Aqueous solution, method for producing the same, and use thereof

Abstract

The present invention relates to an aqueous solution, a method for producing the same, and its use. The aqueous solution contains an alkaline earth metal added in the form of a water-soluble salt, manganese present as a citrate complex of manganese having at least predominantly an oxidation state of +3 or +4, and optionally a lanthanide present in the form of a water-soluble complex. The precursor aqueous solution contains metals in a stoichiometric ratio suitable for forming a film of a composite inorganic metal oxide by chemical solution deposition (CSD). The composite inorganic metal oxide can be used as a memristor material, generally in microelectronic devices, magnetic devices, and spintronic devices, in solid oxide fuel cells, in magnetic refrigeration, and in the field of biomedicine, and as a catalyst.

Description

Technical Field

[0001] The present invention relates to an aqueous solution of a metal compound. In particular, the present invention relates to a novel aqueous solution of a water-soluble metal compound such as a metal salt or a metal complex. This can be used, for example, for the production of metal oxide films by liquid phase precipitation. The present invention also relates to a method for producing an aqueous solution containing a dissolved water-soluble metal compound and a metal complex, as well as the use of the aqueous solution.

Background Art

[0002] WO2121 / 152215A1 discloses the production of a memristor material having the formula Gd l-x Ca x MnO3 (or abbreviated "GCMO"). Here, x is a value greater than 0 and less than 1 from starting materials formed by pulsed laser deposition of specific solid materials, namely gadolinium(III) oxide, calcium carbonate, and manganese(IV) oxide.

[0003] In pulsed laser deposition, typically abbreviated as "PLD", the starting material is evaporated by a high-power pulsed laser beam hitting a target in a vacuum chamber. Inside the chamber, the vapor of the target will deposit as a thin film on a substrate such as a silicon wafer facing the target. PLD is useful for the production of high-quality thin films. However, the PLD technique is energy-intensive and has some limitations for its large-scale application, such as its use at the industrial level.

[0004] Therefore, there is a need for a method for producing perovskite-manganite lanthanide oxides that are useful as thin films that can be easily upscaled according to wafer size, particularly for the industrial production of GCMO and similar complex metal oxides, such as GCMO films.

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention aims to eliminate at least some of the problems in the art by supplying an aqueous solution of a metal compound and a complex useful as a precursor for a metal oxide film. This film can be manufactured by depositing a layer from the solution by chemical solution deposition and thermally curing the layer.

Means for Solving the Problems

[0006] In a first aspect, this specification relates to an aqueous solution. The aqueous solution contains an alkaline earth metal added in the form of a water-soluble salt and manganese present at least mainly as Mn(III) and / or Mn(IV) complexes, exhibits a molar concentration of 0.1 to 5 with respect to the alkaline earth metal, and is formulated in a precursor solution for use in the chemical solution deposition of a thin film of a material having the formula I, R 1-x A x MnO3I and is incorporated into a precursor solution for use in the chemical solution deposition of a thin film of a material having the formula I, In the formula, R represents a lanthanide, A represents an alkaline earth metal, and, x has a value in the range of 0.5 to 1.0, The above-mentioned material is crystalline.

[0007] Optionally, the solution contains a lanthanide present in the form of a water-soluble complex.

[0008] In a second aspect, the present specification relates to a method for preparing an aqueous solution containing an element selected from alkaline earth metals, manganese, and optionally a lanthanide. The method includes supplying a first aqueous solution of a water-soluble salt of an alkaline earth metal, and further includes the following, namely, a second aqueous solution of manganese present as a citrate complex of manganese having at least mainly an oxidation state of +3 and / or +4 and optionally a water-soluble complex of a lanthanide, and a third aqueous solution of manganese present as a citrate complex of manganese having an oxidation state of +3 and / or +4 and a water-soluble complex of a lanthanide, and supplying at least one of them. Then, the first aqueous solution and at least one of the second aqueous solution and the third aqueous solution are mixed at a predetermined ratio to supply an aqueous precursor solution.

[0009] In a third aspect, the present specification relates to the use of an aqueous solution as an aqueous precursor solution for preparing a thin film, particularly an epitaxial thin film, by chemical solution deposition.

[0010] In a fourth aspect, the present specification relates to Formula I, R 1-x A x MnO3I relates to preparing an epitaxial thin film having the formula by depositing it on a substrate for supplying a memristor. Wherein, R represents Eu, Gd, Tb, Sm, Pr, La, and Nd, A represents an alkaline earth metal, and, x has a value in the range of 0.5 to 1.0.

[0011] In a fifth aspect, the present specification relates to a method for manufacturing a thin film on a substrate. The method includes the following. Supplying a substrate, Making the substrate hydrophilic, Preparing a thin film by chemical solution deposition of an aqueous solution as described herein.

[0012] More specifically, the present invention is characterized by what is described in the independent claims.

[0013] Considerable advantages are obtained by the present invention.

[0014] Therefore, by the present invention, an aqueous solution of a metal containing the metal in an appropriate stoichiometric ratio for a solution usable in Chemical Solution Deposition (CSD) can be supplied for manufacturing an inorganic thin film.

[0015] The water-based chemical solution deposition method can be easily scaled to wafer size, enabling the production of industrial-sized metal oxide films containing lanthanides, alkaline earth metals, and at least one manganese or similar material, such as metal oxide films of gadolinium, calcium, and manganese (also abbreviated as GCMO films), and metal oxide films of lanthanum, calcium, and manganese (also abbreviated as LCMO films).

[0016] High solubility of metal compounds and complexes is achieved. This enables reaching high concentrations of metals such that, for example, film thicknesses in the range of 10 to 100 nm can be easily obtained by CSD. Since citrate can be the only complexing agent present in the solution, it allows for the omission of other complexing agents such as EDTA, NTA, or DTPA that would beneficially contribute to the presence of free metal compounds in the solution. The amount of citrate ligands is preferably adjusted such that no free ligands are present. The absence of EDTA, NTA, DTPA and similar complexing agents also means that the aqueous solution does not contain large molecules. Thus, with this aqueous solution, thinner and better quality films can be produced than with known solutions. Therefore, it is not necessary to use the sol - gel method. As a result, the produced film has the further advantage of not containing a significant amount of organic materials that need to be burned before the film is complete. A further advantage is that this aqueous solution does not need to use any toxic substances. Additionally, the supplied solution exhibits viscosity and wetting properties that make it suitable for coating the solution onto a surface or substrate. Examples of useful methods include dip coating, aerosol deposition, inkjet printing, roll - on roll transfer and spin coating.

[0017] All other elements present in the solution, namely H, C, N, and (partially) O, can be removed by heat - treating the deposited layer. Thus, high - purity films are obtained by heating and curing (annealing) the films deposited by CSD as discussed herein.

[0018] Additionally, this precursor solution is water - based. Thereby, the use of environmentally harmful solvents or compounds, especially organic solvents or complexing agents such as EDTA or NTA, can be avoided. This solution leaves no residue of excess elements of the solvents or reagents used in the preparation of the precursor solution, leaving only the desired complex metal oxide. Furthermore, this technology enables the use of non - toxic metal - organic compounds.

[0019] Further features of the preferred embodiments of the present technology will be discussed in more detail in the following detailed description.

Brief Description of the Drawings

[0020]

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Mode for Carrying Out the Invention

[0021] Definition Unless otherwise specified in this specification or obvious from the context, any percentage mentioned in this specification is expressed as a weight percentage based on the total weight of each composition.

[0022] In the context of this specification, "alkaline earth metal" has its conventional meaning. In particular, alkaline earth metals represent beryllium (Be), calcium (Ca), magnesium (Mg), barium (Ba), and strontium (Sr), and combinations thereof.

[0023] In the context of this specification, the term "manganese present as a citrate complex of manganese having an oxidation state of +3 and / or +4" particularly means Mn(III) citrate complex, Mn(IV) citrate complex, and combinations thereof.

[0024] In the context of this specification, "lanthanide" means the elements having atomic numbers from 57 to 71 in the periodic table of elements, i.e., from lanthanum to lutetium. These metal elements have the following symbols: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0025] Embodiments of the present technology relate to "complex" metal oxides. In particular, such composite metal oxides typically include oxides of gadolinium, or other lanthanides, calcium, or other alkaline earth metals, and manganese (including similar metals). Preferably, the present metal oxide is perovskite manganite. In one embodiment, they have an orthogonal perovskite structure.

[0026] As described later, the composite metal oxide can be used as a "memristive material".

[0027] In this context, the term "memristor" refers to an electronic component whose current conduction characteristics depend on previously received electrical conditions.

[0028] These memristive materials can be used, for example, in neuromorphic computing, distributed computing, edge computing, or processing devices related to storage devices, as well as in other applications and uses.

[0029] "Chemical Solution Deposition" refers to a coating or deposition method in which a coating is formed on a substrate from the corresponding components of a solution. In embodiments of the present technology, a solution of a water-soluble compound or complex of a lanthanide, an alkaline earth metal, and Mn is deposited on a substrate. Thereby, the corresponding composite metal oxide can be epitaxially grown on the substrate.

[0030] The solution is referred to as a chemical "precursor solution" and is typically used to create a film on a substrate.

[0031] Embodiments One embodiment of the present technology supplies an aqueous solution. The aqueous solution includes the following. An alkaline earth metal added in the form of a water-soluble salt, Manganese, which exists at least mainly as a manganese citrate complex, and a lanthanide, optionally present in the form of a water-soluble complex.

[0032] The metal compound or complex exists, if not completely, mainly in a dissolved or solvated form in the aqueous phase and is likely to form a clear solution of the metal compound or complex in water. However, a small portion of the metal present in a dispersed phase in water cannot be excluded.

[0033] In one embodiment, the aqueous solution generally contains the following, calculated from the total weight of the solution. by weight of a water-soluble alkaline earth metal salt, 0.5 to 40%, particularly 1 to 25%, or 1.5 to 20%, or 1.7 to 15%, particularly 2 to 10% or 2 to 7.5% or 2 to 5%, by weight of manganese, calculated as MnO2, 0.5 to 20%, particularly 1 to 15%, or 1.5 to 12.5%, or 2 to 10%, or 2.5 to 7.5% or 3 to 5%, and, by weight of a lanthanide, calculated as the corresponding oxide, 0.1 to 15%, particularly 0.2 to 10%, or 0.3 to 7.5%, or 0.4 to 6%, for example 0.5 to 5% or 0.5 to 4.5%.

[0034] In addition, the aqueous solution typically contains, calculated from the total weight of the solution, up to 50% by weight of citric acid, particularly up to about 1 to 40%, or 5 to 35% or 10 to 30%, preferably 15 to 25%, or 17.5 to 22.5%.

[0035] In one particular embodiment, the aqueous solution contains the following, calculated from the total weight of the solution. by weight of a water-soluble alkaline earth metal salt selected from calcium, barium or strontium, 1.5 to 15%, for example 2 to 10%, by weight of manganese, calculated as MnO2, 1 to 15%, for example, 2 to 10%, Calculated as the corresponding oxide, the weight of the lanthanide is 0.2 to 10%, for example 0.3 to 7.5%, and the aforementioned lanthanide is selected from Eu, Gd, Tb, Sm, Pr, La and Nd, particularly Gd, Eu, La or Pr. By weight of citric acid, 1 to 40%, for example 10 to 30%.

[0036] In order to prepare a thin film having the formula I (see below) where the value of x is about 0.7 to 0.8, particularly about 0.75, a solution of the above type containing, calculated as the corresponding oxide, 0.1 to 15%, particularly 1 to 10%, or 1.5 to 7.5%, or 1.8 to 6%, for example 2 to 5% by weight of the lanthanide is preferably used.

[0037] In one embodiment, at least 99% by weight, particularly 99.5 to 100% by weight, or 99.9% by weight or more of the metal is present as a metal compound or complex in dissolved or solvated form.

[0038] In one embodiment, the aqueous solution contains an alkaline earth metal salt selected from the group consisting of calcium, barium, strontium and combinations thereof. In particular, the aqueous solution contains calcium in the form of a water-soluble salt such as calcium nitrate, particularly calcium nitrate hydrate, or calcium acetate. Calcium is added as a salt and dissolved in water. Similarly, strontium or barium can be added as a water-soluble salt and dissolved in water.

[0039] Complexes of alkaline earth metals can also be used as starting materials for alkaline earth metals, instead of or in addition to water-soluble salts. Examples include complexes of alkaline earth metals with EDTA, NTA and DTPA, but preferably EDTA, NTA and DTPA are not used.

[0040] In one embodiment, the manganese is present primarily as a citrate complex of Mn(III) or Mn(IV) or a combination thereof, and shows a citrate ligand (also referred to as a "citrato" ligand) bonded to the Mn cation. In one particular embodiment, an aqueous solution containing manganese in the form of a Mn(III) and / or Mn(IV) citrate complex having, on average, one citrato ligand per Mn cation is supplied. Typically, the manganese citrate complex(es) darken(s) the aqueous solution.

[0041] In addition to manganese, the present technology is suitable for the processing of other metals having similar chemical properties.

[0042] In one embodiment, the aqueous solution contains a lanthanide added in the form of a water-soluble salt such as a nitrate, hydroxide or complex, particularly in the form of a citrate complex or a complex formed by another complexing agent such as EDTA, NTA or DTPA. However, preferably, EDTA, NTA and DTPA are not used.

[0043] In one particular embodiment, the lanthanide complexed with the citrato ligand is water-soluble. In one embodiment, the lanthanide is present in the form of a trivalent lanthanide (oxidation state: +3), although at least some lanthanides may also be present as citrate complexes of divalent or tetravalent lanthanide ions.

[0044] In addition to the citrate complexes of lanthanides, not only water-soluble salts of lanthanides but also other complexes can be used.

[0045] In one embodiment, the lanthanide is selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, particularly Eu, Gd, Tb, Sm, Pr, La and Nd.

[0046] In one preferred embodiment, the lanthanide is Gd. In particular, Gd is present in the form of a citrate complex of a trivalent metal. The aforementioned Gd is used as a starting material, for example, in the form of an oxide (Gd2O3).

[0047] In another preferred embodiment, the lanthanide is La or Eu or Tb or Sm or Pr or Nd.

[0048] The aqueous solution according to the present technology can be incorporated into a precursor solution for use in chemical solution deposition of a thin film of a material having the following formula. R 1-x A x MnO3I In the formula, R represents a lanthanide such as Eu, Gd, Tb, Sm, Pr, La or Nd, A represents an alkaline earth metal, and, x has a value in the range of 0.5 to 1.0, for example 0.55 to 1.0, 0.6 to 1.0, 0.65 to 1.0, 0.7 to 1.0 or 0.75 to 1.0, or 0.5 to 0.99, for example 0.55 to 0.99, 0.6 to 0.99, 0.65 to 0.99, 0.7 to 0.99 or 0.75 to 0.99. Preferably, x has a value in the range of 0.5 to 0.95, for example 0.55 to 0.95, 0.6 to 0.95, 0.65 to 0.95, 0.7 to 0.95 or 0.75 to 0.90.

[0049] In one embodiment, in formula I, x has a value of 0.6 to 0.95, or 0.75 to 0.90.

[0050] The molar concentration of the aqueous solution with respect to the alkaline earth metal is 0.1 to 5. According to one embodiment, the molar concentration is 0.2 to 4, for example 0.3 to 3.5, or 0.5 to 3, particularly about 0.6 to 2, or 0.7 to 1.5 or about 1 ± 0.25, for an alkaline earth metal such as calcium.

[0051] The above composition and the molar concentration in the above range contribute to the formation of an epitaxial thin film having a thickness of at least 10 nm when an aqueous solution is deposited on a substrate by chemical solution deposition and then cured at a particularly elevated temperature to remove elements such as H, C, N, and (partially) O. Citrate ions form complexes with metal cations. In one embodiment, the aqueous solution has no or essentially no free citrate ligands.

[0052] Generally, the water content of the aqueous solution, calculated from the total weight of the solution, is in the range of about 10 to 99% by mass, typically about 15 to 95%, for example 10 to 90%, for example 20 to 85% or 25 to 85% or 30 to 80% or 40 to 80% or 50 to 75%. The solid content of the solution, calculated from the total weight of the solution, is 5 to 85% by mass, such as 10 to 90%, for example 15 to 80% or 15 to 75% or 20 to 70% or 20 to 60% or 25 to 50%.

[0053] In one embodiment, the aqueous solution is suitable for producing a film having a thickness of, for example, about 10 to 200 nm, 25 to 175 nm, 50 to 150 nm, or 75 to 125 nm, for example 100 nm ± 10 nm by CSD, and this aqueous solution has a water content of 40 to 80% by mass, particularly 50 to 75%. Thus, the solid content of the solution, calculated from the total weight of the solution, is generally about 20 to 60% by weight, particularly 25 to 50%.

[0054] In one embodiment, the aqueous solution has a pH in the basic range, particularly 8 or higher, for example 8 to 13 or 9 to 12. If the pH is 8 or higher, it contributes to the stabilization of the citrate complex.

[0055] In one embodiment, the present technology includes a method for preparing an aqueous solution containing elements selected from alkaline earth metals, manganese, and optionally lanthanides.

[0056] In the method, the aqueous solution can be prepared by the following steps. Supplying a first aqueous solution of a water-soluble salt of an alkaline earth metal, and, the following, a second aqueous solution of a manganese citrate complex, and, supplying at least one of a third aqueous solution of a manganese citrate complex and a water-soluble complex of a lanthanide, and, mixing the first, and at least one of the second and third aqueous solutions, in a predetermined ratio to supply the aqueous solution.

[0057] In the first step, typically as a separate stock solution, a first aqueous solution of an alkali metal is supplied. The solution is formed by dissolving a suitable source of calcium, in particular a water-soluble calcium compound, in water to supply a calcium-containing aqueous solution. In one embodiment, the calcium source is selected from calcium nitrate and calcium acetate. In another embodiment, the calcium source is calcium nitrate and / or calcium acetate. When the decomposition of nitrates is too intense in the next step, it is preferable to select calcium acetate, or a mixture of calcium acetate and calcium nitrate.

[0058] In a particular embodiment, calcium nitrate hydrate is used. Calcium nitrate has been found not to interfere with the wetting properties of the final precursor solution. Furthermore, nitrate anions decompose into nitrogen dioxide and oxygen at a temperature considerably lower than 900 °C, for example up to 750 °C. This means that the film formed from the precursor solution containing calcium in the form of dissolved calcium nitrate (or calcium nitrate hydrate) after heat treatment contains no nitrogen impurities. Also, since citrate burns out during heat treatment, in practice, the impurity level is very low or essentially residue-free.

[0059] The calcium concentration in the stock solution is typically in the range of 0.5 to 10 M, particularly about 1 to 8 M or 1.5 to 7 M, or 2 to 6.5 M, for example 2.5 to 6 M.

[0060] In one embodiment, the stock solution of the alkaline earth metal (i.e., the first solution) is mixed with at least one of the second and third solutions at a predetermined volume ratio corresponding to the intended stoichiometry of the composite oxide. For example, the formula R 1-x A x MnO3, where A is an alkaline earth, R is a lanthanide, and x is a value less than 1, to prepare a precursor solution for the composite oxide, the first and third aqueous solutions, and optionally the second aqueous solution, are mixed in a ratio that gives a predetermined content of lanthanide in the precursor solution.

[0061] The order of mixing of the first, second, and third solutions can vary.

[0062] In one embodiment, the first aqueous solution is mixed with a predetermined amount of the second aqueous solution and then optionally with the third aqueous solution.

[0063] In one embodiment, the first aqueous solution is mixed with a predetermined amount of the third aqueous solution and then with the second aqueous solution.

[0064] In yet a further embodiment, the second and third solutions are first mixed together in a predetermined ratio and then the solution thus obtained is mixed with the first solution. The second and third solutions are mixed together in a ratio calculated based on the target ratio of lanthanide and manganese in the metal oxide film produced, for example, from the final aqueous solution.

[0065] Generally, the mixing ratio (volume / volume) of the first solution to the second or third solution, or to the combination of the second and third solutions, varies in the range of 1:99 to 75:25, for example 5:95 to 50:50, particularly 10:90 to 30:70, for example 12:88 to 25:75 or 15:85 to 20:80.

[0066] The mixing ratio (volume / volume) of the second solution to the third solution varies in the range of 0:100 to 100:0, particularly 1:99 to 99:1 such as 5:95 to 95:5, particularly 10:90 to 90:10 or 20:80 to 80:20, for example 30:70 to 70:30.

[0067] In one particular embodiment, in order to supply a water-soluble lanthanide component to the third solution, the method includes the following steps. Dissolving citric acid in water to form an aqueous solution, and Adding a lanthanide oxide to the aqueous solution of citric acid to form the aforementioned water-soluble complex of the lanthanide.

[0068] In one particular embodiment, the method for preparing a dissolved lanthanide component includes the following steps. Supplying a lanthanide oxide, and Dissolving citric acid in an aqueous solution in a molar excess, particularly a 2.5 - to 5-fold molar excess, compared to the lanthanide oxide, and Contacting the lanthanide oxide with citric acid until at least substantially all of the lanthanide oxide is dissolved, and Raising the pH of the aqueous solution to form a stable citrate complex of the lanthanide.

[0069] Lanthanides are typically selected from the group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and combinations thereof, particularly the group of Eu, Gd, Tb, Sm, Pr, La, and Nd, and combinations thereof, for example the group of Eu, Gd, Tb, Sm, Pr, La, and Nd, and combinations thereof. In one embodiment, the lanthanide is Gd.

[0070] In one embodiment, the method includes preparing a dissolved manganese component for the second and / or third solution by the following steps. Adding MnO2 to an aqueous solution of citric acid, and The step of reducing Mn to Mn(II) in an aqueous citric acid solution; The step of raising the pH of the aqueous solution to a value of at least 8 to form Mn(III) or Mn(IV) citrate, or a combination thereof.

[0071] Manganese(III) or manganese(IV) can be reduced to manganese(II) by hydrogen peroxide, which acts as a mild reducing agent. Hydrogen peroxide is added as an aqueous solution and can be mixed with an aqueous solution of citric acid and MnO2. After reducing Mn(IV) (and Mn(III)) to Mn(II), an aqueous solution of manganese citrate is formed.

[0072] The manganese component of the complex can be oxidized to an oxidation state greater than +2 in the next step, for example, by the addition of ammonia. Thus, ammonia can be added to an aqueous solution containing Mn(II) citrate to raise the pH, thereby causing deprotonation of the citric acid and oxidizing the manganese to Mn(III) or Mn(IV), or a combination thereof.

[0073] By raising the pH of the aqueous solution of manganese citrate, for example, by adding ammonia in an amount of 8 or more, complex formation of manganese with the citrate ligand will occur.

[0074] In one embodiment, for example, to form a second aqueous solution, MnO2 is added to an aqueous solution obtained by adding citric acid to water.

[0075] In another embodiment, for example, to form a third aqueous solution, MnO2 is added to an aqueous solution containing lanthanide citrate. The aqueous solution of lanthanide citrate is typically prepared using an excess amount, typically a molar excess of 1.5 to 20 times, such as 2 to 10 times, for example 2.2 to 8 times, especially 2.5 to 5 times, of citric acid. This means that after the lanthanide is dissolved in the solution as a citrate complex, manganese citrate can be easily formed in the solution without adding further citric acid.

[0076] Surprisingly, it has been found that the presence of lanthanide in the aqueous phase contributes to the stabilization of this aqueous solution.

[0077] One embodiment applicable to both the second and third solutions involves preparing a supersaturated solution of Mn(II) citrate with a pH less than 8, especially less than 7, for example less than 6, for example less than 5.5 or less than 5.

[0078] As described above, the molar concentration of alkaline earth metals such as calcium in the aqueous solution is preferably about 0.5 to 2.5, or 0.6 to 2, or 0.7 to 1.8, or 0.8 to 1.6, or about 1 ± 0.25.

[0079] The aqueous solutions disclosed herein can be used as aqueous precursor solutions for preparing thin films, especially epitaxial thin films, by chemical solution deposition.

[0080] Typically, the precursor solution is applied onto a substrate by suitable techniques such as dip coating, aerosol deposition, inkjet printing, 3D printing, roll-on roll transfer or spin coating to form a layer, which is then dried and heat-treated to remove elements not involved in the formation of the desired composite oxide, such as hydrogen, carbon, nitrogen and oxygen. Further methods of applying this precursor solution onto a substrate include tape casting, press printing, screen printing, offset printing, and flexographic printing.

[0081] According to one embodiment, an aqueous solution is used to prepare a thin film on a substrate. The substrate is made hydrophilic prior to the preparation of the thin film.

[0082] The substrate can be pretreated, for example, with a piranha solution to make it hydrophilic and any organic residues can be removed from the substrate before applying the precursor solution to the substrate. Alternatively, a plasma cleaner can be used for the pretreatment, that is, the substrate is made hydrophilic by plasma treatment. The plasma cleaner can actually be used for high-speed etching applications and surface treatment. In the case of the H2O option, this system can be used to hydrophilize the surface of the substrate by adding OH radicals to the surface of the material. This method is relatively rapid and does not generate any extra material that needs to be discarded. Thus, the use of a plasma cleaner for surface hydrophilization is an environmentally friendly method as opposed to traditional piranha treatment.

[0083] Finally, the film is typically annealed to enable crystallization of the thin film. The film can be subjected to etching, sputtering, or lithography such as UV lithography or EUV lithography.

[0084] Also, the annealing may include two steps, a first heat treatment and the annealing itself (second heat treatment) that follows it.

[0085] In one embodiment, the (first) heat treatment is carried out by raising the temperature of the material from the ambient temperature (room temperature) to a target temperature of typically at least 150°C, particularly at least 200°C, for example at least 250°C or at least 300°C, for example about 350°C or for example about 400°C. Annealing (the second heat treatment), as described above, is carried out by raising the temperature of the material typically obtained after the first heat treatment from the ambient temperature (or room temperature) to a target temperature of at least 400°C, 450°C, 500°C, 600°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1100°C or 1200°C. In both the heat treatment and annealing, the ramp-up of the temperature from room temperature to the target temperature takes place over a time of 1 to 24 hours, typically 4 to 16 hours, for example 6 to 14 hours. Extending the time of the temperature ramping contributes to the formation of a film having an even surface. Similarly, it is preferable to use the lowest possible temperature for each step in order to ensure good quality of the final film.

[0086] In one embodiment, the temperature is raised at a rate of about 0.1 to 5°C, or 0.2 to 3°C, for example 0.3 to 1.5°C, or 0.4 to 1°C, for example about 0.5°C / min. Here, a rate of less than 0.5°C / min may be suitable for avoiding damage to the surface of the substrate. The temperature can be raised to a target temperature in the range of for example about 300 to about 600°C, or 300 to 580°C, or 300 to 450°C over a time of 4 to 16 hours such as 1 to 24 hours, or 2 to 20 hours, or 6 to 14 hours. The object thus treated is then held at the target temperature for 1 to 10 hours and then cooled passively.

[0087] In one embodiment, annealing is carried out in air or an oxygen atmosphere at a temperature in the range of 600 to 1200 °C, for example about 750 °C. Some examples of suitable temperature ranges for annealing are 600 to 1000 °C, 700 to 1200 °C, 750 to 1000 °C, 600 to 800 °C and 600 to 750 °C. The suitable temperature for annealing may vary depending on the method of chemical deposition. For example, spin coating typically forms a thinner layer than, for example, dip coating. Thus, when dip coating is used, either a higher temperature, a lower rate of temperature increase, and / or a longer holding time at the target temperature is required compared to the case of spin coating. The same applies to inkjet printing and roll-on roll transfer.

[0088] In one embodiment, the present aqueous precursor solution is suitable for use in the production of a material having Formula I by chemical solution deposition. R 1-x A x MnO3I wherein, R represents a lanthanide, particularly Eu, Gd, Tb, Sm, Pr, La, Nd and combinations thereof, A represents an alkaline earth metal, and, x has a value in the range of 0.5 to 1.0, for example 0.55 to 1.0, 0.6 to 1.0, 0.65 to 1.0, 0.7 to 1.0 or 0.75 to 1.0, or 0.5 to 0.99, for example 0.55 to 0.99, 0.6 to 0.99, 0.65 to 0.99, 0.7 to 0.99 or 0.75 to 0.99. Preferably, x has a value in the range of 0.5 to 0.95, for example 0.55 to 0.95, 0.6 to 0.95, 0.65 to 0.95, 0.7 to 0.95 or 0.75 to 0.95 or 0.75 to 0.9. The aforementioned material is preferably crystalline.

[0089] In one embodiment, the aqueous precursor solution is used to prepare a thin film having a thickness in the range of 20 - 2000 nm, such as 25 - 1000 nm, for example 30 - 750 nm, or 40 - 700 nm, or 50 - 500 nm, and optionally comprises a plurality of overlapping layers having individual thicknesses of at least 1 nm, or at least 1.5 nm, or at least 2 nm, or at least 2.5 nm or at least 3 nm or at least 4 nm, or at least 5 nm, and up to 30 nm, or up to 25 nm, or up to 20 nm, or up to 15 nm, or up to 10 nm. In this context, overlapping means that at least one layer overlaps on top of another layer. In other words, one or more layers may be larger than one or more of the other layers, mainly they cover each other.

[0090] As is apparent, the present technology provides for the manufacture of a thin film having the above formula I. In formula I, R means a lanthanide, especially Eu, Gd, Tb, Sm, Pr, La and Nd, A represents an alkaline earth metal, especially calcium, barium or strontium, and, x has a value in the range of 0.5 - 1.0, for example 0.55 - 1.0, 0.6 - 1.0, 0.65 - 1.0, 0.7 - 1.0 or 0.75 - 1.0, or 0.5 - 0.99, for example 0.55 - 0.99, 0.6 - 0.99, 0.65 - 0.99, 0.7 - 0.99 or 0.75 - 0.99. Preferably, x has a value in the range of 0.5 - 0.95, for example 0.55 - 0.95, 0.6 - 0.95, 0.65 - 095, 0.7 - 0.95 or 0.75 - 0.95 or 0.75 - 0.90.

[0091] Furthermore, in one embodiment, an epitaxial thin film having formula I, wherein, R represents a lanthanide, especially Eu, Gd, Tb, Sm, Pr, La and Nd, A represents an alkaline earth metal such as calcium, barium or strontium, and, x has a value in the range of 0.6 to 0.95, or 0.75 to 0.90, and this epitaxial thin film is deposited on a substrate to supply a memristor.

[0092] In a fifth aspect, the present specification relates to a method for manufacturing a thin film on a substrate. The method includes the following steps. Supplying a substrate, and Making the substrate hydrophilic, and Preparing a thin film by chemical solution deposition of an aqueous solution as described herein.

[0093] The various embodiments and variations described herein are similarly applicable to a method for manufacturing a thin film on a substrate.

[0094] The memristor structures that can use this film are exemplified by the embodiments in the attached drawings.

[0095] FIG. 1 is a schematic cross-sectional view of a so-called planar-type memristor here. The memristor includes a piece of memristor material 1 that constitutes a memristive connection between a first contact 2 and a second contact 3. The expression "memristor material" is used here for the purpose of clearly indicating the material that appears between the contacts. However, the current understanding is that the memristor-type effect occurs not in the bulk of the memristor material itself, but at the junction or interface between the memristor material and the contacts.

[0096] The conductive connection can be used to couple the first contact 2 and the second contact 3 to respective nodes 4 and 5, expose the piece of memristor material 1 to a desired voltage, and measure the resulting electrical characteristics of the piece of memristor material 1.

[0097] A piece of the memristor material 1 forms a film on the substrate 6. In a planar memristor as shown in FIG. 3, the piece of the memristor material 1 can be deposited directly in the form of a film on the surface of the substrate 6, but this is not a requirement, and for example, an intermediate layer of the desired material can be used therebetween if it is advantageous to achieve better compatibility between the crystal structures of the various materials involved.

[0098] FIG. 2 is a schematic cross-sectional view of a memristor. Here, the memristor is of a so-called transverse type and may also be referred to as a capacitor type. Also in this case, a piece of the memristor material 1 forms a memristor connection between the first contact 2 and the second contact 3, and the conductive connection couples them to the nodes 4 and 5 respectively. The piece of the memristor material 1 forms a film on top of the substrate 6, but is not directly on top of the substrate 6 because the second contact 3 forms a layer therebetween. The transverse type memristor as shown here has certain advantageous features compared to the planar type of FIG. 1, for example, the required operating voltage may be lower in some cases. However, its manufacture requires more steps than that of a planar memristor.

[0099] FIG. 3 is a schematic cross-sectional view of a multi-layer memristor. In this case, there are two or more pieces of the memristor material 1, and each piece may be sandwiched between other layers. In the exemplary structure shown in FIG. 3, the intermediate layer is the layer of the first contact 2 and the second contact 3. As a result, each piece of the memristor material 1 has the first contact layer on one side and the second contact layer on the other side in the lateral direction, that is, in a direction perpendicular to the surface of the substrate 6.

[0100] In all of these embodiments where pieces of memristor material form a film on top of the substrate, the thickness d of the film in a direction perpendicular to the surface of the substrate may be, for example, between 1 and 500 nanometers, or in one preferred embodiment between 10 and 100 nanometers, or for example between 20 and 80 nanometers. The thickness d of the film does not have to be constant throughout. However, a constant film thickness can be easily achieved by chemical solution deposition using this precursor solution.

[0101] The first contact 2 and the second contact 3 may be made of the same material or different materials. Stating that a contact is made of a particular material is synonymous with stating that each contact is predominantly composed of that material to the extent that the properties of that material govern the effects observed at the interface between the contact and the piece of memristor material.

[0102] In many embodiments that have been found to exhibit interesting properties, the first material and the second material are different materials. In particular, it has been found that a memristor may exhibit interesting properties when the rectifying properties of the junction between the first material and the memristor material are different from the rectifying properties of the junction between the second material and the memristor material.

[0103] An example of a material that can be used as the first material is aluminum. Other examples of materials that can be used as the first material also exist and include, but are not limited to, titanium.

[0104] Examples of materials that can be used as the second material are gold and silver. Other examples of materials that can be used as the second material also exist and include, but are not limited to, copper, platinum, palladium, indium, and SrRuO3.

[0105] A typical property of a memristor is that its current conduction property depends on the electrical conditions it has been subjected to in the past. A memristor can be "programmed" to have a particular resistance value by applying a "write" pulse of a particular amplitude.

[0106] Even more interesting are memristors having a linear or log-linear response to write pulses of different amplitudes. This concept is described in more detail in WO2021 / 152215A1, the content of which is incorporated herein by reference.

[0107] For the application of GCMO in microchip device manufacturing, it is preferred that the film can be patterned. In embodiments of the present technology, a simple procedure involves wet chemical etching of a GCMO thin film implemented as part of a photolithography process. For that purpose, an aqueous etchant composition containing, for example, hydrochloric acid and potassium iodide, and an antioxidant such as ascorbic acid can be used. Typically, the concentration of hydrochloric acid is about 0.05 - 0.15M, the concentration of potassium iodide is about 0.5 - 5M, and the concentration of ascorbic acid is 0.01 - 2.5M. Preferably, the concentration of hydrochloric acid is about 0.12M, the concentration of potassium iodide is about 5M, and the concentration of ascorbic acid is about 0.1M.

Example

[0108] The following non-limiting examples illustrate further embodiments of the present technology.

[0109] (Example 1) Preparation of the precursor solution for GCMO

[0110] Stock solutions were prepared to obtain a precursor solution having the desired value of x in the metal oxide of formula II for the intended thin film. Gd 1-x Ca x MnO3II

[0111] The precursor solution for the metal oxide with x = 0.95 is designated as "GCMO95". On the other hand, the precursor solution for the metal oxide with x = 0.75 is designated as "GCMO75".

[0112] The starting materials and reagents and their amounts are shown in Table 1.

[0113]

Table 1

[0114] 1.1 Preparation of calcium stock solution A 5M Ca(NO3)2 solution was prepared by dissolving Ca(NO3)2·4H2O in deionized water and carefully filtering to remove trace amounts of solid impurities. Refer to Table 1 for the mass of Ca(NO3)2·4H2O.

[0115] 1.2 Preparation of stock solutions of GCMO95 and GCMO75 Two 25 mL stock solutions corresponding to the compounds of Formula II with x = 0.95 and x = 0.75, respectively, were prepared. The starting materials and reagents are shown in Table 1.

[0116] H3Cit was dissolved in approximately 200 mL of deionized water in a large beaker (500 mL). The weighed amount of Gd2O3 was added. The hot plate was set to 125 - 150 °C and left standing (for about 6 hours), and all the oxides dissolved to form a completely transparent and colorless solution. After cooling the solution to room temperature, it was diluted to 300 mL. Next, after adding MnO2, an H2O2 solution was added dropwise at room temperature (generating oxygen gas). The solution was left standing until it became transparent (for about 12 hours). The solution generally exhibited a pink color, presumably due to the presence of Mn 3+ . Solutions with a higher gadolinium concentration (x = 0.75) exhibited a stronger red color. At this stage, the solution was supersaturated.

[0117] The pH of the transparent solution was raised to about 8 - 9 by dropwise addition of aqueous ammonia. The solution became darker to exhibit a deep red color but remained transparent. Here, the solution was stable and could be evaporated.

[0118] Next, the solution was evaporated on a hot plate set at 75 °C until the volume was less than 25 mL, and the solution temperature was set to about 50 °C (taking 2 days to 1 week).

[0119] Once the solution was cooled, deionized water was added to adjust the total volume to 25 mL, and it was carefully filtered to remove all traces of solid impurities.

[0120] Here, these stock solutions, designated as GCMO95 and GCMO75, can be used to prepare thin films of GCMO at x = 0.95 and x = 0.75, respectively, after adding an appropriate amount of the Ca stock solution.

[0121] 1.3 Mix GCMO95 and GCMO75 with the Ca stock solution to obtain precursors with a predetermined x. The stock solutions were mixed as shown in Table 2 to obtain precursor solutions with a predetermined x.

[0122]

Table 2

[0123] (Example 2) Spin-coating of the precursor solution onto the substrate The above procedure was further tested by preparing precursors with x = 0.75, 0.85, and 0.95 from GCMO95, GCMO85, and GCMO75 and the calcium stock solution. Thin films of these precursors were coated onto 5 mm × 5 mm SrTiO3 (STO) substrates using the following procedure.

[0124] The substrate was cleaned and made hydrophilic by treatment with sulfuric acid-piranha.

[0125] Spin-coating of the solution onto the STO substrate was performed in air at 5000 rpm (accelerating at 3000 rpm / s).

[0126] Drying and thermal decomposition were combined in a furnace. First, the temperature was raised from room temperature to 350 °C at about 0.5 °C / min over 12 hours, and then the coated substrate was held at 350 °C for 3 hours and then passively cooled to room temperature.

[0127] Annealing of the coated substrate was carried out in oxygen in the furnace at 900 °C for 24 hours to enable crystallization of GCMO.

[0128] (Example 3) Spin coating of the precursor solution onto the substrate Example 2 was repeated except that annealing was carried out in oxygen in the furnace at 750 °C for 24 hours to enable crystallization of GCMO.

[0129] (Example 4) Etching of GCMO The GCMO obtained in Example 3 was etched by wet chemical etching as part of a photolithography process. The etchant was based on a chloride that reduces manganese to water-soluble manganese(II) under acidic conditions in the presence of iodide, and provides a method for promoting the reduction of the elemental chlorine formed back to chloride, thereby regulating the rate of the process. The following aqueous etchant composition was used. [HCl] = 0.12 M; [KI] = 5 M; [ascorbic acid] = 0.1 M

[0130] Ascorbic acid was added to prevent the iodide from being oxidized and the solution from becoming opaque. When the etchant was diluted 1:1, the etching time was 1 - 2 minutes depending on the film.

[0131] Results Experimental verification of the film quality for two different x values (0.75 and 0.85 respectively) was carried out by XRD analysis (see Figures 4a to 4d), magnetic measurements (see Figures 5a and 5b), and scanning electron microscopy for the smoothness and thickness of the film (see Figures 6a and 6b). Evaluation of the memristor characteristics was also carried out (Figure 7).

[0132] a. XRD analysis The microstructure attributes were characterized using a PanAnalytical Empyrean X-ray diffractometer with a 5-axis goniometer. The diffractometer was used with an Empyrean Cu LFF HR X-ray tube. The X-ray radiation was filtered to consist mainly of Cu Kα1 and Kα2 X-rays. The incident beam optics were composed of a Bragg-Brentano HD X-ray mirror, a fixed 1 / 4° divergence slit, a 5 mm mask, a 0.04 rad Soller slit, and a 1° anti-scattering slit. The diffracted beam optics were composed of a 7.5 mm divergence slit, a 0.04 rad Soller slit, and a PIXcel detector array. The θ~2θ results shown in Fig. 4a show the (00l) peaks from both the SrTiO3 substrate and the GCMO film. The peak identification is based on (ICSD 12626). Note that the indexing of the b and c parameters is swapped. This is due to labeling convenience.

[0133] The pole figure obtained from the GCMO (204) peak is shown in Fig. 4b.

[0134] This is supported by the detailed data of the θ~φ scan shown in Fig. 4c and the ω~2θ scan shown in Fig. 4d of the GCMO (204) peak. These results indicate that the film is epitaxial, phase-pure, and fully c-axis oriented.

[0135] b. Magnetic measurements The magnetic transitions of spin-coated GCMO thin films with Ca concentrations of x = 0.75 and x = 0.85 were studied by measuring the zero-field-cooled (ZFC) magnetization M and the field-cooled (FC) magnetization M in the temperature range of 10~400 K and an external magnetic field of 50 mT, as shown in Figs. 5a and 5b.

[0136] Figures 5a and 5b show the temperature dependence of zero-field-cooled (ZFC) magnetization M and field-cooled (FC) magnetization M measured in a magnetic field of 50 mT at temperatures from 10 to 400 K for two spin-coated GCMO thin films. For different calcium concentrations, Figure 5a shows the case of the GCMO film with x = 0.75, and Figure 5b shows the case of the GCMO film with x = 0.85.

[0137] As will be described later, complex magnetic behaviors due to the mixing of competing ferromagnetic (FM) and antiferromagnetic (AFM) states can be utilized. In this case, the presence of a peak in the ZFC curve below the Curie temperature can be attributed to spin-glass behavior or cluster-glass behavior. The paramagnetic (PM) increase in the MT curve at low temperatures seems to be stronger for spin-coated thin films than for films prepared by PLD. However, the magnetic ordering temperature was equivalent to the results observed in GCMO ceramic bulk and GCMO thin films ablated by pulsed laser deposition (PLD) at the same Ca doping concentration.

[0138] c. Surface characteristics The smoothness and thickness of the film are shown in Figures 6a and 6b for the film prepared according to Example 2. The figure shows a scanning electron microscope (SEM) image of the x = 0.75 film. The film surface is not atomically smooth (during pyrolysis, exhaust gas escapes from the film. Generally, CSD is not a suitable technique for extremely smooth films), but it demonstrates that the film surface is still smooth enough for the manufacture of microelectronics.

[0139] Furthermore, an etching procedure of the above-described type was performed, including the step of etching stripes into the x = 0.75 film. A rough measurement of the film thickness was obtained by measuring the stripe thickness using an atomic force microscope (AFM).

[0140] As a result, a film thickness of ~100 nm was obtained for the undiluted precursor.

[0141] Figures 7a and 7b show the smoothness and thickness of the films prepared according to Example 3. Figure 7a is an SEM image of the x = 0.75 film, and Figure 7b is an SEM image of the x = 0.95 film. Again, it can be seen that the surface has the smoothness suitable for the manufacture of microelectronics. The thickness measured as described above was also ~100 nm.

[0142] d. Memristor characteristics To verify that the GCMO film prepared using the above CSD exhibits memristive switching, a rough planar memristor was constructed on the GCMO film with x = 0.85 by depositing spots of a thin gold film to be used as the cathode (gold has been shown not to cause memristive behavior as it makes an ohmic contact with GCMO).

[0143] Aluminum wire contacts were made to the gold cathode (which is also an ohmic contact) using wire bonds and directly to the GCMO film to supply the anode. (It is this interface between aluminum and GCMO that functions as the memristor.)

[0144] Next, the planar memristor was connected to a Keithley 2614B SourceMeter, and a pulse voltage sweep from U = -12 V to U = +12 V was performed 5 times to examine its memristive switching behavior. The pulses had a width of 50 ms and a frequency of approximately 7 Hz.

[0145] Figure 8 shows the current measured for each pulse. The memristor was probed at 0.4 V at the midpoint of each pulse. The probe current is also shown in the figure.

[0146] This is only a rough demonstration, but it is clearly understood from the multiplicity of the current I(V) as a function of voltage that the sample has a resistance according to the voltage history received by the sample, and that this sample exhibits memristive switching. In particular, the sample exhibited resistive switching, which is a defining characteristic of a memristor.

Industrial Applicability

[0147] The composite metal oxide materials and composite metal oxide films provided by the present technology find use as memristors and their components, and generally in microelectronic devices, magnetic devices, and spintronic devices, in solid oxide fuel cells, in magnetic refrigeration, in the field of biomedicine, and as catalysts. The films can be used for memory components, transistors, integrated circuits, and other components and devices that impart new functions to them. The present precursor solution can be used with appropriate compositions and compounds of synthetic or natural minerals such as hackmanite, zeolite, scapolite or tugtupite.

Claims

1. An aqueous solution comprising: an alkaline earth metal added in the form of a water-soluble salt; and manganese present as a citrate complex of manganese having at least predominantly the +3 and / or +4 oxidation states; A molar concentration of 0.1 to 5 relative to the alkaline earth metal, represented by formula I: R １－ｘ A ｘ MnO ３ I formulated into a precursor solution for use in chemical solution deposition of a thin film of a material having During the ceremony, R represents a lanthanide; A represents an alkaline earth metal, and x has a value in the range of 0.5 to 1.0; The material is crystalline, an aqueous solution.

2. 10. The aqueous solution of claim 1, further comprising a lanthanide present in the form of a water-soluble complex.

3. Based on the total weight of the solution, 0.5 to 40%, particularly 1 to 25%, or 1.5 to 20%, or 1.7 to 15%, particularly 2 to 10%, or 2 to 7.5%, or 2 to 5%, by weight of water-soluble alkaline earth metal salt; MnO ２ and 0.5 to 20%, in particular 1 to 15%, or 1.5 to 12.5%, or 2 to 10%, or 2.5 to 7.5%, or 3 to 5%, by weight of manganese, calculated as 2. An aqueous solution according to claim 1, comprising 0.1 to 15%, in particular 0.2 to 10%, or 0.3 to 7.5%, or 0.4 to 6%, for example 0.5 to 5% or 0.5 to 4.5%, by weight of lanthanide, calculated as the corresponding oxide.

4. 2. The aqueous solution of claim 1, wherein the alkaline earth metal is selected at least primarily from the group consisting of calcium, barium, strontium and combinations thereof, and the alkaline earth metal is added in the form of a nitrate or acetate, in particular in the form of a nitrate hydrate, such as calcium nitrate hydrate.

5. 2. The aqueous solution of claim 1, comprising Mn(III) or Mn(IV) citrate complexes having, on average, one citrate ligand per Mn cation.

6. 2. Aqueous solution according to claim 1, comprising a lanthanide such as Eu, Gd, Tb, Sm, Pr, La or Nd, in particular Gd, in the form of a citrate complex.

7. 10. The aqueous solution of claim 1, formulated to have a water content of 40 to 80%, in particular 50 to 75%, by weight, for preparing thin films having a thickness of about 10 to 200 nm, 25 to 175 nm, 50 to 150 nm, or 75 to 125 nm, e.g., 100 nm±10 nm.

8. 2. The aqueous solution of claim 1, having a pH of 7 or higher, for example 8-13 or 9-12.

9. 2. The aqueous solution according to claim 1, which exhibits a molar concentration relative to said alkaline earth metal, such as calcium, of between 0.2 and 4, for example between 0.3 and 3.5, or between 0.5 and 3, in particular between about 0.6 and 2, or between 0.7 and 1.5 or about 1±0.

25.

10. In Formula I, R represents Eu, Gd, Tb, Sm, Pr, La, or Nd; 2. The aqueous solution of claim 1, wherein x has a value in the range of 0.55 to 1.0, 0.6 to 1.0, 0.65 to 1.0, 0.7 to 1.0 or 0.75 to 1.0, or 0.5 to 0.99, for example 0.55 to 0.99, 0.6 to 0.99, 0.65 to 0.99, 0.7 to 0.99 or 0.75 to 0.99, preferably x has a value in the range of 0.5 to 0.95, for example 0.55 to 0.95, 0.6 to 0.95, 0.65 to 0.95, 0.7 to 0.95 or 0.75 to 0.95 or 0.75 to 0.

9.

11. 10. The aqueous solution of claim 1, essentially free of citrate ligand.

12. 1. A method for preparing an aqueous solution containing an element selected from alkaline earth metals, manganese and optionally a lanthanide, comprising the steps of: providing a first aqueous solution of a water-soluble salt of an alkaline earth metal; The following, i.e. a second aqueous solution of a citrate complex of manganese having an oxidation state of +2 and / or +3; and providing at least one of a third aqueous solution containing a citrate complex of manganese having an oxidation state of +2 and / or +3 and a water-soluble complex of a lanthanide; mixing the first aqueous solution and at least one of the second aqueous solution and the third aqueous solution in a predetermined ratio to provide said aqueous solution.

13. 13. The method of claim 12, comprising mixing the first aqueous solution with predetermined amounts of the second and third aqueous solutions.

14. dissolving citric acid in water to form an aqueous solution; and b. adding a lanthanide oxide to the aqueous solution of citric acid to form a water-soluble complex of the lanthanide.

15. providing a lanthanide oxide; dissolving a molar excess, particularly a 2.5 to 5 fold molar excess, of citric acid relative to the lanthanide oxide in an aqueous solution; contacting the lanthanide oxide with citric acid until at least essentially all of the lanthanide oxide is dissolved; and increasing the pH of the aqueous solution to form a stable citrate complex of the lanthanide.

16. MnO in citric acid solution ２ and adding reducing Mn to Mn(II) in the aqueous citric acid solution; and raising the pH of the aqueous solution to at least 8 to form Mn(III) or Mn(IV) citrate, or a combination thereof.

17. 17. The method of claim 16, comprising the step of reducing Mn(IV) and / or Mn(III) to Mn(II) using hydrogen peroxide, preferably mixed with a citric acid solution and dissolved in an aqueous solution.

18. 17. The method of claim 16, comprising adding ammonia to an aqueous solution containing Mn(II) citrate to deprotonate the citrate and oxidize the manganese to Mn(III) or Mn(IV), or a combination thereof.

19. Citric acid was added to water to obtain an aqueous solution containing MnO ２ The method of claim 16, wherein

20. MnO in an aqueous solution containing lanthanide citrate ２ 17. The method of claim 16, comprising adding

21. 21. The method of claim 20, wherein the lanthanide is selected from the group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, such as Eu, Gd, Tb, Sm, Pr, La and Nd and combinations thereof, in particular the lanthanide is Gd.

22. 17. The method of claim 16, comprising preparing a supersaturated solution of manganese citrate having a pH of 8 or above, such as 8-13 or 9-12.

23. Formula I, R １－ｘ A ｘ MnO ３ I 13. The method of claim 12, comprising preparing an aqueous precursor solution suitable for use in chemical solution deposition production of a material having During the ceremony, R represents a lanthanide, in particular Eu, Gd, Tb, Nd, Sm, La or Pr, A represents an alkaline earth metal, and x has a value in the range of from 0.5 to 1.0, for example, from 0.55 to 1.0, from 0.6 to 1.0, from 0.65 to 1.0, from 0.7 to 1.0 or from 0.75 to 1.0, or from 0.5 to 0.99, for example, from 0.55 to 0.99, from 0.6 to 0.99, from 0.65 to 0.99, from 0.7 to 0.99 or from 0.75 to 0.99, preferably x has a value in the range of from 0.5 to 0.95, for example, from 0.55 to 0.95, from 0.6 to 0.95, from 0.65 to 0.95, from 0.7 to 0.95 or from 0.75 to 0.95, The method wherein the material is preferably crystalline.

24. 24. The method of claim 23, wherein the first aqueous solution and at least one of the second aqueous solution and the third aqueous solution are mixed in a ratio corresponding to x to provide the aqueous solution.

25. 10. Use of the aqueous solution according to claim 1 as an aqueous precursor solution for preparing thin films, in particular epitaxial thin films, by chemical solution deposition.

26. 26. Use according to claim 25, wherein the aqueous precursor solution is used to prepare a thin film having a thickness of 20 to 2000 nm, such as 25 to 1000 nm, for example 30 to 750 nm, 40 to 650 nm or 50 to 500 nm, optionally comprising a plurality of overlapping layers having individual thicknesses of at least 2 nm, or at least 2.5 nm, or at least 3 nm, or at least 4 nm, or at least 5 nm and up to 30 nm, or up to 25 nm, or up to 20 nm.

27. Formula I, R １－ｘ A ｘ MnO ３ I 26. The use of claim 25, comprising the step of preparing a thin film of During the ceremony, R represents a lanthanide, such as Eu, Gd, Tb, Sm, Pr, La, or Nd; A represents an alkaline earth metal, and Use wherein x has a value in the range of from 0.5 to 1.0, for example, from 0.55 to 1.0, from 0.6 to 1.0, from 0.65 to 1.0, from 0.7 to 1.0 or from 0.75 to 1.0, or from 0.5 to 0.99, for example, from 0.55 to 0.99, from 0.6 to 0.99, from 0.65 to 0.99, from 0.7 to 0.99 or from 0.75 to 0.99, preferably wherein x has a value in the range of from 0.5 to 0.95, for example, from 0.55 to 0.95, from 0.6 to 0.95, from 0.65 to 0.95, from 0.7 to 0.95 or from 0.75 to 0.95 or from 0.75 to 0.

90.

28. To provide a memristor, a compound of Formula I, R １－ｘ A ｘ MnO ３ I 26. The use of claim 25, comprising the step of depositing an epitaxial thin film of the formula (I) on a substrate, During the ceremony, R represents a lanthanide, such as Eu, Gd, Tb, Sm, Pr, La, or Nd; A represents an alkaline earth metal, and Use wherein x has a value in the range of 0.5 to 0.95 or 0.6 to 0.

9.

29. 10. Use of the aqueous solution according to claim 1 for preparing materials for use in microelectronic, magnetic and spintronic devices, in solid oxide fuel cells, in magnetic refrigeration, in the biomedical field and as catalysts.

30. 30. The use according to claim 29, wherein the aqueous solution is used to prepare a thin film on a substrate, the substrate being made hydrophilic prior to the preparation of the thin film.

31. 31. The use according to claim 30, wherein the substrate is made hydrophilic by plasma treatment.

32. 1. A method for producing a thin film on a substrate, comprising: providing the substrate; rendering the substrate hydrophilic; and preparing the thin film by chemical solution deposition of the aqueous solution of claim 1.

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