected]; Fax 49-208 306 2995; Tel 49-208-3062255. ORCID

F. Marlow: 0000-0002-8539-8171 18278

DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279

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DOI: 10.1021/acs.jpcc.7b05590 J. Phys. Chem. C 2017, 121, 18274−18279