Conformal coating on three-dimensional substrates

Abstract

The disclosure relates to a method for forming a conformal coating on a substrate having a topography presenting a relief. One method of the disclosure includes setting the temperature of the substrate within the range 140-275 C., and coating an aqueous solution including a sol-gel precursor on said substrate. The disclosure also relates to a method for fabricating a battery, a capacitor, a catalyst, a photovoltaic cell or a sensor using such a method, and to an aqueous solution for use in such a method.

Claims

1. A method for forming a conformal coating on a substrate having a topography presenting a relief, said method comprising: setting the temperature of the substrate within the range 140-275 C., and coating an aqueous solution comprising a sol-gel precursor on said substrate to form a coated substrate; and annealing the coated substrate to provide the conformal coating on the substrate having the topography presenting the relief, wherein said relief is composed of a plurality of elements and at least one of said elements has an aspect ratio of at least 10, wherein the conformal coating is a phase-pure TiO.sub.2 anatase film, a lithium titanate film, a lithium lanthanum titanate film, a lithium tungstate film, or a lutetium iron oxide film.

2. The method according to claim 1, wherein said aqueous solution further comprises a solvent which is water-miscible and suitable for lowering the surface tension of water, and wherein the solvent and the water are present in a volumetric ratio such as to set the surface tension of a mixture of water with said solvent in said ratio below 40 mM/m.

3. The method according to claim 1, wherein at least one element of said relief has the shape of a pillar or a trench.

4. The method according to claim 1, wherein the step of coating a solution is performed by spraying.

5. The method according to claim 4, wherein said spraying is performed via an ultrasonic nozzle.

6. The method according to claim 1, further comprising before the coating with the aqueous solution comprising the sol-gel precursor, cleaning said substrate with gaseous ozone in the presence of ultra-violet radiation.

7. A method for fabricating a battery, a capacitor, a catalyst, a photovoltaic cell or a sensor, comprising a step of forming a conformal layer on a substrate having a topography presenting a relief, according to the method as defined in claim 1; and assembling the conformal layer on the substrate into the battery, capacitor, catalyst, photovoltaic cell or sensor.

8. The method according to claim 1, wherein the aqueous solution comprises: water, at least one metal oxide sol-gel precursor, said precursor being a metal carboxylate complex comprising at least a metal selected from the group consisting of Ti, Li, Fe, W, La and Lu, and at least an anion selected from the group consisting of citrate, glycolate, malate, lactate, tartrate, oxalate and acetate, and a solvent which is water-miscible and suitable for lowering the surface tension of water, wherein the solvent and the water are present in a volumetric ratio such as to set the surface tension of a mixture of water with said solvent in said ratio below 40 mN/m.

9. The method according to claim 8, wherein said solvent is an alcohol.

10. The method according to claim 9, wherein the alcohol is ethanol.

11. The method according to claim 9, wherein the solvent to water ratio is from 0.7:1.0 to 1.5:1.0.

12. The method according to claim 8, wherein said metal oxide sol-gel precursor comprises at least a metal selected from the group consisting of Li, Fe, W, La and Lu.

13. The method according to claim 12, wherein said metal oxide sol gel precursor is a citrate metal precursor.

14. The method according to claim 1, wherein the conformal coating is a phase-pure TiO.sub.2 anatase film, a Li.sub.4Ti.sub.5O.sub.12 spinel film, a Li.sub.3x,La(.sub.2/3)-x,TiO.sub.3 film, a Li.sub.x,WO.sub.3 film, a phase-pure hexagonal LuFeO.sub.3 film, or a phase-pure orthorhombic LuFeO.sub.3 film.

15. The method according to claim 1, wherein the annealing is performed at a temperature of from 450 C. to 1100 C.

16. The method according to claim 1, wherein said aqueous solution further comprises a solvent which is water-miscible and suitable for lowering the surface tension of water, and wherein the solvent and the water are present in a volumetric ratio such as to set the surface tension of a mixture of water with said solvent in said ratio below 30 mN/m at 25 C.

17. The method according to claim 1, wherein the sol-gel precursor is a metal carboxylate complex comprising at least a metal selected from the group consisting of Ti, Li, Fe, W, La and Lu, and at least an anion selected from the group consisting of citrate, glycolate, malate, lactate, tartrate, oxalate and acetate.

18. The method according to claim 1, wherein the solvent is an alcohol and wherein the solvent and the water are present in a volumetric ratio of from 0.7:1.0 to 1.5:1.0.

19. The method according to claim 1, wherein the conformal coating is a film of a lithium titanate, a lithium lanthanum titanate, a lithium tungstate, or a lutetium iron oxide.

20. The method according to claim 19, wherein the conformal coating is a Li.sub.4Ti.sub.5O.sub.12 spinel film, a Li.sub.3x,La(.sub.2/3)-x,TiO.sub.3 film, a Li.sub.x,WO.sub.3 film, a phase-pure hexagonal LuFeO.sub.3 film, or a phase-pure orthorhombic LuFeO.sub.3 film.

21. The method according to claim 20, wherein the step of coating a solution is performed by spraying and wherein said aqueous solution further comprises a solvent which is water-miscible and suitable for lowering the surface tension of water, and wherein the solvent and the water are present in a volumetric ratio such as to set the surface tension of a mixture of water with said solvent in said ratio below 30 mN/m at 25 C.

22. The method according to claim 1, wherein the conformal coating has a thickness uniformity having a relative standard deviation of less than 5%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(A), 1(B), 1(C) and 1(D) are SEM pictures of Si micropillars with 20 nm TiN coating before application of the method of the present invention.

(2) FIGS. 2(A), 2(B) and 2(C) are SEM pictures of the micropillars of FIGS. 1(A), 1(B), 1(C) and 1(D) after conformal Ti coating according to an embodiment of the present invention, before the subsequent heat treatment.

(3) FIG. 3(A) is graph of the Raman spectrum of the top of a pillar before (bottom) and after (top) conformal coating according to an embodiment of the present invention. As a reference the Raman signals for anatase (.Math.) and rutile (.box-tangle-solidup.) are shown. FIG. 3(B) is a picture of the top of the pillar after the conformal coating.

(4) FIG. 4 shows the XRD patterns of pillars before (bottom) and after (top) conformal coating according to an embodiment of the present invention. As a reference the XRD signals for anatase (.Math.) and rutile (.box-tangle-solidup.) are shown.

(5) FIGS. 5(A), 5(B) and 5(C) are SEM pictures of the micropillars of FIGS. 1(A), 1(B), 1(C) and 1(D) after conformal Ti, Li coating according to an embodiment of the present invention, before the subsequent heat treatment.

(6) FIGS. 6(A), 6(B), 6(C) and 6(D) are SEM pictures of the micropillars of FIGS. 1(A), 1(B), 1(C) and 1(D) after conformal Li.sub.3xLa.sub.(2/3)-xTiO.sub.3 with x=0.117 coating according to an embodiment of the present invention, after the subsequent heat treatment.

(7) FIGS. 7(A), 7(B), 7(C) and 7(D) are SEM pictures of the micropillars of FIGS. 1(A), 1(B), 1(C) and 1(D) after conformal LiWO.sub.3 conformal coating according to an embodiment of the present invention, after the subsequent heat treatment.

(8) FIGS. 8(A), 8(B), 8(C) and 8(D) are SEM pictures of the micropillars of FIGS. 1(A), 1(B), 1(C) and 1(D) after conformal LuFeO.sub.3 coating according to an embodiment of the present invention, before the subsequent heat treatment.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(9) The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting.

(10) Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

(11) Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

(12) It is to be noticed that the term comprising, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression a device comprising means A and B should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

(13) Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

(14) Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

(15) Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

(16) Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

(17) In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

(18) The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.

EXAMPLE 1

Synthesis of a Citrato-peroxo Titanium Precursor

(19) The starting product was liquid Ti(IV)isopropoxide (Ti(OiPr).sub.4, Acros organics, 98+%). The appropriate amount of Ti(IV)isopropoxide to obtain a 0.73 mol/L solution was added to a tenfold volume of water, leading to immediate hydrolysis and condensation and finally to the formation of a white precipitate. This precipitate was filtered and washed with water. The hydrolysis product was processed quickly in order to avoid it becoming insoluble. In a next step, citric acid (CA, C.sub.6H.sub.8O.sub.7, Sigma-Aldrich, 99%) and hydrogen peroxide (H.sub.2O.sub.2, stabilized, Acros, 35% p.a aqueous solution) were added to the fresh, wet precipitate, so that the molar ratio Ti.sup.4+:CA:H.sub.2O.sub.2 was 1:2:1.2. The mixture was then stirred at 80 C., which after a short time yielded a clear, burgundy coloured solution, with a pH of about 1. The solution was cooled to room temperature and in the third step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH (pH electrode, WTW inoLab 740 with Sentix 81 electrode). This was accompanied by intense gas evolution and an increase of the solution's temperature. The solution was again cooled to room temperature and its pH was set to exactly 7. During the third step, the solution's colour changed from burgundy (pH=1) to yellow-orange (pH=7). In the last step, the precursor was diluted with H.sub.2O and at the same time ethanol was added so that the Ti.sup.4+-concentration and the volumetric ethanol:water ratio were, respectively, 0.05 M and 0.9:1.

EXAMPLE 2

Conformal TiO.SUB.2 .Coating of Si Micropillars

(20) Silicon micropillars have been used as high surface area 3D substrate. In view of their application as substrate in a thin-film battery, the Si micropillars were coated with a thin layer of TiN (ca. 20 nm). This layer is not only used as the current collector in the final battery stack, it also acts as blocking layer for Li. FIGS. 1(A), 1(B), 1(C) and 1(D) are SEM images of Si micropillars with 20 nm TiN coating (prior to the conformal coating with the aqueous solution of example 1): FIG. 1(A) shows a 45 tilted view, FIG. 1(B) shows a view of the pillars, FIG. 1(C) shows a view of the bottom of the pillars and FIG. 1(D) shows a view of the top of the pillars. As can be deducted from the SEM images in FIGS. 1(A), 1(B), 1(C) and 1(D), the pillars have a diameter of ca. 1.85-2.85 m (dependent on the height along the pillar on which the measurement is performed). They are about 50 m in height and are positioned 5 m from each other (interpillar distance). The aspect ratio of the silicon pillars is estimated to be 17-27, dependent on the diameter used for the calculation. If the diameter is averaged over the length of the pillars, the aspect ratio is about 22. The substrate comprising the silicon micropillars was cleaned by a UV/O.sub.3 process. The deposition of the precursor on the pillars was performed via spray coating of the solution of example 1 on the substrate set at a temperature of 180 C. in a ExactaCoat ultrasonic coating system. The nozzle was a AccuMist precision ultrasonic spray nozzle. Conformal coverage of the silicon pillars with the Ti precursor could be obtained. The resulting gel is shown in the SEM images of the pillars after deposition (FIGS. 2(A), 2(B), and 2(C)). FIGS. 2(A), 2(B) and 2(C) are SEM images of the Si micropillars coated with 20 nm TiN, covered by a gel, consisting of coordinated Ti complexes: FIG. 2(A) shows a 45 tilted view of the pillars, FIG. 2(B) shows a view of the bottom of the pillars and FIG. 2(C) shows the top of the pillars. Thinner TiO.sub.2 layers were also prepared and showed even better conformality but these layers were more difficult to be distinguished on the SEM images.

(21) Further heat treatment of the pillars on hot plates at first 180 C. (2 min), then 300 C. (2 min) and finally at 600 C. (60 min) lead to the decomposition of the gel and thus to TiO.sub.2 oxide formation. It also triggered the crystallisation of the deposited layer. The morphology change, related to the crystallisation process (SEM image of FIG. 3(B)), and the accompanying Raman spectrum (See FIG. 3(A) top spectrum) can be observed in FIGS. 3(A) and 3(B). It is clear that the TiO.sub.2 layer had been crystallised as a mixture of anatase (bottom pointing triangles) and rutile (top pointing triangles). Further proof of the presence of the phase mixture can be found in the XRD pattern in FIG. 4 (top pattern). A more thorough crystallisation study on planar substrates indicated that phase-pure anatase films could be obtained by lowering the final anneal temperature to 500 C. (data not shown).

EXAMPLE 3

Synthesis of a Citrato-peroxo Titanium-lithium Precursor

(22) First, the mono-metal Ti (IV) and Li (I) ion solutions were prepared separately.

(23) For the Ti (IV) precursor, the starting product was liquid Ti(IV)isopropoxide (Ti(OiPr).sub.4, Acros organics, 98+%). The appropriate amount of Ti(IV)isopropoxide to obtain a 0.73 mol/l solution was added to a tenfold volume of water, leading to immediate hydrolysis and condensation and finally to the formation of a white precipitate. This precipitate was filtered and washed with water. The hydrolysis product was processed quickly in order to avoid it becoming insoluble. In a next step, citric acid (CA, C.sub.6H.sub.8O.sub.7, Sigma-Aldrich, 99%) and hydrogen peroxide (H.sub.2O.sub.2, stabilized, Acros, 35% p.a aqueous solution) were added to the fresh, wet precipitate, so that the molar ratio Ti.sup.4+:CA:H.sub.2O.sub.2 was 1:2:1.2. The mixture was then stirred at 80 C., which after a short time yielded a clear, burgundy coloured solution, with a pH of about 1. The solution was cooled to room temperature and in the third step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH (pH electrode, WTW inoLab 740 with Sentix 81 electrode). This was accompanied by intense gas evolution and an increase of the solution's temperature. The solution was again cooled to room temperature and its pH was set to exactly 7. During the third step, the solution's colour changed from burgundy (pH=1) to yellow-orange (pH=7).

(24) For the Li (I) precursor, lithium hydroxide (LiOH, Sigma-Aldrich, 98%) and citric acid (CA, C.sub.6H.sub.8O.sub.7, Sigma-Aldrich, 99%) were added in water so that the final Li (I) ion concentration and Li.sup.+:CA ratio were, respectively, 0.7 M and 1:1. Then, the mixture was stirred at 80 C., which after a short time yielded a clear, colourless solution, with a pH of about 3.5. The solution was cooled to room temperature and in the third step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH (pH electrode, WTW inoLab 740 with Sentix 81 electrode). This was accompanied by an increase of the solution's temperature. The solution was again cooled to room temperature and its pH was set to exactly 7.

(25) The multi-metal ion Ti/Li precursor was then prepared by mixing the Ti (IV) and Li (I) solutions in the desired amounts (molar ratio=5:4). Water and ethanol were added to the solution so that the total metal ion concentration and the volumetric ethanol:water ratio were set at, respectively, 0.05 M and 0.9:1.

EXAMPLE 4

Conformal Li.SUB.4.Ti.SUB.5.O.SUB.12 .Coating of Si Micropillars.

(26) Example 2 was repeated except that the aqueous solution of example 3 was used instead of the aqueous solution of example 1 and that the further treatment (for decomposition of the gel) of the pillars on hot plates was performed at first 260 C. (2 min), and then 470 C. (2 min). Spray coating of the optimised precursor on Si micropillars, coated with TiN, lead to a complete coverage of the pillars with the precursor. This can be observed in the SEM images in FIGS. 5(A), 5(B) and 5(C), where the gel formation is clearly visible. FIGS. 5(A), 5(B) and 5(C) are SEM images of Si micropillars, coated with 20 nm TiN, covered by a gel, consisting of coordinated Li, Ti complexes obtained by spraying the solution of example 3 thereon. FIG. 5(A) is a 45 tilted view of the pillars, FIG. 5(B) shows the top of the pillars, and FIG. 5(C) shows the bottom of the pillars.

(27) Further heat treatment of the pillars lead to the decomposition of the gel and thus to Li, Ti oxide formation. It also triggered the crystallisation of the deposited layer. The same experiment was repeated on a planar Si.sub.3N.sub.4 surface and on a Pt surface in order to enable an easier crystallisation study of the Li.sub.xTi.sub.yO.sub.z films. It showed that phase-pure Li.sub.4Ti.sub.5O.sub.12 (spinel phase) could be achieved.

EXAMPLE 5

Synthesis of a Citrato Lithium-lanthanum-titanium Precursor

(28) First, the mono-metal Li (I), La (III) and Ti (IV) ion solutions were prepared separately.

(29) For the Li (I) precursor, lithium citrate hydrate (C.sub.3H.sub.5OH(COOLi).sub.3 xH.sub.2O, Sigma Aldrich, 99%) was added to water, yielding a clear, colorless solution with a Li.sup.+-ion concentration 1 M and pH 8 (pH electrode, WTW inoLab 740 with Sentix 81 electrode).

(30) For the La (III) precursor, lanthanum oxide (La.sub.2O.sub.3, Alfa Aesar, 99.9%) and citric acid hydrate (CA, C.sub.6H.sub.8O.sub.7 H.sub.2O, Sigma-Aldrich, 99%) were added to a minimum of water so that the final La.sup.3+:CA ratio was 1:3. The mixture was refluxed at 120 C. for 24 h, after which it was cooled to room temperature and in the next step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH to 12 (pH electrode, WTW inoLab 740 with Sentix 81 electrode). Then, the whole was refluxed at 110 C. for 24 h and water was added, yielding a clear, yellowish 0.075 M solution with a pH of 8.

(31) For the Ti (IV) precursor, the starting product was liquid Ti(IV)isopropoxide (Ti(OiPr).sub.4, Sigma-Aldrich, 97%). The appropriate amount of Ti(IV)isopropoxide to obtain a 0.9 mol/l solution was added to a tenfold volume of water, leading to immediate hydrolysis and condensation and finally to the formation of a white precipitate. This precipitate was filtered and washed with water. The hydrolysis product was processed quickly in order to avoid it becoming insoluble. In a next step, citric acid (CA, C.sub.6H.sub.8O.sub.7, Sigma-Aldrich, 99%) and hydrogen peroxide (H.sub.2O.sub.2, stabilized, Acros, 35% p.a aqueous solution) were added to the fresh, wet precipitate, so that the molar ratio Ti.sup.4+:CA:H.sub.2O.sub.2 was 1:3:1.2. The mixture was then stirred at 80 C., which after a short time yielded a clear, burgundy coloured solution, with a pH of about 1. The solution was cooled to room temperature and in the third step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH (pH electrode, WTW inoLab 740 with Sentix 81 electrode). This was accompanied by intense gas evolution and an increase of the solution's temperature. The solution was again cooled to room temperature and its pH was set to exactly 7. During the third step, the solution's colour changed from burgundy (pH=1) to yellow-orange (pH=7).

(32) The multi-metal ion Li/La/Ti precursor was then prepared by mixing the Li (I), La (III) and Ti (IV) solutions in the desired amounts (molar ratio=0.35:0.55:1). Water and ethanol were added to the solution so that the total metal ion concentration and the volumetric ethanol:water ratio were set at, respectively, 0.01 M and 0.9:1.

EXAMPLE 6

Conformal Li.SUB.3x.La.SUB.(2/3)-x.TiO.SUB.3 .x=0.117 Coating of Si Micropillars

(33) Example 2 was repeated except that the aqueous solution of example 5 was used instead of the aqueous solution of example 1, that the substrate was set at a temperature of 200 C. instead of 180 C. and that the further treatment of the pillars on hot plates was performed at first 180 C. (2 min), then 300 C. (2 min), then 600 C. (2 min), and finally 700 C. (60 min in a furnace). Spray coating of the optimised precursor on Si micropillars, coated with TiN, and subsequent annealing in order to decompose the precursor and trigger the Li/La/Ti oxide formation lead to a complete coverage of the pillars. This can be observed in the SEM images in FIGS. 6(A), 6(B), 6(C) and 6(D), where the complete coating is visible. FIGS. 6(A), 6(B), 6(C) and 6(D) are SEM images of Si micropillars, coated with 20 nm TiN, covered by Li.sub.3xLa.sub.(2/3)-xTiO.sub.3 (x=0.117), obtained by spraying the solution of example 5 thereon. FIG. 6(A) is an overview of the pillars, FIG. 6(B) shows the bottom of the pillars, FIG. 6(C) the middle and FIG. 6(D) shows the top of the pillars. Furthermore, it was shown that a crystalline coating could be achieved.

EXAMPLE 7

Synthesis of a Citrato Lithium-tungsten Precursor

(34) First, the mono-metal Li (I) and W (VI) ion solutions were prepared separately.

(35) For the Li (I) precursor, lithium citrate hydrate (C.sub.3H.sub.5OH(COOLi).sub.3.xH.sub.2O, Sigma Aldrich, 99%) was added to water, yielding a clear, colorless solution with a Li.sup.+-ion concentration 1 M and pH 8 (pH electrode, WTW inoLab 740 with Sentix 81 electrode).

(36) For the W (VI) precursor, tungstic acid (H.sub.2WO.sub.4, Sigma Aldrich, 99% and citric acid hydrate (CA, C.sub.6H.sub.8O.sub.7 H.sub.2O, Sigma-Aldrich, 99%) were added to a minimum of water so that the final W.sup.6+:CA ratio was 1:4. The mixture was refluxed at 120 C. for 24 h, after which it was cooled to room temperature and in the next step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH to 12 (pH electrode, WTW inoLab 740 with Sentix 81 electrode). Then, the whole was refluxed at 110 C. for 24 h and water was added, yielding a clear, slightly grey colored 0.15 M solution with a pH of 8.

(37) The multi-metal ion Li/W precursor was then prepared by mixing the Li (I) and W (VI) solutions in the desired amounts (molar ratio=1:1). Water and ethanol were added to the solution so that the total metal ion concentration and the volumetric ethanol:water ratio were set at, respectively, 0.01 M and 0.9:1.

EXAMPLE 8

Conformal LiWO.SUB.3 .Coating of Si Micropillars

(38) Example 2 was repeated except that the aqueous solution of example 7 was used instead of the aqueous solution of example 1, that the substrate was set at a temperature of 200 C. instead of 180 C. and that the further treatment of the pillars on hot plates was performed at first 180 C. (2 min), then 300 C. (2 min), then 600 C. (2 min), and finally 700 C. (60 min in a furnace). Spray coating of the optimised precursor on Si micropillars, coated with TiN, and subsequent annealing on hot plates in order to decompose the precursor and trigger the Li/W oxide formation lead to a complete coverage of the pillars. This can be observed in the SEM images in FIGS. 7(A), 7(B), 7(C) and 7(D), where the complete coating is visible. FIGS. 7(A), 7(B), 7(C) and 7(D) re SEM images of Si micropillars, coated with 20 nm TiN, covered with the aforementioned oxide, obtained by spraying the solution of example 7 thereon. FIG. 7(A) is an overview of the pillars, FIG. 7(B) shows the bottom of the pillars, FIG. 7(C) the middle and FIG. 7(D) shows the top of the pillars.

EXAMPLE 9

Synthesis of a Citrato Lutetium-iron Precursor

(39) First, the mono-metal Lu (III) and Fe (III) ion solutions were prepared separately.

(40) For the Lu (III) precursor, lutetium (III) oxide (Lu.sub.2O.sub.3, Alfa Aesar, 99.9%) and citric acid (CA, C.sub.6H.sub.8O.sub.7, Sigma-Aldrich, 99%) were added to a minimum of water so that the final Lu.sup.3+:CA ratio was 1:1. The mixture was refluxed at 120 C. for 24 h, after which it was cooled to room temperature and in the next step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH to 11 (pH electrode, WTW inoLab 740 with Sentix 81 electrode). Then, the whole was refluxed at 110 C. for 24 h and water was added, yielding a clear, colorless 0.1 M solution with a pH of 7.5.

(41) For the Fe (III) precursor, iron (III) citrate hydrate (FeC.sub.6H.sub.5O.sub.7.H.sub.2O, Acros, 98%, 18-20% Fe) was added in water so that the final Fe (III) ion concentration was 0.1 M. Then, the mixture was stirred at 80 C. overnight, which yielded a clear, brown solution with a pH of about 1.5. The solution was cooled to room temperature and in the next step, ammonia (NH.sub.3, Merck, 32% aqueous solution, extra pure) was added dropwise to the solution in order to increase the pH to 7 (pH electrode, WTW inoLab 740 with Sentix 81 electrode).

(42) The multi-metal ion Lu/Fe precursor was then prepared by mixing the Lu (III) and Fe (III) solutions in the desired amounts (molar ratio=1:1). Water and ethanol were added to the solution so that the total metal ion concentration and the volumetric ethanol:water ratio were set at, respectively, 0.01 M and 0.9:1.

EXAMPLE 10

Conformal LuFeO.SUB.3 .Coating of Si Micropillars

(43) Example 2 was repeated except that the aqueous solution of example 9 was used instead of the aqueous solution of example 1, that the substrate was set at a temperature of 200 C. instead of 180 C. and that the further treatment of the pillars on hot plates was performed at first 320 C. (2 min), then 510 C. (2 min), then 800 C. (60 min in a furnace) or 1000 C. (60 min in a furnace). 800 C. led to a pure hexagonal phase while 1000 C. lead to a pure orthorhombic phase. Spray coating of the optimised precursor on Si micropillars, coated with TiN, and subsequent annealing on hot plates in order to decompose the precursor and trigger the Lu/Fe oxide formation lead to a complete coverage of the pillars. This can be observed in the SEM images in FIGS. 8(A), 8(B), 8(C) and 8(D), where the complete coating is visible. FIGS. 8(A), 8(B), 8(C) and 8(D) are SEM images of Si micropillars, coated with 20 nm TiN, covered with the aforementioned oxide, obtained by spraying the solution of example 9 thereon. FIG. 8(A) is an overview of the pillars, FIG. 8(B) shows the bottom of the pillars, FIG. 8(C) the middle and FIG. 8(D) shows the top of the pillars. Further heat treatment triggered the crystallisation of the deposited layer. The same experiment was repeated on a planar Si.sub.3N.sub.4 surface and in order to enable an easier crystallisation study of the LuFeO.sub.3 films. It showed that both phase-pure hexagonal LuFeO.sub.3 and orthorhombic LuFeO.sub.3 could be achieved in function of the annealing temperature used.

(44) It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.