Method for cutting nanosheets along the crystal axis direction, and cut nanosheets

Abstract

To provide a method for cutting a nanosheet along the crystallographic axis orientation, and a nanosheet obtained thereby.SOLUTION: A method for cutting a nanosheet along the crystallographic axis orientation includes preparing a nanosheet dispersion liquid having nanosheets dispersed in an organic solvent, preparing a substrate having an uneven structure, and forming, on the substrate, a nanosheet monolayer film composed of nanosheets, using the nanosheet dispersion liquid. The nanosheet and the substrate satisfy a predetermined relationship.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for cutting a nanosheet along a crystal axis direction, and to the cut nanosheet. [Background technology]

[0002] Layered compounds have a structure in which layers formed by chemical bonds are stacked by intermolecular forces, and many layered compounds can be mechanically or chemically exfoliated layer by layer. The nanosheets obtained by this exfoliation are two-dimensional crystals with thicknesses of one to several atoms and dimensions of several tens of micrometers in the in-plane direction. Nanosheets of a wide variety of materials, including graphene, chalcogenides, oxides, and hydroxides, have been synthesized to date, and are known to exhibit a variety of fascinating properties, such as high electron mobility, ferroelectricity, catalysis, chromism, and ferromagnetism, depending on their composition and structure.

[0003] Techniques for forming a nanosheet monolayer film using such nanosheets are known (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses a method for forming a nanosheet monolayer film using a spin coating method. Patent Document 2 discloses a method for forming a thin film made of a photocatalytic nanosheet consisting of NbO6 and / or TiO6 octahedral units on a substrate by the Langmuir-Blodgett (LB) method.

[0004] On the other hand, secondary processing (such as cutting or perforating) of nanosheets (two-dimensional materials) is an important process for improving the functionality of nanosheets and for fabricating devices using nanosheets (see, for example, Non-Patent Documents 1 to 4).

[0005] As methods for cutting nanosheets, Non-Patent Document 1 discloses a method in which a substrate to which a nanosheet has been adsorbed is pulled laterally to apply a mechanical external force, Non-Patent Document 2 discloses a method in which substrates are partially adsorbed above and below the nanosheet and the two substrates are then separated vertically, Non-Patent Document 3 discloses a cutting method using a scanning probe microscope, and Non-Patent Document 4 discloses a lithography method using a laser.

[0006] However, the mechanical external force application methods described in Non-Patent Documents 1 and 2 do not allow for control of the shape or size of the nanosheets being cut. Furthermore, no methods for cutting nanosheets by controlling their crystal axis orientation have been reported. In principle, it would be necessary to first clarify the crystal orientation of the nanosheet and then apply an external force perpendicular to that orientation. However, various technical challenges exist at both the steps of determining the crystal axis orientation on the substrate and applying the corresponding external force, making this method unrealistic at present. Additionally, the lithography methods described in Non-Patent Documents 3 and 4 require a long time to process a single nanosheet, making them unsuitable for large-scale processing. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-253009 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-292680 [Non-patent literature]

[0008] [Non-Patent Document 1] Seung Ryul Na et al.,ACS Nano,2016,10,9616 [Non-patent document 2] Kyounghwan Kim et al., Nano Lett., 2016, 16, 1989 [Non-patent document 3] Hongyuan Li et al.,Nano Lett.,2018,18,8011 [Non-patent document 4] Xu-Dong Chen et al., Adv. Mater., 2016, 28, 2563 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above, an object of the present invention is to provide a method for cutting a nanosheet along the crystal axis direction, and a nanosheet obtained thereby. [Means for solving the problem]

[0010] The method of the present invention for cutting nanosheets along the crystal axis direction comprises preparing a nanosheet dispersion in which nanosheets are dispersed in an organic solvent, preparing a substrate having a textured structure, and using the nanosheet dispersion on the substrate to form a nanosheet monolayer film consisting of the nanosheets, wherein the nanosheets, the organic solvent, and the substrate satisfy the following formula, thereby solving the above-mentioned problem. nL>(x×E×L) / (S×(A1-A2)×sinθ) nL>d Here, L is the length (m) of one side of the unit cell of the nanosheet, nL is the length (m) of the short side of the nanosheet, and S is the area (m 2 ), E is the bond energy (J / mol) between atoms constituting the nanosheet, x is the number (mol) of chemical bonds crossing the crystal plane perpendicular to the horizontal plane of the nanosheet when the nanosheet is assumed to be the same size as the unit cell, and A1 is the adhesion energy (J / m 2 ), and A2 is the adhesion energy between the nanosheet and the organic solvent (J / m 2 ) where θ is the angle between the side surface of the concave-convex structure of the substrate and the main surface of the substrate, and is greater than 0° and less than 90°, and d is the distance (m) between convex portions of the concave-convex structure. The uneven structure may be mountain-shaped or pyramidal. The nanosheets may be exfoliated from an inorganic layered material selected from the group consisting of titanium oxide, niobium oxide, tantalum oxide, perovskite oxide, manganese oxide, cobalt oxide, molybdenum oxide, tungsten oxide, ruthenium oxide, phosphate, carbon, boron nitride, phosphorus, transition metal chalcogenides, double hydroxides, and rare earth hydroxides. The organic solvent may be at least one selected from the group consisting of dimethyl sulfoxide, formamide, propanol, dimethylformamide, tetrahydrofuran, N,N-dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, and N,N'-dimethylpropyleneurea. nL is 460 x 10 -9 m or larger, 500 x 10 -6 It may be in the range of m or less. The angle θ may be in the range of 10° to 30°. The concentration of the nanosheets in the nanosheet dispersion may be in the range of 0.05% by mass to 1% by mass. The formation of the nanosheet monolayer film may involve forming a liquid film made of the organic solvent on the substrate, floating the nanosheet on the surface of the liquid film, and transferring the nanosheet to the side surface of the uneven structure as the organic solvent evaporates. The nanosheet monolayer film may be formed using a spin coating method or a Langmuir-Blodgett (LB) method. The substrate may be polymer coated. The aspect ratio of the nanosheet may be in the range of 800 to 8000. Following the formation of the nanosheet monolayer film, the method may further comprise drying the nanosheet monolayer film. The nanosheet according to the present invention has edges along the crystal axis direction and has a rectangular or triangular shape that reflects the crystal structure, thereby solving the above-mentioned problems. The edges may be

[01] and

[10] oriented. The longitudinal length may be in the range of 500 nm to 5 μm. [Effects of the Invention]

[0011] The method for cutting nanosheets along the crystal axis orientation of the present invention includes preparing a nanosheet dispersion in which nanosheets are dispersed in an organic solvent, preparing a substrate having a textured structure, and using the nanosheet dispersion to form a nanosheet monolayer film composed of nanosheets on the substrate, wherein the nanosheets and substrate satisfy the above formula. By selecting the nanosheets, organic solvent, and substrate so as to satisfy the above formula, the nanosheets are adsorbed to the textured structure of the substrate, generating tensile stress within the nanosheet plane. As a result, the nanosheets are cut along the crystal axis orientation.

[0012] The nanosheets cut in this way have edges aligned with the crystal axis and have rectangular or triangular shapes that reflect the crystal structure. The edges can exhibit high ferroelectricity, high catalytic activity, high chromism, and high ferromagnetism, making them promising for application in a variety of devices. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a flowchart showing a process for cutting a nanosheet along a crystal axis direction according to the present invention. [Figure 2] Schematic diagram showing the unit cell of a nanosheet [Figure 3] Schematic diagram showing a cross section of a substrate having a concave-convex structure [Figure 4] Schematic diagram showing the principle of nanosheet cutting [Figure 5] FIG. 1 is a diagram schematically illustrating an example of a substrate having a concave-convex structure. [Figure 6] FIG. 10 is a diagram schematically illustrating another example of a substrate having a concave-convex structure. [Figure 7] Graph showing the relationship between the energy required to cut titanium oxide nanosheets and the adhesive energy [Figure 8] AFM image of substrate 1 [Figure 9] AFM image of substrate 2 [Figure 10] A diagram showing cross sections of the surface shapes of substrates 1 and 2 [Figure 11] Figure showing an AFM image of NS1 / substrate 1 in Example 1 [Figure 12] Figure showing an AFM image of NS1 / substrate 2 in Example 2 [Figure 13] A diagram showing a cross section of the surface shape of NS1 / substrate 1 in Example 1 and NS1 / substrate 2 in Example 2. [Figure 14] 1 shows an SEM image of NS1 / substrate 1 in Example 1. [Figure 15] Figure showing SEM image of NS1 / substrate 2 in Example 2 [Figure 16] SEM image of NS2 / substrate 1 in Example 5 and a diagram showing the length distribution of nanosheets on the substrate DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted. FIG. 1 is a flowchart showing the process of cutting a nanosheet along the crystal axis direction according to the present invention.

[0015] The present inventors have found that by appropriately selecting the nanosheet, organic solvent, and substrate, and performing the following steps S110 to S130, the nanosheet can be cut along the crystal axis direction. Step S110: Prepare a nanosheet dispersion in which nanosheets are dispersed in an organic solvent. Nanosheets are two-dimensional sheet-like materials with thicknesses ranging from one atom to several dozen atoms. Typically, the thickness is between 0.3 nm and 5 nm. Step S120: Preparing a substrate having a relief structure. Step S130: Forming a nanosheet monolayer film made of nanosheets on a substrate using the nanosheet dispersion liquid. The nanosheet monolayer film refers to a monolayer film formed by arranging nanosheets.

[0016] Here, the nanosheet, organic solvent and substrate are selected to satisfy the following relationship: nL>(x×E×L) / (S×(A1-A2)×sinθ)...(1) nL>d (2) Here, L is the length of one side of the nanosheet unit cell (m), nL is the length of the nanosheet in the short direction (m), and S is the area of ​​the nanosheet unit cell (m 2 ), E is the bond energy (J / mol) between the atoms that make up the nanosheet, x is the number (mol) of chemical bonds that cross the crystal plane perpendicular to the horizontal plane of the nanosheet, assuming a nanosheet of the same size as the unit cell, and A1 is the adhesion energy (J / m 2 ), and A2 is the adhesion energy between the nanosheet and the organic solvent (J / m 2 ) where θ is the angle between the side surface of the uneven structure of the substrate and the main surface of the substrate, and is greater than 0° and less than 90°, and d is the distance (m) between the convex portions of the uneven structure.

[0017] The inventors of the present invention have discovered the relationship between the nanosheet, the organic solvent, and the substrate, and have succeeded in cutting the nanosheet along the crystal axis direction when certain conditions are met. Formulas (1) and (2) are explained below with reference to the drawings.

[0018] FIG. 2 is a schematic diagram showing a unit cell of a nanosheet. FIG. 3 is a schematic diagram showing a cross section of a substrate having a concave-convex structure. FIG. 4 is a schematic diagram showing the principle of how a nanosheet is cut.

[0019] <Nanosheet unit cell> Nanosheet 210 is a sheet exfoliated from inorganic layered material 220, and has a thickness of several atomic layers. Types of inorganic layered material 220 and nanosheets will be described later. Nanosheet 210 of inorganic layered material 220 has a different unit cell shape depending on the type of inorganic layered material 220, but is classified into four types: two-dimensional rectangular lattice, two-dimensional square lattice, two-dimensional hexagonal lattice, and two-dimensional quadrilateral lattice.

[0020] FIG. 2 shows an inorganic layered material 220 having a unit cell 230 of a two-dimensional rectangular lattice, and a nanosheet 210 exfoliated from the material. For example, such an inorganic layered material includes titanium oxide, in which the larger atom in the unit cell is O (oxygen) and the smaller atom is Ti (titanium). For reference, FIG. 2 also shows a titanium oxide nanosheet (Ti 0.87 O2 0.52- The lattice length in the case of a nanosheet is shown.

[0021] In equation (1), L (m) may be the length of the unit cell 230 in the short direction as shown in FIG. 2, and the area S (m 2 ) is expressed as L×L×c (c is a constant, c>1). When the nanosheet is a two-dimensional square lattice, L is the length of one side, and S is L×L. When the nanosheet is a two-dimensional hexagonal lattice, L is the length of one side, and S is L×L×sin60°. When the nanosheet is a two-dimensional quadrilateral lattice, L is the length in the short direction, and S is L×L×c×sinα (c and α are constants, c≧1, 0°<α<90°, except when c=1 and α=60°).

[0022] <Shape and size of nanosheets> The nanosheet 210 is composed of a plurality of unit cells 230, and is basically a similar shape to the unit cells 230. Therefore, the nanosheet 210 has a size of n times the size of each side. In this case, the length of one side of the nanosheet 210 (the length in the short direction in the case of a two-dimensional rectangular lattice) is n×L (m), and its area is n 2 × S. For example, nL is 460×10 -9 m or larger, 500 x 10 -6 It is advisable to select a range of m or less, which will facilitate cutting of the nanosheet. Furthermore, although this is related to the number x of chemical bonds, which will be described later, it is advisable to use nanosheets with an aspect ratio of preferably 800 or more and 8000 or less, which will allow the nanosheets to be cut efficiently.

[0023] <Chemical bonds inside nanosheets> In Figure 2, x is the number (mol) of chemical bonds that cross the crystal plane perpendicular to the horizontal plane of the nanosheet, assuming a nanosheet of the same size as the unit cell. Therefore, using the bond energy E (J / mol) between the atoms that make up the nanosheet and the length L (m) of the unit cell 230 (the length in the short direction in the case of a two-dimensional rectangular lattice), the total bond energy per unit cell length is E × x / L.

[0024] <Substrate surface shape> As shown in FIG. 3, in the present invention, substrate 300 has a concave-convex structure 310. The angle θ formed by the side of concave-convex structure 310 (also referred to as a slope for ease of understanding) with respect to main surface 320 of substrate 300 (i.e., the horizontal surface of substrate 300) is greater than 0° and less than 90°. When θ is 0°, concave-convex structure 310 is not formed. When θ is 90°, the nanosheet cannot adhere to concave-convex structure 310, and therefore cannot be cut. The angle θ is preferably in the range of 10° to 30°. Within this range, the nanosheet adheres to the slope of concave-convex structure 310, generating a tensile stress, as described below, which promotes cutting.

[0025] Since nanosheet 210 needs to be adsorbed beyond the distance d between the peaks of uneven structure 310, the length of nanosheet 210 (the length in the short direction in the case of a two-dimensional rectangular lattice) must be greater than d (see equation (2) above). If nL≦d, nanosheet 210 exceeds the distance d and cannot be adsorbed onto the slopes of uneven structure 310, and nanosheet 210 cannot be cut.

[0026] <Adhesion energy> Next, the adhesive energy of the nanosheet 210 is calculated. Here, the adhesive energy of the nanosheet 210 includes the adhesive energy A1 (J / m 2 ), and the adhesion energy A2 (J / m 2 ) exists, and this difference becomes the adhesion energy when the nanosheet 210 present on the organic solvent surface adheres to the substrate 300.

[0027] The adhesion energies A1 and A2 can be calculated using the surface free energies of the nanosheet 210, the substrate 300, and the organic solvent. The surface free energy may be a literature value or a value measured by contact angle measurement. In this specification, water and methylene iodide were used as probe liquids, and contact angle measurements were performed using a contact angle meter (DMe-210, manufactured by Kyowa Interface Science Co., Ltd.).

[0028] The surface free energies of the nanosheet 210, the substrate 300, and the organic solvent are G1, G2, and G3 (J / m), respectively. 2 ), A1 and A2 can be expressed as follows: A1=2×√(G1×G2) A2=2×√(G1×G3) Here, A1>A2 is satisfied. As a result, the nanosheet 210 is transferred from the organic solvent to the substrate 300.

[0029] <Tensile stress acting on nanosheets when adsorbed to a substrate> By the above-mentioned step S130, a nanosheet monolayer film 420 (FIG. 4) is formed on the surface of the substrate 300, in which the nanosheets floating on the surface of the organic solvent 410 are accumulated, and as the organic solvent 410 evaporates, the nanosheet monolayer film 420 is transferred to the substrate 300. Note that in FIG. 4, the details are enlarged for clarity, so the nanosheet monolayer film 420 is a single nanosheet, but it should be understood that the nanosheet monolayer film is composed of nanosheets arranged horizontally across the entire substrate.

[0030] That is, the adhesive energy A2 between the nanosheet 210 and the organic solvent 410 is lost, and adhesive energy A1 between the nanosheet 210 and the substrate 300 is generated. As described above, A1>A2 is satisfied, so the adhesive energy difference at this time is A1-A2. The adhesive force based on this adhesive energy difference acts in a direction perpendicular to the slope of the concave-convex structure 310 of the substrate 300. This horizontal component becomes the tensile stress T(J) acting on the nanosheet 210. The tensile stress T is expressed by the following equation as the product of the horizontal component of the adhesive energy difference and the area of ​​the nanosheet. T = (A1-A2) × sinθ × n 2 ×S ···(A)

[0031] When θ = 0° (substrate without a concave-convex structure), the tensile stress T is 0 and the nanosheet is not cut. Also, when θ = 90°, the nanosheet cannot follow the concave-convex structure of the substrate and the adhesive area with the concave-convex structure is small, so sufficient adhesive energy is not generated and the nanosheet is not cut.

[0032] <Energy required to cut nanosheets> To cut a nanosheet along a certain crystal axis, it is necessary to cut the chemical bonds that cross the crystal plane perpendicular to the horizontal plane of the nanosheet, including that crystal axis, from one end of the nanosheet to the other. The total energy D(J) is expressed as follows, using the sum of bond energies per unit cell length (E×x / L) and the length of the nanosheet nL: D=E×x / L×nL (B)

[0033] <Conditions for cutting nanosheets by adsorption onto the slope of a concave-convex structure> When a nanosheet is adsorbed onto the slope of the concave-convex structure 310 of the substrate 300, the tensile stress T acting on the nanosheet must exceed the energy required to cut the nanosheet along the crystal axis direction, i.e., T>D. Therefore, from formulas (A) and (B), it can be deduced that the nanosheet can be cut when the above formula (1) is satisfied.

[0034] Next, the components used in carrying out the method of the present invention will be described in detail. As described above, the nanosheets to be cut are not particularly limited as long as they are nanosheets exfoliated from an inorganic layered material. However, they are preferably exfoliated from an inorganic layered material selected from the group consisting of titanium oxide, niobium oxide, tantalum oxide, perovskite oxide, manganese oxide, cobalt oxide, molybdenum oxide, tungsten oxide, ruthenium oxide, phosphate, carbon, boron nitride, phosphorus, transition metal chalcogenides, double hydroxides, and rare earth hydroxides. Nanosheets exfoliated from these inorganic layered materials are well known and exhibit or are expected to exhibit excellent properties such as electronic and ionic conductivity, semiconductivity, insulation, high dielectric constant, ferroelectricity, ferromagnetism, fluorescence, and photocatalysis, making them advantageous for practical use. Those skilled in the art will be able to select the appropriate nanosheet depending on the application.

[0035] The titanium oxide may be any layered titanium oxide that can be exfoliated to produce titanium oxide nanosheets. The exfoliated titanium oxide nanosheets may be, for example, those having the general formula (Ti,M)O, as specifically shown in Table 1. 2-d (M is a metal element or a vacancy, and 0≦d≦1). Examples of layered titanium oxides that can produce such titanium oxide nanosheets include Na2Ti3O7, K2Ti4O9, and Cs2Ti5O 11 , A y (Ti,M)2O4 (A is at least one alkali metal element, and M is a monovalent to trivalent metal element), etc.

[0036] Niobium oxide is any layered niobium oxide that can be exfoliated to produce niobium oxide nanosheets. As shown in Table 1, exemplary niobium oxide nanosheets include Nb3O8 - , NbO 17 4- , TiNbO5 - , TiNbO 14 3- , Ti2NbO7 - Layered niobium oxides that produce such niobium oxide nanosheets include KNb3O8, K4Nb6O 17 , A[TiNbO5], A3[Ti5NbO 14], A[Ti2NbO7] (A is at least one alkali metal element), etc.

[0037] The tantalum oxide is any layered tantalum oxide that can be exfoliated to produce tantalum oxide nanosheets. Exemplary tantalum oxide nanosheets include TaO3, as shown in Table 1. - , TiTaO5 - Layered tantalum oxides that produce such tantalum oxide nanosheets include RbTaO3 and A[TiTaO5] (A is at least one alkali metal element).

[0038] The perovskite oxide is any layered perovskite oxide that can be exfoliated to produce perovskite nanosheets. The exfoliated perovskite nanosheets include, for example, perovskite oxides having the general formula A, as shown in Table 1. n-1 M n O 3n+1 - (A = Ca, Sr, Ba, Na, K, rare earth elements, M = Nb, Ta, Ti, 2≦n≦7). Layered perovskite oxides that produce such perovskite nanosheets have the formula A'[A n-1 M n O 3n+1 ], represented by Dion-Jacobson type (A': alkali metal), A'2[A n-1 M n O 3n+1 ], and (Bi2O2)[A n-1 M n O 3n+1 This compound system has a variety of chemical compositions, but typical layered perovskite oxides include KLaNbO, KCaNbO, and 10 , KSr2Nb3O 10 , CsCa2Nb3O 10 , KCa2NaNb4O 13 , KCa2Na2Nb5O 16 , KCa2Na3Nb6O 19 , Li2Eu 2 / 3 Ta2O7, K2La2Ti3O10 , (K 1.5 Eu 0.5 )Ta3O 10 etc.

[0039] Manganese oxide is any layered manganese oxide that produces manganese oxide nanosheets by monolayer exfoliation. Manganese oxide nanosheets include, for example, MnO2 as shown in Table 1 0.4- , general formula Mn 1-x M x O2 δ- (M is a metal element, for example, Ru, Co, 0 < x ≤ 0.5). There are layered manganese oxides that produce such manganese oxide nanosheets, which are represented by the general formula A p MnO2, A p Mn 1-x M x O2 (A is at least one kind of alkali metal element, 0 < p ≤ 1).

[0040] Cobalt oxide is any layered cobalt oxide that produces cobalt oxide nanosheets by monolayer exfoliation. Cobalt oxide nanosheets include, for example, those represented by CoO2 - . There are layered cobalt oxides that produce such cobalt oxide nanosheets, which are represented by the general formula A p CoO2 (A is at least one kind of alkali metal element, alkaline earth metal element or transition metal element, 0 < p ≤ 1).

[0041] Molybdenum oxide is any layered molybdenum oxide that produces molybdenum acid nanosheets by monolayer exfoliation. Here, as shown in Table 1, molybdenum acid nanosheets include those represented by MoO2 δ- . There are layered molybdenum oxides that produce such molybdenum acid nanosheets, which are represented by the general formula AMoO2 (A is at least one kind of alkali metal element and M is at least one kind of alkali metal element).

[0042] The tungsten oxide is any layered tungsten oxide that can be exfoliated to produce tungstate nanosheets. The tungstate nanosheets contain Rb as shown in Table 1. 2.4 W 11 O 35 1.6- 、 Cs4W 11 O 36 2- The layered tungsten oxides that produce such tungstate nanosheets include, for example, Rb4W 11 O 35、 Cs 6+z W 11 O 36 (0≦z≦0.31), etc.

[0043] The ruthenium oxide can be any layered ruthenium oxide that can be exfoliated to produce ruthenium oxide nanosheets. Exemplary ruthenium oxide nanosheets include RuO2, as shown in Table 1. 0.2- and RuO 2.1 0.2- The layered ruthenium oxide that produces such ruthenium oxide nanosheets is represented by the general formula ARuO2, A 0.2 RuO 2.1 (A is at least one alkali metal element).

[0044] The phosphoric acid can be any layered phosphoric acid that produces phosphorus oxide nanosheets by monolayer exfoliation. Here, an exemplary phosphorus oxide nanosheet is SbPO, as shown in Table 2. 14 3- , TaP2O8 - The layered phosphoric acid that produces such phosphorus oxide nanosheets is represented by the general formula A3Sb3P2O 14 , ATaP2O8 (A is at least one alkali metal element).

[0045] Carbon is any layered carbon that can be exfoliated to produce graphene (0.3 nm thick) or graphene oxide nanosheets, and graphite is known. Phosphorus is known to exfoliate to produce layered black phosphorus nanosheets. Transition metal chalcogenides include layered MX2 and layered Bi2Te3, which can be exfoliated to produce single-layer MX2 nanosheets (M is a transition metal, X is a chalcogen), and single-layer Bi2Te3 nanosheets. Boron nitride is known to exfoliate to produce single-layer hBN nanosheets (0.3 nm thick). For double hydroxides (LDHs) and rare earth hydroxides, see Table 2.

[0046] The process for producing nanosheets of titanium oxide, niobium oxide, tantalum oxide, perovskite oxide, manganese oxide, cobalt oxide, molybdenum oxide, tungsten oxide, ruthenium oxide, etc. is called soft chemical treatment, which is a combination of acid treatment and peeling treatment.

[0047] For example, if the powder of the inorganic layered material is brought into contact with an aqueous solution of an acid 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 obtained hydrogen ion exchanger is treated with tetrabutylammonium ions (TBA + When the nanoparticles are stirred in an aqueous solution containing basic substances such as tetrapropylammonium ions, tetraethylammonium ions, tetramethylammonium ions, n-propylamine, n-ethylamine, and ethanolamine, they become colloidal. In this way, the layers that made up the layered structure are peeled off one by one, yielding nanosheets.

[0048] Graphite oxide is 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 ultrasonic treatment. Graphene oxide contains oxygen functional groups on its surface, such as hydroxyl, carboxylic acid, epoxy, and carbonyl groups, and is hydrophilic, so it can be obtained as a colloidal aqueous solution.

[0049] [Table 1]

[0050] [Table 2]

[0051] In this specification, the side length, thickness, and aspect ratio of nanosheets before cutting are calculated as follows. Scanning electron microscope images were obtained for 160 nanosheets adsorbed on a substrate without a textured structure, and the longitudinal and lateral lengths were measured, with the average values ​​taken as the side lengths in the longitudinal and lateral directions. Cross-sectional profiles of 30 nanosheets were measured using a scanning probe microscope, and the average value was taken as the thickness. The aspect ratio was calculated by dividing the average side length (longitudinal length and lateral length) by the average thickness.

[0052] In step S110, the organic solvent is not particularly limited, but is preferably selected from the group consisting of dimethyl sulfoxide (DMSO), formamide, propanol, dimethylformamide, tetrahydrofuran, N,N-dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, and N,N'-dimethylpropyleneurea. These organic solvents are polar solvents that allow nanosheets to be easily dispersed. Among them, DMSO is preferred from the viewpoints of its moderate viscosity (which easily spreads to form a thin liquid film when forming the nanosheet monolayer film in step S130) and vapor pressure (which is easy to control because it dries relatively slowly).

[0053] In step S110, the concentration of nanosheets in the nanosheet dispersion is not particularly limited as long as a nanosheet monolayer film is formed in step S130, but is preferably in the range of 0.05% by mass to 5% by mass. This range promotes the formation of a nanosheet monolayer film. From the viewpoint of the dispersibility of nanosheets in the nanosheet dispersion, the concentration of nanosheets in the nanosheet dispersion is more preferably in the range of 0.05% by mass to 1% by mass.

[0054] In step S120, the substrate having the uneven structure may be prepared by vapor deposition, sputtering, chemical vapor deposition, sol-gel method, etc., as long as it satisfies the above-mentioned conditions, or may be surface-processed by photolithography, electrochemical etching, etc.

[0055] FIG. 5 is a diagram schematically illustrating an example of a substrate having a concave-convex structure. FIG. 6 is a diagram schematically illustrating another example of a substrate having a concave-convex structure.

[0056] The uneven structure of the substrate is not particularly limited as long as it satisfies the above-mentioned conditions, but is preferably pyramidal as shown in FIG. 5 or mountain-shaped as shown in FIG. 6. For example, in the case of a substrate made of indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), it is easy to form a triangular pyramidal uneven structure among pyramidal structures from the viewpoint of crystal structure. Furthermore, from the viewpoint of processing, the mountain-shaped uneven structure shown in FIG. 6 is easily obtainable. Note that the substrate material is not limited to the above, and any substrate can be used, such as a metal substrate such as Au or Pt, a semiconductor substrate such as Si or GaAs, a transparent substrate such as quartz or glass, an oxide single crystal substrate such as sapphire, MgO, SrTiO3, SrTiO3:Nb (Nb-doped SrTiO3), or SrRuO3, or an organic substrate such as plastic.

[0057] In step S120, the surface of the substrate may be subjected to a hydrophilic treatment 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, rinsing with ultrapure water, immersing in concentrated sulfuric acid for 15 to 45 minutes, and then rinsing with ultrapure water again. The hydrophilic treatment may be performed by irradiating with oxygen plasma or by irradiating with ultraviolet light in an ozone atmosphere for 15 to 30 minutes.

[0058] In step S120, the substrate surface may be coated with a polymer. It is preferable to select such a polymer that has the same charge as the selected nanosheet. This prevents the nanosheet from adhering to the slope of the substrate's uneven structure before tensile stress is applied, thereby enabling the nanosheet to be efficiently cut. Examples of polymers with cationic charges include poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), and polyallylamine hydrochloride (PAH). PDDA and PEI are preferred because of their versatility. Examples of polymers with anionic charges include poly(sodium styrene sulfonate) (PSS) and poly(sodium acrylate) (PAA). PSS is preferred because of its versatility.

[0059] In step S130, as explained with reference to Fig. 4, any method can be used to form a nanosheet monolayer film on a substrate having a textured structure via a liquid film made of an organic solvent in a nanosheet dispersion, but more specifically, a liquid film made of an organic solvent is formed on the substrate, and nanosheets floating on the liquid film surface are transferred to the slopes of the textured structure as the organic solvent evaporates, thereby forming a nanosheet monolayer film. As such a method, preferably, a spin coating method or a Langmuir-Blodgett (LB) method can be used.

[0060] The amount of nanosheet dispersion to be dropped, the rotation speed, and the spin coating time in the spin coating method vary depending on the type, size, and concentration of the selected nanosheet. However, within the above-mentioned concentration range, for example, 5 μL / cm 2 More than 30μL / cm 2The following ranges of drop amount, rotation speed of 500 rpm to 8000 rpm, and spin coating time of 1 minute to 15 minutes can be used.

[0061] The resting time, compression speed, surface pressure, and lifting speed of the substrate in the LB method vary depending on the type, size, and concentration of the selected nanosheet. However, within the above-mentioned concentration range, the resting time is, for example, between 10 and 60 minutes, and the lifting speed is 0.1 mm / sec. -1 More than 1mmsec -1 Compression speed below 5mNm -1 More than 20mNm -1 Surface pressure in the range below, 0.5mmsec -1 More than 5mmsec -1 The following lifting speeds can be used:

[0062] Simply forming a nanosheet monolayer film in this manner cuts the nanosheet along its crystal axis orientation, but the nanosheet monolayer film may be dried following step S130, which can facilitate cutting of the nanosheet. Alternatively, step S120 can usually be performed at room temperature (a temperature range of 10°C to 35°C) in the atmosphere, but the substrate may be heated during spin coating, or the trough may be heated during LB method.

[0063] Nanosheets cut in this way have edges aligned with the crystal axis direction, and have rectangular or triangular shapes that reflect the crystal structure. Because all sides have edges aligned with the crystal axis direction, the properties of the nanosheets (e.g., ferroelectricity, catalytic activity, chromism, ferromagnetism, etc.) can be further enhanced, making them suitable for device applications.

[0064] These edges are preferably oriented in the

[01] and

[10] directions, and the longitudinal length of the nanosheet after cutting is preferably in the range of 500 nm to 5 μm. This range allows for use in devices. The edge orientation can be measured using high-resolution electron microscopy or electron diffraction, but it can also be determined more simply from the crystal structure and unit cell of the nanosheet used. For example, if the nanosheet used is titanium oxide, it grows along the c-axis, so the longitudinal direction of the unit cell can be determined as the

[01] orientation parallel to the c-axis, and the transverse direction perpendicular to this can be determined as the

[10] orientation parallel to the a-axis. Furthermore, the size of the nanosheet after cutting can be determined from a scanning electron microscope image of the nanosheet on the substrate, as described above.

[0065] Although the present specification has described in detail a method for cutting nanosheets using a nanosheet dispersion in which nanosheets are dispersed in an organic solvent, if a nanosheet monolayer film can be formed by transferring nanosheets to the slope of a textured surface via a liquid film made of an organic solvent, it is not necessary to use a nanosheet dispersion. For example, if a liquid film made of an organic solvent is formed by dropping an organic solvent onto a substrate having a textured structure, and then a nanosheet produced on another substrate by chemical vapor deposition (CVD) is transferred onto the liquid film, the nanosheet can be similarly cut as the organic solvent evaporates.

[0066] Next, using typical nanosheets, we investigated nL that satisfies the above formula (1). Typical nanosheets used were graphene, molybdenum sulfide, titanium oxide, and niobium oxide. Indium tin oxide (ITO) with a triangular pyramidal uneven structure was used as the substrate, and dimethyl sulfoxide (DMSO) was used as the organic solvent. The angle θ between the main surface of the substrate and the slope (side) of the uneven structure was θ = tan -1 The surface free energy G2 of the substrate and the surface free energy G3 of DMSO were 74.7 × 10 -3 J / m 2 (calculated value from contact angle measurement) and 43.5 × 10 -3 J / m 2(H. Lawrence Clever et al., The Journal of Physical Chemistry, 1963, 67, 918).

[0067] The unit cell shape of graphene, molybdenum sulfide, titanium oxide, and niobium oxide, the length of one side of the unit cell L (m), the bond energy E (J / mol), the number of chemical bonds x (mol) crossing the crystal plane perpendicular to the horizontal plane of the nanosheet when a nanosheet of the same size as the unit cell is assumed, and the surface free energy G1 (J / m 2 The length of one side of the nanosheet, nL (m), was calculated from equation (1) using the surface free energies G2 and G3 of the substrate and organic solvent. The results are shown in Table 3. [Table 3]

[0068] According to Table 3, the substrate having a predetermined uneven structure (here, ITO, and θ=tan -1 It was shown that when using an organic solvent (DMSO in this case) and a sieving angle (θ(0.4)≒22°) to cut a nanosheet along its crystal axis, there is a limit to the length of one side of the nanosheet, nL (m), and that this length varies depending on the type of nanosheet.

[0069] Next, we investigated the relationship between the adhesive energy and the energy required for cutting using titanium dioxide nanosheets. Here, titanium dioxide nanosheets are attached to an ITO substrate via an organic solvent (DMSO). FIG. 7 is a diagram showing the relationship between the energy required to cut a titanium oxide nanosheet and the adhesive energy.

[0070] Figure 7 shows the unit cell (0.38 nm × 0.30 nm) of the titanium dioxide nanosheet shown in Figure 2 again. Here, we assume that all vacancies at Ti sites in the titanium dioxide nanosheet are occupied by Ti atoms. The number of Ti-O bonds crossing the lattice plane perpendicular to the horizontal plane of the nanosheet, including the

[01] axis within the unit cell, is 2, which corresponds to a density of 6.67 nm per length in the

[01] axis direction. -1 is equivalent to

[0071] Using the bond energy of the Ti-O bond (670 kJ / mol), the energy required for cutting is the density per unit length along the

[01] axis (6.67 nm ) because the nanosheet is a similar shape to the unit cell. -1 ), and therefore increases linearly with the length of the nanosheet, as shown in Figure 7.

[0072] On the other hand, the adhesion energy between the nanosheet and the ITO substrate is 148 mJ / m 2 The adhesive energy between the nanosheet and DMSO is 113 mJ / m 2 The difference in adhesive energy between these two (35 mJ / m 2 ) acts as a tensile stress, and since this tensile stress is proportional to the area of ​​the nanosheet, it increases quadratically with the length of the nanosheet, as shown in Figure 7.

[0073] According to Figure 7, the size of the nanosheet in the short direction is 460 × 10 -9 At less than 460 × 10 m, the adhesion energy is less than the energy required for scission, so the nanosheet is not scissioned along the

[01] axis. -9 Above m, the adhesion energy exceeds the energy required for scission, and the nanosheet is scissed along the

[01] axis.

[0074] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples. [Example]

[0075] [Nanosheet] 1. Large titanium oxide nanosheets (Ti 0.87 O2 0.52- Preparation of NS1 Layered titanium oxide (K 0.8 Ti 1.73 Li 0.27 O4) as a starting material, large titanium oxide nanosheets (Ti 0.87 O2 0.52- ) was prepared.

[0076] Layered titanium oxide (K 0.8 Ti 1.73 Li 0.27 O4) was synthesized by solid-phase synthesis. The 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 out based on the stoichiometric ratio of TiO2:K2CO3:Li2CO3 = 1:0.23:0.078 and ground and mixed in an alumina mortar for 60 minutes. During firing, a portion of the alkali metal carbonate, K2CO3, was evaporated, so a 5% molar excess was added. The ground and mixed raw material powders were placed in a platinum crucible and pre-fired in an electric furnace at 900°C for 1 hour. The pre-fired raw material powders were then ground and mixed again in the alumina mortar for 30 minutes. The ground and mixed raw material powders were then placed in a platinum crucible and fired at 1000°C for 20 hours to produce layered titanium oxide K2CO3. 0.8 Ti 1.73 Li 0.27 Got O4.

[0077] The obtained layered titanium oxide (0.2 g) and a hydrochloric acid solution (1 mol / L, 200 mL) were stirred in a beaker and subjected to acid treatment for 72 hours to obtain a hydrogen ion exchanger (H 1.07 Ti 1.73 The hydrogen ion exchanger was then transferred to tetrabutylammonium ion (TBA + ) / proton (H + The mixture was mixed at a ratio of 4 g / L with an aqueous solution of tetrabutylammonium hydroxide, the concentration of which was adjusted so that the ratio of Ti to Ti was 1, and the mixture was allowed to react at room temperature for 2 weeks. 0.87 O20.52- A milky white colloidal aqueous solution was prepared in which nanosheets with a thickness of approximately 1.2 nm and a length of 5 μm to 15 μm were dispersed. The concentration of the colloidal aqueous solution (stock solution) containing titanium oxide nanosheets was 0.25 mass %.

[0078] The colloidal aqueous solution was centrifuged (1500 rpm) to remove unexfoliated layered titanium oxide and re-stacked nanosheets, and the supernatant was collected and centrifuged again (10000 rpm). The precipitate was dispersed in dimethyl sulfoxide (DMSO) to prepare a nanosheet dispersion. The concentration of nanosheets in the nanosheet dispersion was determined by the absorbance of the dispersion, the molar absorption coefficient of the nanosheets dispersed in DMSO, and the density of DMSO (ρ = 1.10 g cm). -3 The nanosheet monolayer film was formed on a substrate without a textured structure by spin coating, and the length and thickness of the nanosheet were evaluated using a scanning electron microscope and a scanning probe microscope.

[0079] The results were as follows: Longitudinal length: average 9.0 μm, standard deviation 3.8 μm, maximum 20.1 μm, minimum 2.0 μm Short side length: average 4.9 μm, standard deviation 2.4 μm, maximum 16.8 μm, minimum 1.2 μm Thickness: 1.2 nm Aspect ratio (average length / thickness): Longitudinal 7500, Transverse 4080

[0080] 2. Small titanium oxide nanosheets (Ti 0.87 O2 0.52- Preparation of NS2 Layered titanium oxide (K 0.8 Ti 1.73 Li 0.27 O4) as a starting material, small titanium oxide nanosheets (Ti 0.87 O2 0.52-) was prepared. The preparation of NS1 was the same as that of NS1, except that the mixture was shaken horizontally at 180 rpm when reacting with the tetrabutylammonium hydroxide aqueous solution, and therefore a detailed explanation will be omitted. In this way, a milky white colloidal aqueous solution was prepared in which nanosheets with a thickness of approximately 1.2 nm and lengths of 200 to 800 nm were dispersed. The concentration of the colloidal aqueous solution (stock solution) containing titanium oxide nanosheets was 0.31% by mass. The colloidal aqueous solution was centrifuged (15,000 rpm), and the precipitate was dispersed in DMSO to obtain a nanosheet dispersion. The size of the nanosheets was evaluated in the same manner, and the results were as follows:

[0081] Side length: average 439 nm, standard deviation 148 nm, maximum 940 nm, minimum 145 nm (Since the ratio of the longitudinal length to the lateral length is close to 1, the area of ​​the nanosheet was measured, and the length of one side of a square with the same area was taken as the length of the nanosheet.) Thickness: 1.2 nm Aspect ratio (average length / thickness): 366

[0082] 3. Perovskite nanosheets (Ca2Nb3O 10 - Preparation of NS3 Layered perovskite oxide (KCa2Nb3O 10 ) as a starting material, perovskite nanosheets (Ca2Nb3O 10 - ) was prepared.

[0083] Layered perovskite oxide (KCa2Nb3O 10) was synthesized by solid-phase synthesis. Raw material powders of K2CO3 (Rare Metallic, purity 99.99%), CaCO3 (Rare Metallic, purity 99.99%), and Nb2O5 (Rare Metallic, purity 99.99%) were weighed based on the stoichiometric ratio of K2CO3:CaCO3:Nb2O5 = 1:4:3 and ground and mixed in an alumina mortar for 60 minutes. Note that, because some of the alkali metal carbonate K2CO3 evaporates during firing, a 10% molar excess was added. The ground and mixed raw material powders were placed in a platinum crucible and pre-fired in an electric furnace at 900°C for 1 hour, followed by firing at 1200°C for 12 hours to produce the layered perovskite oxide KCa2Nb3O 10 obtained.

[0084] The obtained layered perovskite oxide (5 g) and a nitric acid aqueous solution (5 mol / L, 200 mL) were stirred in a beaker and subjected to acid treatment for 72 hours to obtain a hydrogen ion exchanger (HCaNbO 10 Then, hydrogen ion exchanger (0.4 g) was added to TBA + / H + The mixture was mixed with 100 mL of tetrabutylammonium hydroxide solution, the concentration of which was adjusted so that the ratio of 1 was 1, and the mixture was allowed to react at room temperature for 2 weeks, resulting in a composition of CaNbO. 10 - A milky white colloidal aqueous solution was prepared in which nanosheets with a thickness of approximately 2.2 nm and lengths of 1 μm to 4 μm were dispersed. After removing unexfoliated material by centrifugation (3000 rpm), the concentration of the colloidal aqueous solution (stock solution) containing perovskite nanosheets was 0.26 mass%. The colloidal aqueous solution was then centrifuged (15000 rpm), and the precipitate was dispersed in DMSO to obtain a nanosheet dispersion. The size of the nanosheets was similarly evaluated, with the results shown below.

[0085] Side length: average 1.9 μm, standard deviation 0.7 μm, maximum 4.5 μm, minimum 0.5 μm (Since the ratio of the longitudinal length to the lateral length is close to 1, the area of ​​the nanosheet was measured, and the length of one side of a square with the same area was taken as the length of the nanosheet.) Thickness: 2.2nm Aspect ratio (average length / thickness): 864

[0086] [substrate] Substrates 1 to 4 were a glass plate (Furuuchi Chemical Co., Ltd.) with unpolished ITO (200 nm thick, surface roughness Ra = 5.0 nm), a glass plate (Kuramoto Manufacturing Co., Ltd.) with polished ITO (180 nm thick, surface roughness Ra = 0.38 nm), a polished Si(100) wafer (SUMCO Corporation) with surface roughness Ra = 0.060 nm, and a glass plate (AGC Corporation) with unpolished FTO (1 μm thick, surface roughness Ra = 27 nm). All substrates were 30 mm × 30 mm in size. The surfaces of these substrates were observed in tapping mode using a scanning probe microscope (SPM, Hitachi High-Tech Corporation, E-sweep) equipped with a cantilever (SI-DF20). AFM images were obtained, and the surface roughness was measured. The results are shown in Figures 8 to 10.

[0087] FIG. 8 is a diagram showing an AFM image of the substrate 1. FIG. 9 is a diagram showing an AFM image of the substrate 2. FIG. 10 is a diagram showing cross sections of the surface shapes of the substrate 1 and the substrate 2. As shown in FIG.

[0088] 8 and 9 show grayscale AFM images of substrates 1 and 2. Unpolished substrate 1 had an uneven structure with a maximum height difference of 20 nm, while polished substrate 2 had a flat surface with a height difference of only about 1 nm. Although not shown, substrate 4 had a height difference of about 100 nm, but showed an AFM image similar to that of substrate 1, and substrate 3 showed an AFM image similar to that of substrate 2.

[0089] The uneven structure of the substrate 1 was investigated in detail from the cross-sectional profile of the substrate 1 in Figure 10. The cross-sectional profile can be considered as having 20 nm high mountain peaks arranged at 100 nm intervals (d). Therefore, the angle between the main surface of the substrate 1 and the slope of the peak is θ = tan -1 (20 / 50)=tan -1Although not shown, the substrate 4 can be considered as having 100 nm high mountain peaks arranged at intervals (d) of 500 nm, so the angle between the main surface of the substrate 4 and the slope of the peaks is θ=tan -1 (100 / 250)=tan -1 The result is (0.4).

[0090] Although not shown, it was confirmed using a scanning electron microscope that ITO has a cubic crystal structure and forms a triangular pyramidal uneven structure on the surface of the glass plate. This indicated that the uneven structure was triangular pyramidal. FTO has a tetragonal crystal structure and forms a triangular pyramidal uneven structure on the surface of the glass plate. The above results are summarized in Table 4.

[0091] [Table 4]

[0092] [Example 1 to Example 6] In Examples 1 to 6, a nanosheet monolayer film was formed on a substrate using a nanosheet dispersion. Specifically, a nanosheet dispersion was prepared by dispersing nanosheets NS1 to NS3 in DMSO (Step S110 in Figure 1). Next, a substrate was prepared (Step S120 in Figure 1). The above-mentioned substrate was washed with acetone and subjected to oxygen plasma treatment using a plasma ion bombarder (PiB-20, manufactured by Vacuum Device Co., Ltd.) to make the surface hydrophilic. The hydrophilized clean substrate was placed in the sample holder of a spin coater (MS-B100, manufactured by Mikasa Co., Ltd.), and the nanosheet dispersion was dropped onto the substrate surface to form a nanosheet monolayer film (Step S130 in Figure 1). The nanosheet concentrations in the nanosheet dispersion and the spin coating conditions are summarized in Table 5 for simplicity. The samples of Examples 1 to 6 thus obtained may be referred to as NS1 / substrate 1, NS1 / substrate 2, NS1 / substrate 3, NS1 / substrate 4, NS2 / substrate 1, and NS3 / substrate 4, respectively.

[0093] [Table 5]

[0094] The surfaces of the samples of Examples 1 to 6 were observed using an SPM. The obtained AFM images and cross sections of the surface shapes are shown in FIGS. FIG. 11 shows an AFM image of NS1 / substrate 1 in Example 1. FIG. 12 shows an AFM image of NS1 / substrate 2 in Example 2. FIG. 13 is a diagram showing cross sections of the surface shapes of NS1 / substrate 1 in Example 1 and NS1 / substrate 2 in Example 2.

[0095] The AFM images in Figures 11 and 12 are similar to those in Figures 8 and 9, respectively. Looking closely at Figure 12, rectangular nanosheets are observed, suggesting that a nanosheet monolayer film in which nanosheets are densely arranged was obtained by the spin coating method.

[0096] The cross-sectional profiles of the samples of Example 1 and Example 2 in Figure 13 were similar to those of Substrate 1 and Substrate 2 in Figure 10, respectively. This indicates that the nanosheet monolayer film formed by spin coating conforms to the shape of the substrate. Although not shown, the AFM images and cross-sectional profiles of the samples of Example 3 and Example 4 also reflected the shape of the substrate.

[0097] The surfaces of the samples of Examples 1 to 6 were observed using a scanning electron microscope (SEM, manufactured by JEOL Ltd., JSM-7001F). The accelerating voltage during observation was 5 kV. The observation results are shown in FIGS. 14 to 16.

[0098] FIG. 14 is a diagram showing an SEM image of NS1 / Substrate 1 in Example 1. FIG. 15 is a diagram showing an SEM image of NS1 / substrate 2 in Example 2. FIG. 16 shows an SEM image of NS2 / substrate 1 in Example 5 and the length distribution of nanosheets on the substrate.

[0099] Figure 14 shows SEM images of NS1 / Substrate 1 in Example 1 at various magnifications. Focusing on the high magnification of Figure 14, it can be seen that gaps (cracks) have appeared within each nanosheet, and that the nanosheet has been cut into a lattice pattern.

[0100] It is noteworthy that cracks occurred perpendicular to each of the four edges of the nanosheet before cutting, while all nanosheets maintained their rectangular shape after cutting. Since layered titanium oxide is known to grow most rapidly along the c-axis, the longitudinal direction of the nanosheet is parallel to the c-axis (

[01] orientation), and the transverse direction is parallel to the a-axis (

[10] orientation). Furthermore, the longitudinal length of the nanosheet after cutting was an average of 1,000 nm.

[0101] Although not shown, the SEM images of NS1 / substrate 4 in Example 4 and NS3 / substrate 4 in Example 6 were similar to those in Figure 14, and showed that the nanosheets were cut. After cutting, each nanosheet had a rectangular shape, with edges oriented in the

[01] and

[10] directions, and an average longitudinal length of 1000 nm.

[0102] On the other hand, when a substrate without a concave-convex structure was used, the nanosheet was not cut, as shown in Figure 15. Similarly, in the case of NS1 / substrate 3 in Example 3, the nanosheet was not cut.

[0103] The length distribution of nanosheets on substrates in Figure 16 also shows the results when small titanium oxide nanosheets (NS2) were spin-coated onto a substrate without a textured structure (Substrate 2). As in Example 2, nanosheets were not cut on Substrate 2, corresponding to the length distribution of nanosheets in the dispersion. On the other hand, on substrates with a textured structure, when the average nanosheet side length (439 nm) was below the cuttable length threshold (460 nm), some nanosheets were not cut. Upon adsorption onto the textured substrate, the proportion of nanosheets decreased when the side length was in the 400-500 nm range and longer, suggesting that nanosheets were cut. This length range coincided with the cuttable length threshold calculated by theoretical calculation, demonstrating the effectiveness of satisfying the above formulas (1) and (2). The above results are summarized in Table 6.

[0104] [Table 6]

[0105] Furthermore, it was found that Examples 1, 4, and 6 satisfy formula (1) and formula (2), but in Example 5, some of the nanosheets do not satisfy formula (1). From the above, it was shown that nanosheets that satisfy formula (1) and formula (2), an organic solvent, and a substrate are used to prepare a nanosheet dispersion in which nanosheets are dispersed in an organic solvent, a substrate with a textured structure is prepared, and a nanosheet monolayer film made of nanosheets is formed on the substrate, thereby enabling the nanosheets to be cut along the crystal axis direction.

[0106] [Example 7] Example 7 was the same as Example 1, except that a substrate 1 coated with an anionic polymer was used. The anionic polymer was sodium polystyrene sulfonate (PSS), and the hydrophilically treated substrate 1 was immersed in an aqueous PSS solution to coat it with the polymer. When an SEM image of the NS1 / polymer / substrate 1 obtained in Example 7 was observed, a similar appearance to that shown in Figure 14 was observed, indicating that the nanosheets had been cut. [Industrial Applicability]

[0107] The method of cutting nanosheets along their crystal axis orientation according to the present invention allows for the secondary processing of many nanosheets at the same time on a substrate-size scale. The ability to cut nanosheets along their crystal axis orientation will enable the fabrication of more precise devices and the study of physical properties arising from nanosheet edges, which is expected to lead to the development of new applied research into nanosheets. [Explanation of symbols]

[0108] 210 nanosheets 220 Inorganic layered materials 230 unit cells 300 boards 310 Uneven structure 320 Main Surface 410 Organic Solvents 420 Nanosheet monolayer film

Claims

1. A method for cutting a nanosheet along a crystal axis direction, comprising: Preparing a nanosheet dispersion in which nanosheets are dispersed in an organic solvent; providing a substrate having a relief structure; forming a nanosheet monolayer film made of the nanosheets on the substrate using the nanosheet dispersion; It encompasses The method, wherein the nanosheets, the organic solvent, and the substrate satisfy the following formula: nL>(x×E×L) / (S×(A1-A2)×sinθ) nL>d Here, L is the length (m) of one side of the unit cell of the nanosheet, nL is the length (m) of the nanosheet in the short direction, and S is the area (m ２ ), E is the bond energy (J / mol) between atoms constituting the nanosheet, x is the number (mol) of chemical bonds crossing the crystal plane perpendicular to the horizontal plane of the nanosheet when the nanosheet is assumed to be the same size as the unit cell, and A1 is the adhesion energy (J / m ２ ), and A2 is the adhesion energy between the nanosheet and the organic solvent (J / m ２ ) where θ is the angle between the side surface of the uneven structure of the substrate and the main surface of the substrate, and is greater than 0° and less than 90°, and d is the distance (m) between convex portions of the uneven structure.

2. The method according to claim 1 , wherein the uneven structure is a mountain or pyramid shape.

3. 3. The method of claim 1 or 2, wherein the nanosheets are exfoliated from an inorganic layered material selected from the group consisting of titanium oxide, niobium oxide, tantalum oxide, perovskite oxide, manganese oxide, cobalt oxide, molybdenum oxide, tungsten oxide, ruthenium oxide, phosphate, carbon, boron nitride, phosphorus, transition metal chalcogenides, double hydroxides, and rare earth hydroxides.

4. The method according to any one of claims 1 to 3, wherein the organic solvent is at least one selected from the group consisting of dimethyl sulfoxide, formamide, propanol, dimethylformamide, tetrahydrofuran, N,N-dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, and N,N'-dimethylpropyleneurea.

5. The nL is 460 × 10 －９ Larger than m, 500 x 10 －６ The method according to any one of claims 1 to 4, wherein the range is m or less.

6. The method according to any one of claims 1 to 5, wherein the angle θ is in the range of 10° to 30°.

7. The method according to any one of claims 1 to 6, wherein the concentration of the nanosheets in the nanosheet dispersion is in the range of 0.05% by mass or more and 1% by mass or less.

8. The method according to any one of claims 1 to 7, wherein the formation of the nanosheet monolayer film comprises forming a liquid film made of the organic solvent on the substrate, floating the nanosheet on the surface of the liquid film, and transferring the nanosheet to the side surface of the uneven structure as the organic solvent evaporates.

9. The method according to any one of claims 1 to 8, wherein the nanosheet monolayer film is formed using a spin coating method or a Langmuir-Blodgett (LB) method.

10. The method according to any one of claims 1 to 9, wherein the substrate is polymer coated.

11. The method according to any one of claims 1 to 10, wherein the aspect ratio of the nanosheet is in the range of 800 or more and 8000 or less.

12. The method according to any one of claims 1 to 11, further comprising drying the nanosheet monolayer film following the formation of the nanosheet monolayer film.

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