Colloid means a state where colloidal particles having a size of several nm to several μm are dispersed in a medium, and has a broad industrial application in fields such as paints and medical supplies.
When a suitable condition is selected, the colloidal particles are regularly disposed in a colloidal dispersion and form a structure referred to as a “colloidal crystal”. There are two types of the colloidal crystal. The first type is a crystal formed on a condition that a particle volume fraction is about 0.5 (a concentration=50% by volume) or more in a colloidal system (hard sphere system) having no particular interaction between particles. This is similar to a phenomenon that when macroscopic balls are stuffed into a restricted space, the balls are regularly disposed. The second type is a crystal structure formed by an electrostatic interaction acting between particles in a dispersion system (charged colloidal system) of charged colloidal particles. For example, the crystal is formed by a colloidal system obtained by dispersing silica particles (SiO2) and other particles made of a polymer (polystyrene, poly (methyl methacrylate), or the like) having a surface having a dissociable group into a polar medium such as water. Since the electrostatic interaction extends to a long distance, a crystal can be produced even when a particle concentration is low (a distance between particles is long) or a particle volume fraction is about 0.001.
Similarly to usual crystals, the colloidal crystals show the Bragg diffraction electromagnetic waves. The diffraction wavelength can be set to a visible light range by selecting production conditions (a particle concentration and a particle diameter). Consequently, application development to an optical element or the like including a photonic material has been actively considered both nationally and internationally. The present mainstream of a producing process of an optical material includes a multilayer thin film process and a lithography process. Both of the techniques have excellent periodic accuracy. However, the former provides only a one-dimensional periodic structure, and the latter provides only the one-dimensional periodic structure or a two-dimensional periodic structure. A three-dimensional crystal structure (opal structure) obtained by precipitation of fine particles has a plane distance having good evenness when particles having a uniform particle diameter are used. However, an area having good single crystallinity is limited to about ten periods. It is difficult for a process for precipitating fine particles to construct a macroscopic three-dimensional structure (that is, a large colloidal single crystal).
Usually, the colloidal crystals are obtained as polycrystal bodies in which about 1 mm microcrystals assemble together. However, when the colloidal crystals are used as the optical element, a single crystal of cm order is often required. Various lattice defects and unevenness usually exist in the colloidal crystals. This may obstruct the use of the colloidal crystals as the optical element. As described above, establishment of a process for producing colloidal crystals has been required, which (1) has high quality (that is, free from lattice defects and unevenness as much as possible) and (2) can produce a large single crystal.
As a technique of controlling generation of colloidal crystals derived from charged colloidal system, hitherto, a technique (Non-Patent Document 1) and a process (Non-Patent Document 2) have been reported hitherto. The technique (Non-Patent Document 1) obtains a single crystal by shearing orientation of charged colloidal polycrystals to an ionic polymer latex/water dispersion system in a region between parallel plates, the region having a gap of about 0.1 mm. The process (Non-Patent Document 2) applies an electric field for crystallization. However, these processes have drawbacks: for example, a special device for applying a shear field is required in the former case, and crystallization is hindered due to impurity ions produced by an electrode reaction in the latter case. In addition, there is a report (Non-Patent Document 3) in which charged colloidal crystals are solidified by a polymer gel, and a distance of crystal planes is controlled using a change in gel volume caused by a change in temperature. However, this requires a complicated process, and no attempt to generate a crystal from an unregulated particle disposal state has been made.
The present inventors have developed a producing process of colloidal crystals for making a specific ionization material coexist in a charged colloidal dispersion system to form the colloidal crystals by a change in temperature (Patent Document 1). The process can produce the colloidal crystals comparatively easily without requiring a particular device and a complicated process from various kinds of charged colloidal systems. However, it is difficult for the process to produce a large-sized single crystal exceeding 1 cm.
The present inventors have succeeded in producing the largest colloidal crystals in the world such as columnar crystals having a length of several cm and cubic crystals having one side of about 1 cm, using a novel technique of diffusing a base from one end of a sample as the process for producing colloidal crystals capable of obtaining the large-sized single crystal. In the technique, the inventors have used a silica colloidal particles/water system in which charge number is increased with pH to be crystallized (Non-Patent Document 4, Patent Document 2). However, the process has a fault that it takes an extremely long time to grow crystals. It was found by spectrometry that large unevenness (inclination and fluctuation) exist in the lattice spacing of the large-sized crystals thus obtained. This is considered to be caused by time/spatial unevenness of a base concentration and disorder of diffuse or the like essential to a diffuse phenomenon. The unevenness of the plane distance is decreased with time. However, even if the base concentration is approximately uniform, distribution of about 10% exists in the plane distance. Consequently, the application is limited as the optical element.
Consequently, the present inventors have conducted earnest studies to develop a process for producing colloidal crystals, which can easily and inexpensively produce a large-sized single crystal having fewer lattice defects and less unevenness (Patent Document 3). In the process, a colloidal dispersion in which pyridine is added to silica colloid is prepared. A degree of dissociation of pyridine changes depending on a temperature. When the higher the temperature of the colloidal dispersion is, the larger the charge density of the silica particles is, and thereby, the colloidal dispersion has a character in which the colloidal crystals form. The colloidal dispersion is put into a vessel in a state where the colloidal crystals do not form. A temperature of one end side of the vessel is locally set to a temperature at which the colloidal crystals form by warming. A range set to the temperature at which the colloidal crystals form is gradually expanded to grow the colloidal crystals. The colloidal crystals thus obtained are changed to an extremely large single crystal that has fewer lattice defects and less unevenness. Consequently, a half-value width in an absorption spectrum and a reflection spectrum can be set to an extremely narrow range of 20 nm or less. Space unevenness of diffraction wavelength having extremely high quality of 2.0% or less can be obtained. Herein, the space unevenness is obtained by denoting by percentage a value obtained by dividing standard deviation of spatial distribution of the diffraction wavelength of the colloidal crystals measured by reflective spectroscopy or transmission spectroscopy by weighted average efficiency of the diffraction wavelength (hereafter the same)    Non-Patent Document 1: B. J. Ackerson and N. A. Clark, Phys. Rev. A 30, 906, (1984)    Non-Patent Document 2: T. Palberg, W. Moench, J. Schwarz and P. Leiderer, J. Chem. Phys. 102, 5082, (1995)    Non-Patent Document 3: J. M. Weissman, H. B. Sunkara, A. S. Tse and S. A. Asher, Science, 274, 959, (1996)    Non-Patent Document 4: N. Wakabayashi, J. Yamanaka, M. Murai, K. Ito, T. Sawada, and M. Yonese Langmuir, 22, 7936-7941, (2006)    Patent Document 1: Japanese Patent Application Laid-Open No. 11-319539    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-89996    Patent Document 3: Japanese Patent Application Laid-Open No. 2008-93654