JP7504445B2 - Method for producing a thin film consisting of a nanosheet monolayer film - Google Patents

Description

The present invention relates to a method for producing a thin film consisting of a nanosheet monolayer film.

Nanosheets obtained by exfoliating a single layer of inorganic layered materials are known to be materials with high two-dimensional anisotropy, with lateral sizes ranging from several hundred nm to several tens of μm for a thickness equivalent to a few atoms. Layered oxides are known as typical starting materials, and oxide nanosheets obtained by exfoliation of these materials have been reported to reflect their chemical composition and structure and exhibit excellent properties such as electronic and ionic conductivity, semiconductivity, insulation, high dielectric properties, ferroelectricity, ferromagnetism, fluorescence properties, and photocatalysis.

To fully utilize these properties, it is necessary to arrange the nanosheets tightly on the substrate to form a dense monolayer film, and then repeat this process to construct a multilayer film or superlattice.

One such technique is, for example, a method that combines layer-by-layer adsorption and ultrasonic treatment (see, for example, Non-Patent Document 1). According to Non-Patent Document 1, a substrate precoated with positively charged polydiallyldimethylammonium (PDDA) ions is immersed in a negatively charged nanosheet colloidal dispersion for 20 minutes, and the nanosheets are adsorbed onto the substrate by electrostatic attraction. Next, the overlapping portions are removed by ultrasonic treatment, resulting in a well-aligned nanosheet monolayer film. Furthermore, by repeating this process of preparing a monolayer film, it is possible to construct a multilayer film or a superlattice.

Another technique known is the Langmuir-Blodgett (LB) method (see, for example, Non-Patent Document 2). According to Non-Patent Document 2, after a nanosheet colloidal dispersion is spread in a trough, surface pressure is applied to adsorb nanosheets to the gas-liquid interface, which is then transferred to a substrate, resulting in a well-aligned nanosheet monolayer film. Furthermore, by repeating this process of preparing a monolayer film, it is possible to construct multilayer films and superlattices.

Furthermore, as another technique, spin coating is known (see, for example, Non-Patent Document 3 and Patent Document 1). According to Non-Patent Document 3 and Patent Document 1, a small amount of an organic solvent sol in which nanosheets are dispersed in an organic solvent is dropped onto a substrate and spin-coated, which is a simple procedure that allows a densely-arranged monolayer film of nanosheets to be obtained in an extremely short time of about one minute. Furthermore, by repeating this procedure for producing a monolayer film, it is possible to construct a multilayer film or a superlattice.

However, the techniques of Non-Patent Documents 1 and 2 have problems such as a long film formation time (for example, 2 hours in Non-Patent Document 1 and 1 hour in Non-Patent Document 2), complicated film formation operations, and a small area of substrate on which a film can be formed. On the other hand, the spin coating method of Non-Patent Document 3 and Patent Document 1 is a very simple operation, and enables large-area film formation in an extremely short time of about 1 minute, which is equivalent to about one-tenth of the film formation time of Non-Patent Documents 1 and 2, and has improved productivity compared to the techniques of Non-Patent Documents 1 and 2. The techniques of Non-Patent Document 3 and Patent Document 1 require pretreatment of the solution and control of the spin coating, so there was a demand for the development of a new film formation technique that would eliminate the problems of the conventional techniques.

JP 2013-253009 A

T. Tanaka et al., Adv. Mater., 16, 872 (2004) K. Akatsuka et al., ACS Nano, 3, 1097 (2009) K. Matsuba et al., Sci. Adv., 3, e1700414 (2018)

In light of the above background, the objective of the present invention is to provide a novel method for producing a thin film consisting of a nanosheet monolayer film.

As a result of intensive research into solving the above problems, the inventors have found that the problems associated with conventional thin film manufacturing methods that use nanosheets can be solved by dropping an aqueous colloidal solution in which nanosheets of exfoliated inorganic layered materials are dispersed onto a substrate and then aspirating the solution to produce a thin film. Based on these findings, the present inventors have completed the present invention.
That is, the present invention is characterized as follows.

The method for producing a thin film consisting of a nanosheet monolayer film according to the present invention comprises preparing a colloidal aqueous solution in which nanosheets obtained by exfoliating an inorganic layered material into a single layer are dispersed in a dispersion medium containing water and a lower alcohol having 5 or less carbon atoms, dripping the colloidal aqueous solution onto a substrate, and aspirating the dripped colloidal aqueous solution from the substrate, thereby solving the above-mentioned problem.
The lower alcohol may be at least one selected from the group consisting of methanol, ethanol, propanol and butanol.
The content of the lower alcohol in the dispersion medium may be in the range of 0.5% by volume to 15% by volume.
The content of the lower alcohol in the dispersion medium may be in the range of 1% by volume or more and 10% by volume or less.
The concentration of the nanosheets in the aqueous colloidal solution may be in the range of 0.01 g/L to 1 g/L.
The concentration of the nanosheets in the aqueous colloidal solution may be in the range of 0.01 g/L to 0.5 g/L.
The concentration of the nanosheets in the aqueous colloidal solution may be in the range of 0.02 g/L to 0.2 g/L.
In the dropping, the amount of the colloidal aqueous solution dropped onto the substrate may be in the range of 0.1 μL/mm ² to 5 μL/mm ² .
In the dropping, the amount of the colloidal aqueous solution dropped onto the substrate may be in the range of 0.3 μL/mm ² to 2.5 μL/mm ² .
The inorganic layered material may be selected from the group consisting of layered titanium oxide, layered perovskite oxide, layered manganese oxide, layered cobalt oxide, layered tungsten oxide, layered niobium oxide, layered tantalum oxide, layered titanium-niobium oxide, layered titanium-tantalum oxide, layered molybdenum oxide, layered ruthenium oxide, and graphite oxide.
The method may further include heating the substrate.
The substrate may be heated at a temperature in the range of 50° C. to 150° C.
The method may further include repeating the dropping and the suction.
The repeating may further include heating the thin film consisting of the nanosheet monolayer film on the substrate obtained by the suction, and washing the thin film consisting of the nanosheet monolayer film on the substrate with pure water.
Heating the thin film may involve heating the thin film made of the nanosheet monolayer film on the substrate at a temperature in the range of 150° C. or more and 250° C. or less.
The method may further include irradiating the thin film consisting of the nanosheet monolayer film on the substrate obtained by the suction with ultraviolet light.

The method for producing a thin film consisting of a nanosheet monolayer film according to the present invention includes preparing a colloidal aqueous solution in which nanosheets exfoliated from an inorganic layered material are dispersed in a dispersion medium containing water and a lower alcohol having 5 or less carbon atoms, dropping this onto a substrate, and aspirating the solution. By using the method of the present invention, a thin film consisting of a nanosheet monolayer film can be obtained in a short time with simple operations. In addition, since the colloidal aqueous solution contains a lower alcohol as a dispersion medium, the surface tension of the colloidal aqueous solution is reduced, and the colloidal aqueous solution can easily spread on the substrate when dropped. In addition, since the convection of nanosheets can be promoted in the colloidal aqueous solution, a high-quality nanosheet monolayer film in which nanosheets are densely arranged can be obtained.

A flow chart showing the steps for producing a nanosheet monolayer film according to the present invention. A procedure illustrating the production of a nanosheet monolayer according to the present invention. A flow chart showing the steps of producing a nanosheet multilayer film according to the present invention. FIG. 1 shows the results of observation of the aqueous colloidal solution for dropping in Example 1. FIG. 1 shows a CFLM image of the thin film of Example 4. FIG. 1 shows a CFLM image of the thin film of Example 1. FIG. 1 shows a CFLM image of the thin film of Example 5. FIG. 1 shows a CFLM image of the thin film of Example 6. FIG. 1 shows a CFLM image of the thin film of Example 7. FIG. 1 shows a CFLM image of the thin film of Example 8. FIG. 1 shows an AFM image of the thin film of Example 4. FIG. 1 shows AFM images of various locations of the thin film of Example 4. FIG. 1 shows an AFM image of the thin film of Example 9. FIG. 1 shows an AFM image of the thin film of Example 11. FIG. 1 shows an AFM image of a thin film of Example 17. FIG. 2 shows an AFM image of a thin film of Example 23. FIG. 1 shows the UV-vis absorption spectrum of the thin film of Example 1. FIG. 2 shows the appearance of the thin film of Example 28. FIG. 2 shows the UV-vis absorption spectrum of the thin film of Example 25. FIG. 2 shows the UV-vis absorption spectrum of the thin film of Example 25. FIG. 2 shows the layer number dependence of absorbance of the thin film of Example 25. FIG. 2 shows the layer number dependence of absorbance of the thin film of Example 25. FIG. 2 shows an XRD pattern of the thin film of Example 25. FIG. 1 shows a HRTEM image of a cross section of a thin film of Example 27. FIG. 2 shows the wavelength dependence of the refractive index and extinction coefficient of the thin film of Example 25. FIG. 2 shows the frequency dependence of the dielectric constant and dielectric loss of the thin film of Example 27. FIG. 2 shows the leakage current characteristics of the thin film of Example 27. FIG. 2 shows the sheet resistance of the thin film of Example 28.

A method for producing a thin film made of the nanosheet monolayer film of the present invention will be described in detail. In this specification, the term "nanosheet monolayer film" refers to a monolayer film formed by densely arranging nanosheets obtained by peeling off a single layer of an inorganic layered material without overlapping. The term "thin film made of nanosheet monolayer film" is used when referring to the nanosheet monolayer film itself, or when referring to a multilayer film (also called a nanosheet multilayer film) in which the nanosheet monolayer film is multilayered. It should be noted that the term "superlattice made of nanosheet monolayer film" refers to a thin film formed by combining and stacking two or more different types of nanosheet monolayer films.

In addition, in this specification, if the coverage (the ratio of nanosheet overlap in the total film area and the gap between nanosheets in the total film area) calculated using atomic force microscope image analysis software (Nano Navi, version 5.02, manufactured by SII Nano Technology) is in the range of 0% to 10% overlap and 0% to 10% gap, it can be determined to be a nanosheet monolayer film.

The case of producing a nanosheet monolayer film as a thin film made of the nanosheet monolayer film of the present invention will be described.
FIG. 1 is a flow chart showing the steps of producing a nanosheet monolayer film according to the present invention.
FIG. 2 is a procedure showing how to fabricate a nanosheet monolayer film according to the present invention.

Step S110: A colloidal aqueous solution 230 (FIG. 2) is prepared in which a nanosheet 210 (FIG. 2) in which an inorganic layered material has been exfoliated into a single layer is dispersed in a dispersion medium 220 (FIG. 2) containing water and a lower alcohol having 5 or less carbon atoms.
Step S120: The colloidal aqueous solution 230 prepared in step S110 is dropped onto the substrate 240 (FIG. 2). The colloidal aqueous solution 230 dropped onto the substrate 240 spreads to the edge of the substrate 240, and then the droplets become stable due to surface tension.
Step S130: The colloidal aqueous solution dropped in step S120 is aspirated from the substrate 240. In Fig. 2, a pipette 250 is shown as the aspirating device.

The inventors of the present application have found that the nanosheet monolayer film 260 can be easily manufactured by the above-mentioned process. In particular, because a lower alcohol is used in addition to water as a dispersion medium for the nanosheets, the surface tension of the colloidal aqueous solution is reduced, and the dropped colloidal aqueous solution can be efficiently spread on any substrate. In addition, the lower alcohol promotes convection of the nanosheets in the colloidal aqueous solution, which has the effect of allowing the nanosheets to be densely arranged simply by suction.

Each step will be described in detail.
In step S110, the inorganic layered material is any material that can be peeled into nanosheets, but preferably, the inorganic layered material is selected from the group consisting of layered titanium oxide, layered perovskite oxide, layered manganese oxide, layered cobalt oxide, layered tungsten oxide, layered niobium oxide, layered tantalum oxide, layered titanium-niobium oxide, layered titanium-tantalum oxide, layered molybdenum oxide, layered ruthenium oxide, and graphite oxide. All of these materials exhibit or are expected to exhibit excellent properties such as electron/ion conductivity, semiconductor properties, insulation properties, high dielectric properties, ferroelectric properties, ferromagnetism, fluorescent properties, and photocatalytic properties, and nanosheets peeled from these materials can also have the above properties, which is advantageous for practical use.

The layered titanium oxide is any layered titanium oxide that produces a titanium oxide nanosheet by exfoliation. Here, the exfoliated titanium oxide nanosheet is represented, for example, by the general formula (Ti,M)O2 _-d (where M is a metal element or a vacancy, and 0≦d≦1). Examples of layered titanium oxides _{that produce such titanium oxide nanosheets include Na2Ti3O7, K2Ti4O9, Cs2Ti5O11, and Ax(Ti,M)2O4 ( where A is at least one alkali} metal element, _and M is _a monovalent to trivalent _metal element).

The layered perovskite oxide is any layered perovskite oxide that produces perovskite nanosheets by exfoliation, for example, the exfoliated perovskite nanosheets represented by the general formula A _n-1 M _n O _3n+1 (A=Ca, Sr, Ba, Na, K, rare earth elements, M=Nb, Ta, Ti, 2≦n≦7). Known layered perovskite oxides that produce such perovskite nanosheets include the Dion-Jacobson type (A': alkali metal) represented by A'[A _{n-1 M n} O _3n +1 ], the Ruddlesden-Popper type represented by A' ₂ [A n-1 M _n O _3n+1 ], and the Aurivillius type represented by (Bi ₂ O ₂ )[A _n-1 M _n O _3n+1 ]. _{This compound system has a} variety of chemical compositions _{, but} representative layered perovskite oxides _{include KLaNb2O7 , KCa2Nb3O10 , KSr2Nb3O10 , CsCa2Nb3O10 , KCa2NaNb4O13 , KCa2Na2Nb5O16 , KCa2Na3Nb6O19 , Li2Eu2 / 3Ta2O7 , K2La2Ti3O10 , ( K1.5Eu0.5 ) Ta3O10 , etc.}

The layered manganese oxide is any layered manganese oxide that produces manganese oxide nanosheets by exfoliation. Here, the exfoliated manganese oxide nanosheets are represented, for example, by the general formula MnO ₂ , Mn _1-y M _y O ₂ (where M is a metal element, for example, Fe, Co, 0<y<1). The layered manganese oxide that produces such manganese oxide nanosheets is represented by the general formula A _x MnO ₂ , A _x Mn _1-y M _y O ₂ (where A is at least one alkali metal element, 0<x≦1).

The layered cobalt oxide is any layered cobalt oxide that produces cobalt oxide nanosheets by exfoliation. The exfoliated cobalt oxide nanosheets are represented by the general formula _CoO2 , for example. The layered cobalt oxide that produces such cobalt oxide nanosheets is represented by the general formula _AxCoO2 (A is at least one alkali metal element, alkaline earth metal element, or transition metal element, _and 0<x≦1).

Layered tungsten oxide is _any layered tungsten oxide _{that can be exfoliated to produce tungstic acid nanosheets. Layered tungsten oxides include Rb4W11O35, Cs6+zW11O36 ( 0 ≦ z ≦} 0.31), _Cs8.5W15O48 , _etc.

The layered niobium oxide is any layered niobium oxide that produces niobium oxide nanosheets by exfoliation _, where exemplary niobium oxide nanosheets are _Nb3O8 , _Nb6O17 , etc. Layered niobium oxides _that produce such _niobium oxide nanosheets include _{KNb3O8 , K4Nb6O17} , _etc.

The layered tantalum oxide is any layered tantalum oxide that can be exfoliated to produce tantalum oxide nanosheets, where an exemplary tantalum oxide nanosheet is _TaO3 , etc. The layered tantalum oxide that produces such tantalum oxide nanosheets can be _RbTaO3 , etc.

The layered titanium niobium oxide is any layered titanium niobium oxide that can be exfoliated to produce titanium niobate nanosheets, where exemplary titanium niobate nanosheets are _TiNbO5 , _Ti5NbO14 , _Ti2NbO7 , etc. Examples of layered titanium niobium oxides that _can produce such titanium niobate nanosheets include A[ _TiNbO5 ], _A3 [ _Ti5NbO14 ], A[ _{Ti2NbO7 ] (} wherein A is at _least one alkali metal element), etc.

The layered titanium-tantalum oxide is any layered titanium-tantalum oxide that can be exfoliated to produce titanium-tantalate nanosheets, where exemplary titanium-tantalate nanosheets are _TiTaO5 , _Ti5TaO14 , _{Ti2TaO7 ,} etc. Layered titanium-tantalum oxides _that produce such titanium-tantalate nanosheets include A[ _TiTaO5 ], _A3 [ _Ti5TaO14 ], A[ _{Ti2TaO7 ]} (A _is at least one alkali metal element), etc.

The layered molybdenum oxide is any layered molybdenum oxide that produces molybdic acid nanosheets by exfoliation. Here, an exemplary molybdic acid nanosheet is represented by _MoO2 . The layered molybdenum oxide that produces such molybdic acid nanosheets is represented by the general formula _AMoO2 (A is at least one alkali metal element, and M is at least one alkali metal element).

The layered ruthenium oxide is any layered ruthenium oxide that produces ruthenium oxide nanosheets by exfoliation. Exemplary ruthenium oxide nanosheets are represented by _RuO2 and _RuO2.1 . The layered ruthenium oxide that produces such ruthenium oxide nanosheets is represented by the general formula _ARuO2 (A is at least one alkali metal element).

The process for the single layer peeling of layered titanium oxide, layered perovskite oxide, layered manganese oxide, layered cobalt oxide, layered tungsten oxide, layered niobium oxide, layered tantalum oxide, layered titanium-niobium oxide, layered titanium-tantalum oxide, layered molybdenum oxide, layered ruthenium oxide, etc. is called soft chemical treatment, which is a combination of acid treatment and colloidal treatment.

For example, when the powder of the inorganic layered material described above is contacted with an aqueous acid solution such as hydrochloric acid, and the product is filtered, washed, and then dried, all of the alkali metal ions present between the layers before the treatment are replaced with hydrogen ions or oxonium ions, and a hydrogen ion exchanger is obtained. Next, the hydrogen ion exchanger obtained is placed in an aqueous solution containing a basic substance such as tetrabutylammonium ion (TBA ⁺ ), tetrapropylammonium ion, tetraethylammonium ion, tetramethylammonium ion, n-propylamine, n-ethylamine, and ethanolamine, and stirred to form a colloid. In this way, the layers that constituted the layered structure are peeled off one by one, and nanosheets are obtained.

Graphite oxide can be obtained by chemically oxidizing graphite. After oxidizing graphite with a strong acid such as sulfuric acid, potassium persulfate, or potassium permanganate, graphene oxide (GO) nanosheets can be synthesized by exfoliation. Graphene oxide contains oxygen functional groups such as hydroxyl, carboxylic acid, epoxy, and carbonyl groups on its surface and is water-soluble, so it can be obtained as a colloidal aqueous solution and is suitable for the thin film production of the present invention.

The general formula of the inorganic layered material described above is merely an example, and as long as the layered structure is maintained, dissimilar metal elements such as additive elements may be dissolved in the material, and the material is not necessarily composed only of the above elements or the above composition.

In step S110, the lower alcohol has good affinity with water as long as it has 5 or less carbon atoms and is not particularly limited, but preferably at least one type is selected from the group consisting of methanol, ethanol, propanol, and butanol. These are easy to obtain and easy to handle. Propanol may be either 1-propanol or 2-propanol. Butanol may be either 1-butanol, 2-methyl-1-propanol, 2-butanol, or 2-methyl-2-propanol. Among them, ethanol is particularly suitable from the viewpoints of reducing the surface tension of the colloidal aqueous solution, increasing the evaporation rate of the dispersion medium in the dropped colloidal aqueous solution, and promoting convection of the nanosheets in the dispersion medium.

In step S110, the content of the lower alcohol in the dispersion medium is preferably in the range of 0.5 mass% to 15 volume%. This range promotes the production of a nanosheet monolayer film. The content of the lower alcohol in the dispersion medium is more preferably 1 volume% to 10 volume%, and even more preferably 1 volume% to 5 volume%. This promotes the reduction in surface tension and the convection of the nanosheets even with the addition of a small amount of alcohol, and makes it possible to obtain a dense nanosheet monolayer film.

In step S110, the concentration of the nanosheets in the aqueous colloidal solution is preferably in the range of 0.01 g/L or more and 1 g/L or less. This allows a dense nanosheet monolayer film to be obtained with high efficiency. The concentration of the nanosheets in the aqueous colloidal solution is more preferably in the range of 0.01 g/L or more and 0.5 g/L or less. This allows a dense nanosheet monolayer film to be obtained with higher efficiency. The concentration of the nanosheets in the aqueous colloidal solution is even more preferably in the range of 0.02 g/L or more and 0.2 g/L or less. This allows a dense nanosheet monolayer film to be obtained with even higher efficiency.

In step S120, the substrate onto which the colloidal aqueous solution is dropped is not particularly limited, but examples of the substrate that can be used include metal substrates such as Au and Pt, semiconductor substrates such as Si and GaAs, transparent substrates such as quartz and glass, oxide single crystal substrates such as sapphire, MgO, ITO, _SrTiO3 , _SrTiO3 :Nb (Nb-added _SrTiO3 ), and _SrRuO3 , and organic substrates such as plastic.

The surface of the substrate is preferably made hydrophilic by a cleaning process. The cleaning process involves wiping the surface with acetone, immersing the substrate from which organic matter has been removed in a mixed solution of methanol and hydrochloric acid (mixed at a volume ratio of 1:1) for 15 to 45 minutes, cleaning with ultrapure water, immersing in concentrated hydrochloric acid for 15 to 45 minutes, and then cleaning again with ultrapure water. The cleaning process can also be substituted by irradiating the substrate with ultraviolet light for 15 to 30 minutes in an ozone atmosphere.

In step S120, the colloidal aqueous solution can be dripped using any means capable of dripping a solution equipped with a nozzle, but examples of dripping devices that can be used include a micropipette or other pipette capable of measuring the amount of liquid, a dropper, an inkjet printer, or the like.

In step S120, the amount of the colloidal aqueous solution dropped onto the substrate is not limited as long as it is dropped enough to cover the substrate, but is preferably in the range of 0.1 μL/ ^mm2 or more and 5 μL/ ^mm2 or less. Within this range, a nanosheet monolayer film can be efficiently produced. The amount of the drop is more preferably in the range of 0.3 μL/ ^mm2 or more and 2.5 μL/mm2 or ^less . Within this range, a nanosheet monolayer film can be efficiently produced with a small amount of colloidal aqueous solution. The amount of the drop is even more preferably in the range of 1 μL/ ^mm2 or more and 2.5 μL/ ^mm2 or less. Within this range, a nanosheet monolayer film can be efficiently produced with a small amount of colloidal aqueous solution without having to choose a substrate.

In step S130, the dropped colloidal aqueous solution can be aspirated using any means capable of aspirating a solution equipped with a nozzle, but examples of the means include a pipette such as a micropipette capable of measuring the amount of liquid, a dropper, and an aspirating nozzle.

In step S130, the dropped colloidal aqueous solution is aspirated in an amount substantially equal to the amount dropped, excluding the amount that naturally evaporates, but since step S130 is performed immediately after step S120, the amount that naturally evaporates may be ignored. The time required for aspirating varies depending on the type of inorganic layered material selected, the concentration of the colloidal aqueous solution, and the type of substrate, but is illustratively in the range of 15 seconds to 1 minute. Therefore, the entire manufacturing process is short and efficient.

In step S130, the mixture may be held for a certain period of time after suction. This allows the remaining dispersion medium to be dried and removed. The holding time is, for example, 3 seconds or more and 10 seconds or less.

The nanosheet monolayer film of the present invention can be produced at room temperature (temperature range of 10°C to 35°C) in the atmosphere, but the substrate may also be heated. This allows precise control of the evaporation rate of the dispersion medium in the dropped colloidal aqueous solution and the convection of the nanosheets. The substrate may be heated prior to step S110 or after step S110.

When heating the substrate, the heating temperature is preferably in the range of 50°C or higher and 150°C or lower. This allows a nanosheet monolayer film to be obtained in a short time. More preferably, the heating temperature is in the range of 90°C or higher and 130°C or lower. This allows a nanosheet monolayer film to be obtained with a good yield. For heating, a normal heating device such as a hot plate can be used.

As explained above, according to the method of the present invention, a nanosheet monolayer film can be produced in an extremely short time by a simple operation of dropping an aqueous colloidal solution using a dispersion medium containing water and alcohol onto a substrate and then aspirating it.

By repeating the above steps, a superlattice consisting of a nanosheet multilayer film or a nanosheet monolayer film can be produced as a thin film.
FIG. 3 is a flow chart showing the steps of producing a nanosheet multilayer film according to the present invention.

Following the process for producing the nanosheet monolayer film illustrated in FIG. 1, the process of dropping the colloidal aqueous solution onto the substrate and the process of sucking the dropped colloidal aqueous solution from the substrate can be repeated (step 310 in FIG. 3). This makes it possible to obtain a nanosheet multilayer film in which the nanosheet monolayer film is multilayered.

Step S310 preferably further includes a step of heating the thin film consisting of the nanosheet monolayer film on the substrate obtained by step S130, and a step of washing the thin film consisting of the nanosheet monolayer film on the substrate with pure water.

The heating step removes unnecessary organic substances (e.g., TBA ⁺ , TMA ⁺ , etc., used for peeling off the nanosheets). The heating temperature is not particularly limited as long as it is a temperature at which the organic substances can be removed, but is preferably in the range of 150° C. or more and 250° C. or less. The heating time is not particularly limited, and may be at least 5 minutes or more. This removes the organic substances remaining in the thin film made of the nanosheet monolayer film, and a strong multilayer film in which the nanosheet monolayer film and the nanosheet monolayer film are closely attached to each other is obtained.

The washing process after heating makes the surface more compatible with the colloidal aqueous solution, facilitating the next lamination process.

By repeating the above steps multiple times (n) as one set, it is possible to produce a multilayer film in which n nanosheet monolayer films are stacked.

In addition, by combining two or more different types of nanosheet monolayer films and laminating them together using the above process as one set, it is possible to produce a superlattice.

The obtained nanosheet multilayer film or superlattice may be irradiated with ultraviolet light (light having a wavelength in the range of 200 nm to 400 nm). This removes organic matter between the nanosheet monolayer films. Depending on the selected layered inorganic substance, the nanosheet monolayer film may induce a photocatalytic effect by ultraviolet light irradiation. This decomposes and removes organic matter (e.g., TBA ⁺ , TMA ⁺ , etc., used for peeling off the nanosheets) between the nanosheet monolayer films. As a result, the nanosheet multilayer film or superlattice can be made into an inorganic thin film that does not contain organic matter. The irradiation of ultraviolet light may be combined with heating for removing organic matter.

The ultraviolet light irradiation conditions are appropriately modified depending on the number of layers of the nanosheet multilayer film and superlattice. For example, if the nanosheet multilayer film has 10 layers, irradiation with 1 mW/ ^cm2 ultraviolet light for about 72 hours or more can sufficiently remove organic matter.

Such nanosheet multilayer films are stable because organic matter has been removed. In addition, nanosheet multilayer films are advantageous in that they allow oxide nanosheets to stably and effectively utilize their excellent properties, such as electronic and ionic conductivity, semiconductivity, insulation, high dielectric properties, ferroelectricity, ferromagnetism, fluorescent properties, and photocatalytic properties.

As explained above, according to the present invention, a nanosheet monolayer film can be multilayered simply by repeating the steps shown in FIG. 1, making the operation simple and allowing thin and thick films to be produced in a short time.

The thin film obtained by the method of the present invention can provide a thin film that has the excellent properties of oxide nanosheets, such as excellent electronic and ionic conductivity, semiconductivity, insulating properties, high dielectric properties, ferroelectric properties, ferromagnetic properties, fluorescent properties, and photocatalytic properties. The manufacturing method of the present invention allows such thin films to be manufactured easily, with a small amount of solution, and with high quality, thereby significantly reducing the costs of thin film manufacturing.

Next, the present invention will be described in detail using specific examples, but it goes without saying that the present invention is not limited to these examples.

[Preparation of nanosheets used in production]
1. Preparation of titanium _oxide nanosheet ( _Ti0.87O2 ) Using _layered titanium oxide ( _{K0.8Ti1.73Li0.27O4} ) as _a starting material, _{a titanium oxide nanosheet (Ti0.87O2 ) was} prepared.

Layered titanium oxide ( _{K0.8Ti1.73Li0.27O4} ) was synthesized by solid-phase synthesis. _Raw material _powders of _TiO2 (Rare Metallic, purity 99.99%), _K2CO3 (Rare Metallic, purity _99.99 % ₎ and _Li2CO3 (Rare Metallic, purity 99.99%) were weighed based on the stoichiometric ratio _TiO2 : _{K2CO3 : Li2CO3} = 1:0.23:0.078, and were ground and mixed for ₆₀ minutes using an alumina mortar. During firing, part of the _alkali metal carbonates _K2CO3 and _Li2CO3 evaporates, so _they were added in a molar ratio of 5 _% excess. The ground and mixed raw material powder was placed in a platinum crucible and pre-fired in an electric furnace at 900° C. for 1 hour. The pre-fired raw material powder was then ground and mixed again in an alumina mortar for 30 minutes. The ground and mixed raw material powder was then placed in a platinum crucible and fired at 1000° C. for 20 hours to obtain layered titanium oxide K _0.8 Ti _1.73 Li _0.27 O ₄ .

The obtained layered titanium oxide (0.2 g) and an aqueous hydrochloric acid solution (200 mL) were stirred in a beaker and subjected to an acid treatment for 72 hours to obtain a hydrogen ion exchanger (H _1.07 Ti _1.73 O _{4.H 2} O). Next, the hydrogen ion exchanger was mixed at a ratio of 4 g/L with an aqueous tetrabutylammonium hydroxide solution whose concentration was adjusted so that the ratio of tetrabutylammonium cation (TBA ⁺ )/proton (H ⁺ ) was 1, and the mixture was allowed to react at room temperature for 2 weeks to produce a milky white colloidal aqueous solution in which nanosheets with a thickness of about 1 nm and a lateral size of 5 μm to 10 μm, represented by the composition formula Ti _0.87 O ₂ , were dispersed. The concentration of the colloidal aqueous solution (undiluted solution) containing titanium oxide nanosheets was 0.36 mass%.

2. Preparation of Perovskite Nanosheets (Ca ₂ Nb ₃ O ₁₀ ) Perovskite nanosheets (Ca ₂ Nb _{3 O 10 ) were prepared using layered perovskite oxide (KCa 2 Nb 3 O 10 ) as a} starting material.

The layered perovskite oxide (KCa ₂ Nb ₃ O ₁₀ ) was synthesized by solid-phase synthesis. Raw material powders of K ₂ CO ₃ (Rare Metallic, purity 99.99%), CaCO ₃ (Rare Metallic, purity 99.99%) and Nb ₂ O ₅ (Rare Metallic, purity 99.99%) were weighed based on the stoichiometric ratio of K ₂ CO ₃ :CaCO ₃ :Nb ₂ O ₅ = 1:4:3, and were ground and mixed for 60 minutes using an alumina mortar. During firing, part of the alkali metal carbonate K ₂ CO ₃ evaporates, so it was added in a molar ratio of 10% excess. The ground and mixed raw material powder was placed in a platinum crucible, pre-fired in an electric furnace at 900° C. for 1 hour, and then fired at 1200° C. for 12 hours to obtain a layered perovskite oxide KCa ₂ Nb ₃ O ₁₀ .

The obtained layered perovskite oxide (5 g) and a nitric acid aqueous solution (200 mL) were stirred in a beaker and subjected to acid treatment for 72 hours to obtain a hydrogen ion exchanger (HCa ₂ Nb ₃ O ₁₀ ·1.5H ₂ O). Next, the hydrogen ion exchanger (0.4 g) was mixed with a tetrabutylammonium hydroxide aqueous solution (100 mL) whose concentration was adjusted so that the TBA ⁺ /H ⁺ ratio was 2, and the mixture was allowed to react at room temperature for 2 weeks to produce a milky white colloidal aqueous solution in which nanosheets with a thickness of about 1.5 nm and a lateral size of 2 μm to 5 μm, represented by the composition formula Ca ₂ Nb ₃ O ₁₀ , were dispersed. The concentration of the colloidal aqueous solution (stock solution) containing the perovskite nanosheets was 0.4 mass%.

3. Preparation of Ruthenium Oxide Nanosheets (RuO _2.1 ) Ruthenium oxide nanosheets (RuO _2.1 ) were prepared using layered ruthenium oxide (K _0.2 RuO _2.1 ) as a starting material.

Layered ruthenium oxide ( _K0.2RuO2.1 ) was synthesized by solid-phase synthesis _. Raw material powders of _K2CO3 (Rare Metallic, purity 99.99%) and _RuO2 (Rare Metallic, purity 99.99%) were weighed based on the stoichiometric ratio _K2CO3 : _RuO2 = _0.625 :1, and ground and mixed for 60 _minutes using an alumina mortar. The ground and mixed raw material powders were placed in a platinum crucible and fired at 900°C for 12 hours in _{an Ar atmosphere to obtain layered ruthenium oxide K0.2RuO2.1 .}

The obtained layered ruthenium oxide (5 g) and hydrochloric acid solution (200 mL) were stirred in a beaker and acid-treated for 72 hours to obtain a hydrogen ion exchanger (H _0.2 RuO _2.1 ·0.9H ₂ O). Next, the hydrogen ion exchanger (0.4 g) was mixed with a tetrabutylammonium hydroxide solution (100 mL) whose concentration was adjusted so that the TBA ⁺ /H ⁺ ratio was 5, and the mixture was reacted at room temperature for 2 weeks to produce a brown colloidal aqueous solution in which nanosheets with a thickness of about 1 nm and a lateral size of 1 μm to 5 μm, represented by the composition formula RuO _2.1, were dispersed. The concentration of the colloidal aqueous solution (stock solution) containing ruthenium oxide nanosheets was 0.4 mass%.

4. Preparation of Graphene Oxide Nanosheets (GO) Graphene oxide nanosheets (GO) were prepared using graphite as a starting material.

Graphite powder (Wako Pure Chemical Industries, Ltd.) was ground in an alumina mortar for 60 minutes to produce a fine powder. A solution of sodium nitrate (0.75 g) and potassium permanganate (4.5 g) dissolved in concentrated sulfuric acid (100 mL) was prepared, to which graphite powder (1 g) was added and stirred at room temperature for 7 days. The reaction solution was cooled, and 5% sulfuric acid (100 mL) was slowly added, followed by 30% hydrogen peroxide (3 mL), and the mixture was stirred at room temperature for 2 hours. This prepared a solution containing graphite oxide.

A mixed solution of 3% sulfuric acid and 0.5% hydrogen peroxide was added to the solution containing graphite oxide, and the precipitate was redispersed, then centrifuged (7000 rpm, 60 minutes), and the supernatant was discarded. This redispersion/centrifugation procedure was repeated 15 times. Next, ultrapure water was added, and the procedure of redispersion, centrifugation (7000 rpm, 30 minutes), and discarding the supernatant was repeated 15 times. The supernatant was then collected, and ultrapure water was added and stirred to obtain a black colloidal aqueous solution in which graphene oxide nanosheets were dispersed. The concentration of the colloidal aqueous solution (undiluted solution) containing graphene oxide nanosheets was 0.2% by mass.

[Examples 1 to 9]
In Examples 1 to 9, a stock solution of a colloidal aqueous solution containing titanium oxide nanosheets was used to prepare colloidal aqueous solutions of various concentrations for thin films, and titanium oxide nanosheet monolayer films were produced as thin films on various substrates.

The above-mentioned titanium oxide ( _{Ti0.87O2 )} nanosheet-containing colloidal aqueous solution (stock solution) was weighed out in the amount shown in Table 1, diluted with 5 mL of ultrapure water (Ultrapure water device Milli-Q Element manufactured by Japan Millipore), and then various amounts of ethanol were added to prepare a colloidal aqueous solution for dropping (coating solution) (step S110 in FIG. 1). Table 1 also shows the ethanol content (volume %) in the dispersion medium of each coating solution, and the nanosheet concentration (g/L) in the coating solution.

Figure 4 shows the observation results of the colloidal aqueous solution for dropping in Example 1.

As shown in FIG. 4, the colloidal aqueous solution exhibited a light milky white color, and it was found that the Ti _0.87 O ₂ nanosheets were uniformly dispersed.

As shown in Table 2, the substrates used were a quartz glass substrate (30 mmφ), a silicon (Si) wafer (30 mmφ), a Si substrate (1 inch square) with a 90 nm native oxide film, a Pt substrate (30 mmφ), a SrTiO ₃ :Nb substrate (15 mm square), and a PET substrate (50 mm square).

The surfaces of the quartz glass substrate, the Si wafer, and the Si substrate with a native oxide film were wiped with acetone to remove organic matter. Next, each substrate was immersed in a mixed solution of methanol and concentrated hydrochloric acid (mixed at a volume ratio of 1:1) for 30 minutes and washed with ultrapure water. After that, each substrate was immersed in concentrated hydrochloric acid for 30 minutes and washed with ultrapure water. This resulted in a hydrophilic substrate. The Pt substrate, the SrTiO ₃ :Nb substrate, and the PET substrate were irradiated with ultraviolet light for 15 minutes in an ozone atmosphere and washed.

Next, the cleaned substrate was placed on a hot plate heated to the temperature shown in Table 2 and held there for 3 minutes. After that, the coating liquid was dripped onto the substrate in the amount shown in Table 2 (step S120 in FIG. 1). Next, substantially the entire amount of the dripped coating liquid was aspirated using a micropipette (step S130 in FIG. 1). Note that the thin film of Example 5 was held there for 3 seconds after aspirating.

The surfaces of the thin films of Examples 1 to 9 were observed using a confocal laser microscope (CFLM, Olympus Corporation, OLS4000). The observation results are shown in Figures 5 to 10.

The surfaces of the thin films of Examples 1 to 9 were observed using an atomic force microscope (AFM, manufactured by SII Nano Technology, Inc., E-Sweep scanning probe microscope system). A silicon cantilever (spring constant k = 20 N/m) was used as the probe, and observations were performed in tapping mode. The observation results are shown in Figures 11 to 13.

The UV-Vis absorption spectra of the thin films of Examples 1 to 3 and 9 were measured using a spectrophotometer (Hitachi U-4100). The samples were placed so that the incident light was perpendicular to the quartz glass substrate. The measurement results are shown in Figure 17.

[Examples 10 to 15]
In Examples 10 to 15, a stock solution of a colloidal aqueous solution containing ruthenium oxide nanosheets was used to prepare colloidal aqueous solutions of various concentrations for thin films, and ruthenium oxide nanosheet monolayer films were produced as thin films on various substrates.

The above-mentioned colloidal aqueous solution (stock solution) containing ruthenium oxide (RuO _2.1 ) nanosheets was weighed out in the amounts shown in Table 3, diluted with 5 mL of ultrapure water (Ultrapure water device Milli-Q Element manufactured by Japan Millipore), and then various amounts of ethanol were added to prepare colloidal aqueous solutions for dropping (coating solutions) (step S110 in FIG. 1). Table 3 also shows the ethanol content (volume %) in the dispersion medium of each coating solution, as well as the concentration (g/L) of nanosheets in the coating solution.

Various substrates were cleaned in the same manner as in Examples 1 to 9. The cleaned substrates were placed on a hot plate heated to the temperature shown in Table 4 and held there for 3 minutes. Then, the coating liquid was dripped onto the substrate in the amount shown in Table 4 (step S120 in FIG. 1). Next, substantially the entire amount of the dripped coating liquid was aspirated using a micropipette (step S130 in FIG. 1). Note that the thin film of Example 12 was held there for 3 seconds after aspirating.

The thin films of Examples 10 to 15 were observed using a confocal laser microscope and an atomic force microscope in the same manner as in Examples 1 to 9, and the UV-vis absorption spectrum of the thin film of Example 10 was measured using a spectrophotometer. The results are shown in Figure 14.

[Examples 16 to 21]
In Examples 16 to 21, a stock solution of a colloidal aqueous solution containing perovskite nanosheets was used to prepare colloidal aqueous solutions for thin films of various concentrations, and perovskite nanosheet monolayer films were produced as thin films on various substrates.

The above-mentioned colloidal aqueous solution (stock solution) containing the perovskite nanosheet (Ca ₂ Nb ₃ O ₁₀ ) nanosheet was weighed out in the amount shown in Table 5, diluted with 5 mL of ultrapure water (Ultrapure water device Milli-Q Element manufactured by Japan Millipore Co., Ltd.), and then various amounts of ethanol were added to prepare a colloidal aqueous solution (coating solution) for dropping (step S110 in FIG. 1). Table 5 also shows the content (volume %) of ethanol in the dispersion medium of each coating solution, and the concentration (g/L) of nanosheet in the coating solution.

Various substrates were cleaned in the same manner as in Examples 1 to 9. The cleaned substrates were placed on a hot plate heated to the temperature shown in Table 6 and held there for 3 minutes. After that, the coating liquid was dripped onto the substrate in the amount shown in Table 6 (step S120 in FIG. 1). Next, substantially the entire amount of the dripped coating liquid was aspirated using a micropipette (step S130 in FIG. 1).

The thin films of Examples 16 to 21 were observed using a confocal laser microscope and an atomic force microscope in the same manner as in Examples 1 to 9, and the UV-vis absorption spectrum of the thin film of Example 17 was measured using a spectrophotometer. The results are shown in Figure 15.

[Examples 22 to 24]
In Examples 22 to 24, a stock solution of a colloidal aqueous solution containing graphene oxide nanosheets was used to prepare colloidal aqueous solutions of various concentrations for thin films, and graphene oxide nanosheet monolayer films were produced as thin films on various substrates.

The colloidal aqueous solution (stock solution) containing the above-mentioned graphene oxide (GO) nanosheets was weighed out in the amounts shown in Table 7 and diluted with 5 mL of ultrapure water (Ultrapure water system Milli-Q Element manufactured by Japan Millipore Corporation), and then various amounts of ethanol and tetrabutylammonium hydroxide (TBAOH) aqueous solution were added to prepare a colloidal aqueous solution for dropping (coating solution) (step S110 in Figure 1). Table 7 also shows the ethanol content (volume %) in the dispersion medium of each coating solution, as well as the concentration of nanosheets (g/L) in the coating solution.

Various substrates were cleaned in the same manner as in Examples 1 to 9. The cleaned substrates were placed on a hot plate heated to the temperature shown in Table 8 and held there for 3 minutes. After that, the coating liquid was dripped onto the substrate in the amount shown in Table 8 (step S120 in FIG. 1). Next, substantially the entire amount of the dripped coating liquid was aspirated using a micropipette (step S130 in FIG. 1).

The thin films of Examples 22 to 24 were observed using a confocal laser microscope and an atomic force microscope in the same manner as in Examples 1 to 9, and the UV-vis absorption spectrum of the thin film of Example 23 was measured using a spectrophotometer. The results are shown in Figure 16.

The results of the thin films of Examples 1 to 24 will be summarized below.
FIG. 5 shows a CFLM image of the thin film of Example 4.
FIG. 6 shows a CFLM image of the thin film of Example 1.
FIG. 7 shows a CFLM image of the thin film of Example 5.
FIG. 8 shows a CFLM image of the thin film of Example 6.
FIG. 9 shows a CFLM image of the thin film of Example 7.
FIG. 10 shows a CFLM image of the thin film of Example 8.

According to Fig. 5, no black areas originating from the Si wafer were observed in the thin film of Example 4, and it was found that the entire Si wafer was covered with _Ti0.87O2 nanosheets. According to Figs. 6 to 10, the entire substrate was covered with _{Ti0.87O2 nanosheets} regardless of the type of substrate. Although not shown, the CFLM images of the thin films of Examples 10 to 24 also had the same appearance _.

FIG. 11 shows an AFM image of the thin film of Example 4.
FIG. 12 shows AFM images of various locations of the thin film of Example 4.
FIG. 13 shows an AFM image of the thin film of Example 9.
FIG. 14 shows an AFM image of the thin film of Example 11.
FIG. 15 shows an AFM image of the thin film of Example 17.
FIG. 16 shows an AFM image of the thin film of Example 23.

According to FIG. 11, the thin film of Example 4 was composed of Ti _0.87 O ₂ nanosheets, and covered the entire Si wafer. It was confirmed that the thin film of Example 4 was a nanosheet monolayer film in which the nanosheets were arranged densely without overlapping each other and without gaps. When the coverage was calculated by performing image analysis (Nano Navi, version 5.02, manufactured by SII Nano Technology), it was found that the overlap was 0% and the gap was 3%, and it was revealed that the nanosheets were arranged densely. Although not shown, the thin films of Examples 1 to 3 and Examples 5 to 8 also showed a similar state, with the overlap being 0% to 10% and the gap being in the range of 0% to 10%.

Figure 12 shows the results of evaluating the position dependence of the coverage of the thin film of Example 4 from the center 1 to the edge 3 of the Si wafer. It was confirmed that at all points, near the center 1, at the edge 3, and in between 2, a monolayer film of nanosheets was obtained in which the nanosheets overlapped with each other and were densely arranged without gaps.

According to FIG. 14, the thin film of Example 11 was composed of RuO _2.1 nanosheets, and covered the entire Si wafer. It was confirmed that the thin film of Example 11 was a nanosheet monolayer film in which the nanosheets were arranged densely without overlapping each other and without gaps. When the coverage was calculated by performing image analysis, it was found that the overlap was 3% and the gap was 5%, and it was revealed that the nanosheets were arranged densely. Although not shown, the thin films of Examples 10 and 12 to 15 also showed a similar state, with the overlap being 0% to 10% and the gap being in the range of 0% to 10%.

According to FIG. 15, the thin film of Example 17 was composed of Ca ₂ Nb ₃ O ₁₀ nanosheets, and covered the entire Si wafer. It was confirmed that the thin film of Example 17 was a nanosheet monolayer film in which the nanosheets were arranged densely without overlapping each other and without gaps. When the coverage was calculated by performing image analysis, it was found that the overlap was 2% and the gap was 8%, and it was revealed that the nanosheets were arranged densely. Although not shown, the thin films of Examples 16 and 18 to 21 also showed a similar state, and all of them had an overlap of 0% to 105% and a gap in the range of 0% to 10%.

According to FIG. 16, the thin film of Example 23 was composed of graphene oxide nanosheets, and covered the entire Si wafer. It was confirmed that the thin film of Example 23 was a single layer of nanosheets in which the nanosheets were densely arranged without overlapping or gaps. Image analysis was performed to calculate the coverage rate, which revealed that the overlap was 1% and the gap was 8%, making it clear that the nanosheets were densely arranged. Although not shown, the thin films of Examples 22 and 24 also showed a similar appearance, with both overlapping between 0% and 10% and gaps in the range between 0% and 10%.

On the other hand, according to Figure 13, in the thin film of Example 9, in which a lower alcohol was not used as a dispersion medium, the nanosheets were visually observed to overlap, had many gaps, and were not densely arranged. When the coverage rate was calculated, both the overlaps and gaps exceeded 10%.

Figure 17 shows the UV-vis absorption spectrum of the thin film of Example 1.

According to Fig. 17, in the absorption spectrum of the thin film of Example 1, an absorption band originating from _{Ti0.87O2 nanosheets} was observed at a wavelength of 265 nm. In addition, the absorbance at a wavelength of 265 nm was approximately 0.065 at any location, regardless of the position of the quartz glass substrate. This value was within the range of absorbance corresponding to a _{Ti0.87O2 nanosheet} monolayer film, which also showed that the thin film of Example 1 was a _{Ti0.87O2 nanosheet} monolayer film. The thin films of Examples 2 and 3 also showed similar absorbance and were nanosheet monolayer films.

Although not shown, in the absorption spectrum of the thin film of Example 10, an absorption band originating from RuO _2.1 nanosheets was observed at a wavelength of 380 nm, and its absorbance was about 0.071. In the absorption spectrum of the thin film of Example 16, an absorption band originating from Ca ₂ Nb ₃ O ₁₀ nanosheets was observed at a wavelength of 260 nm, and its absorbance was about 0.068. In the absorption spectrum of the thin film of Example 22, an absorption band originating from GO nanosheets was observed at a wavelength of 235 nm, and its absorbance was about 0.011.

The above results show that a nanosheet monolayer film can be obtained by the method of the present invention, in which a colloidal aqueous solution in which nanosheets are dispersed using a lower alcohol with 5 or less carbon atoms as a dispersion medium, as shown in Figure 1, is dropped onto a substrate and then sucked up. It was also shown that the method of the present invention is effective for any substrate and any nanosheet in which a layered inorganic compound has been exfoliated into a single layer. It was shown that by employing the method of the present invention, a uniform nanosheet monolayer film can be obtained over the entire surface of a large-area substrate in an extremely short time.

[Examples 25 to 28]
In Example 25, a stock solution of a colloidal aqueous solution containing titanium oxide nanosheets or ruthenium oxide nanosheets was used to prepare a colloidal aqueous solution for thin films, and nanosheet multilayer films were produced as thin films on various substrates.

The above-mentioned aqueous colloidal solution (stock solution) containing _titanium oxide ( _Ti0.87O2 ) nanosheet or ruthenium oxide nanosheet ( _RuO2.1 ) was weighed out in the amounts shown in Table 9 and diluted with 5 mL of ultrapure water (Ultrapure water system Milli-Q Element manufactured by Japan Millipore Corporation). Various amounts of ethanol were then added to prepare aqueous colloidal solutions (coating solutions) for dropping (step S110 in FIG. 1).

As shown in Table 10, the substrates used were a quartz glass substrate (30 mmφ), a silicon (Si) wafer (30 mmφ), a SrTiO ₃ :Nb substrate (15 mm square), and a PET substrate (50 mm square). The pretreatment of the substrates was the same as in Examples 1 to 8.

The cleaned substrate was then placed on a hot plate heated to the temperature shown in Table 10 and held for 3 minutes. The coating liquid was then dripped onto the substrate in the amount shown in Table 10 (step S120 in FIG. 1). Next, substantially the entire amount of the dripped coating liquid was aspirated using a micropipette (step S130 in FIG. 1). As shown in Table 10, the dripping and suction steps were repeated a total of n times (n is a maximum of 100 times, and n corresponds to the number of layers in which the nanosheets are densely arranged) (step S310 in FIG. 3). When n=1, for example, this corresponds to the titanium oxide nanosheet monolayer film of Example 1 in which the dripping and suction steps were performed only once.

In detail, when n≦2, the thin film was heated at 200° C. for 15 min after each suction. This removed the TBA ⁺ remaining in the nanosheet monolayer film. The heated thin film was then washed with pure water. This allowed the coating liquid to be more easily absorbed.

The process was repeated a predetermined number of times to produce the following thin films: a multilayer film made of _{Ti0.87O2 nanosheets} on a _quartz glass substrate as a thin film of Example 25 (a nanosheet multilayer film of up to 100 layers (n = 1 to 100)); a multilayer film made of _Ti0.87O2 nanosheets on a Si wafer as a thin film of Example 26 (a nanosheet multilayer film of up to 10 layers (n = 1 to 10)); a multilayer film made of _Ti0.87O2 nanosheets on a _SrTiO3 : _Nb substrate as a thin film of Example 27 (a nanosheet multilayer film of up to 50 layers (n = 1 to 50)); and a multilayer film made of _RuO2.1 nanosheets on a PET substrate as a thin film of Example 28 (a nanosheet multilayer film of up to 5 layers (n = 1 to 5)). The obtained nanosheet multilayer film was irradiated with ultraviolet light (wavelength: 200 to 300 nm, intensity: 1 mW/cm ² ) for 72 hours to remove organic matter present between the nanosheet monolayer films.

The appearance of the thin film of Example 28 was observed. The results are shown in Figure 18. For the thin film of Example 25, the UV-vis absorption spectrum was measured in the same manner as in Example 1, and the absorbance was calculated. The results are shown in Figures 19 to 22. Powder X-ray diffraction (XRD) was performed using CuKα radiation for the thin films of Examples 25 to 28. For the XRD measurement, a Rint-2000 manufactured by Rigaku Corporation was used. The results are shown in Figure 23. The cross sections of the thin films of Examples 25 to 28 were observed with a high-resolution transmission electron microscope (HRTEM). For the HRTEM observation, an H-9000 manufactured by Hitachi High-Technologies Corporation was used. The results are shown in Figure 24.

Next, the optical properties, dielectric properties, and conductive properties of the thin films of Examples 25 to 28 were evaluated. The refractive index and extinction coefficient of the thin film of Example 25 were evaluated using an epipsometer. For the epipsometer measurement, M-2000 manufactured by J. A. Woollam was used. The measurement results are shown in FIG. 25. The dielectric properties of the thin film of Example 27 were measured. Gold (100 μmφ) was evaporated onto the thin film as an upper electrode, and probes were applied to the SrTiO ₃ :Nb substrate and the upper electrode, respectively, to measure the dielectric properties. The results are shown in FIG. 26 and FIG. 27. The conductive properties of the thin film of Example 28 were evaluated using a four-terminal method. The measurement results are shown in FIG. 28.

The above results are summarized below.
FIG. 18 is a diagram showing the appearance of the thin film of Example 28.

Figure 18 (A) shows the appearance of the thin film of Example 28 (corresponding to n = 1), and Figure 18 (B) shows the appearance of the thin film of Example 28 (n = 2). As shown in Figure 18, both the nanosheet monolayer film and the nanosheet multilayer film were transparent to visible light and had high transmittance, and were stable even when bent. This shows that the thin film obtained by the method of the present invention, whether it is a nanosheet monolayer film or a nanosheet multilayer film, is a uniform film over the entire substrate. It was also shown that by selecting a flexible substrate, a flexible thin film can be provided.

FIG. 19 is a diagram showing the UV-vis absorption spectrum of the thin film of Example 25.
FIG. 20 shows the UV-vis absorption spectrum of the thin film of Example 25.
FIG. 21 is a graph showing the layer number dependence of absorbance of the thin film of Example 25.
FIG. 22 is a graph showing the layer number dependence of absorbance of the thin film of Example 25.

Figures 19 and 21 show the results of thin films of Example 25 where n=1 to 10 (n=1 is the same as the thin film of Example 1), and Figures 20 and 22 show the results of thin films of Example 25 where n=10, 20, 30, 40, 50, 60, 70, 80, 90, and 100. "L" in Figure 19 represents a layer; for example, "1L" represents a single nanosheet monolayer film where n=1, and "2L" represents a nanosheet multilayer film consisting of two nanosheet monolayer films where n=2. The same applies to the other figures.

Figures 19 to 22 show that the absorbance at a wavelength of 265 nm increases linearly with an increase in the number of layers n. In particular, Figures 19 and 21 show that the method of the present invention can produce a nanosheet monolayer film that is reliably multilayered layer by layer. Furthermore, Figures 20 and 22 show that the method of the present invention can produce a thick film at the 100 nm level, which was difficult to achieve with conventional film formation techniques.

Figure 23 shows the XRD pattern of the thin film of Example 25.

23 shows the XRD patterns of the thin films (n = 1 to 10) of Example 25. In each _XRD pattern, the basic diffraction series derived from _Ti0.87O2 nanosheets was observed, and it was found that the intensity of the diffraction peak increased with an increase in the number of layers n. This indicates that the nanosheet monolayer films are regularly stacked at regular intervals, and it was confirmed that a nanosheet multilayer film having an ordered layered structure was formed by the method of the present invention.

Figure 24 shows an HRTEM image of a cross section of the thin film of Example 27.

Figure 24 shows an HRTEM image of the cross section of the thin film of Example 27 (n = 10). Figure 24 confirms that the nanosheets are stacked in parallel on the substrate at the atomic level, and it can be said that a high-quality multilayer film has been realized that is stacked layer-by-layer while maintaining the density and smoothness of a single nanosheet. It is also noteworthy that no interfacial reaction layer (dead layer) is formed at the interface between the substrate and the nanosheets, which is often a problem in conventional thin film processes due to interfacial deterioration caused by thermal annealing or compositional deviation. This is a groundbreaking effect that is due to the manufacturing method of the present invention using a low-temperature solution process that is not affected by interfacial deterioration.

Figure 25 shows the wavelength dependence of the refractive index and extinction coefficient of the thin film of Example 25.

Figure 25 shows the wavelength dependence of the refractive index and extinction coefficient of the thin film of Example 25 (n = 10). According to Figure 25, the nanosheet multilayer film consisting of 10 nanosheet layers, although it is an extremely thin film with a thickness of 10 nm, has a high refractive index of 2.5 or more over the entire measured wavelength range and exhibits an excellent extinction coefficient of essentially 0. This confirmed that the thin film obtained by the method of the present invention is suitable for application in optical filters. Although not shown, a thin film with n = 50 also exhibited a similarly high refractive index and an extinction coefficient close to 0.

FIG. 26 is a graph showing the frequency dependence of the dielectric constant and dielectric loss of the thin film of Example 27.
FIG. 27 shows the leakage current characteristics of the thin film of Example 27.

26 and 27 show the dielectric properties and leakage current properties of the thin films (n=10, 20, 50) of Example 27. According to FIG. 26, the nanosheet multilayer film (e.g., 10 layers or 20 layers) obtained by the method of the present invention exhibits a high relative dielectric constant of 115 to 125 and good dielectric loss properties of 3% or less, and it was found that the relative dielectric constant has a value larger than the relative dielectric constant (20 to 60) of rutile-type TiO ₂ , which is considered to be the highest among binary simple oxides. Furthermore, it was found that the nanosheet multilayer film of the present invention has almost no frequency dependence or film thickness dependence of the relative dielectric constant and has good dielectric properties.

27, the nanosheet multilayer film obtained by the method of the present invention showed good leakage current characteristics of 10 ^-7 A/cm2 or ^less at an applied electric field of 1 MV/cm, as well as excellent voltage resistance characteristics of 3.8 MV/cm, despite having only 10 layers. It was also confirmed that the insulation characteristics and voltage resistance characteristics can be improved by increasing the number of layers to 20 or 50 layers.

This indicates that the nanosheet multilayer film obtained by the method of the present invention has a high dielectric constant and functions as a dielectric material.

Figure 28 shows the sheet resistance of the thin film of Example 28.

Figure 28 shows the sheet resistance of the thin films (n = 1, 2, 4) of Example 28, as well as the sheet resistance of a tin oxide-doped indium oxide transparent conductive film (ITO, 40 nm to 300 nm) and graphene (0.35 nm to 1.5 nm). The sheet resistance values of ITO and graphene are based on Figure 2b in F. Bonaccorso et al., Nat. Photon., 4, 611, 2010.

As shown in Figure 28, the single-layer and multi-layer films made of nanosheets are extremely thin, with a thickness of 10 nm or less, but they exhibit a low sheet resistance of 300 to 900 Ω/□, and it has been confirmed that they have high conductivity comparable to that of ITO, which is used as a transparent electrode, and graphene, which has the highest conductivity among two-dimensional materials.

As shown in Figure 18, it was shown that flexible transparent electrode materials can be provided by forming a nanosheet monolayer film or a nanosheet multilayer film on a flexible substrate.

The manufacturing method of the present invention allows thin films consisting of nanosheet monolayer films to be produced easily, in a short time, and with a small amount of solution, with a large area and high quality, which significantly reduces the costs of thin film production and is effective as an industrial film formation method. In addition, the thin films obtained by the manufacturing method of the present invention can provide thin films that have the excellent properties of oxide nanosheets, such as excellent electronic and ionic conductivity, semiconductivity, insulation, high dielectric properties, ferroelectricity, ferromagnetism, fluorescent properties, and photocatalytic properties, and are expected to be used as important components of various functional materials and devices.

210 Nanosheet 220 Dispersion medium 230 Colloidal aqueous solution 240 Substrate 250 Pipette 260 Nanosheet monolayer film

Claims (16)

A method for producing a thin film consisting of a nanosheet monolayer film, comprising the steps of:
preparing a colloidal aqueous solution in which nanosheets obtained by exfoliating an inorganic layered material are dispersed in a dispersion medium containing water and a lower alcohol having 5 or less carbon atoms;
Dropping the colloidal aqueous solution onto a substrate;
and sucking the dropped aqueous colloidal solution from the side of the substrate on which the aqueous colloidal solution was dropped . The method of claim 1, wherein the lower alcohol is at least one selected from the group consisting of methanol, ethanol, propanol, and butanol. The method according to claim 1 or 2, wherein the content of the lower alcohol in the dispersion medium is in the range of 0.5% by volume or more and 15% by volume or less. The method according to claim 3, wherein the content of the lower alcohol in the dispersion medium is in the range of 1% by volume or more and 10% by volume or less. The method according to any one of claims 1 to 4, wherein the concentration of the nanosheets in the aqueous colloidal solution is in the range of 0.01 g/L or more and 1 g/L or less. The method according to claim 5, wherein the concentration of the nanosheets in the aqueous colloidal solution is in the range of 0.01 g/L to 0.5 g/L. The method according to any one of claims 1 to 6 , wherein the sucking comprises sucking the dropped aqueous colloid solution for a period of time ranging from 15 seconds to 1 minute . The method according to any one of claims 1 to 7, wherein the amount of the colloidal aqueous solution dropped onto the substrate in the dropping step is in the range of 0.1 µL/ ^mm2 to 5 µL/ ^mm2 . The method according to claim 8 , wherein the amount of the solution dispensed onto the substrate is in the range of 0.3 μL/mm ² to 2.5 μL/mm ² . The method according to any one of claims 1 to 9, wherein the inorganic layered material is selected from the group consisting of layered titanium oxide, layered perovskite oxide, layered manganese oxide, layered cobalt oxide, layered tungsten oxide, layered niobium oxide, layered tantalum oxide, layered titanium-niobium oxide, layered titanium-tantalum oxide, layered molybdenum oxide, layered ruthenium oxide, and graphite oxide. The method of any one of claims 1 to 10, further comprising heating the substrate. The method of claim 11, wherein the substrate is heated to a temperature range of 50°C or more and 150°C or less. The method according to any one of claims 1 to 12, further comprising repeating the dropping and the suction. The repeating step comprises:
heating the thin film formed of the nanosheet monolayer film on the substrate obtained by the suction;
The method according to claim 13 , further comprising: washing the thin film consisting of the nanosheet monolayer film on the substrate with pure water. The method according to claim 14, wherein heating the thin film comprises heating the thin film consisting of the nanosheet monolayer film on the substrate at a temperature range of 150°C or more and 250°C or less. The method according to any one of claims 1 to 15, further comprising irradiating the thin film of the nanosheet monolayer film of the substrate obtained by the suction with ultraviolet light.