# Patent

## Title
Precursors for the polymer-assisted deposition of films

## Abstract

A polymer assisted deposition process for deposition of metal oxide films is presented. The process includes solutions of one or more metal precursor and soluble polymers having binding properties for the one or more metal precursor. After a coating operation, the resultant coating is heated at high temperatures to yield metal oxide films. Such films can be epitaxial in structure and can be of optical quality. The process can be organic solvent-free.

## Background
This application is a continuation-in-part of U.S. Ser. No. 10/616,479, filed Jul. 8, 2003, by McCleskey et al., and issued on Apr. 29, 2008 as U.S. Pat. No. 7,365,118, which is a continuation-in-part of U.S. Ser. No. 09/629,116, filed Jul. 31, 2000, by Li et al., and issued Jul. 8, 2003 as U.S. Pat. No. 6,589,457.


STATEMENT REGARDING FEDERAL RIGHTS
This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention.


FIELD OF THE INVENTION
The present invention relates to a deposition technique for metal oxide films and more particularly to the polymer assisted solution deposition of metal oxide films, especially for deposition of metal oxide films or epitaxial metal oxide films.
BACKGROUND OF THE INVENTION
Metal oxide films are widely used in the electronics industry. Preparation of such metal oxide films has been accomplished by physical vapor deposition techniques such as sputtering, electron beam (e-beam) evaporation, thermal evaporation, molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), by chemical vapor deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), and metalorganic chemical vapor deposition (MOCVD), and by sol-gel techniques and other chemical solution deposition techniques. Chemical solution deposition techniques have been generally viewed as less capital intensive (see, Lange, “Chemical Solution Routes to Single-Crystal Thin Films”, Science, vol. 273, pp. 903-909, 1996 and Schwartz, “Chemical Solution Deposition of Perovskite Thin Films”, Chemical Materials, vol. 9, pp. 2325-2340, 1997). Also, chemical solution techniques are not generally limited to flat surfaces.
Sol-gel techniques are not desirable for many industrial production processes. The sol-gel process uses the high reactivity of organometallic precursors and hydrolyzes these organometallic compounds to make various oligomers. These metal oxo oligomers have suitable viscosity to allow spinning into thin films, which can be fired into ceramic materials at high temperatures. The complication in such a sol-gel process is the uncontrollable polymerization of the metal oxo oligomers because of complex reactive species in the precursor solution. Therefore, the reproducibility of sol-gel processes can be poor which hinders the applications in industrial processes despite the low costs.
The use of organometallic compounds in chemical solution deposition techniques can be a drawback. As metal salts are typically insoluble in organic solvents, organic moieties have been added to the metal complexes in order to make such metal compounds soluble. This is often undesirable as new reactions and techniques must be developed to incorporate such soluble organic groups onto metal ions. In addition, the to resulting organometallic compounds are usually difficult to handle because of their relatively higher reactivity than metal salts.
One problem with the processing of metal salts into ceramic thin films involves the hydrolytic properties of metal ions. Transition metal ions, such as titanium, niobium and tantalum, react with water violently to form metal oxides or metal hydroxides and precipitate out of solution during processing. A more desired methodology to achieve metal oxide films would be a chemical solution having the following properties: clean decomposition to pure ceramics; stable chemical solutions (no gelling) without any reactions before the firing stage; and, the desired viscosity for spin coating, spray coating, or film casting.
U.S. Pat. No. 6,589,457 by Li et al. is directed to deposition of metal oxides from aqueous solutions of water-soluble metal precursors and water-soluble polymers. While none of the examples included a polymer other than polyvinyl alcohol, Li et al. illustrate the continuing efforts in the development of chemical solution deposition processes for production of metal oxide films. The present invention is a continuation of those efforts.
An object of the present invention is to provide compositions of matter for use in a chemical solution deposition method of forming metal oxide films, such a chemical solution deposition method including the deposition of a metal precursor and a soluble polymer where the polymer has binding properties for the metal precursor. In addition to metal oxide films, other films such as metal films, metal nitride films, metal phosphate films, metal boride films, metal silicate films, metal fluoride films, metal chalcogenide films and metal pnictogens films can be prepared.
Another object of the present invention is to provide a chemical solution deposition method of forming high purity metal oxide films or epitaxial metal oxide films.
Yet another object of the present invention is to provide metal precursor solutions, e.g., metal oxide precursor solutions, having a long shelf-life time in comparison to typical sol-gel solutions.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides for a composition of matter including a solution of at least two metal precursors and a soluble polymer, said polymer characterized as having binding properties for the at least two metal precursors, wherein the binding constant for binding between the polymer and the metal precursors is at least about 10000 mol−1 L. In one embodiment, the solution further includes a metal-binding ligand or salts thereof.
The present invention still further provides a composition of matter including an aqueous homogenous solution of one or more metal precursors, a soluble polyethylenimine or polyethylenimine derivative, and a metal binding ligand or salt thereof, the polyethylenimine or polyethylenimine derivative characterized as having binding properties for the one or more metal precursors and the metal selected from alkali metals, alkaline earth metals, main group metals, transition metals other than copper, and lanthanide metals.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the generic temperature profile of post thermal treatment for epitaxial growth of different oxide films deposited by the polymer assisted deposition technique.
FIG. 2 shows the x-ray diffraction 2θ-scan of a europium oxide (Eu2O3) film deposited by the process of the present invention on a lanthanum aluminate (LaAlO3) substrate. The film is preferentially oriented out of the plane.
FIG. 3 shows the x-ray diffraction φ-scan of (440) Eu2O3 deposited by the process of the present invention on a LaAlO3 (110) substrate. The film is also oriented in the plane.
FIG. 4 shows a cross-sectional photomicrograph of a high resolution transmission electron microscopy (TEM) of a Eu2O3 film deposited by the process of the present invention on a LaAlO3 substrate. The interface between the substrate and the film is very sharp. No voids and second phases were detected in the film.
FIG. 5 shows a selected electron diffraction pattern of a Eu2O3 film deposited by the process of the present invention on a LaAlO3 substrate. The epitaxial nature of the film can be clearly seen from this diffraction pattern.
FIG. 6 shows the x-ray diffraction 2θ-scan of a titanium oxide (TiO2) film deposited by the process of the present invention on a R-cut sapphire substrate. The film has a rutile structure and is preferentially oriented out of the plane.
FIG. 7 shows the x-ray diffraction 2θ-scan of a titanium oxide (TiO2) film deposited by the process of present invention on a lanthanum aluminate (LaAlO3) substrate. The film has an anatase structure and is preferentially oriented out of the plane.
FIG. 8 shows the x-ray diffraction φ-scan of anatase (101) TiO2 and (101) LaAlO3 substrate. The anatase phase is formed by the process of present invention. The film is oriented in the plane.
FIG. 9 shows the x-ray diffraction 2θ-scan of a YBa2Cu3O7, (YBCO) film deposited by the process of present invention on a LaAlO3 substrate. The film is highly c-axis oriented.
FIG. 10 shows the x-ray diffraction φ-scan of (102) YBCO and (101) LaAlO3 substrate. The film was deposited by the process of present invention. The YBCO is epitaxial as seen by FIGS. 9 and 10.
FIG. 11 shows the x-ray diffraction 2θ-scan of a BaTiO3 film deposited by the process of present invention on a LaAlO3 substrate. The film is highly oriented out of the plane.
FIG. 12 shows the x-ray diffraction φ-scan of (101) BaTiO3 and (101) LaAlO3 substrate. The film was deposited by the process of present invention. The BaTiO3 is epitaxial as seen by FIGS. 11 and 12.
FIG. 13 shows the x-ray diffraction 2θ-scan of a SrTiO3 film deposited by the process of present invention on a LaAlO3 substrate. The film is highly oriented out of the plane.
FIG. 14 shows the x-ray diffraction φ-scan of (202) SrTiO3 and (202) LaAlO3 substrate. The film was deposited by the process of present invention. The SrTiO3 is epitaxial as seen by FIGS. 13 and 14.



DETAILED DESCRIPTION
The present invention is concerned with a process for preparing metal oxide films from solutions, optionally in an organic solvent-free process. The metal oxide films can be prepared with an epitaxial structure. The elimination of organic solvents from chemical solution deposition techniques can be preferred in some instances.
The process of the present invention uses a soluble polymer to assist in the deposition of the desired metal oxide. Thus, the process can be referred to as a polymer assisted deposition process. Inclusion of a soluble polymer with a single metal precursor or multiple metal precursors promotes better distribution of the materials during the deposition. The polymer can be removed subsequently by heating at sufficiently high temperatures to eliminate the polymer and leave a metal oxide film. The resultant metal oxide film can be prepared with orientation, i.e., the film can be prepared with an epitaxial structure. By using a soluble polymer in conjunction with one or more metal precursors, single or mixed compound/complex metal oxide films can be prepared. In one embodiment, the overall process can be an aqueous process that can be organic solvent free. Depending upon the selection of precursor and atmosphere during heating, other films such as metal films, metal nitride films, metal phosphate films, metal boride films, metal silicate films, metal fluoride films, metal chalcogenide films and metal pnictogenide films can be prepared.
The heating of the polymer and metal layer is generally carried out under an inert atmosphere, a reducing atmosphere, or an oxygen-containing atmosphere. Of example, in the case where it is desired to reduce a metal precursor to obtain a metal film, the reducing atmosphere can include, e.g., hydrogen, ammonia, formaldehyde, carbon monoxide, formic acid or other reducing agents well known to those skilled in the art. A reducing atmosphere can also be used in the preparation of metal nitride, metal boride, metal chalocogenide, and metal pinctogenide films. Suitable inert atmospheres can generally be argon nitrogen and the like. For preparation of metal oxides, an oxygen-containing atmosphere will generally be used.
While a metal oxide film can be prepared with an highly ordered structure, e.g., an epitaxial structure, by the process of the present invention, it may also be prepared with an amorphous structure or a nanocrystalline structure or a polycrystalline structure by suitable treatment after deposition of the polymer and metal containing layer upon a substrate or by suitable selection of the substrate. By “highly ordered” is meant ordering with a preferred orientation as can be seen in XRD measurements. Such amorphous or polycrystalline structures may be preferred for some applications.
The metal-containing films (the metal, the oxide, the nitride and the like) of the present invention are uniform films, i.e., they are continuous films covering the target substrate. They can also be readily formed as conformal films upon non-planar substrates or surfaces.
The soluble polymer used in the present process has binding properties for the metal precursors used to form the metal oxide film and can be, e.g., polyethylenimine (PEI), a substituted PEI or PEI derivative such as a carboxylated-polyethylenimine (PEIC), a phosphorylated-polyethylenimine (PEIP), a sulfonated-polyethylenimine (PEIS), an acylated-polyethylenimine, hydroxylated water-soluble polyethylenimines and the like or a polymer such as polyacrylic acid, polypyrolidone, and poly(ethylene-maleic acid). PEI or substituted PEIs such as PEIC are generally the preferred polymers. Substituted or PEI derivatives are post modified after formation of the base polymer. Typically, the molecular weight of such polymers is greater than about 30,000. By “binding” it is meant that the polymer and the metal are bound through any of various mechanisms such as electrostatic attraction, hydrogen bonding, covalent bonding and the like.
In order to have good processing characteristics, a precursor solution for metal oxide films must have a suitable viscosity. In the present invention, polymer plays dual functions. In other words, a polymer is also used to assist the viscosity desired for metal oxide film processing.
The solutions used in depositing the polymer and metal to the substrates are homogeneous solutions. By “homogeneous” is meant that the solutions are not dispersions or suspensions, but are actual solutions of the polymer, metal complexes and any metal binding ligands.
The criteria on the choice of polymers are that they be soluble, undergo a clean decomposition upon heating at high temperatures, e.g., temperatures over about 250° C., and are compatible with the metal compounds. By aiding in the desired viscosity, such polymers can allow processing of the metal oxide precursor solution into desired configurations such as films. The desired viscosity can be achieved through controlling the solution concentration of the soluble polymers and by controlling the molecular weight of the polymer. For high quality homogeneous films, polymer concentrations and the polymer ratio to metal components should be maintained at a proper balance. The rheology of the metal oxide precursor solution can also be important for the morphology and quality of the final metal oxide films. In order to form smooth films, the polymer solution must have suitable rheological properties so that any spin-coated film has no undesired patterns associated with polymer rheological properties.
The polymer further functions as binding agent to the metals within the precursor solution in assisting the formation of an intermediate deposited polymer-metal composite film and ultimately a metal oxide film. This requires that the polymer should also have suitable interactions to metal ions such that no phase separation occurs during the deposition processes. Thereafter, the deposited polymer-metal composite films are heated at high temperatures (calcined), e.g., at temperatures above about 250° C. to obtain the final metal oxide films. Thus, the soluble polymer selection should also have suitable decomposition characteristics, e.g., a clean decomposition under such calcination conditions, so that the final metal oxide film can be free of side products.
The general approach of the present invention can be applied to main group metals, transition group metals and lanthanide metals in forming resultant metal oxides, metal nitrides, metal phosphides, metal borides, metal silicides, metal chalcogenides, metal pnictogenides or metals. Among the main group metals are included aluminum, gallium, germanium, indium, tin, antimony, lead, and bismuth.
Among the transition metals are included titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, and iridium.
Among the lanthanide metals are included lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
Other metals such as alkaline earth metals including calcium, strontium and barium can generally be one metal component in mixed metal oxides such as calcium titanate, calcium ruthenate, barium titanate, barium ruthenate, strontium titanate, strontium ruthenate, yttrium barium copper oxide. Simple oxides such as MgO and SrO can be also prepared by this method.
Still other metals such as alkali metals including sodium, lithium and potassium may generally be one metal component in mixed metal oxides such as lithium tantalate (LiTaO3), lithium niobate (LiNbO3), Fe- or Ti-doped lithium niobate, potassium barium niobate (KBa2Nb5O15), potassium lithium niobate (K3Li2Nb5O15), potassium sodium tantalate ((K1-xNax)TaO3), K3Li2(TaxNb1-x)5O15 and the like.
The present invention may also employ fluoride complexes as precursors in the process of the present invention. For example, a gallium hexafluoride anion (GaF63−) can be formed by the addition of gallium nitrate and ammonium bifluoride and can be used as the metal precursor.
The metal oxide films prepared by the present process can include a metal oxide with a single metal, can be a metal oxide with two metals or three metals or may be a metal oxide including four or more metals. Among the metal oxides preparable by the present process are included titanium oxide, magnesium oxide, zinc oxide, ruthenium oxide and the like. Among the mixed metal oxides preparable by the present process are included barium titanium oxide (barium titanate), strontium titanium oxide (strontium titanate), barium strontium titanium oxide (barium strontium titanate), strontium ruthenium oxide (strontium ruthenate), lanthanum-strontium manganese oxide, yttrium-barium-copper oxide (YBa2Cu3O7) and the like. The metal oxide films prepared by the present process can be insulating, resistive, conductive, ferroelectric, ferromagnetic, piezoelectric, and even superconductive depending upon the chemical compositions and microstructures.
The composition, e.g., solution, used for the deposition includes the soluble polymer and the metal precursors. In addition, other metals can be included through addition of appropriate metal salts. For example, barium can be added through a barium salt such as barium acetate. Other suitable metal salts may include metal nitrates, metal oxalates, metal acrylates, and metal coordination complexes.
Metal nitride films preparable by the present process can include a metal nitride with a single metal, can be a metal nitride with two metals or three metals or may be a metal nitride including four or more metals. Among the metal nitrides that can be prepared by the present process are included titanium nitride, gallium nitride, zirconium nitride, vanadium nitride and aluminum nitride and the like. Among the mixed metal nitrides that can be prepared by the present process are included titanium aluminum nitride, magnesium vanadium nitride and the like. The metal nitride films prepared by the present process can be insulating, resistive, conductive, ferroelectric, ferromagnetic, and piezoelectric, depending upon the chemical compositions and microstructures. The preparation of metal nitride films can be accomplished by depositing a polymer and metal containing layer upon a suitable substrate as with a metal oxide, but the heating can then be conducted under an ammonia-containing atmosphere, a reducing atmosphere or an inert atmosphere to yield the desired nitride materials.
Metal phosphate films preparable by the present process can include a metal phosphate with a single metal, can be a metal phosphate with two metals or three metals or may be a metal phosphate including four or more metals. Among the metal phosphates that can be prepared by the present process are included zirconium phosphate, molybdenum phosphate and the like. The metal phosphate films prepared by the present process can be insulating, resistive, conductive, ferroelectric, ferromagnetic, and piezoelectric and may be useful as catalytic materials, depending upon the chemical compositions and microstructures. The preparation of metal phosphate films can be accomplished by depositing a polymer and metal containing layer upon a suitable substrate as with a metal oxide, but the heating can then be conducted under an inert atmosphere to yield the desired phosphate materials. Suitable phosphate precursors for inclusion in a homogenous solution of the present invention can include phosphoric acid or may include alkylphosphates or ammonium phosphate and the like.
Metal boride films preparable by the present process can include a metal boride with a single metal, can be a metal boride with two metals or three metals or may be a metal boride including four or more metals. Among the metal borides preparable by the present process are included magnesium boride, lanthanum boride and the like. Among the mixed metal borides that can be prepared by the present process are included YRh4B4 and the like. The metal boride films prepared by the present process can be semiconductors, superconductors, diamagnetic, paramagnetic, ferromagnetic, anti-ferromagnetic, insulating, resistive, conductive, ferroelectric, and piezoelectric, depending upon the chemical compositions and microstructures. The preparation of metal boride films can be accomplished by depositing a polymer and metal containing layer upon a suitable substrate as with a metal oxide, but the heating can then be conducted under an inert atmosphere or a reducing atmosphere to yield the desired boride materials. Suitable boride precursors for inclusion in a homogenous solution of the present invention may include borohydrides and the like.
In addition to metal nitride films, similar or other metal pnictogenides such as metal arsenides (e.g., gallium arsenide) or metal phosphides (e.g., gallium phosphide) may be preparable by the present process. The preparation of metal pnictogenide films can be accomplished by depositing a polymer and metal containing layer upon a suitable substrate as with a metal oxide, but the heating can then be conducted under an inert atmosphere to yield the desired pnictogenide materials. The metal pnictogenide films prepared by the present process can be insulating, resistive, conductive, ferroelectric, ferromagnetic, and piezoelectric, depending upon the chemical compositions and microstructures. Suitable pnictogenide precursors for inclusion in a homogenous solution of the present invention can include arsenide trichloride or may include thiourea and the like.
In addition to metal oxide films, similar or other metal chalcogenide films that can be prepared by the present process can include a metal chalcogenide with a single metal (e.g., lead selenide, zinc sulfide and the like), can be a metal chalcogenide with two metals or three metals (e.g., copper indium gallium selenide and the like) or may be a metal chalcogenide including four or more metals. The metal chalcogenide films prepared by the present process can be insulating, resistive, conductive, ferroelectric, ferromagnetic, and piezoelectric, depending upon the chemical compositions and microstructures. The preparation of metal chalcogenide films can be accomplished by depositing a polymer and metal containing layer upon a suitable substrate as with a metal oxide, but the heating can then be conducted under an inert atmosphere or reducing atmosphere to yield the desired chalcogenide materials. The chalcogenides can include selenides, tellurides and the like. Suitable chalcogenide precursors for inclusion in a homogenous solution of the present invention may include dialkyl selenide, dissolved selenium metal and the like.
Metal oxide clusters can be employed as anions bound to a polymer in the present invention. In this manner, metal silicides may be preparable by the present process using a sodium silicon water glass (e.g., a 50/50 weight % solution of Na and Si containing negatively charged silicate oligomers). Other metal oxide clusters such as sodium orthovanadate or sodium tungstate can also be used.
In one aspect of the present invention, composites can be prepared including the various metal-containing films as described with various additional additives to provide tailoring of the material properties. Among the additives can be nanoparticles, especially nanoparticles of various metals such as transition metals, lanthanide metals or main group metals, nanoparticles of various metal oxides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, nanoparticles of various metal nitrides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, nanoparticles of various metal carbides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, nanoparticles of various metal chalcogenides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, nanoparticles of various metal pnictogenides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, nanoparticles of various metal borides including one or more metal such as a transition metal, a lanthanide metal or a main group metal, or nanoparticles of various metal silicides including one or more metal such as a transition metal, a lanthanide metal or a main group metal. Examples of such nanoparticles can include titanium dioxide, strontium oxide, erbium oxide and the like, such nanoparticles suitable for modifying the electronic properties of metal oxide films of a different material.
Other additives may include silicon beads, which may provide void properties to the various metal containing films in accordance with the present invention. Also, various quantum dot materials, e.g., cadmium selenide dots having a coating of zinc sulfide, such quantum dot materials being well known to those skilled in the art, may be added to the various metal containing films in accordance with the present invention.
The composition, e.g., solution, used for the deposition includes the soluble polymer and the metal precursors. In addition, other metals can be included through addition of appropriate metal salts. For example, barium can be added through a barium salt such as barium acetate. Other suitable metal salts may include metal nitrates, metal oxalates, metal acrylates, and metal coordination complexes.
The solvent for dissolution of the soluble polymer can be, e.g., water, lower alcohols such as methanol, ethanol, propanol and the like, acetone, propylene carbonate, tetrahydrofuran, acetonitrile, acetic acids and mixtures thereof such as water and ethanol and the like. As the soluble polymer used in the present invention includes binding properties for the metals or metal precursors used in formation of the metal oxide films, the polymer can help provide the necessary solubility to the respective metals, e.g., metal precursors. In some instances, the metal can initially be in a metal complex such as a complex of the respective metal with a metal binding ligand or salt thereof such as ethylenediaminetetraaceticacid (EDTA) or salts thereof such as dipotassium ethylenediaminetetraaceticacid. EDTA-metal complexes are generally soluble within solutions including a soluble polymer with binding properties for the metal precursors such as PEI and the like.
Among suitable metal binding ligands besides EDTA and salts thereof can be included other carboxylic acid ligands such as ethylenediaminediaceticacid (EDDA), trans-1,2-diamino-cyclohexan-N,N,N′,N′-tetraacetic acid (CDTA), ethyleneglycol-O,O′-bis-(2-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA), diethylenetriamine-pentaacetic acid (DTPA), N-(2-hydroxyethyl)-ethylenediamine-N,N′,N′-triacetic acid (HEDTA), nitrilotriacetic acid (NTA), triethylentetramine-N,N,N′,N′′,N′′′,N′′′′-hexaacetic acid (TTHA) and the like, polypyridyl ligands such as terpyridine, 2,2′-bypyridine, 1,10-phenanthroline and the like, beta-diketone (acetylacetonate) ligands such as 2,4-propanedione and derivatives thereof, catecholate and aryl oxide or alkyl oxide ligands, macrocyclic ligands such as cyclam, cyclen, triazacyclononane and derivatives thereof, or other simple ligands such as aquo (H2O) and amines (NH3), i.e., Co(NH3)62+. Shiff-base ligands such as trimethylenediamainetetramethylglyoximato ligand or the salen type ligands may also be used.
The starting solution is typically maintained at ambient temperatures from about 15° C. to about 30° C., more usually from about 20° C. to about 25° C. Within those temperature ranges, the materials added to the solution are soluble. In preparation of solutions used in the present process, the solutions using a polyethylenimine as the metal binding polymer can be filtered prior to use to remove any non-soluble components. One exemplary process in the preparation of the solutions involves filtering the precursor solution through an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane. Such a filter is designed to pass materials having a molecular weight of less than about 10,000 g/mol (e.g., unbound metal, smaller polymer fragments and the like) while retaining the desired materials of a larger size. Ultrafiltration allows for removal of any unwanted salts such as cations, anions or other impurities. The binding constant of the metal to the polymer is critical to the success of the polymer filtration. Metals with binding constants lower than about 10000 mol−1 L will be washed from the polymer by the ultrafiltration process and are not useful in the invention. In general, the larger the binding constant, the better results can be obtained. For example lutetium solutions of PEI can be prepared, but the binding constant of the lutetium to the PEI is too low and all lutetium is lost from the polymer during ultrafiltration. Concentrations of the metal ion that exceed the binding capacity of the polymer or system will result in unbound metals in the polymer solution. This effects the formation of good quality films. Solutions where the metal is not bound to the polymer with binding constants lower than about 10000 mol−1 L exhibit lower stability, often demonstrated by precipitation of the metal. For example, zirconium precipitates from the solutions if the concentration of the metal ions exceeds the binding capacity. Films prepared from solutions where the binding constant for the metal was below about 10000 mol−1 L often result in low density or powdery films. For example, films prepared using zinc solutions where the concentration is above the binding capacity of the polymer result in powdery films that do not adhere well to the substrates. In mixed metal systems when the binding constant for either metal is below about 10000 mol−1 L, the formation of low quality materials through phase separation can result.
The metal ratio can be controlled through appropriate addition of metal precursors to the solvent used in the deposition. Such solutions can generally have a shelf life of more than a year. As a general practice, solutions with metal concentrations of greater than 20 mmol are preferred.
In one embodiment of the present invention, the starting solution can be deposited on a desired substrate, e.g., by spray coating, dip coating, spin coating, ink jet printing and the like. After deposition of the starting solution on a substrate, the deposited coating must be calcined or heated at high temperatures of from about 250° C. to about 1300° C., preferably from about 400° C. to about 1200° C. for a period of time sufficient to remove the polymer and to form only the metal oxide film. FIG. 1 shows a general temperature profile of the thermal treatment for growth of metal oxide films deposited by the process of this invention. Heating times may be varied and may be longer depending upon the thickness of the deposited film.
Optionally, the deposited coating can be initially dried by heating to temperatures of from about 50° C. to about 150° C. for from about 15 minutes to several hours, preferably for less than one hour. The deposited polymer-metal oxide film undergoes removal of a percentage of volatile species during such an initial drying stage.
The resultant metal oxide films from the present process have been optical quality films in that they are highly smooth films with a mirror-like appearance. Many of the films have been found to be epitiaxial in structure.
The present invention enables the processing of metal oxide films with convenience and flexibility required in industrial fabrication. This process involves making metal oxide films from solutions—optionally in an organic solvent-free process. Barium titanate (BaTiO3) and strontium titanate (SrTiO3) films have been prepared using polymer-assisted aqueous deposition (PAD) techniques. X-ray diffraction measurement indicates that the barium titanate and strontium titanate films on LaAlO3 are preferentially oriented along the (100). They are also epitaxial as confirmed from x-ray (φ-scans of the (101) diffraction of the films and transmission electron microscopy.
The polymer is used to bind metals and metal precursors. The binding of the metal precursor to the polymer enables the formation of metal bound polymer solutions that are stable for months, allows the removal of any unwanted anions or cations by filtration, e.g., through an Amicon ultrafiltration unit, and brings multiple metals together in a homogeneous manner at a molecular level without unwanted precipitation. For some solutions, the Amicon filtration is required to remove anions or cations, such as sulfate from a vanadyl sulfate precursor or sodium from a sodium orthovanadate precursor, that cannot be removed during the thermal treatment of the film. For other solutions, Amicon filtration may not required, for example, simple nitrate or chloride metal precursors can readily decompose to oxides with no chloride or nitrate in the final film since they can be removed as gaseous products during the thermal treatment. When the solution is not treated with Amicon filtration such as in example GG for zirconyl nitrate, the metal must still have a binding constant greater than about 10000 mol−1 L and the concentration of the metal cannot exceed the binding capacity of the polymer. Stable solutions can be prepared within these constraints to yield high quality films. Low binding constants such as zirconyl nitrate added to polyethylenimine with no EDTA or solutions with the metal precursor in excess of the binding capacity of the polymer can lead to unstable solutions in which the metal precipitates as an oxide or results in poor quality films as a result of metal oligomerization prior to film formation during thermal treatment.
The polymer metal precursor binding prevents selective precipitation of unwanted metal oxide phases even when multiple metal solutions are mixed together. A portion of the water can be removed and the metals concentrated within the remaining solution. Even at the extreme of just polymer and metal, the dried solution (a gel) includes only well dispersed metal atoms bound to the polymer. The present invention has extended the potential of controlling relative metal concentrations at the molecular level for mixed metal oxides such as superconductors (YBa2Cu3O7-x). This can be done in one of three ways.
In a first manner, a single polymer (such as carboxylated polyethylenimine) can be added to a solution containing simple salts (such as nitrate) of two or more metals in the correct ratio. If the binding constant is high for both metals then they will remain in the correct ratio during filtration and concentration of the polymer.
In a second manner, the metals can be bound individually to one or more polymers and concentrated. The resulting solution can be examined by ICP to determine metal content and then mixed appropriately prior to spin coating. Different polymers and different solvents can be used for different metals in this system.
In a third manner, metal complexes such as a metal-EDTA complex can be prepared and mixed in the desired ratios. These complexes can then be bound to a polymer (such as polyethylenimine) and concentrated.
The present invention is more particularly described in the following examples which are intended as illustrative only, since numerous modifications and variations will be apparent to those skilled in the art.
Examples A-NN describe the preparation of solutions used in the deposition and formation of the metal oxide films, metal films or other metal containing films. Examples 1-21 describe the deposition of such metal oxide films, metal films or other metal containing films using such solutions. Polyethylenimine was obtained from BASF as a water free, branched, polymer with an average MW of 50,000. Water was deionized via reverse osmosis (having a resistivity >16 Ohms).
EXAMPLE A
A solution including zinc chloride and polyethylenimine was prepared as follows. An amount of 4.4 grams of polyethylenimine was dissolved in 40 mL of water and the pH was adjusted to pH 6 with addition of 10% HCl. To this solution was added 2.2 grams of ZnCl2 and the solution was stirred. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 45 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 21.1 mg/mL of Zn.
EXAMPLE B
A solution including zinc nitrate and polyethylenimine was prepared as follows. An amount of 2.0 grams of polyethylenimine was dissolved in 40 mL of water and the pH was adjusted to pH 6 with addition of 10%. HCl. To this solution was added 2.5 grams of zinc nitrate hexahydrate and the solution was stirred. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 24.2 mg/mL of Zn.
EXAMPLE C
A solution including zinc chloride, dipotassium ethylenediaminetetraaceticacid (EDTA K2) and polyethylenimine was prepared as follows. An amount of 2.0 grams of dipotassium ethylenediaminetetraaceticacid was dissolved in 30 mL of water. To this solution was added 0.75 grams of zinc chloride and the solution was stirred. After stirring, 2 grams of polyethylenimine were added and the pH was adjusted to 9 with addition of 10% HCl. The solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 24.2 mg/mL of Zn.
EXAMPLE D
A solution including copper nitrate and polyethylenimine was prepared as follows. One gram of polyethylenimine (from BASF) was placed in a 50 mL Falcon tube and dissolved in 25 mL of water. Then, 0.85 grams of copper nitrate trihydrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 15.2 mg/mL of Cu.
EXAMPLE E
A solution including copper nitrate, ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. One gram of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. One gram of polyethylenimine (from BASF) was added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 0.85 grams of copper nitrate trihydrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 15.2 mg/mL of Cu.
EXAMPLE F
A solution including yttrium nitrate, ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. One gram of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. One gram of polyethylenimine (from BASF) was added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 1.36 grams of yttrium nitrate hexahydrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 15.3 mg/mL of Y.
EXAMPLE G
A solution including barium nitrate, ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. One gram of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. One gram of polyethylenimine (from BASF) was added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 0.90 grams of barium nitrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 29.5 mg/mL of Ba.
EXAMPLE H
A solution including barium hydroxide, ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. An amount of 1.4 grams of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. 1.4 grams of polyethylenimine (from BASF) were added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 1.50 grams of water insoluble barium hydroxide octahydrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 14 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 28.8 mg/mL of Ba.
EXAMPLE I
A solution including titanium and peroxide and PEIC was prepared as follows. An amount of 1.0 gram of PEIC (PEI with 1⁄2 of the amine sites functionalized into carboxylates) was dissolved in 30 mL of water. A solution of soluble titanium was prepared by placing 2.5 grams of 30% peroxide into 30 mL of water and then slowly adding 2.5 grams of titanium tetrachloride. Small aliquots of 1 mL of the titanium solution were then added to the PEIC solution and the pH was monitored as the pH increased above 3.5 aliquots of 10% NaOH were added to lower the pH to 7.5. This process was repeated until addition of the titanium solution resulted in precipitate that would not dissolve. The solution was then filtered and was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 4.6 mg/mL of Ti.
EXAMPLE J
A solution including strontium nitrate and ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. An amount of 1.7 grams of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. 1.7 grams of polyethylenimine (from BASF) were added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 1.22 grams of strontium nitrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 17 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 16.0 mg/mL of Sr.
EXAMPLE K
A solution including cadmium nitrate and ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. An amount of 2.0 grams of polyethylenimine was dissolved in 40 mL of water and the pH was adjusted to pH 6 with addition of 10% HCl. 2.5 grams of cadmium nitrate tetrahydrate were added and the solution was stirred. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 37.6 mg/mL of Cd.
EXAMPLE L
A solution including indium nitrate, ethylenediaminetetraaceticacid and polyethylenimine was prepared as follows. One gram of ethylenediaminetetraaceticacid was placed in a 50 mL Falcon tube and 25 mL of water were added. The ethylenediaminetetraaceticacid does not dissolve at this stage. One gram of polyethylenimine was added to the solution and the solution was agitated until the ethylenediaminetetraaceticacid and the polyethylenimine were in solution. Then 1.00 grams of indium nitrate were added. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 14.2 mg/mL of In.
EXAMPLE M
A solution including tin (II) chloride and PEIC was prepared as follows. An amount of 1.0 gram of PEIC (PEI with 1⁄2 of the amine sites functionalized into carboxylates) was dissolved in 30 mL of water. To this solution was added 0.65 grams of tin(II) chloride. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 15.5 mg/mL of Sn.
EXAMPLE N
A solution including gallium chloride and polyethylenimine in ethanol was prepared as follows. An amount of 5 grams of polyethylenimine were dissolved in 95 grams of ethanol. The solution was dried over molecular sieves for two days and then filtered through a 0.49 micron filter. About 20 mL of the solution were placed in a 50 mL Falcon tube and 0.6 grams of gallium chloride were added in an inert atmosphere. A precipitate appears and then dissolves into solution. This solution was placed in an Amicon ultrafiltration unit containing a YM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL with absolute ethanol and then concentrated to 10 mL in volume. After filtration, the viscosity of the solution was further increased by removing a small amount of solvent by rotary evaporation under reduced pressure. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 14.3 mg/mL of Ga.
EXAMPLE 0
A solution including indium and tin was prepared as follows. An amount of 5.0 grams of the final solution from example L was mixed with 5.0 grams of the final solution from example M to yield a solution equal molar in In and Sn.
EXAMPLE P
A solution including barium and titanium was prepared as follows. An amount of 5.0 grams of the final solution from example I was mixed with 2.21 grams of the final solution from example G to yield a solution equal molar in Ti and Ba.
EXAMPLE Q
A solution including strontium and titanium was prepared as follows. An amount of 5.0 grams of the final solution from example I was mixed with 2.59 grams of the final solution from example J to yield a solution equal molar in Ti and Sr.
EXAMPLE R
A solution including yttrium barium and copper was prepared as follows. An amount of 5.0 grams of the final solution from example F was mixed with 8.0 grams of the final solution from example G and 10.0 grams of the final solution from example E to yield a solution with molar ratios of 1:2:3 for Y:Ba:Cu.
EXAMPLE S
A solution including yttrium barium and copper was prepared as follows. A YBa2Cu3 solution was also prepared by addition of 2.0 grams polyethylenimine and 2.0 grams ethylenediaminetetraacetic acid to a solution of the metal nitrates having an a ratio of metal of Y:Ba:Cu equal to 1:2:3 (0.47 grams yttrium nitrate hexahydrate, 0.64 grams barium nitrate and 0.89 grams copper nitrate trihydrate). The solution was then concentrated by rotary evaporation under reduced pressure. This solution can be used for coating without filtration since nitrates decompose readily to gases. Alternatively hydroxide and other salts that thermally degrade can also be used.
EXAMPLE T
A solution including cadmium and zinc and polyethylenimine was prepared as follows. An amount of 2.0 grams of polyethylenimine was dissolved in 40 mL of water and the pH was adjusted to pH 6 with addition of 10% HCl. To this solution was added 1.6 grams of zinc nitrate hydrate and 0.4 grams of cadmium nitrate tetrahydrate and the solution was stirred. After stirring the solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 18.5 mg/mL of Zn and 6.9 mg/mL of Cd.
EXAMPLE U
A solution including gallium-doped zinc was prepared as follows. An amount of 200 mg of the concentrated solution from example N was mixed with 3.8 grams of the solution from example B. The resulting solution was clear and homogenous. This method can be used to generate a wide variety of gallium doped zinc solutions by simply mixing in the appropriate ratios.
EXAMPLE V
A solution including europium chloride and PEIC was prepared as follows. A europium solution with a carboxylated form of polyethylenimine (PEIC) was prepared by mixing 2.39 g of EuCl3 6H2O with 1.52 g of the polymer. It was then filtered by the Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane with 200 mL of water and concentrated to 11.5 mL. The resulting solution of 13 wt % PEI and 0.49 wt % Eu was used for spin coating
EXAMPLE W
A solution including zirconyl nitrate and polyethylenimine was prepared as follows. An amount of 1.0 grams of dipotassium ethylenediaminetetraacetic acid was dissolved in 30 mL of water. To this solution was added 2.0 grams of zirconyl nitrate (35 wt % in water) and the solution was stirred. Polyethylenimine (1 gram) was then added to the solution and the solution was stirred. The resulting solution is clear and has a pH of 8.0. This solution was placed in an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL with absolute ethanol and then concentrated to 10 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 19.3 mg/mL of Sr.
EXAMPLE X
A solution including titanium catecholate and polyethylenimine was prepared as follows. Titanium solutions were prepared using a titanium catecholate precursor. A solution of 2 g of PEI in 40 mL of deionized water was added to a mixture containing 2 g of Ti(cat)3(NH4)2 dissolved in 20 mL of deionized water. This deep red solution was then heated for 20 minutes at 80° C. Upon cooling a precipitate was observed. The solution was stirred for a further 24 hrs at room temp then filtered through Celite. The deep red solution was filtered using an Amicon ultrafiltration unit containing a PM 10 ultrafiltration membrane, to give clean solution. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution had 1.4 mg/mL of Ti.
EXAMPLE Y
A solution including barium titanium catecholate and polyethylenimine was prepared as follows. An amount of 500 mg of BaTi(cat)3 was dissolved in 40 mL of water. This was then added to a solution of PEIC (2 g PEIC dissolved in 40 mL of water). This deep red solution was stirred for 24 hrs and then all of the solvent was removed at reduced pressure. The resultant oil was resdisolved in water (40 mL) and filtered through Celite to give a clear solution.
EXAMPLE Z
A solution including yttrium, barium and copper was prepared as follows. Three equimolar solutions of yttrium nitrate (1.36 grams yttrium nitrate hexahydrate, 1.0 grams ethylenediaminetetraacetic acid, 1.2 grams polyethylenimine, 30 mL water), barium nitrate (0.90 grams barium nitrate, 1.0 grams ethylenediaminetetraacetic acid, 1.2 grams polyethylenimine, 30 mL water), and copper nitrate (0.85 grams copper nitrate trihydrate, 1.0 grams ethylenediaminetetraacetic acid, 1.2 grams polyethylenimine, 30 mL water) with ethylenediamine tetraacetic acid and polyethylenimine were prepared and then mixed in a 1:2:3 ration Y:Ba:Cu. The final solution was concentrated by rotary evaporation under reduced pressure.
EXAMPLE AA
Metals can be directly bound by adding metal salts to the polymer solution. This has been demonstrated with PEI and Co, Ni, Pd, Zn and Cu as follows and with the PEIC polymer and Ti, Sn, Eu (as described above). A palladium chloride and polyethylenimine polymer solution was prepared by dissolving 1.0 grams of polyethylenimine in 40 mL of water and adjusting the pH to 6 with addition of 10% HCl. To this polymer solution was added 1.0 gram of palladium chloride and the solution was stirred. The solution pH was adjusted to pH 1 to get complete dissolution and then titrated back to pH 6 with NaOH. After stirring, the solution filtered with a 0.45 micron filter and then placed in an Amicon filtration unit containing a PM 10 filter designed to pass materials having a molecular weight <10,000 g/mol. The retained solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution 425 mM Pd. Solutions of Co, Ni, Cu, and Zn bound to PEI were made in a similar manner although they did not require acidic conditions to dissolve. In these cases the metal salt was added to PEI and the pH was adjusted to near 7 with 10% HCl. The solutions were then purified by ultrafiltration through the Amicon filtration unit containing a PM 10 filter. A metal concentration greater than 10 mM indicates substantial binding to the polymer and the particular metal concentrations of these solutions are shown in Table 1. Final metal concentrations can be readily altered by adding water or concentrating by rotary evaporation.











TABLE 1







Metal salt
Polymer
Final [metal]









1.0 g PdCl2
1.0 g PEI
425 mM



0.85 g Cu(NO3)2
1.2 g PEI
330 mM



3H2O





1.0 g ZnCl2
1.0 g PEI
350 mM



1.8 g CoCl2
1.8 g PEI
181 mM



2.0 g NiCl2
2.0 g PEI
441 mM










EXAMPLE BB
Metal oxide clusters can be bound as anions to the polymer. This has been demonstrated with PEI and Si, W, and V. A silicon and polyethylenimine polymer solution was prepared by dissolving 7.0 grams of polyethylenimine in 70 mL of water followed by the addition of 8 g of water glass (50/50 wt % solution of Na and Si containing negatively charge silicate oligomers). The solution was titrated to pH 6 using 10% HCl. After stirring the solution (at pH 6) filtered through CeliteTM diatomaceous earth and then placed in an Amicon filtration unit containing a PM 10 filter designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 80 mL in volume. This process was repeated three times to remove the unwanted sodium. Inductively coupled plasma-atomic emission spectroscopy showed that the final was 473 mM in Si. Vanadium and tungsten were bound as metal oxide clusters in a similar manner except that the metal oxide clusters are only stable at acidic pH. Solutions of the metal oxide clusters at pH 4 were added to PEI in solution at pH 4 and stirred. The pH can then be adjusted to 6 followed by purification by Amicon filtration. Alternatively the solution can be purified directly by ultrafiltration with no pH adjustment. The particular metal concentrations of these solutions are shown in Table 2.











TABLE 2







Metal complex
Polymer
Final [metal]









1.9 g sodium 
2.0 g PEI 
139 mM



orthovanadate at pH 4
at pH 4




1.5 g sodium 
1.5 g PEI 
257 mM



tungstate at pH 4
at pH 4




  8 g water glass 
  7 g PEI
473 mM



(50/50 wt % Si/Na)












EXAMPLE CC
Metals can be bound to PEI as fluoride complexes as follows. Gallium nitrate (1.0 g) was dissolved in 20 mL of water followed by the addition of 0.67 g of ammonium bifluoride to generate the GaF63− anion. PEI (1.0 g) was then added and the mixture was purified by Amicon filtration.
EXAMPLE DD
Metals can also be bound to PEI as EDTA complexes of the metals. This has been demonstrated with zinc (Zn), hafnium (Hf), gallium (Ga), neodymium (Nd), yttrium (Y), barium (Ba), strontium (Sr), bismuth (Bi), manganese (Mn), cerium (Ce), indium (In), europium (Eu), lanthanum (La), aluminum (Al), calcium (Ca), and lead (Pb). For a solution of zinc chloride, dipotassium ethylenediaminetetraacetic acid and polyethylenimine, 2.0 grams of dipotassium ethylenediaminetetraacetic acid were dissolved in 30 mL of water. Then, 0.75 grams of zinc chloride were added and the solution was stirred. After stirring, 2 grams of polyethylenimine were added. The solution was placed in an Amicon filtration unit containing a PM 10 filter designed to pass materials having a molecular weight <10,000 g/mol. The solution was diluted to 200 mL and then concentrated to 20 mL in volume. Inductively coupled plasma-atomic emission spectroscopy showed that the final solution was 373 mM Zn. The other metals including Hf, Ga, Nd, Y, Ba, Sr, Bu, Mn, Ce, In, Eu, La, Al, Ca, and Pb were each bound to PEI as EDTA complexes of the metals in a similar manner. The particular metal concentrations of these solutions are shown in Table 3.










TABLE 3





Metal Salt
Polymer
EDTA
Final [metal]







1.0 g HfPOCl2
1.0 g PEI
1.0 g
155 mM


1.0 g Ga(NO3)3
1.0 g PEI
1.0 g
230 mM


1.5 g PbCl2
1.5 g PEI
1.5 g
234 mM


3.0 g La(NO3)3
2.0 g PEI
2.0 g
172 mM


1.3 Y(NO3)3 6H2O
1.0 g PEI
1.0 g
115 mM


1.5 g Nd(NO3)3 6 H2O
1.0 g PEI
1.0 g
158 mM


1.5 g CeCl3 7 H2O
1.2 g PEI at pH 4 
1.2 g
134 mM


1.0 g EuCl3
1.0 g PEI
1.0 g
136 mM


0.5 g Ca(OH)2
4.0 g PEI
4.0 g
152 mM


1.7 g MnCl2 hydrate
1.8 g PEI
1.7 g
216 mM


1.7 g Al2(SO4)3 hydrate
1.9 g PEI
1.7 g
182 mM


2.0 g Al(NO3)3 hydrate
2.0 g PEI
2.0 g
245 mM


1.0 g Bi(NO3)3
1.0 g PEI
1.0 g
101 mM









EXAMPLE EE
A solution including zinc nitrate and polyethylenimine was prepared as follows. An amount of 1.0 grams of polyethylenimine was dissolved in 40 mL of water and the pH was adjusted to pH 6 with addition of 10% HCl. To this solution was added 2.5 grams of zinc nitrate hexahydrate and the solution was stirred. The solution was then concentrated to 20 mL in volume to give a clear solution.
EXAMPLE FF
A solution including copper nitrate and polyethylenimine was prepared as follows. One gram of polyethylenimine (from BASF) was placed in a 50 mL Falcon tube and dissolved in 25 mL of water. Then, 0.85 grams of copper nitrate trihydrate were added. After stirring the solution was then concentrated to 10 mL in volume, to give a clear blue solution.
EXAMPLE GG
A solution of zirconium bound polyethylenimine was prepared as follows: 1.34 gram of ethylenediaminetetraacetic acid was placed in a 50 mL Falcon tube and 20 mL of water were added followed by 0.715 mLs of concentrated NH4OH solution. Next 2.94 g of a 30 wt % zirconyl nitrate solution were added followed by the dropwise addition of concentrated NH4OH until the solution cleared. Finally, two grams of polyethylenimine (from BASF) was added.
EXAMPLE HH
A solution of zirconium bound polyethylenimine was prepared as follows: 1.34 gram of ethylenediaminetetraacetic acid was placed in a 50 mL Falcon tube and 20 mL of water were added followed by 0.715 mLs of concentrated NH4OH solution. Next 2.94 g of a 30 wt % zirconyl nitrate solution were added followed by the dropwise addition of concentrated NH4OH until the solution cleared. Two grams of polyethylenimine (from BASF) was added. The final solution was then purified by Amicon filtration.
EXAMPLE II
A solution of uranium bound polyethylenimine was prepared as follows: 1.72 mLs of concentrated HCl were added to 3.28 mLs of water followed by 3.66 grams of polyethylenimine (from BASF) The pH of the solution was then adjusted to pH 4 by dropwise addition of concentrated HCl. A uranium stock solution was prepared by dissolving 0.459 g of uranyl nitrate hydrate in 3 mLs of water. The final uranium bound polyethylenimine solution was formed by mixing an equal volume, 1 mL each, of the uranium stock solution and the acidic PEI solution.
EXAMPLE JJ
A solution of vanadium bound polyethylenimine was prepared as follows: 1.0 g of vanadyl sulfate and 1.0 g of ethylenediaminetetraacetic acid were mixed together with 30 mL of water in a 50 mL Falcon tube. 1.0 g of polyethylenimine (from BASF) was then added resulting in a clear blue solution. The solution was then purified by Amicon filtration to remove the sulfate anions and concentrated to 10 mL of volume.
EXAMPLE KK
A mixed metal solution containing yttrium and uranium was prepared by combining solution F and solution II in a ratio such that the yttrium:uranium molar ratio was 6:1 based on ICP analysis of the final solution from Example F and the final solution from Example II.
EXAMPLE LL
A mixed metal solution containing yttrium and vanadium was prepared by combining solution F and solution JJ in a ratio such that the yttrium:vanadium molar ratio was 1:1 based on ICP analysis of the final solutions from Examples F and JJ.
EXAMPLE MM
The precursor solution for the deposition of BiVO4 films was obtained from a mixture of separate solutions of Bi and V bound to polymers. Specifically, 2 g of Bi(NO3)3 was dissolved in 40 mL of H2O, followed by addition of 2 g of ethylenediaminetetraacetic acid (EDTA) and 2 g of polyethylenimine (PEI) into the solution. To prepare the V solution, 2 g of NaVO3 was dissolved in 40 mL of H2O followed by addition of 2 g of PEI. The two solutions were separately purified by repeated Amicon filtration that retains the high molecular weight polymer with bound metal atoms while allowing any low molecular weight (<10 000) species to pass through. This effectively removes nitrate ions from the bismuth precursor and Na+ from the vanadium precursor, since Na+ does not bind PEI. The final concentrations of the solutions were 0.216 M for Bi and 0.134 M for V as determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Mixed metal solutions of Bi and V were then made by combining the separate Bi and V solution to obtain the desired stoichiometry.
EXAMPLE NN
Solutions containing Cu and Al were prepared by mixing separate solutions of copper and aluminum. The solution containing copper was prepared by dissolving 3 g Cu(NO3)2 in 40 mL H2O, followed by adding 3 g PEI polymer into the solution. The aluminum solution was prepared with by dissolving 2 g Al(NO3)3 in 40 mL H2O, followed by adding 3 g of a fluorinated PEI (PEIF) into the solution. The Cu and Al solutions were then separately filtrated and concentrated in an Amicon filtration unit to give a final concentration of 471 mM for Cu and 201 mM for Al, respectively. The final Cu/Al solutions were prepared by mixing the Cu and Al solutions in a ratio to obtain the desired Cu:Al stoichiometry
EXAMPLE 1
The europium metal containing solution from Example V was used to spin coat films onto substrates of lanthanum aluminum oxide (LaAlO3). Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds. (Spin Coater Model 100, from Cost Effective Equipment, a division of Brewer Science, Inc., Rolla, Mo.)
The resultant coating of polyethylenimine and europium metal was gradually heated from room temperature to about 1000-1200° C. over a period of about one hour under an oxygen atmosphere. FIG. 1 shows the general temperature profile of the thermal treatment for epitaxial growth of films deposited by the process of this invention. This heating process yielded polymer-free metal oxide films on the LaAlO3 substrates. An x-ray diffraction 2θ-scan of the resultant europium oxide (Eu2O3) film is shown in FIG. 2. The film was preferentially oriented out of the plane. FIG. 3 shows the φ-scans of (440) Eu2O3 and (101) LaAlO3. The film is also oriented in the plane. The epitaxial nature of the film deposited using this invention is further evidenced by the transmission electron microscopy as shown in FIG. 4 (high-resolution transmission electron microscopy) and FIG. 5 (selected electron diffraction pattern of a Eu2O3 film on the LaAlO3 substrate). The interface between the substrate and the Eu2O3 film is very sharp. No voids and second phases were detected in the film.
EXAMPLE 2
The titanium metal containing solution from Example I was used to spin coat films onto R-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of polyethylenimine and titanium metal was gradually heated from room temperature to about 1100-1200° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films on the sapphire substrates. The x-ray diffraction 2θ-scan of the titanium oxide (TiO2) film on the R-cut sapphire substrate is shown in FIG. 6. The film has a rutile structure and was preferentially oriented out of the plane. The epitaxial nature of the rutile TiO2 on R-cut sapphire is further evidenced by the high-resolution transmission electron microscopy and selected electron diffraction pattern (not shown over here). The interface between the substrate and the TiO2 film is very sharp. No voids and second phases were detected in the film.
EXAMPLE 3
The titanium metal containing solution from Example I was used to spin coat films onto LaAlO3 substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of polyethylenimine and titanium metal was gradually heated from room temperature to about 980° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films on the sapphire substrates. The x-ray diffraction 2θ-scan of the titanium oxide (TiO2) film on the LaAlO3 substrate is shown in FIG. 7. The film has an anatase structure and was preferentially oriented out of the plane. The epitaxial nature of the anatase TiO2 on LaAlO3 is further evidenced by the φ-scans of (101) TiO2 and (101) LaAlO3 as shown in FIG. 8.
EXAMPLE 4
The zinc metal containing solutions from Example A, B or C were used to spin coat films onto c-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coatings of polyethylenimine and zinc metal were gradually heated from room temperature to about 750-1200° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films on the sapphire substrates. The epitaxial nature of the zinc oxide on c-cut sapphire was evidenced by the high-resolution transmission electron microscopy and selected electron diffraction pattern (not shown over here). The interface between the substrate and the zinc oxide film is very sharp. No voids and second phases were detected in the film.
It should be noted that no significant difference (in terms of the structure) was found between the solutions prepared from example A, B or C. However, the zinc oxide film has better surface morphology as detected from the optical scope where solution C was used.
EXAMPLE 5
The yttrium, barium and copper metal containing solution from Example Z was used to spin coat films onto LaAlO3 substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of polyethylenimine and yttrium, barium and copper metal was gradually heated from room temperature to about 800-980° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films of YBCO on the substrates. The x-ray diffraction 2θ-scan of the YBCO film on the LaAlO3 substrate is shown in FIG. 9. The film is highly c-axis oriented and shows no detectable second phase. The epitaxy nature of the YBCO on LaAlO3 is further evidenced by the φ-scans of (102) YBCO and (101) LaAlO3 as shown in FIG. 10.
EXAMPLE 6
The barium and titanium metal containing solution from Example P was used to spin coat films onto LaAlO3 substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of related polymer and barium and titanium metal was gradually heated from room temperature to about 1200° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films of BaTiO3 on the substrates. The x-ray diffraction 2θ-scan of the BaTiO3 film on the LaAlO3 substrate is shown in FIG. 11. The film is highly oriented out of the plane and shows no detectable second phase. The epitaxy nature of the BaTiO3 on LaAlO3 is further evidenced by the φ-scans of (101) BaTiO3 and (101) LaAlO3 as shown in FIG. 12.
EXAMPLE 7
The strontium and titanium metal containing solution from Example Q was used to spin coat films onto LaAlO3 substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of related polymer and strontium and titanium metal was gradually heated from room temperature to about 1200° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free metal oxide films of SrTiO3 on the substrates. The x-ray diffraction 2θ-scan of the SrTiO3 film on the LaAlO3 substrate is shown in FIG. 13. The film is highly oriented out of the plane and shows no detectable second phase. The epitaxial nature of the SrTiO3 on LaAlO3 is further evidenced by the φ-scans of (202) SrTiO3 and (202) LaAlO3 as shown in FIG. 14.
EXAMPLE 8
An amorphous glass substrate was coated with a conducting film of indium tin oxide. A glass slide was dipped into the solution from example O. The solution coated half of the glass slide. The glass slide was then placed into a furnace and gradually heated at 10° C./minute up to 300° C., held at 300° C. for 1 hour, heated at 10° C./minute to 400° C., held at 400° C. for 30 minutes and then allowed to cool. Resistance measurements showed that the uncoated glass was non-conductive (resistance>Mohms), and the coated half had resistances as low as 20,000 Ohms.
EXAMPLE 9
A porous Anodisc membrane (aluminum oxide) was coated with zinc oxide to make the membrane more stable in a corrosive environment. An Anodisc membrane with 200 nm pores was wetted with a drop of solution from example W. The membrane was allowed to dry for two hours. The membrane was then placed into a furnace and gradually heated at 10° C./minute to 300° C., held at 300° C. for 1 hour, heated at 10° C./minute to 400° C., held at 400° C. for 30 minutes, heated at 10° C./minute to 500° C., held at 500° C. for 1 hour and then allowed to cool. Corrosion tests with concentrated hydrochloric acid showed that the zirconium oxide coated membrane was able to withstand 5 hours of concentrated hydrochloric acid with no adverse effects. An uncoated membrane breaks apart and dissolves turning the solution yellow in the same period of time. The solution with the zirconium coating showed no yellowing and the membrane retained its porosity as evidenced by the transparency when wet.
The results of these examples for films of europium oxide, zinc oxide, titanium dioxide, and mixed complex oxides such as BaTiO3, SrTiO3, and YBa2Cu3O7-x demonstrate that epitaxial films can be achieved using the process or technique of the present invention. This technique could therefore provide a cost effective and easy route to such complex metal oxides as YBCO used in the superconducting field as well as a wide range of applications in other areas such as doped zinc oxide and indium tin oxide films for transparent conductors, zirconium oxide as a hardening/anticorrosion layer, Eu2O3 as a buffer layer or as a dielectric materials, BaTiO3 and SrTiO3 as dielectric layer for capacitors and microwave devices. The use of metal oxides is extensive. Epitaxial growth means that the technique could be used for demanding electronic applications. The low decomposition temperature of PEI and PEIC makes such polymers desirable for glass coatings for optics/electrooptics applications. As these films can be made readily by spin coating or dip coating means that a wide variety of material types and shapes can be readily used. This may extend the range of applications to include metal oxide catalysts for catalytic converters and protective coatings for metal-based membranes.
EXAMPLE 10
A palladium metal film was prepared as follows by spin coating a surface with the polymer and metal solution followed by removal of the polymer under a 5% hydrogen atmosphere. A palladium solution prepared as described in example AA was spin coated onto a glass slide at 2000 rpm. The slide was then heated in a 5% hydrogen atmosphere at VC/min to 120° C. with a dwell time of 30 min followed by heating to 350° C. at 1° C./minute with a dwell time of 30 min followed by heating to 450° C. at 1° C./minute with a dwell time of 1 hour and then cooling to 25° C. The resulting film was shown to be a continuous palladium film by XRD analysis and conductivity.
EXAMPLE 11
A zirconium phosphate film was prepared by adding a stoichometric amount of phosphate in the form of phosphoric acid to a solution of zirconium bound to PEI. The solution was then spin coated onto a glass slide and thermally treated at 1° C. to 120° C. with a dwell time of 30 minutes followed by heating to 350° C. at 1° C./minute with a dwell time of 30 minutes followed by heating to 450° C. at 1° C./minute with a dwell time of 1 hour and then cooling to 25° C.
EXAMPLE 12
An epitaxial film of strontium titanate with amorphous silicon nanoparticles was prepared as follows. A solution of silica particles was prepared by mixing 4 g of PEI with 2.5 g of Ludox AS-30 to yield a homogeneous solution. This solution was then combined with a solution of Ti bound to PEIC and a solution of strontium bound to PEI to yield a solution with a metal ratio of Ti:Sr:Si of 1:1:0.3. This solution was spin coated onto a crystal of lanthanum aluminum oxide and thermally treated to yield an epitaxial film with amorphous silicon particle as evidenced by TEM and XRD.
EXAMPLE 13
Porous epitaxial films of strontium titanate were prepared by heating the spin coated lanthanum aluminum oxide to 110° C. at a rate of VC/minute with a dwell time of 60 minutes followed by heating to 300° C. at 1° C./minute with a dwell time of 30 minutes followed by heating to 500° C. at 1° C./minute with a dwell time of 60 mM followed by heating to 800° C. at 20° C./minute with a dwell time of 60 minutes and then cooling to room temperature at 20° C./minute.
EXAMPLE 14
A polymer solution was coated onto and into a porous substrate as follows. Alumina anodises with 200 nm pores were coated with titanium dioxide using a solution of Ti bound to PEIC and with zirconium oxide using a solution of zirconium bound to PEI. The optimal metal concentration in the solution was between 100 and 300 mM. The solution was allowed to fill the pores and the alumina supports were then thermally treated at 1° C. to 120° C. with a dwell time of 30 minutes followed by heating to 350° C. at 1° C./minute with a dwell time of 30 mM followed by heating to 500° C. at 1° C./minute with a dwell time of 1 hour and then cooling to 25° C. XRF analysis shows a uniform coating on the alumina disc and none of the pores are blocked.
EXAMPLE 15
Gallium nitride is prepared by an aqueous or ethanol route using GaF63− bound to the PEI. The solution from example CC is readily spin coated and is then thermally treated in an ammonia atmosphere to generate the gallium nitride film.
EXAMPLE 16
The resulting precursor solution with the desired stoichiometric molar ratio of Bi/V (1:1) from Example MM was spin-coated on quartz, ITO-glass, mesoporous silica covered quartz, and mesoporous silica covered ITO-glass substrates at 2000 rpm for 30 s.
The films were annealed at 500° C. for 1 hour in flowing O2 at different ramping rates (1, 5, 10, 15, and 20° C./min). BiVO4 films with a thickness of 100 nm determined by a Dektak profilometer were obtained by one spin-coat. The thickness of the films can be increased by multiple spin-coats.
EXAMPLE 17
The precursor with desired stoichiometric ratio of Cu:Al of 1:1 from Example NN was spin coated onto c-plane sapphire substrate at 2000 rpm for 30 seconds. The films were heated at 940° C. for 1 hour in flowing N2 gas to obtain films of CuAlO2 as determined by XRD analysis.
EXAMPLE 18
The zirconium metal containing solution from Example GG was used to spin coat films onto C-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of polyethylenimine and zirconium metal was gradually heated from room temperature to from about 900 to about 1100° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free zirconium oxide films as identified by XRD.
EXAMPLE 19
The zirconium metal containing solution from Example HH was used to spin coat films onto C-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coating of polyethylenimine and zirconium metal was gradually heated from room temperature to from about 900 to about 1100° C. C over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free zirconium oxide films as identified by XRD similar in quality to that obtained in example 18.
EXAMPLE 20
The yttrium:uranium metal containing solution from Example KK with a Y:U ratio of 6:1 was used to spin coat films onto C-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coated substrate was gradually heated from room temperature to from about 900 to about 1100° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free Y6UO12 films as identified by XRD.
EXAMPLE 21
The yttrium:vanadium metal containing solution from Example LL with a Y:V ratio of 1:1 was used to spin coat films onto C-cut sapphire substrates. Spin coating was readily achieved with a spinning speed of 1500 rpm over 30 seconds.
The resultant coated substrate was gradually heated from room temperature to from about 900 to about 1100° C. over a period of about one hour under an oxygen atmosphere. This heating process yielded polymer-free YVO4 films as identified by XRD.
Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.

## Claims

1. A composition of matter comprising a solution of at least two metal precursors, ethylenediaminetetraacetic acid or salts of ethylenediaminetetraacetic acid, and a soluble polymer selected from the group consisting of polyethylenimine, carboxylated polyethylenimine, polypyrolidone, and poly(ethylene-maleic acid), said polymer characterized as having binding properties for said at least two metal precursors, wherein the binding constant for the polymer-metal complex interaction is greater than about 10000 mol−1 L, said at least two metal precursors are present in a pre-selected ratio, wherein said at least two metals are selected from the group consisting of lanthanide metals.

2. A composition of matter comprising a solution of at least two metal precursors, ethylenediaminetetraacetic acid or salts of ethylenediaminetetraacetic acid, and a soluble polymer selected from the group consisting of polyethylenimine, carboxylated polyethylenimine, polypyrolidone, and poly(ethylene-maleic acid), said polymer characterized as having binding properties for said at least two metal precursors, wherein the binding constant for the polymer-metal complex interaction is greater than about 10000 mol−1 L, said at least two metal precursors are present in a pre-selected ratio, wherein said at least two metals are indium and tin.

3. A composition of matter comprising a solution of at least two metal precursors, ethylenediaminetetraacetic acid or salts of ethylenediaminetetraacetic acid, and a soluble polymer selected from the group consisting of polyethylenimine, carboxylated polyethylenimine, polypyrolidone, and poly(ethylene-maleic acid), said polymer characterized as having binding properties for said at least two metal precursors, wherein the binding constant for the polymer-metal complex interaction is greater than about 10000 mol−1 L, said at least two metal precursors are present in a pre-selected ratio, wherein said at least two metals are indium and titanium.