US20070023289A1

US20070023289A1 - Mesoporous film, laser emission assembly, and process for producing mesoporous film

Info

Publication number: US20070023289A1
Application number: US11/437,787
Authority: US
Inventors: Hirokatsu Miyata; Masatoshi Watanabe; Takashi Noma; Kazuyuki Kuroda; Takashi Suzuki; Ayumu Fukuoka
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2005-05-24
Filing date: 2006-05-22
Publication date: 2007-02-01
Also published as: JP2006327854A

Abstract

A novel structure is provided in which mesopores are oriented. Further a mesoporous film is provided which has two-branched diffraction peaks at intervals or 180° according to in-plane X-ray diffraction.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a mesoporous film, specifically to a mesoporous film useful as a catalyst carrier, an adsorbent, a separator, and the like. The present invention relates also to a process for producing the mesoporous film. More specifically, the present invention relates to a technique for controlling orientation of fine pores formed by self-organization macroscopically in a mesoporous film by utilizing structural anisotropy of a surface of a substrate; a technique for controlling orientation of polymer molecule chains by utilizing the oriented pores; and a technique for controlling a physical property such as light emission.
2. Related Background Art
Semiconductor working techniques are progressing remarkably rapidly, and will achieve working accuracy of 100 nm very soon. Miniaturization of semiconductor elements enables increase of the switching speed and decrease of the power consumption. Therefore, the miniaturization of the semiconductor elements is indispensable for high-performance LSI. Until now, the integration degree of the semiconductor elements is increasing linearly with the year. However, the working by conventional photolithography will reach a working limit in near future, so that development of a novel process to replace the conventional photolithography is wanted urgently.
A process utilizing self-organization is attracting attention as a working process exceeding the working limit of conventional photolithography. In this process, a fine structure is formed spontaneously by utilizing an inherent property of a material. Various types of fine structures can be formed by self-organization, including structures of layer types, fiber types, column types, sphere types, and porous material types. Promising uses are suggested for the fine structure. Among them, porous thin films formed on a substrate are useful in various industrial application fields and are most promising.
One of the porous thin films attracting attention nowadays is a thin alumina nanohole film formed by anodization of aluminum. In the film, fine pores are formed, perpendicular to the face of the thin aluminum film by anodization under certain conditions, by focusing the electric field. Various applications are proposed for the alumina nanohole films, including electron-releasing elements, and recording mediums having magnetic substance introduced therein.
Another type of materials attracting attention are mesoporous thin films prepared by a sol-gel process or a like process by using micelles of a surfactant as a template. These films can be formed to have a regular pore structure by a simple process like dip coating. This technique is described comprehensively in Angewandte Chemie International Edition, vol.38, pp. 56-57. Of the mesporous films, the most stable and industrially useful are mesoporous silica thin films. Many applications thereof are proposed, including catalysts, and light-emitting materials.
The aforementioned mesoporous films have highly regular pore structure locally, but have generally no long-period structural regularity in planes, with pores directed at random macroscopically in planes. Several methods have been proposed regarding the orientation control of the pores in a macroscopic scale.
The aforementioned methods for controlling the pore orientation have problems. The method described in Chemistry of Materials: vol.9, pp 1505-1507 utilizes shear stress caused by flow of a reaction solution for control of pore orientation. However, this method cannot achieve high conrollability in the pore orientation, and cannot readily form a uniform film over a large area, being not suitable inherently for an industrial process.
In another method described in U.S. Pat. No. 6,004,444, an elastic resin stamp having a fine capillary is pressed against a substrate and a reaction solution is allowed to flow through a groove in the stamp by electro-osmosis. In this method, the pore orientation is controlled by shear stress in the solution flowing through the narrow capillary and formation of a mesoporous silica film is promoted by the generated joule heat. In this method, the mesoporous silica film should have a fine pattern for control of the pore orientation, and the uniform orientation over a large area cannot be achieved in principle.
In both of these two methods, since the control of the pore orientation direction depends on the flow of the reaction solution, the orientation control is limited to one direction.
The present invention intends to provide a method for controlling arbitrarily the orientation direction of pores in a mesoporous silica thin film beyond the limit of the conventional techniques. The present invention intends also to provide a novel mesoporous film having pores in a controlled pore orientation.

SUMMARY OF THE INVENTION

The present invention provides a simple method for controlling the direction of pores in a mesoporous material throughout the entire substrate. The present invention enables orientation of the pores, not in one direction, but in two independent directions.
According to an aspect of the present invention, there is provided a mesoporous film formed on a substrate face and having tubular mesopores arranged in a state of a honeycomb-packed pore structure, wherein the substrate face has a structural anisotropy, the mesopores are controlled to orient in two directions in planes, and the tubular mesopores are arranged parallel to each other and parallel to the substrate face.
In the mesoporous film, micelles of amphiphillic molecules are preferably filled in the mesopores.
The wall of the mesopores is preferably formed from a material containing silica.
The size distribution of the mesopores measured by nitrogen gas adsorption preferably has a single maximum, and 60% or more of the mesopores are distributed in the size distribution range of breadth of 10 nm.
In the mesoporous film, of the two in-plane orientation directions, the region of a first orientation direction and the region of a second orientation direction are preferably and substantially the same in area.
The in-plane orientation direction of the mesopores is preferably given by a rubbing treatment, and is controlled by the structural anisotropy of the face of the substrate in two directions, and the direction of the mesopore orientation is the same as the direction of the rubbing treatment. The in-plane orientation of the mesopores is preferably controlled to two directions by a Langmuir-Blodgett film of a polymer compound having structural anisotropy formed on the face of the substrate, and the orientation direction of the mesopores is the same as the direction of lifting-up of the substrate in the process of the Langmuir-Blodgett film formation.
In the mesoporous film, a conjugated polymer compound is preferably held in a part or the entire of the mesopores.
According to another aspect of the present invention, there is provided a laser emission assembly, comprising the mesoporous film just mentioned above.
According to still another aspect of the present invention, there is provided a laser emission assembly, constituted of the mesoporous film just mentioned above and a medium having a refractivity nearly the same as the refractivity of the substrate carrying the mesoporous film.
According to a further aspect of the present invention, there is provided a process for producing a mesoporous film including a step of preparing a substrate having a surface structural anisotropy, and a step of bringing the substrate into contact with an aqueous solution containing an inorganic oxide precursor and an amphiphillic substance to form a thin film of a composite of an inorganic oxide and a surfactant having a structure of tubular micelles of the surfactant regularly arranged therein on the substrate face,
wherein the composition of the aqueous solution is adjusted to control the tubular molecule micelles to be oriented in two directions in planes.
The process preferably comprises a step of removing the surfactant from the formed thin film of the inorganic oxide-surfactant composite to make the pores of the mesoporous film empty.
According to a further aspect of the present invention, there is provided a process for producing a mesoporous film including a step of forming a thin film of a polymer compound on a substrate, a step of rubbing treatment of the polymer thin film, and a step of bringing the substrate having the polymer thin film having been rubbing-treated into contact with an aqueous solution containing an inorganic oxide precursor and an amphiphillic substance to form a thin film of a composite of an inorganic oxide and a surfactant having a structure of tubular micelles or a surfactant on the face of the substrate holding the rubbing=treated polymer thin film,
wherein the composition of the aqueous solution is adjusted to control the tubular mesopores to be oriented in two directions in planes.
The process preferably comprises a step of removing the surfactant from the thin film containing the formed inorganic oxide-surfactant composite to make the mesopores of the mesoporous film empty.
According to a further aspect of the present invention, there is provided a mesoporous film in which the in-plane X-ray diffraction spectrum has two-branched diffraction peaks at intervals of 180°.
The present invention provides also a laser device having principal emission directions controlled in two preferred directions in one plane.
The present invention provides also a process for producing a mesoporous film in which the inorganic compound is silica, and the inorganic oxide precursor is a silica precursor
According to the present invention, a mesoporous film is produced which has tubular mesopores directed in two orientation directions on a substrate having an anisotropic surface structure, under suitable reaction conditions. This mesoporous film having a controlled pore structure is capable of orienting a guest conjugated polymer compound introduced into the mesopores, and is useful as optical thin films and optical elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing for explaining a pore structure of a mesoporous film of the present invention.
FIG. 2 is a schematic drawing for explaining two orientation directions of tubular mesopores in the mesoporous film of the present invention.
FIG. 3 is a schematic drawing for explaining a conventional mesoporous film having uniaxially oriented tubular mesopores.
FIG. 4 is a schematic drawing of an apparatus for forming an LB film used in the present invention.
FIG. 5 is a schematic drawing of a reaction vessel for producing a mesoporous film by irregular nucleus formation-growth.
FIG. 6 is a schematic drawing of a film-forming apparatus for dip-coating for mesoporous film formation by a solvent evaporation method.
FIG. 7 is a schematic drawing for explaining a structure of a symmetric wave guide of a laser of the present invention.
FIG. 8 is a profile of an in-plane X-ray rocking curve of a mesoporous silica film produced in Example 1 of the present invention.
FIG. 9 is a transmission electron micrograph of a mesoporous silica film produced in Example 1 of the present invention.
FIG. 10 shows dependency, on polarization direction, of the polarized absorption spectrum of a mesoporous silica film containing MEH-PPV introduced into the fine pores produced in Example 3 of the present invention.
FIG. 11 is a schematic drawing for explaining an arrangement for laser emission measurement prepared according to the present invention.
FIG. 12 shows decrease of the emission spectrum breadth with increase of the exciting light intensity by the laser prepared in Example 4 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained below in detail.
FIG. 1 is a schematic drawing of a mesoporous film of the present invention. In the mesoporous film of the present invention, bent tubular mesopores are arranged in plains. The tubular mesopores are bent (or zigzagged), as shown in FIG. 1, in plains, basically parallel to the substrate face. The bent tubular mesopores are packed in the closest state, in a honeycomb state at the cross-section of the film. Ideally, the packed state of the mesopores is hexagonally symmetrical: The imaginary lines connecting the centers of the mesopores at the cross-section would be regularly hexagonal. However, in the present invention, the arrangement of the mesopores in the mesoporous film need not be in the complete hexagonally symmetrical state, but may be in a deformed hexagonal state having a short structural period in the film thickness direction. This signifies that, in small regions of the film, the mesopores are arranged in a uniform orientation direction in the film thickness direction.
The pores of the mesoporous film are formed by surfactant micelles. Since, under certain conditions, the micelles are formed uniformly by association of the same number of molecules, mesopores are formed to have the same number of molecules, mesopores are formed to have the same pore diameter. Various types of the micelles are known, including spheres, tubes, and layers. Essentially in the present invention, the micelles for forming the pores of the mesoporous film are tubular.
The mesopores in the present invention are fine pores having a diameter ranging from 2 nm to 50 nm in accordance with the definition by IUPAC. The pore diameters are substantially uniform in the mesoporous film of the present invention. Here, the uniform pore diameter signifies that, in the pore diameter distribution measured by nitrogen gas adsorption measurement, the distribution has one maximum, and 60% or more of the mespores distribute within a diameter range of a 10-nm breadth.
Strictly, the term “mesoporous film” indicates a thin film having empty mesopores. However, in the present invention, the mesoporous film includes those which contain surfactant micelles before surfactant removal. The mesoporous film in the present invention may be a film after removal of the surfactant, or a film filled with the surfactant, insofar as the film has structural features described below.
In the mesoporous film of the present invention, the tubular mesopores are bent (or zigzagged) in planes in a controlled bending state.
Specifically, as shown schematically in FIG. 1 and FIG. 2, the bending angles are controlled to be constant. As shown in FIG. 2, the tubular mesopores are controlled to align locally, but in different two orientation directions over the entire film face.
In the pore structure of the mesoporous film of the present invention having pores oriented in two controlled directions in planes, the total area of the regions of the mesopores oriented in one direction and the total area of the other regions of the mesopores oriented in the other direction are substantially equal to each other. This is confirmed by X-ray diffraction analysis as mentioned later.
FIG. 3 shows a conventional mesoporous film in which the orientation direction of the tubular micelles is controlled in planes. As shown in FIG. 3, the conventional mesoporous film has tubular mesopores controlled to align in one orientation direction, which is different in structure from the thin film having tubular mesopores controlled to direct in two orientation directions of the present invention.
In the mesoporous film of the present invention, the material for forming the pore wall of the porous material is not limited at all, provided that the material is capable of forming the pore structure of the present invention. The materials containing silica, particularly silica itself, are preferred.
In the mesoporous film of the present invention, the directions of the mesopore orientation in planes are defined by structural anisotropy of the substrate surface. The structural anisotropy of the substrate surface herein signifies various types of anisotropy, including anisotropy in atom arrangement on a specific crystal face of a crystalline substrate, anisotropy in a corrugated structure on a substrate, structural anisotropy in a high polymer thin film formed on a substrate, and the like. The substrate having an anisotropic surface is not specially limited, provided that the substrate enables the formation of the mesoporous film having the pore structure of the present invention.
The substrate useful in the present invention is explained below in more detail.
Firstly, a crystalline substrate is explained which has anisotropy in atomic arrangement on the surface. The preferred examples include a (110)-face of a single crystal substrates having diamond crystal structure, or of single crystal substrates having a sphalerite type crystal structure. Especially preferred is the (110)-face of silicon. On the face of such a crystalline substrate, the direction of the specific arrangement of atoms is decided definitely, whereby the surfactant micelles are oriented. The inventors of the present invention found a method of control of pore orientation in a silica meso-structure formed on a silicon single crystal face (110) having twofold symmetry in the surface atomic arrangement, and disclose it in Japanese Patent Application Laid-Open No. 2000-233995. For such a use of the substrate, the clean surface of the substrate should be bared. For example, a silicon substrate should be treated for removing the natural oxide film from the surface. This can be achieved by relatively simple treatment of the surface in dilute hydrofluoric acid for several minutes.
Next, a substrate is explained which has a high polymer thin film having structural anisotropy. Here, a Langmuir-Blodgett method, and a rubbing method are explained. The method is not limited thereto, provided that a high polymer film having structural anisotropy can be formed on a substrate.
Of the methods, the Langmuir-Blodgett method is explained firstly which is employed for forming a Langmuir-Blodgett film (LB film) of a polymer compound. The LB film is prepared by transferring a monomolecular film developed on a water surface onto a substrate. By repeating the film transfer steps, an intended number of film layers can be built up in superposition. In the present invention, the LB film includes modified monomolecular built-up films derived by treating the LB film formed on a substrate to change the chemical structure with the built-up structure retained.
The LB film can be formed by a conventional method. FIG. 4 illustrates schematically a conventional apparatus for formation of an LB film. In FIG. 4, pure water 42 is filled in water container 41. Fixed barrier 43 holds a surface pressure sensor not shown in the drawing. Monomolecular layer 46 is formed on the water surface by dropping a liquid containing an objective substance or a precursor of an objective substance on the water surface in a region between fixed barrier 43 and movable barrier 44, and a constant surface pressure is applied by movement of movable barrier 44. The position of movable barrier 44 is controlled by the surface pressure sensor to apply a constant surface pressure to the film on the water surface during the film transfer onto the substrate. The pure water is kept pure invariably by a water-feeding device and water-draining device not shown in the drawing. Water container 41 has a pit. Substrate 45 is held at the position of the pit, and can be driven vertically in direction of arrow 401 at a constant speed by a driving device not shown in the drawing. The film on the water surface is transferred onto the substrate by inserting and lifting the substrate into and from the water.
The LB film used in the present invention can be formed by use of such an apparatus by inserting a substrate into the water and listing it from the water to transfer a monomolecular film, layer by layer, onto the substrate with application of a surface pressure to the monomolecular layer developed on the water surface. The form and properties of the film depend on the surface pressure, the inserting and lifting speed of the substrate, and the layer number. The surface pressure in the film formation is decided optimally according to a surface area-surface pressure curve. Generally the surface pressure ranges from several to several ten mN/m. The speed of the movement of the substrate ranges from several to several hundred mm/min. The number of the layers is suitably decided in the range from several layers to several hundred layers. A usual LB film formation method is described above. However, the method of the LB film formation is not limited thereto. For example, a method is applicable in which water as a sub-phase is allowed to flow.
The material for the LB film is not specially limited, provided that the material is stable in the process for forming of the thin film of the meso-structure mentioned later and the material enables uniaxial orientation of the mesopores in the formed meso-structure. Polyimides are preferred therefore. An LB film of a polyimide can be formed by preparing an alkylamine salt of a polyamic acid as the precursor of the intended polyimide, forming a LB film from this material according to the aforementioned method, and heat-treating the formed film to cause a dehydrating ring closure reaction to for imidation and deamination to form a polyimide LB film.
In the high-polymer LB film produced in the above process, the polymer molecule chains can be oriented in the substrate lifting direction in the LB film formation process. This anisotropy in the molecular orientation enables orientation of the surfactant micelles in the mesoporous film.
Next, a substrate is explained which has a high-polymer thin film formed thereon and having been subjected to a rubbing treatment. A substrate is coated by a polymer by spin coating or a like coating method, and the coating polymer film is rubbed in one direction with cloth or a like material. The rubbing cloth is wound around a roller, and the roller rotating is brought into contact with the substrate surface and a stage holding the substrate is moved in one direction relative to the roller.
The suitable rubbing cloth is selected for the high polymer material to be rubbed, the useful material including usual materials such as nylon and rayon. The rubbing intensity is optimized by adjusting parameters such as the roller rotation speed, the pressure for pushing the roller against the substrate, and movement speed of the substrate-holding stage. The high polymer compound to be rubbing-treated is not specially limited, provided that the polymer is resistant in the later process for forming the meso-structure thin film and is capable of controlling the orientation of fine pores in the meso-structure. Polyimides are preferred as the high polymer.
Two types of structural anisotropy can be induced in the high polymer thin film on the substrate surface by the rubbing treatment. One type is a corrugated state caused on the polymer film surface by rubbing the polymer film surface with the cloth. The corrugation has high anisotropy owing to one-directional rubbing with the wound cloth. Another type is anisotropy in arrangement of the high polymer chains in the polymer film: This is caused by heating and stretching the polymer by the heat generated during the rubbing treatment to heat the polymer compound above the glass transition temperature. The former, the corrugated state, can be formed on nearly all of the polymer thin film by the rubbing treatment. On the other hand, the latter, the anisotropy in the polymer chain arrangement is considered to be caused in a certain relation between the polymer molecular chain structure and the rubbing conditions.
Next, the method of formation of a thin meso-structure film on a substrate is explained. Methods of the thin film formation are roughly classified into two methods. A first method is based on non-uniform nucleus generation-growth from a solution onto the substrate surface. A second method is a solvent evaporation method based on a sol-gel process. The former method is disclosed, for example, in Chemistry of Materials, vol.14, pp. 766-772. The latter method is disclosed, for example, in Nature, vol.389, pp. 364-368.
Firstly, the above first method based on non-uniform nucleus generation-growth from a solution onto the substrate surface is explained below. This method is employed mainly for preparation of a mesoporous silica thin film. This method is analogous to a crystal growth method, and is employed for preparing a thin film of a meso-structure. In this method, the substrate is kept in an aqueous surfactant solution containing a precursor of an intended pore wall-constituting material to form a mesoporous film.
In this method of formation of the meso-structure thin film, a reaction vessel shown in FIG. 5, for example, may be employed. Reaction vessel 51 may be made of any material, provided that the material does not adversely affect the reaction: polypropylene Teflon®, and the like may be used. The reaction vessel may be enclosed in a container made of rigid material like stainless steel to protect from a pressure during the reaction. In the reaction vessel, a substrate holder 53 is placed, for example, as shown in FIG. 5 for holding substrate 55. During the reaction, the meso-structure will be formed not only on the substrate but also in the solution. This meso-structure can deposit from the solution onto the substrate. To prevent this, the substrate is held in the solution with the face for the film formation kept directed downward during the reaction.
The solution for the film formation reaction contains a surfactant and a source material such as an alkoxide for the intended inorganic material. Depending on the material of the pore wall, a catalyst like an acid for hydrolysis of the inorganic source material may be added in a suitable amount. When an alkoxide is used as the source material, the alkoxide is preferably selected which releases a water-soluble alcohol by the hydrolysis. For example, for forming the silica pore wall, the reaction solution may be prepared by adding tetraethoxysilane or tetramethoxysilane to an aqueous acidic solution of a surfactant.
The useful surfactant includes cationic surfactant such as quaternary ammonium, and nonionic surfactant having polyethylene oxide as a hydrophilic moiety, but is not limited thereto. The length of the surfactant molecule employed is decided depending on the pore diameter of the invented meso-structure. For enlarging the surfactant micelle diameter, an additive such as mesitylene may be added thereto. The acid as the catalyst may be a usual acid such as hydrochloric acid and nitric acid. The shape and structure of the film deposited on the substrate are affected greatly not only by the surfactant, the acid, and the source material for the inorganic component, but also by the properties of the substrate surface. Therefore, for the film formation, the composition of the reaction solution should be optimized for the substrate employed.
Under the aforementioned conditions, the material is deposited in a mesoporous state on the substrate. The temperature for the deposition is not specially be limited, preferably being selected in the range from room temperature to about 100° C. The reaction time ranges from several hours to several months. A longer reaction time gives a thicker mesoporous film.
The mesoporous film formed on the substrate as above is washed with pure water and air-dried to obtain a final thin film.
From the mesoporous film filled with the surfactant micelles prepared as above, an empty mesoporous film can be obtained by removing the template surfactant micelles. The removal of the surfactant can be conducted by a conventional method, the method including baking, oxidation-decomposition by ozone formed by UV irradiation, extraction with a solvent, extraction with a fluid in a supercritical state, and so forth. For example, from the filled mesoporous silica film, the surfactant can be removed completely without destroying the meso-structure by baking in the air at 550° C. for 10 hours. The temperature and time for the baking are suitably selected depending on the material for the pore wall material and the surfactant employed. In the case where a high polymer film is formed on the substrate surface for control of the mesopore orientation, the orientation-controlling polymer film between the mesoporous film and the substrate is also removed by the baking operation to give a structure in which the orientation-controlled mesoporous film is formed directly on the substrate. On the other hand, solvent extraction or a like method, although the surfactant cannot be removed completely, enables formation of the mesoporous film on a substrate incapable of withstanding the baking treatment.
Next, the solvent evaporation method for the film formation is explained below. According to the solvent evaporation method, an aqueous solution or organic solvent/water mixed solution containing a surfactant at a concentration lower than the critical micelle concentration and a precursor for the pore wall-constituting inorganic substance is applied to form a coating layer on a substrate by spin coating, dip coating, mist coating, or a like coating method, and a meso-structure is formed with increase of the surfactant concentration by evaporation of the solvent from the coating layer. An alcohol is useful as the organic solvent. This method is advantageous in that the substrate material can be selected from a variety of materials since the reaction conditions are mild and the film can be formed in a relatively short time.
The spin coating or dip coating can be conducted with a conventional coating apparatus without limitation. In some cases, a means for controlling the temperature of the solution, a means for controlling the temperature or humidity of the coating atmosphere may be employed, if necessary.
Formation of a mesoporous film by a dip coating method is explained as an example. FIG. 6 shows schematically an apparatus for dip coating. In FIG. 6, the numerals indicate members as follows: 61, a vessel; 62, a substrate; 63, a precursor solution. Precursor solution 63 is an aqueous solution, an organic solution, or an aqueous organic solution which contains a surfactant at a concentration lower than the critical micelle concentration and a precursor of the inorganic component, and may contain a hydrolysis-polycondensation catalyst like an acid if necessary. For example, for preparation of a mesoporous silica film, the solution is prepared by dissolving a surfactant in an aqueous alcohol and adding thereto an acid as the hydrolysis catalyst.
The useful surfactant includes cationic surfactants like quaternary ammonium compounds, and nonionic surfactants containing polyethylene oxide as the hydrophilic moiety similarly as in the nonuniform nucleus generation-growth method, but is not limited thereto. The length of the surfactant molecule employed is decided depending on the pore diameter of the intended meso-structure. For enlarging the surfactant micelle diameter, an additive such as mesitylene may be added thereto.
Substrate 62 for the mesoporous film is fixed by holder 64 to rod 65 and is moved vertically by z-stage 66. During the film formation, the temperature of reaction solution 63 is controlled with heater 68 and thermocouple 67. For precise temperature control, the entire vessel may be placed in a heat-insulating chamber not shown in the drawing. The substrate after application of the reaction solution is preferably dried in an apparatus for controlling the temperature and humidity. After the drying step, the substrate may be aged in a high humidity atmosphere.
Besides the dip-coating and spin-coating, the effective method for mesoporous material preparation by solvent evaporation includes a pen lithography described in Nature, vol.405, p. 56, and inkjet methods. These methods enable patterning of a mesoporous film on an intended position on the substrate.
The pen lithography employs a pen to draw lines by using the reaction solution instead of an ink. The line breadth can be changed as desired by changing the shape of the pen, the movement speed of the pen or the substrate, the rate of fluid-supply to the pen, and so forth. The line breadth can be changed from a μm order to mm order. An arbitrary pattern, like a straight line and a curved line can be drawn. A planar pattern can be drawn by overlapping the spread of the reaction solution applied on the substrate.
Further, the inkjet method is effective for drawing a discontinuous dot-shaped pattern. The inkjet method applies a reaction solution by ejecting the reaction solution just instead of an ink as droplets through an inkjet nozzle onto a substrate. By overlapping the spreading portions of the deposited reaction solution on the substrate, both a line pattern and a planar pattern can be formed.
The meso-structure prepared by the solvent evaporation method can be converted to an empty mesoporous film by removal of the surfactant from the mesopores in the same manner as in the case of the film formed by nonuniform nucleus generation-growth method.
The pore structure in the mesoporous film can be evaluated generally by transmission electron microscopy, and X-ray diffraction analysis. In the mesoporous film of the present invention, tubular mesopores are aligned parallel to the substrate face, so that in-plane X-ray diffraction analysis should be employed for evaluation of the orientation in a plane. In the evaluation of the mesoporous film of the present invention by the in-plane X-ray diffraction analysis, two-branched diffraction peaks are observed at intervals of 180° in the in-plane rocking curve. The interval between the two-branched diffraction peaks corresponds to the angle difference between the two orientation directions. In the mesoporous film of the present invention, the two orientation directions of the mesopores are defined by the directions of the anisotropy of the substrate: The two orientation directions are on the both sides at the same angle to the rubbing treatment direction or the substrate-lifting direction in the LB film formation.
The in-plane X-ray diffraction peaks observed as paired two peaks have substantially the same diffraction intensity. This signifies that, in one plane, the area of the mesopores oriented in one direction is the same as the area or the mesopore oriented in another direction.
The mesoporous film prepared in the present invention has substantially uniform mesopores characteristically. The size and size-distribution of the mesopores can be measured by adsorption isotherm measurement with nitrogen gas.
From the nitrogen gas adsorption isotherm measurement, the pore size distribution according to Barret-Joyner-Halenda (BJH) method has a single peak within the range from 2 nm to 50 nm, and not less than 60% of the pores distribute within the pore size range of 10 nm breadth.
The present invention further includes a composite material having tubular mespores directed in two directions and containing a conjugated polymer compound introduced into the mesopores. This composite thin film is explained below.
The inventors of the present invention investigated introduction of a conjugated polymer into mesopores oriented in planes of a mesporous silica film. The investigation is disclosed in Journal of the American Chemical Society, vo.126, pp. 4476-4477. This disclosure describes polymer chains highly oriented in the mesopores oriented in one direction.
A conjugated polymer can be introduced also into the two-directionally oriented tubular mesopores of the present invention.
For introduction of the conjugated polymer, the inside surface of the mesopores is preferably treated for hydrophobicity. The hydrophobicity of the inside of the mesopores can promote remarkably the introduction of the polymer into the mesopores. For example, treatment of the mesporous film with phenyldimethylchlorosilane or 1,1,1,3,3,3-hexamethyldisilazane causes bonding of the organic compound to the silanol groups in the mesopores to make hydrophobic effectively the inside of the mesopores. The agent for the hydrophobicity is not limited thereto. An agent other than the silane coupling agent can be used, provided that it gives a similar effect. The surface treatment of the inside of the mesopores signifies specifically a treatment like immersion of the mesoporous silica film into a silane-coupling agent, or a like treatment. However, the method of the modification is not limited thereto. For example, a reaction in a gas phase can be applicable. In the coupling reaction, a substance serving as a catalyst for the reaction may be added to the reaction system. The catalyst is exemplified by trimethylsilane.
After the treatment for the hydrophobicity of the inside of the mesopores, a conjugated polymer is introduced into the mesopores. Various conjugated polymers are useful therefor, including polymers having a polyphenylene-vinylene skeleton, polymers having a polythiophene skeleton, polymers having a polypyrrole skeleton, and polymers having a polyfluorene skeleton, but is not limited thereto. The conjugated polymer can be introduced into the mesopore by various methods. In one method, a mesporous silica film having an orientation-controlled pore structure is immersed into a solution of the conjugated polymer. In another method, a solution of the conjugated polymer is applied by dropping onto a substrate and the substrate is heated. Any introduction method may be employed in the present invention, provided that the method can introduce the conjugated polymer into the mesopores. After the polymer material is introduced into the mespores by contact with a solution of the polymer material, a superfluous conjugated polymer material is removed from the surface of the film.
The composite thin film holding a conjugated polymer in the mespores prepared as above is observed to be colored in specific two directions when viewed under polarized light, which shows that the polymer compound is oriented in the mespores. The orientation directions coincide with the mesopore orientation directions as measured by in-plane X-ray diffraction analysis.
The anisotropy of the conjugated polymer is observable not only in the light absorption but also in light emission. A highly fluorescent conjugated polymer introduced into the oriented mesopores emits fluorescence polarized in two directions, as well as the dependence of the light absorption of polarized light. This polarized fluorescence with a fluorescence spectrum detector having a polarizer in front of the detector by changing the polarization angle. From the mesoporous film of the present invention, fluorescence light is emitted in the two directions same as the two directions of mesopore orientation observed by the in-plane X-ray diffraction analysis.
The present invention further provides a laser emission assembly containing a composite thin film having tubular mesopores oriented in two directions and a conjugated polymer introduced into the mesopores. These are explained below. A highly fluorescent conjugated polymer, which is introduced into the mesopores of the present invention, will cause stimulated emission when the exciting light intensity is increased above a certain threshold, resulting in significant decrease of spectrum line breadth and remarkable increase of the emission intensity. This phenomenon is observed more obviously with a symmetric wave guide as shown in FIG. 7. In FIG. 7, the surface of the composite thin film is covered with liquid 74 having the same refractive index as that of underlying substrate 71 of the thin film. With such a construction, even at a smaller thickness of the film, laser oscillation, namely the phenomenon of light emission according to the stimulated emission and the like phenomenon, can be induced. Arrow 701 shows the mesopore orientation direction. The substrate material and the liquid having the same refractivity are not specially limited. The laser beam is emitted from the edge face of the film. The emission is directed to two directions perpendicular to the two directions detected by in-plane X-ray diffraction analysis. This is caused by high level of control of the polymer orientation.
The present invention is explained below in more detail by reference to Examples without limiting the invention.

EXAMPLE 1

In this Example, a mesoporous silica film was prepared on a rubbing-treated polyimide coating a substrate, the film having tubular mesopores oriented in two in-plane orientation directions at equal angles on both sides of the rubbing direction.
A quartz glass substrate was washed successively with acetone, isopropyl alcohol, and water, and the surface of the substrate was cleaned in an ozone-generating apparatus. On the substrate, a solution of polyamic acid A in NMP was applied by spin coating, and the coating layer was baked at 200° C. for one hour to obtain a film of a polyimide A having the chemical structure shown below. The polyimide-A film had a thickness of 100 nm.
The film was rubbing-treated under the conditions shown in Table 1 for use as the substrate.

TABLE 1

Conditions of Rubbing of Polyimide A

Cloth material Nylon

Roller diameter (mm) 24

Pushing depth (mm) 0.4

Rotation frequency (rpm) 1000

Stage speed (mm/min) 600

Repetition times 2
After the rubbing treatment, the polyimide molecular chains are found to be oriented in the rubbing direction in the polyimide film by polarized infrared absorption spectroscopy. This film was found to have fine grooves of a breadth of several to tens of nanometers aligned in the rubbing direction by examination by atomic force microscopy.
A mesoporous silica film was formed on this substrate. A nonionic surfactant, polyoxyethylene-10-cetyl ether (abbreviated as C₁₆EO₁₀, trade name: Brij56) was used as the surfactant. This surfactant was dissolved in pure water, and hydrochloric acid and tetraethoxysilane (TEOS) were added thereto to obtain a final solution of the component ratio:
TEOS:H₂O:HCl:C₁₆EO₁₀=0.10:100:3.0:0.010.
In this solution, the aforementioned substrate coated with the rubbing-treated polyimide film was held with the coated face kept downward to allow the reaction to proceed at 80° C. for three days to obtain a mesoporous silica film. The substrate taken out from the reaction solution was washed sufficiently with pure water and air-dried.
A transparent film was formed on the substrate, and the film was observed to have a uniform interference color. This film was confirmed to have a periodic structure of a 5.1-nm period in the film thickness direction by X-ray diffraction analysis. This thin film had a structure having tubular mesopores in a honeycomb packing construction by examination of the cross-section of the film by transmission electron microscopy.
This film was subjected to in-plane X-ray analysis, and was confirmed to have an in-plane periodic structure of a 7.4-nm period. The in-plane rocking curve was measured to investigate the orientation distribution in the plane. The observed profile is shown in FIG. 8: two diffraction peaks appeared at an interval of 180°. From this, obviously the tubular mesopores are oriented in two directions in the film formed in the present invention. The two directions are at equal angle θ from rubbing direction 801. Specifically, in the film prepared in this Example, the tubular mesopores were in a zigzag state with angles of ±19° from the rubbing direction. In the X-ray diffraction profile shown in FIG. 8, the paired two diffraction peaks had equal intensities. This signifies that the total area of the region of the mesopores oriented in the one direction is substantially equal to that of the mesopores oriented in the other direction.
This structure the film was confirmed by examination of the film by transmission electron microscopy. FIG. 9 is an electron micrograph, showing obviously the controlled orientation of the mesopores in two directions in planes.
In FIG. 9, the indicated two angles are equal to each other. Since the electron micrograph of FIG. 9 was taken from the thin film peeled off from the substrate, the mesopores are observed to be curved owing to the film strength.
This film was baked at 550° C. in the air to remove the surfactant to obtain empty mesopores. The removal of the surfactant was confirmed by infrared absorption spectroscopy. The baking causes condensation of the silanol groups to induce shrinkage of the structural period length by about 20% in the thickness direction. This was confirmed by X-ray diffraction analysis. On the other hand, the structure in the plane direction is little affected by the baking, which was confirmed by in-plane X-ray diffraction analysis.
The film on the substrate was subjected to nitrogen adsorption isotherm measurement. The adsorption was of type IV. The result was analyzed by a BJH method, and the pore size distribution was found to be narrow, having a single peak at 3.2 nm, and 80% or more of the mesopores were distributed in a distribution range of 10 nm breadth.

EXAMPLE 2

In this Example, a mesoporous silica film was prepared on a substrate having an LB-film of a polyimide. The mesoporous silica film had tubular mesopores oriented in two in-plane orientation directions at equal angles on both sides from the rubbing direction.
Polyamic acid B, a precursor of the polyimide B having the structure shown by the formula below, and N,N-dimethylhexadecylamine were mixed at a molar ration of 1:2 to form an N,N-dimethylhexadecylamine salt of the polyamic acid B. This mixture was dissolved in N,N-dimethylacetamide to prepare a 0.5-mM solution. This solution was dropped onto a water face kept at 20° C. of an LB film-forming apparatus. The monomolecular film formed on the water surface was transferred onto a substrate by applying a constant surface pressure of 30 mN/m at a substrate dipping rate of 5.4 mm/min.
The substrate employed was a quartz glass substrate. This substrate was washed with acetone, isopropyl alcohol, and pure water successively, and the surface was cleaned in an ozone-generating apparatus. On the substrate, an LB film of alkylamine salt of polyamic acid was formed in 30 layers. The formed LB film was baked at 300° C. for 30 minutes under a nitrogen gas flow to obtain an LB film of the polyimide B having the structure shown by the formula below. The imidation by dehydrating ring closure of the polyamic acid and the elimination of the alkylamine were confirmed by infrared absorption spectroscopy.
In the polyimide thin film prepared in this Example, the polymer chains were found to be oriented in the direction of lifting of the substrate in the film formation by polarized infrared absorption spectroscopy.
On this substrate, a silica meso-structure film was formed as below. In this Example, the mesoporous silica film was formed from the same materials and under the same reaction conditions as in Example 1.
After the film formation reaction, the substrate was taken out from the reaction vessel, and was washed. A transparent film was found to be formed on the substrate, and the film was observed to have a uniform interference color. This film was confirmed to have a periodic structure of a 5.1-nm period in the film thickness direction by X-ray diffraction analysis. This thin film had a structure having tubular mesopores in a honeycomb packing construction by transmission electron microscopy of the cross-section of the film.
This film was subjected to in-plane X-ray analysis, and was confirmed to have an in-plane periodic structure of a 7.3-nm period. The in-plane rocking curve was measured to investigate the orientation distribution in the plane. Thereby nearly the same profile as that measured in Example 1 shown in FIG. 8 was obtained although the half width was broader than that. This shows obviously that the tubular mesopores are oriented in two directions in the film formed in this Example also. The two directions are at equal angles from the substrate lifting direction. Specifically, in the film prepared in this Example, the tubular mesopores were in a zigzag structure with angles of ±18.5° from the substrate lifting direction in the LB film formation. In the profile of in-plane X-ray diffraction rocking curve, the paired two diffraction peaks had equal intensities. This signifies that the total area of the region of the mesopores oriented in one direction was substantially equal to that of the mesopores oriented in the other direction.
The film prepared in this Example was examined by transmission electron microscopy, and a zigzag pore structure of the film was confirmed to be substantially the same as shown in FIG. 9.
This film was baked at 550° C. in the air to remove the surfactant to obtain empty mesopores. The removal of the surfactant was confirmed from infrared absorption spectrum. The baking causes condensation of the silanol groups to result in shrinkage of the structural period by about 20% in the thickness direction. This was confirmed by X-ray diffraction analysis. On the other hand, the structure in the plane direction is little affected by the baking, which was confirmed by in-plane X-ray diffraction analysis.
The film on the substrate was subjected to nitrogen adsorption isotherm measurement. The adsorption was of a type IV. The result was analyzed by a BJH method, and was found that the pore size distribution was found to be narrow, having a single peak at 3.1 nm, and 80% or more of the mesopores were distributed in a distribution range of 10 nm breadth.

EXAMPLE 3

In this Example, into mesopores of the mesoporous silica oriented in independent two orientation directions prepared in Example 1, a fluorescent conjugated polymer, poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (hereinafter referred to as MEH-PPV) was introduced to prepare a fluorescent thin film which emits polarized fluorescence light in different two directions.
The mesoporous silica film having fine pores made empty by baking in Example 1 was treated for silane coupling to make the inner face of the mesopores hydrophobic. Specifically, the film immediately after the baking was immersed into a 1:1 mixture of trimethylchlorosilane and 1,1,1,3,3,3-hexamethyldisilazane for two hours, and the treated thin film was washed with ethanol to remove superfluous silane coupling agent and dried.
Then, MEH-PPV was introduced into the mesopores as below. A portion of 0.12 g of MEH-PPV free from a low molecular component was dissolved in 9 mL of chlorobenzene. In this solution, the above mesoporous silica film having been treated for silane coupling was immersed. The mesoporous silica film kept immersed in the solution was heated to 80° C. in a closed container to introduce the MEH-PPV into the pores. After five days of contact of the substrate with the MEH-PPV solution, the substrate was washed with chloroform to remove the superfluous polymer compounds and was dried.
The dried thin film had a red color, which shows introduction of the MEH-PPV into the fine pores. FIG. 10 shows the polarized light absorption spectrum in the visible region. The polarized light absorbance was very high in the polarization direction parallel to the mesopore orientation direction as shown by solid line 1001 in FIG. 10, whereas the polarized light absorbance was very low in the polarization direction parallel to the rubbing direction as shown by broken line 1002 in FIG. 10. The absorbance maximums were observed in two directions in which the light polarization direction is parallel to the mesopore orientation direction, and the absorbance was lower between the maximums. This shows that the conjugated polymer molecular chains are controlled to orient in the two directions of the mesopore orientation.
The properties of the polarized fluorescence light emitted from the thin film were measured by changing independently the polarization direction of the exciting light and the polarization direction of the fluorescence. The light emission from the MEH-PPV-orienting mesoporous silica film prepared in this Example was most strong when the polarizer at the excitation light side was set at an angle parallel to the mesopore orientation direction and the polarizer at the fluorescent light side was set at the same angle. In contrast, the detected fluorescence emission was weak when the polarizer at the excitation light side was set at the angle parallel to one of the mesopore orientation directions and the polarizer at the fluorescent light side was set at the angle parallel to the other one of the mesopore orientation directions. This shows that two different orientation states exist locally. Very little fluorescence was detected when the polarizer at the fluorescent light side was set at the angle parallel to the rubbing direction.
This Example shows that the direction of the polarized light emission can be controlled precisely in two directions by controlling the orientation of the tubular mesopores in two directions and that optical properties can be controlled by nano-space control.

EXAMPLE 4

In this Example, a laser was constructed by employing the mesoporous silica film prepared in the above Example 3, the mesoporous silica film holding a conjugated polymeric MEH-PPV capable of emitting light polarized in two directions.
On the mesoporous silica film prepared in Example 3 holding the conjugated polymer MEH-PPV capable of emitting light polarized in two directions, a glass substrate as a cover was placed with interposition of spacer 73 as shown in FIG. 7. In the space between the composite film and the covering substrate, oxygen-free glycerin 74 was filled and sealed. Incidentally the glycerin was selected since it is a transparent liquid and has nearly the same refractivity as the quartz glass forming the composite film. Thus a symmetric waveguide structure was prepared in which the mesoporous silica film holding a conjugated polymer is held between materials of the same refractivity.
FIG. 11 illustrates schematically an apparatus for laser oscillation. This wave guide structure was enclosed in vacuum vessel 1105, and the vessel was evacuated by vacuum pump 1106. Incident light 1101 was introduced through transparent window 1104 to the composite thin film in the symmetric waveguide structure. The exciting light employed was second harmonic wave of Nd:YAG laser (wavelength: 532 nm). The light component of light 1102 emitted from the edge face of the composite thin film was measured with the arrangement shown in FIG. 11.
With this constitution, the emission spectrum was measured by increasing the exciting light intensity. Thereby remarkable change was observed in the emission spectrum. FIG. 12 shows the change. With increase of the exciting light intensity, the width of the emission spectrum line decreased remarkably, and the intensity of a specific wave component increased selectively. This is caused by mirrorless laser oscillation by stimulated emission. In the composite thin film of the present invention, the threshold of the exciting light intensity for the spectrum line breadth decrease was as low as about 0.05 mJ/mm². This is presumably due to effectiveness of the emitted light for induction of stimulated emission resulting from the highly controlled orientation of the conjugated polymer in the composite thin film of the present invention.
Another feature of the laser of the present invention is a high degree of polarization of the light. The mesopores are distributed in two directions in planes, but are oriented entirely parallel to the substrate surface. Therefore, the light emitted from the edge of the film is highly polarized in the direction parallel to the substrate, irrespective of the zigzag state of the mesopores.
Still another feature of the present invention is defined direction of the emission of the laser beams. As shown in FIG. 11, in this Example, the composite thin film having a symmetric waveguide structure is made turnable in a plane in the direction of arrow 1111, and the distribution of the emitted laser beam intensity can be measured. As the result of the measurement, the light is emitted only little in the direction parallel to the rubbing direction, but is emitted with high intensity in the directions perpendicular to the mesopore directions. This effect is due to the high degree of the orientation of the conjugated polymer molecule chains in two directions.
According to the present invention, in a thin film of a mesoporous material, the in-plane orientation of the tubular mesopores can be controlled in two directions. This thin film is promising for use as optical thin films and optical elements.
This application claims priority from Japanese Patent Application No. 2005-150986 filed on May 24, 2005, which is hereby incorporated by reference herein.

Claims

1. A mesoporous film formed on a substrate face and having tubular mesopores arranged in a state of a honeycomb-packed pore structure, wherein the substrate face has a structural anisotropy, the mesopores are controlled to orient in two directions in planes, and the tubular mesopores are arranged parallel to each other and parallel to the substrate face.

2. The mesoporous film according to claim 1, wherein micelles of amphiphillic molecules are filled in the mesopores.

3. The mesporous film according to claim 1, wherein the wall of the mesopores is formed from a material containing silica.

4. The mesporous film according to claim 1, wherein the size distribution of the mesopores measured by nitrogen gas adsorption has a single maximum, and 60% or more of the mesopores are distributed in the size distribution range of breadth of 10 nm.

5. The mesporous film according to claim 1, wherein, of the two in-plane orientation directions, the region of a first orientation direction and the region of a second orientation direction are substantially the same in area.

6. The mesporous film according to claim 1, wherein the in-plane orientation direction of the mesopores is given by a rubbing treatment, and is controlled by the structural anisotropy of the face of the substrate in two directions, and the direction of the mesopore orientation is the same as the direction of the rubbing treatment.

7. The mesporous film according to claim 1, wherein the in-plane orientation of the mesopores is controlled to two directions by a Langmuir-Blodgett film of a polymer compound having structural anisotropy formed on the face of the substrate, and the orientation direction of the mesopores is the same as the direction of lifting-up of the substrate in the process of the Langmuir-Blodgett film formation.

8. The mesoporous film according to claim 1, wherein a conjugated polymer compound is held in a part or the entire of the mesopores.

9. A laser emission assembly, comprising the mesoporous film set forth in claim 8.

10. A laser emission assembly, constituted of the mesoporous film set forth in claim 9 and a medium having a refractivity nearly the same as the refractivity of the substrate carrying the mesoporous film.

11. A process for producing a mesoporous film including a step of preparing a substrate having a surface structural anisotropy, and a step of bringing the substrate into contact with an aqueous solution containing an inorganic oxide precursor and an amphiphilic substance to form a thin film of a composite of an inorganic oxide and a surfactant having a structure of tubular micelles of the surfactant regularly arranged therein on the substrate face,

wherein the composition of the aqueous solution is adjusted to control the tubular molecule micelles to be oriented in two directions in planes.

12. The process for producing a mesoporous film according to claim 11, wherein the process comprises a step of removing the surfactant from the formed thin film of the inorganic oxide-surfactant composite to make the pores of the mesoporous film empty.

13. A process for producing a mesoporous film including a step of forming a thin film of a polymer compound on a substrate, a step of rubbing treatment of the polymer thin film, and a step of bringing the substrate having the polymer thin film having been rubbing-treated into contact with an aqueous solution containing an inorganic oxide precursor and an amphiphilic substance to form a thin film of a composite of an inorganic oxide and a surfactant having a structure of tubular micelles of a surfactant on the face of the substrate holding the rubbing-treated polymer thin film,

wherein the composition of the aqueous solution is adjusted to control the tubular mesopores to be oriented in two directions in planes.

14. The process for producing a mesoporous film according to claim 13, wherein the process comprises a step of removing the surfactant from the thin film containing the formed inorganic oxide-surfactant composite to make the mesopores of the mesoporous film empty.

15. A mesoporous film in which the in-plane X-ray diffraction spectrum has two-branched diffraction peaks at intervals of 180°.