Colloidal crystals and methods for producing the same

Abstract

[Problem] To provide: a colloidal crystal which has a four-fold symmetrical structure and can exist stably even in a space that is geometrically unconstrained; and a production method therefor. [Solution] A colloidal crystal according to the present invention is characterized by existing in a space that is geometrically unconstrained and having a four-fold symmetrical structure. The colloidal crystal according to the present invention can be produced by: filling a dispersion of colloidal particles into a space between a substrate 1 and a counter plate 2 facing the substrate 1 to precipitate a charged colloidal crystal having a four-fold symmetrical structure (crystallization step S1); and fixing the charged colloidal crystal having a four-fold symmetrical structure by electrostatically adsorbing the same onto the substrate 1 (fixing step S2).

Description

[Technical Field]

[0001] The present invention relates to a colloidal crystal having a four-fold symmetric structure and a method for producing the same. [Background technology]

[0002] A colloid is a state in which a dispersed phase is dispersed in a dispersion medium, and when the dispersion medium is liquid, it is called a colloidal dispersion. Charged colloidal particles, which have a charge on their surface, spontaneously arrange themselves in a regular pattern with a distance between them in a colloidal dispersion when appropriate conditions are selected due to the electrostatic repulsion that acts between the particles. This structure is called a charged colloidal crystal. Charged colloidal crystals are known to adopt either a body-centered cubic (BCC) or face-centered cubic (FCC) structure, depending on the conditions (Non-Patent Document 1).

[0003] The formation of charged colloidal crystals from colloidal dispersions occurs through self-organization, as colloidal particles attempt to form a thermodynamically stable structure. This has the advantage of not requiring precise processing techniques, unlike lithography and other methods. Furthermore, by selecting the diameter of the colloidal particles, they can be used as photonic materials that respond to various wavelengths. For this reason, much research has been conducted on the production of colloidal crystals.

[0004] Typically, the (111) plane of a face-centered cubic (FCC) lattice with six-fold symmetry is oriented on the walls of a container or a substrate. A method using a constrained space has been developed as a technique for imparting four-fold symmetry to the planes oriented on the substrate (Non-Patent Document 2). This method utilizes the phenomenon that a six-fold symmetric structure or a four-fold symmetric structure is formed by adjusting the ratio of particle diameter to gap length in a constrained space (i.e., a geometrically restricted space) with gaps of about 1 to 100 μm (see Figure 20).

[0005] However, in the manufacturing method of colloidal crystals with a four-fold symmetric structure that utilizes such a confined space, there is a problem in that colloidal crystals with a four-fold symmetric structure cannot be maintained unless the special environment of a confined space with gaps of approximately 1 to 100 μm is maintained, which has been an obstacle when using colloidal crystals as photonic materials, etc. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] "Colloidal Crystals, Formation, and Their Applications" edited by Hiroshi Nakamura and Junpei Yamanaka, CMC Publishing, 2020 [Non-patent document 2] Pieranski, P.; Strzelecki, L.; Pansu, B. Thin Colloidal Crystals. Phys. Rev. Lett. 1983, 50 (12), 900-903. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in view of the above-mentioned conventional situation, and aims to provide a colloidal crystal having a four-fold symmetric structure that can exist stably even in a space that is not geometrically constrained, and a method for producing the same. [Means for solving the problem]

[0008] The colloidal crystals of the present invention exist in a geometrically unconstrained space and have a four-fold symmetric structure. As described above, it is known that a four-fold symmetric structure is generated when a dispersion of colloidal particles is placed in a geometrically constrained closed space (for example, a constrained space between two opposing planes). However, the colloidal crystals of the present invention can stably adopt a four-fold symmetric structure even in an unconstrained space, not in such a geometrically constrained closed space. Therefore, when used in optical elements, the colloidal crystals do not need to exist in a geometrically constrained closed space, and have the advantage of being easy to use.

[0009] The colloidal crystal of the present invention can be a two-dimensional colloidal crystal consisting of a single layer, or a three-dimensional colloidal crystal consisting of multiple layers. Furthermore, in a three-dimensional colloidal crystal, a first layer consisting of a first colloidal particle and a second layer consisting of a second colloidal particle are alternately repeated to form multiple layers, and the refractive index of the first colloidal particle or the second colloidal particle is the same as the refractive index of the dispersion medium. In this case, light passing through the colloidal crystal is not refracted by the second colloidal particle constituting the second layer, and the second layer becomes transparent to light. That is, if a BCC crystal is stacked on a substrate with the (100) plane as the first layer and the second layer is made optically transparent, the particles in the non-transparent first and third layers will have a simple cubic lattice (SC). This provides the special effect of forming a structure optically identical to the structure of colloidal particles consisting of a simple cubic lattice (SC), which cannot be formed by self-assembly alone.

[0010] The colloidal crystal of the present invention can be produced as follows. That is, this method for producing colloidal crystals comprises a crystallization step of filling the space between a substrate and an opposing plate facing the substrate with a dispersion of first colloidal particles to precipitate charged colloidal crystals with a four-fold symmetric structure made of the first colloidal particles, and a fixation step of electrostatically adsorbing and fixing the charged colloidal crystals with a four-fold symmetric structure made of a single layer of the first colloidal particles to the substrate.

[0011] In the colloidal crystal manufacturing method of the present invention, after the immobilization step, a second layer formation step can be performed by bringing a dispersion of second colloidal particles having an opposite charge to that of the first colloidal particles into contact with the monolayer of the first colloidal particles, and electrostatically adsorbing the second colloidal particles onto the monolayer of the first colloidal particles, thereby forming a charged colloidal crystal with a four-fold symmetric structure in which two layers are stacked.

[0012] Furthermore, after the second layer formation process, a third layer formation process can be carried out by bringing a dispersion of third colloidal particles having an opposite charge to that of the second colloidal particles into contact with the monolayer of the second colloidal particles, electrostatically adsorbing the third colloidal particles onto the monolayer of the second colloidal particles, thereby forming a charged colloidal crystal with a four-fold symmetric structure in which three layers are stacked.

[0013] Furthermore, by alternately repeating the second layer formation step and the third layer formation step, it is possible to obtain a charged colloidal crystal with a four-fold symmetric structure in which four or more layers are stacked.

[0014] Alternatively, the surface of the substrate or the colloid particles in the colloid particle dispersion may be chemically modified with a modifying group capable of imparting an electric charge, and the immobilization step may be performed by eliminating ions in the colloid particle dispersion present between the substrate and the opposing plate. Ion exclusion makes it easier for the colloid particles to electrostatically adsorb to the substrate. Ions in the dispersion of colloidal particles can be removed by, for example, placing an ion exchange resin around the substrate and the opposing plate to adsorb the ions, or by immersing the substrate and the opposing plate in pure water to diffuse the ions present between the substrate and the opposing plate by diffusion or convection. Another method for the immobilization process is to change the pH of the colloidal particle dispersion between the substrate and the counterplate to alter the magnitude and sign of the surface charge of the colloidal particles and the substrate, resulting in electrostatic adsorption with opposite surface charges. For example, the silane coupling agent aminopropyltriethoxysilane (APTES) contains weakly basic amino groups and is positively charged at pH < 7.8. Silica contains weakly acidic silanol groups and is negatively charged. Glass substrates surface-modified with APTES are negatively charged at pH above approximately 7 and positively charged at pH below 7 (see Aoyama, Y.; Toyotama, A.; Okuzono, T.; Yamanaka, J., Langmuir, 2019, 135 (28), 9194-9201). In this way, by positively charging the initially negatively charged substrate, the negatively charged colloidal particles can be electrostatically adsorbed onto the substrate. One method for decreasing the pH of the colloidal particle dispersion is to immerse the substrate and the opposing plate in a hydrochloric acid solution and introduce hydrochloric acid into the colloidal dispersion by diffusion and convection. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of a colloidal crystal according to a first embodiment. [Figure 2] 1 is a process diagram showing the steps of producing colloidal crystals according to the first embodiment. [Figure 3] Schematic diagram of a colloidal crystal with a four-fold symmetry structure (a) and a colloidal crystal with a six-fold symmetry structure (b) [Figure 4] This is a phase diagram obtained from theoretical calculations. [Figure 5] FIG. 1 is a schematic diagram of a colloidal crystal according to a second embodiment. [Figure 6] FIG. 1 is a process diagram showing the colloidal crystal production process of the second embodiment. [Figure 7]1A is a schematic diagram of a colloidal crystal according to a third embodiment, and FIG. 1B is a schematic diagram of a colloidal crystal according to a third embodiment formed on a substrate 1 immersed in a dispersion medium having the same refractive index as the colloidal particles 13b of the second layer. [Figure 8] FIG. 2 is a cross-sectional view showing a colloidal crystal preparation cell 20 and its periphery. [Figure 9] This is a three-dimensional image of a colloidal crystal obtained by LSM. [Figure 10] This is a phase diagram obtained from a three-dimensional image. [Figure 11] 1 shows a microscopic image of Example 1 and the radial distribution function calculated from the image (top: microscopic image and radial distribution function of a colloidal crystal with a four-fold symmetric structure; bottom: microscopic image and radial distribution function of a colloidal crystal with a six-fold symmetric structure). [Figure 12] 1 shows microscope images of colloidal crystals for Example 2 (left: colloidal crystals with a four-fold symmetric structure, right: colloidal crystals with a six-fold symmetric structure). [Figure 13] 1 shows microscope images of colloidal crystals for Example 3 (left: colloidal crystals with a four-fold symmetric structure, right: colloidal crystals with a six-fold symmetric structure). [Figure 14] 1 shows microscope images of colloidal crystals for Example 4 (left: colloidal crystals with a four-fold symmetric structure, right: colloidal crystals with a six-fold symmetric structure). [Figure 15] 1 is a microscope image of colloidal crystals for Example 5. [Figure 16] 1 is a microscope image of colloidal crystals for Example 6. [Figure 17] Adsorption curves due to decreasing base concentration in the confined space (bars indicate standard deviations in experimental values, curves indicate calculated values ​​based on the diffusion equation for various initial ion concentrations C*). [Figure 18] 10 shows optical microscope images of each layer in the colloidal crystal of Example 7. [Figure 19]10 shows cross-sectional images taken by a confocal optical microscope of the colloidal crystal of Example 7, when the medium is ethylene glycol and when the medium is a water-ethylene glycol mixed solution adjusted to have the same refractive index as the polystyrene of the second layer. [Figure 20] FIG. 1 is a schematic diagram showing that different colloidal crystals are formed by adjusting the ratio of particle diameter to gap length. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. <Embodiment 1> A schematic diagram of a colloidal crystal having a four-fold symmetric structure according to embodiment 1 is shown in Figure 1(a). This colloidal crystal is a two-dimensional colloidal crystal with a four-fold symmetric pattern, and is composed of a single layer of the (100) face of an FCC (face-centered cubic) structure (see Figure 1(b)). The colloidal particles that make up the colloidal crystal are not in contact with each other, but are kept at a constant distance. This colloidal crystal can be produced by a series of steps (crystallization step S1 and fixation step S2) shown in Figure 2.

[0017] (Crystallization step S1) A substrate 1 and an opposing plate 2 are prepared, facing each other in parallel. A colloidal particle dispersion liquid, in which colloidal particles 3 are dispersed in a dispersion medium 4, is dropped onto the substrate 1, and then the opposing plate 2 is placed on top of it (see Figure 2(a)). The type of colloidal particles 3 is not particularly limited; inorganic particles (e.g., SiO2 particles, TiO2 particles, alumina particles, etc.), organic particles (e.g., polystyrene particles, acrylic polymer particles, etc.), and metal particles (e.g., precious metal particles such as Au particles, Pt particles, Pd particles, rhodium particles, iridium particles, ruthenium particles, osmium particles, and rhenium particles) can be used. The colloidal particle dispersion liquid can be prepared by dispersing commercially available colloidal particles in an appropriate dispersion medium such as water, by using inorganic particles synthesized by the sol-gel method, or by using relatively uniformly sized particles obtained by polymerizing monomers such as styrene by emulsion polymerization. Colloidal particles can also be prepared by coating the surfaces of nonmetallic particles with a metal (e.g., ceramic or polymer particles coated with a precious metal such as Au). The dispersion medium can be, for example, water, but liquids other than water can also be used. For example, formamides (e.g., dimethylformamide) and alcohols (e.g., ethylene glycols) can be used. These can also be used as a mixture with water.

[0018] Charged colloidal crystals are formed over time from colloidal particles 3 in the colloidal particle dispersion between substrate 1 and counter plate 2 (Figure 2(b)). In this case, the type of charged colloidal crystal formed varies depending on the ratio of the distance (gap) h between substrate 1 and counter plate 2 to the particle diameter (=2a) of colloidal particles 3. This is described in Non-Patent Document 2 and can be derived from theoretical calculations. Specifically, colloidal particles are assumed to be rigid bodies, and to have either the (111) or (100) face of the FCC structure so that the density of colloidal particles in the confined space is maximized. Furthermore, the interactions between colloidal particles are calculated by considering only the hard-sphere potential, and further approximating the high-pressure limit, in which the pressure p depends only on the gap size h, as shown in Equation (1).

[0019]

number

[0020] Here, g is the acceleration of gravity, Δμ is the density difference between the colloid particle and the dispersion medium, and ρ is the volume fraction of the colloid particles. In this model, the volume fraction is calculated geometrically for four-fold symmetric structures and six-fold symmetric structures. Let r be the interparticle distance and a be the radius of the colloid particle, and let d = r / 2a. A schematic diagram of a four-fold symmetric structure is shown in Figure 3. When the interparticle distance > particle diameter in the same layer, particles in different layers are in contact, but particles in the same layer are not in contact (Figure 3(a)). On the other hand, when the interparticle distance = particle diameter in the same layer (Figure 3(a)), the maximum packing ratio of colloid particles is calculated using the following equation (2): 0.74.

[0021]

number

[0022] Here, □ indicates a four-fold symmetric structure. As a result, the volume fraction ρn□ of colloidal particles forming a four-fold symmetric structure can be calculated using the following equation (3):

[0023]

number

[0024] Similarly, in a hexagonal symmetric structure, when the interparticle distance is greater than the colloid particle diameter in the same layer, colloid particles in different layers are in contact, but particles in the same layer are not in contact (Figure 3(b)). When the interparticle distance is equal to the colloid particle diameter in the same layer (Figure 3(b)), d_(n△) is expressed by the following equation (4), where △ indicates a hexagonal symmetric structure.

[0025]

number

[0026] As a result, the volume fraction ρn△ of colloidal particles when a six-fold symmetric structure is formed can be calculated using the following equation (5):

[0027]

number

[0028] Using equations (3) and (5), the volume fractions that can be obtained by changing the size ratio d are calculated and plotted in the graph shown in Figure 4. The closely spaced dotted lines represent the volume fraction change with d for the four-fold symmetric structure, and the coarsely spaced dotted lines represent the volume fraction change with d for the six-fold symmetric structure. The higher the particle density, the more stable the crystalline structure becomes. For this reason, a phase diagram (see Figure 4) can be obtained by color-coding the areas where the dense crystalline structure of colloidal particles changes between the four-fold symmetric structure and the six-fold symmetric structure. From the above results, it can be seen that as the value of h / 2a increases, the phase transition occurs in the following order: 6-fold symmetric structure consisting of two layers (2△) → 4-fold symmetric structure consisting of three layers (3□) → 6-fold symmetric structure consisting of three layers (3△) → 4-fold symmetric structure consisting of four layers (4□) → 6-fold symmetric structure consisting of four layers (4△). Therefore, by maintaining the substrate 1 and the counter plate 2 parallel and the h / 2a ratio such that a 4-fold symmetric structure is formed, colloidal crystals with a 4-fold symmetric structure can be formed. Note that even if the substrate 1 and the counter plate 2 are not parallel but tilted, or if the substrate 1 or the counter plate 2 are bent, and the value of h / 2a varies depending on the location, it is possible to partially form a 4-fold symmetric structure.

[0029] The presence of trace amounts of salt (ionic impurities) can alter the surface charge of colloidal crystals, inhibiting their formation. Therefore, it is preferable to thoroughly demineralize the dispersion medium when preparing a dispersion of colloidal particles. For example, when using water, the dispersion medium is first dialyzed against purified water until the electrical conductivity of the water is approximately the same as that before use. Then, the sample is desalted and purified by storing it in the presence of a thoroughly washed ion exchange resin (a mixed bed of cation and anion exchange resins) for at least one week. However, after this desalting and purification, salts can be added and desalted in the crystallization step S2 described below to precipitate colloidal crystals.

[0030] The particle size and distribution of the colloidal particles must also be considered. The particle size of the colloidal particles is preferably 600 nm or less, and more preferably 300 nm or less. This is because colloidal particles with a particle size greater than 600 nm tend to settle due to the effects of gravity, reducing the stability of the colloidal particle dispersion. Furthermore, the coefficient of variation of the particle size of the colloidal particles (i.e., the value obtained by dividing the standard deviation of the particle size by the average particle size) is preferably 20% or less, more preferably 10% or less, and most preferably 5% or less. A large coefficient of variation of the particle size makes it difficult for colloidal crystals to precipitate, and increases lattice defects and non-uniformity in the colloidal crystals, making it difficult to obtain high-quality colloidal crystals.

[0031] The materials for the substrate 1 and the counter plate 2 are not particularly limited, but for example, a smooth glass plate, a ceramic plate, a plastic plate, a metal plate, or the like can be used.

[0032] (Imobilization step S2) The precipitated charged colloidal crystals with four-fold symmetry are immobilized by electrostatic adsorption (see Figure 2(c)). One immobilization method is to modify the surface of the substrate 1 or colloidal particles 3 with a silane coupling agent containing amino groups and add an alkali such as NaOH, sodium bicarbonate, or sodium carbonate to the dispersion medium. In this case, the substrate 1 and the counter plate 2 can be immersed in water, and cations present between the substrate 1 and the counter plate 2 can be removed by diffusion or convection. Adding an ion-exchange resin to the immersion water can further expedite cation removal. The removal of cations lowers the pH of the colloidal particle dispersion, ionizing the amino groups. This electrostatic attraction between the colloidal particles 3 (which have a negative surface charge but are not chemically modified with amino groups) and the substrate 1 immobilizes a single layer of charged colloidal crystals with four-fold symmetry on the surface of the substrate 1. The resulting charged colloidal crystals are adsorbed to the substrate 1 by electrostatic attraction and remain stable even when immersed in pure water.

[0033] <Embodiment 2> As shown in FIG. 5, the colloidal crystal of embodiment 2 is composed of two layers: a monolayer A having a four-fold symmetric structure and composed of colloid particles 13a spaced a certain distance from each other; and a monolayer B composed of colloid particles 13b positioned directly above the center of the square unit cell of monolayer A and stacked in contact with four colloid particles 13a in monolayer A. This colloidal crystal can be manufactured according to the process diagram shown in FIG. 6. First, the crystallization step S1 and immobilization step S2 described in embodiment 1 are performed to form a monolayer colloidal crystal having a four-fold symmetric structure on substrate 1. Next, counter plate 2 is removed, and substrate 1 is immersed in pure water to wash away any adhering dispersion medium. A dispersion of colloid particles 13b having an opposite charge to that of colloid particles 13a is then brought into contact with the monolayer of colloid particles 13a, thereby electrostatically adsorbing a second layer composed of colloid particles 13b onto the first layer composed of colloid particles 13a (second layer formation step S3). At this time, colloid particle 13b is placed directly above the center of the square crystal lattice made up of colloid particles 13a while being in contact with colloid particles 13a due to electrostatic attraction. In this way, as shown in Figure 5, a colloidal crystal with a four-fold symmetric structure is obtained, which is formed by two layers: single layer A made up of four colloid particles 13a and single layer B made up of colloid particles 13b that are in contact with and stacked on top of each other. For example, when colloid particles 13a have a negative surface charge such as silica, silica whose surface is modified with a silane coupling agent having an amino group can be used as colloid particles 13b having an opposite charge to colloid particles 13a. When colloid particles 13a have a positive surface charge such as silica modified with a silane coupling agent having an amino group, colloid particles 13b can be unmodified silica that has a negative surface charge, silica modified with a polymer having a negative surface charge, or polystyrene that has a negative surface charge.

[0034] <Embodiment 3> The colloidal crystal of embodiment 3 is a colloidal crystal formed by stacking a third layer of colloidal particles on the two-layer colloidal crystal of embodiment 2. First, a two-layer colloidal crystal having a four-fold symmetric structure is formed by the method of embodiment 2. Then, the counter plate 2 is removed, and the substrate 1 is immersed in pure water to wash away any adhering dispersion medium. A dispersion of colloidal particles 13c having an opposite charge to colloidal particles 13b is then brought into contact with the layer of colloidal particles 13b, and colloidal particles 13c are electrostatically adsorbed onto the second layer to form a third layer (third layer formation step S4). In this way, a colloidal crystal having a four-fold symmetric structure in which three layers are stacked can be obtained (see FIG. 7(a)).

[0035] Furthermore, by immersing the substrate 1 on which the colloidal crystal of embodiment 3 has been formed in a dispersion medium having the same refractive index as the colloidal particles 13b of the second layer (transition from FIG. 7(a) to FIG. 7(b)), the colloidal particles 13b become optically transparent. Therefore, optically, the colloidal crystal becomes one consisting of only the first and third layers (see FIG. 7(b)). [Example]

[0036] 1) Preparation of colloidal dispersion An aqueous dispersion of silica particles (KEP-50 manufactured by Nippon Shokubai Co., Ltd., average particle size = 0.53 μm, charge number Z = -6420, coefficient of variation of particle size = 5% and KEP-100, average particle size = 1.1 μm, Z = -19488, coefficient of variation of particle size = 5%) was prepared as a colloidal dispersion. Furthermore, a colloidal dispersion to which a predetermined amount of Na2CO3 or NaOH was added was prepared. In this way, the colloidal dispersions used in Examples 1 to 4 were prepared. Example 1: (KEP-50) Silica particle volume fraction = 0.3, NaOH concentration 50 mM Example 2: (KEP-50) Silica particle volume fraction = 0.1, Na2CO3 concentration 0.1 mM Example 3: (KEP-50) Silica particle volume fraction = 0.2, Na2CO3 concentration 1 mM Example 4: (KEP-100) Silica particle volume fraction = 0.3, no alkali added

[0037] 2) Preparation of a cell for colloidal crystal preparation Cleaning the cover glass Optical microscope cover glass (Matsunami Glass Industrial Co., Ltd., 35 mm x 55 mm x 0.15 mm) was prepared and subjected to UV irradiation and ozone treatment on the front and back surfaces for 10 minutes each using an ASM401N UV-ozone treatment device manufactured by Asumi Giken Co., Ltd. (hereafter referred to as UV / O3 treatment). The cover glass was then immersed in a concentrated HCl + MeOH mixture (volume ratio 1:1) for 30 minutes, washed with Milli-Q water, immersed in concentrated sulfuric acid for 2 hours, and then thoroughly washed with Milli-Q water. Milli-Q water is ultrapure water obtained using a Milli-Q® water production system manufactured by Merck Ltd. Surface modification of cover glass with APTES The cleaned cover glass was immersed in a solution of 3-aminopropyltriethoxysilane (APTES) dissolved in 90% EtOH to a concentration of 1 vol.% (hereafter referred to as the APTES solution) and left at room temperature for 1 hour. The cover glass was then removed and dried overnight in an oven at 70°C to chemically modify the silanol groups on the glass surface with APTES. The cover glass was then washed overnight in Milli-Q water and then dried. 20. Preparation of Colloidal Crystal Preparation Cells A colloidal crystal preparation cell 20 shown in Figure 8 was prepared as a cell for precipitating colloidal crystals. This cell consists of a cover glass 21 chemically modified with APTES, to which a 5 mm thick silicon sheet 22 with a 2 cm x 2 cm square hole is attached.

[0038] 3) Colloidal crystallization step S1 500 μL of colloidal dispersion was dropped into a recess formed by the square hole in the silicon sheet 22 and the cover glass 21. Then, a cover glass 23 was placed inside the square hole, and a quartz glass 24 was placed on top of that, and a 100 g weight 25 was placed on top of that. After leaving it to stand for 30 minutes, the objective lens 26 of a confocal laser microscope was placed close to below the cover glass 21 for observation. That is, the colloidal dispersion was placed in the colloidal crystal preparation cell 20, and left to stand for 30 minutes or more, and then a three-dimensional image was obtained using a laser scanning microscope (LSM) (see Figure 9). From the LSM image analysis, the gap size h and the number of particles Nn in each layer were determined, and the volume fraction φ was calculated. However, because a charged colloidal system was used, the effective radius (a eff ) was calculated and corrected. eff The apparent volume fraction was calculated using the formula, and the phase diagram shown in Figure 10 was created (d = h / 2a eff ).

[0039]

number

[0040] where a is the particle radius, u(r) is the interaction potential between two particles (u(σ)=0), and k Bis the Bolzmann constant, and T is the absolute temperature. As shown in Figure 10, the corrected experimental results were superimposed on the theoretical phase diagram for comparison. Square symbols represent fourfold symmetric structures, and triangle symbols represent sixfold symmetric structures. Filled symbols represent the case without added salt, while open symbols represent the case with added NaCl (NaCl concentration Cs = 0.25 mM). The experimental results were confirmed to be consistent with the theoretical phase diagram. Furthermore, the trend was consistent with the theoretical phase diagram, regardless of whether salt was added or not, demonstrating the usefulness of effective radius correction. The overlap of the fourfold and sixfold symmetric structures at small d values ​​is likely due to the accuracy limitations of LSM. The LSM images were scanned at 0.15 μm / step, and the smaller the gap, the larger the error. The smaller ρ values ​​at large d values ​​is likely due to the blurring of the image as the total number of particles increases, resulting in errors in particle counting. As described above, three-dimensional image analysis by LSM confirmed the phase behavior of the crystal structure in the confined space. That is, as the gap size h increases, a phase transition occurs: 2△ → 3□ → 3△ → 4□ → 4△ → …. Furthermore, using an optical microscope, the transition from a fourfold symmetric structure to a sixfold symmetric structure was observed when the weight was removed. Similar trends were also observed in experiments with Cs = 0.050 mM and 0.10 mM.

[0041] 4) Immobilization step S2 1.5 g of ion exchange resin (a mixed bed of cation and anion exchange resins, manufactured by Bio-Rad Laboratories, Inc., AG501 X-8(D)) was placed on top of the colloidal crystal preparation cell 20. To prevent water evaporation, the colloidal crystal preparation cell 20 was covered with a plastic container (not shown) and allowed to stand for several days. After confirming through microscopic observation that the silica particles had been adsorbed onto the cover glass 21 substrate, the weight 25, quartz glass 24, and cover glass 23 were removed. The cover glass 21 was then thoroughly washed with Milli-Q water, and a sufficient amount of water was added. Microscopic images of the adsorbed silica particles were taken. The microscopic images thus obtained were analyzed using the image analysis software Image J to determine the average distance r between particle centers, and the volume fraction φ was calculated from equation (7). v (where a represents the radius of the silica particles.) The particle radial distribution function g(r) was calculated from the binarized image obtained by binarizing the image to black and white using the image processing software Image J.

[0042]

number

[0043] <Results> 1) Example 1 FIG. 11 shows a microscopic image of Example 1, which used a colloidal dispersion with a silica particle volume fraction of 0.3 and an NaOH concentration of 50 mM, and a radial distribution function calculated from the image. Two microscopic images (both with an image size of 50 μm×50 μm) are shown here because the gap distance varies depending on the deflection of the cover glasses 21 and 23, resulting in colloidal crystals with a four-fold symmetric structure (upper left in FIG. 11) and colloidal crystals with a six-fold symmetric structure (lower left in FIG. 11) depending on the location (the same applies to Examples 2 to 4). In the colloidal crystals with a four-fold symmetric structure, the interparticle distance r was found to be 623±7 nm (volume fraction φ v ,4= 0.46). In addition, for the colloidal crystal with a six-fold symmetry structure, r=600±24 nm (φ v ,6= 0.51).

[0044] 2) Regarding Example 2 FIG. 12 shows a microscopic image of Example 2, which used a colloidal dispersion with a silica particle volume fraction of 0.1 and a Na2CO3 concentration of 0.1 mM (image size: 30 μm × 30 μm). In the colloidal crystal with a four-fold symmetric structure, the interparticle distance r was found to be 695±13 nm (φ v4 = 0.33). In addition, for the colloidal crystal with a six-fold symmetry structure, r = 690 ± 25 nm (φ v6 =0.34).

[0045] 2) Regarding Example 3 FIG. 13 shows a microscopic image of Example 3, which used a colloidal dispersion with a silica particle volume fraction of 0.2 and a Na2CO3 concentration of 1 mM (image size: 30 μm × 30 μm). In the colloidal crystal with a four-fold symmetric structure, the interparticle distance r was 644 ± 15 nm (φ v4 =0.41). In addition, for the colloidal crystal with a six-fold symmetry structure, r = 638 ± 8 nm (φ v6 =0.42).

[0046] 2) Regarding Example 4 FIG. 14 shows a microscopic image of Example 4, which used a colloidal dispersion with a volume fraction of silica particles of 0.3 and no alkali added (image size: 50 μm×50 μm). From the microscopic image, the interparticle distance r of the colloidal crystal with a four-fold symmetric structure was found to be 1.244±0.019 μm (φ v4 = 0.49). In addition, for colloidal crystals with a six-fold symmetry structure, r = 1.269 ± 0.010 μm (φ v6 =0.46).

[0047] From the microscopic images of Examples 1 to 4 and the radial distribution functions calculated from them, it was found that various parameters of the colloidal crystals formed with four-fold symmetric structure or six-fold symmetric structure can be controlled by the particle size and particle concentration of the colloidal particles. In Examples 1 to 4, the colloidal crystal structure differs depending on the location, presumably because the gap distance varies due to the bending of the cover glasses 21 and 23 and the non-uniformity of the load applied by the weight. By replacing the thin cover glasses with glass blocks to make the gap distance uniform, or by adopting a structure that applies the load uniformly, it is possible to control the colloidal crystals to have a four-fold symmetric structure or a six-fold symmetric structure.

[0048] Example 5 1) Preparation of colloidal dispersion In Example 5, an aqueous dispersion of polystyrene particles (manufactured by Thermo, average particle diameter = 600 nm, negative charge, zeta potential = -48 mV) was concentrated under reduced pressure, and then an aqueous NaOH solution was added to prepare a colloidal dispersion with a particle concentration of 40 vol% and an NaOH concentration of 4 mM. 2) Colloidal crystal preparation cell The colloidal crystal preparation cell was the same as the cell used in Examples 1 to 4, and a description thereof will be omitted. 3) Colloidal crystallization step S1 Next, the colloidal crystallization step S1 was carried out in the same manner as in Examples 1 to 4. However, the amount of colloidal dispersion dropped into the recesses was 176 μL. 4) Immobilization step S2 Furthermore, the same immobilization step S2 as in Examples 1 to 4 was carried out. However, the desalination time by the ion exchange resin was set to 24 hours. When the glass substrate was observed under a microscope, colloidal crystals having a four-fold symmetric structure and consisting of a single layer of polystyrene particles were confirmed, as shown in Figure 15. The glass substrate on which these four-fold symmetric colloidal crystals had been formed was stored in pure water.

[0049] Example 6 1) Preparation of colloidal dispersion Example 6 is the same as Example 5 except that the NaOH concentration in the colloidal dispersion was 5 mM, and therefore a description thereof will be omitted. 2) Colloidal crystal preparation cell The colloidal crystal preparation cell was the same as the cell used in Example 5, and a description thereof will be omitted. 3) Colloidal crystallization step S1 Next, the colloidal crystallization step S1 was carried out in the same manner as in Example 5. However, the amount of colloidal dispersion dropped into the recesses was 206 μL. 4) Immobilization step S2 Furthermore, the same immobilization step S2 as in Example 5 was carried out. Then, when the glass substrate was observed under a microscope, colloidal crystals having a four-fold symmetric structure and consisting of a single layer of polystyrene particles were confirmed, as shown in Figure 16. The glass substrate on which these colloidal crystals had been formed was stored in pure water.

[0050] <Adsorption curve of colloidal crystals with four-fold symmetry> The experimental adsorption curves of colloidal crystals with four-fold symmetry were obtained by the following procedure, and compared with the theoretically calculated values. 1) Preparation of silica particle colloidal dispersion An aqueous dispersion of silica particles (KEP-50, manufactured by Nippon Shokubai Co., Ltd.; average particle size = 0.53 μm, charge number Z = -6420, coefficient of variation of particle size = 5%; and KEP-100; average particle size = 1.1 μm, Z = -19488, coefficient of variation of particle size = 5%) was prepared as a colloidal dispersion. A predetermined amount of Na2CO3 was then added to prepare a silica particle colloidal dispersion (volume fraction of silica particles = 0.3, Na2CO3 concentration = 10 mM).

[0051] 2) Preparation of a cell for colloidal crystal preparation Cleaning the cover glass Optical microscope cover glass (Matsunami Glass Industry Co., Ltd., 35 mm x 55 mm x 0.15 mm) was prepared and subjected to UV irradiation and ozone treatment (hereafter referred to as UV / O3 treatment) on the front and back sides for 10 minutes each using an ASM401N UV-ozone treatment device manufactured by Asumi Giken Co., Ltd. The cover glass was then immersed in a concentrated HCl + MeOH mixture (volume ratio 1:1) for 30 minutes, washed with Milli-Q water, immersed in concentrated sulfuric acid for 2 hours, and then thoroughly washed with Milli-Q water. Surface modification of cover glass with APTES The cleaned cover glass was immersed in a solution of 3-aminopropyltriethoxysilane (APTES) dissolved in 90% EtOH to a concentration of 1 vol.% (hereafter referred to as the APTES solution) and left at room temperature for 1 hour. The cover glass was then removed and dried overnight in an oven at 70°C to chemically modify the silanol groups on the glass surface with APTES. The cover glass was then washed overnight in Milli-Q water and then dried. - Preparation of cells for colloidal crystal preparation To prepare colloidal crystals, a cell similar to the colloidal crystal preparation cell 20 used in Examples 1 to 3 was used (see FIG. 8). This cell had a cover glass 21 chemically modified with APTES, to which a 5 mm thick silicon sheet 22 with a 2 cm × 2 cm square hole was attached.

[0052] 3) Colloidal crystallization step S1 500 μL of colloidal dispersion was dropped into a recess formed by the square hole in silicon sheet 22 and cover glass 21. Then, cover glass 23 was placed inside the square hole, and quartz glass 24 was placed on top of that, and a 100 g weight 25 was placed on top of that, and after leaving it to stand for 30 minutes, the objective lens 26 of a confocal laser microscope was placed close to below the cover glass 21 and observed. That is, the colloidal dispersion was placed in colloidal crystal preparation cell 20 and left to stand for 30 minutes or more, and then a three-dimensional image was obtained with a laser scanning microscope (LSM), and it was confirmed that colloidal crystals with a four-fold symmetric structure had been formed.

[0053] 4) Immobilization step S2 1.5 g of ion-exchange resin (a mixed bed of cation and anion exchange resins, manufactured by Bio-Rad Laboratories, Inc., AG501 X-8(D)) was placed on top of the colloidal crystal preparation cell 20. To prevent water evaporation, the colloidal crystal preparation cell 20 was covered with a plastic container (not shown). The adsorption of a monolayer of colloidal crystals with a four-fold symmetry structure onto the cover glass 21, which served as the substrate for silica particles, was observed at regular intervals using a confocal laser microscope. The length from the edge of the cover glass 21 to the growing edge of the monolayer of four-fold symmetry colloidal crystals was measured. The results are shown in Figure 17. The data are the average of three experimental results, and the bars indicate the standard deviation. The results showed that after the addition of the ion-exchange resin beads, ions diffused outward from the edge of the cover glass 21, which in turn caused electrostatic adsorption inward from the edge of the cover glass 21, resulting in the growth of a monolayer of colloidal crystals. After 30 hours, the crystal size reached 1 mm.

[0054] 5) Adsorption curve obtained from theoretical calculation The adsorption process is considered to be due to one-dimensional unidirectional diffusion of the base. The ion concentration C(x, t) (x and t are position and time) is given by the diffusion equation (8), where D is the apparent diffusion coefficient of the ion ( = 1.16 × 10-5 cm2 / s).

[0055]

number

[0056] The ion concentration at any position and time is given by equation (9), where Ci is the initial salt concentration and erf is the error function, assuming that the ion exchange resin has a sufficiently high exchange capacity to satisfy C=0 at x=0.

[0057]

number

[0058] C i= 10 mM, (x, t) was calculated for various values ​​of C = C* using equation (9). The results are shown in Figure 17. The experimental values ​​were in good agreement with the values ​​calculated from the diffusion equation when C* = 4 mM. This result suggests that electrostatic adsorption occurs at C* = approximately 4 mM.

[0059] <Creating multilayer colloidal crystals> Example 7 In Example 7, a colloidal crystal with a four-fold symmetric structure composed of three layers was prepared. The details are shown below. 1) Formation of the first layer A concentrated aqueous dispersion of 440 nm diameter polystyrene particles (Thermo, particle size coefficient of variation = 4%, negative charge, zeta potential = -48 mV) was used. An aqueous NaOH solution was added to this aqueous dispersion to adjust the particle concentration to 33 vol% and the NaOH concentration to 1 mM. A square plastic frame (inner dimensions 20 mm × 20 mm) was placed on the surface of the APTES-modified glass substrate, allowing it to be filled with liquid. 300 μL of the solution was dropped onto the APTES-modified glass substrate. A plastic plate (15 mm x 15 mm) was then placed on top, and 1.5 g of ion exchange resin (a mixed bed of cation and anion exchange resins, Bio-Rad, AG501 X-8(D)) was added to the periphery of the plastic plate, and the plate was desalted for 2 hours. After the desalting process was completed, the glass substrate was washed with water to remove excess particles, and when the glass substrate was observed under a microscope, colloidal crystals with a four-fold symmetric structure and consisting of a single layer of polystyrene particles were confirmed. The glass substrate on which this single layer colloidal crystal had formed was stored in pure water.

[0060] 2) Formation of the second layer A red fluorescent dye (rhodamine isothiocyanate) was adsorbed onto the surface of silica particles KE-P30 (particle diameter 300 nm) manufactured by Nippon Shokubai Co., Ltd., and a silica coating layer was then formed using the sol-gel method. The silica particles, whose outermost surfaces were then covered with a silica coating layer, were then treated with an ethanol solution of polyethyleneimine-based silane coupling agent (manufactured by Gelest Co., Ltd.) to introduce amino groups into the silica coating layer, producing positively charged red fluorescent silica particles with a particle diameter of 430 nm (coefficient of variation of particle diameter = 4%). The zeta potential of these particles was measured to be +58 mV. An aqueous dispersion of these positively charged red fluorescent silica particles (particle concentration = 0.1%) was prepared. 200 μL of this aqueous dispersion was added to 3 mL of 100 μM NaCl aqueous solution to form a saline dispersion, which was then dropped onto the glass substrate on which the first layer of colloidal crystals had been formed. The glass substrate was then contacted with a semipermeable membrane bag filled with ion exchange resin for 2 hours for desalination, and then observed by LSM. The results confirmed that a single layer of red fluorescent silica particles was adsorbed onto a single layer of polystyrene particle colloidal crystals. The glass substrate was then washed with water to remove excess dispersion and then stored in pure water. In this way, a colloidal crystal consisting of a single layer of red fluorescent silica particles stacked on a single layer of polystyrene particles with a four-fold symmetry structure was obtained.

[0061] 3) Formation of the third layer A saline dispersion of 2 μL of the 440 nm diameter negatively charged polystyrene particles (particle concentration = 10 vol%) prepared in the first layer was added to 3 mL of 100 μM NaCl aqueous solution and dispersed. The resulting solution was then dropped onto the second layer. The crystal was then placed in a semipermeable membrane bag filled with ion-exchange resin for 2 hours for desalination. Optical microscopy of the surface confirmed that the polystyrene particles were adsorbed onto the red fluorescent silica particles. Finally, the glass substrate was washed with water to remove excess liquid and then stored in pure water. This resulted in a three-layer colloidal crystal with a four-fold symmetry structure: a monolayer of red fluorescent silica particles layered on top of a monolayer of polystyrene particles, which was then layered on top of that.

[0062] (Microscopic observation) The colloidal crystals of Example 7 thus obtained were photographed using an optical microscope. As a result, as shown in Figure 18, a four-fold symmetric structure in which polystyrene particles were arranged at equal intervals was clearly observed in the microscope image of the first layer. Furthermore, the red fluorescent silica particles of the second layer were located directly above the center of the square unit cell made up of the polystyrene particles of the first layer. Furthermore, the polystyrene particles of the third layer were located directly above the polystyrene particles of the first layer (see Figure 19).

[0063] <Replacement of the dispersion medium in the colloidal crystal of Example 7> 3 mL of ethylene glycol was added to the substrate 1 on which colloidal crystals were formed, as prepared in Example 7, and the substrate was left to stand for 2 hours. Furthermore, instead of ethylene glycol, 3 mL of a water-ethylene glycol mixed solution adjusted to the same refractive index as the silica particles in the second layer was added and the substrate was left to stand for 30 minutes. Images of each sample were then taken with a confocal laser scanning microscope (Nikon, C2 model). As a result, when the medium was ethylene glycol, the first, second, and third layers were clearly observed, whereas when the medium was a water-ethylene glycol mixed solution adjusted to the same refractive index as the silica particles in the second layer, the second layer was transparent and could not be observed.

[0064] The present invention is not limited to the above-described embodiments and examples, and various modifications within the scope of the claims and within the scope that can be easily conceived by a person skilled in the art are also included in the present invention. [Industrial Applicability]

[0065] The colloidal crystal of the present invention can be used as a photonic material that can accommodate a variety of wavelengths by selecting the diameter of the colloidal particles. [Explanation of symbols]

[0066] 1...substrate, 2...opposing plate, 3, 13a, 13b, 13c...colloid particles, 4...dispersion medium, S1...crystallization process, S2...immobilization process, S3...second layer formation process, S4…Third layer formation process, 20... Cell for preparing colloidal crystals, 21... Cover glass, 22... Silicon sheet, 23... Cover glass, 24... Quartz glass, 25... Weight, 26... Objective lens

Claims

1. A colloidal crystal fixed to a substrate, comprising: The colloidal crystal has a four-fold symmetric structure, A colloidal crystal in which the colloidal particles constituting the colloidal crystal are separated from each other without coming into contact with each other.

2. 2. The colloidal crystal according to claim 1, which consists of a single layer.

3. 2. The colloidal crystal according to claim 1, which is composed of multiple layers.

4. 4. The colloidal crystal according to claim 3, wherein a first layer made of the first colloidal particles and a second layer made of the second colloidal particles are alternately repeated to form multiple layers, and the refractive index of the first colloidal particles or the refractive index of the second colloidal particles is the same as the refractive index of the dispersion medium.

5. a crystallization step in which a dispersion of first colloidal particles is filled between a substrate and an opposing plate facing the substrate, and a charged colloidal crystal having a four-fold symmetric structure made of the first colloidal particles is precipitated by adjusting the distance between the opposing plates; and an immobilization step of electrostatically adsorbing and immobilizing the charged colloidal crystal having a four-fold symmetric structure, which is made of a single layer of the first colloidal particles, onto the substrate.

6. 6. The method for producing colloidal crystals according to claim 5, further comprising a second layer formation step, after the immobilization step, of bringing a dispersion of second colloidal particles having an opposite charge to that of the first colloidal particles into contact with the monolayer of first colloidal particles, thereby electrostatically adsorbing the second colloidal particles onto the monolayer of first colloidal particles.

7. 7. The method for producing colloidal crystals according to claim 6, further comprising a third layer formation step, after the second layer formation step, of bringing a dispersion of third colloidal particles having an opposite charge to that of the second colloidal particles into contact with the monolayer of second colloidal particles, thereby electrostatically adsorbing the third colloidal particles onto the monolayer of second colloidal particles.

8. 8. The method for producing colloidal crystals according to claim 7, wherein the second layer forming step and the third layer forming step are repeated alternately.

9. the surface of the substrate or the colloid particles in the dispersion of colloid particles is chemically modified with a modifying group capable of imparting an electric charge; 9. The method for producing colloidal crystals according to claim 5, wherein the immobilization step is carried out by eliminating ions in a dispersion of colloidal particles present between the substrate and the opposing plate.

Images