IE912355A1 - Process for the preparation of inorganic microstructures¹from Langmuir-Blodgett films - Google Patents

Abstract

Method for obtaining inorganic microstructures from Langmuir- Blodgett layers, one or more layers of salts of organic acids being transferred to a support while preserving their order. The adjacent layers are here built up from identical or different salts. The organic component is then thermally desorbed by heating, so that the inorganic component remains on the support.

Description

Description Process for the preparation of inorganic microstructures from Langmuir-Blodgett films Coated supports are gaining increasing importance in industrial technology. Thus, for example, optical waveguide systems or filters for optical purposes are coated with thin films, which, owing to their low critical surface tension, are also suitable for improving the friction properties of these materials, for the preparation of protective layers and for further relevant applications. The preparation of monomolecular layers and the construction of systems composed of monomolecular layers of inorganic metals or metal ions is important for studying surface reactions, such as catalysis or corrosion, but in particular also for the construction of magnetic information stores and for optical applications.

The most simple method seems to be to start with monomolecular layers as subunits for the construction of complex systems. These layers are obtained by transfer of monomolecular films from a liquid surface to a suitable solid support. On a water surface, films of this type, for example composed of molecules of a fatty acid having 20 carbon atoms (arachidic acid, CH3-(CH2) 18-COOH), can be easily formed and tightly packed. To this end, a solution of the organic acid is added dropwise to a water surface which is limited by rigid barriers and a movable float. The solvent evaporates, and the remaining molecules are compressed and tightly packed by a defined force acting on the float. Since the molecules have a hydrophilic (for example a carboxyl group) and a hydrophobic portion (for example a hydrocarbon chain) and are very sparingly soluble in water, they are oriented at the water surface. The hydrophilic groups remain in the water, and the hydrophobic chains point upwards. - 2 To transfer (apply) a tightly packed Langmuir-Blodgett film from the water surface to a solid support, the latter is immersed and again withdrawn. When it is withdrawn, the hydrophilic groups of the film adhere to the hydrophilic support surface, and the sheet is thus covered with a monomolecular layer. In contrast, a hydrophobic substrate is already coated during the initial immersion. In each further dipping process (immersion and withdrawal), two further monomolecular layers are transferred. The interaction between the hydrophilic groups or hydrophobic end groups causes each layer transferred to adhere to the previous one.

For years, Langmuir-Blodgett (LB) films were only considered to be of interest because of their monomolecular layer structures. In 1967 Kuhn (Kuhn, Naturwissenschaften 1967, 54, 429) caused a sensation with his new strategy of arranging multilayers in which clever molecules were disposed in an inert matrix of fatty acid molecules. This was the starting point for a large number of functionally designed LB films. Polymerization, polycondensation, redox reactions and in-situ syntheses in this ordered state were developed (Ruaudel-Teixier, Rosilio, Barraud, Thin Solid Films, 1980, 68, 7). In the organized solid state, reactivity is closely related to structure, so that it is of great interest to prepare films of this type selectively for the desired areas of application. Molecular arrangements of this type have new interesting properties, which in some cases differ considerably from those of the starting substances.

The heat treatment of Langmuir-Blodgett films on the water surface in order to prepare high-quality calcium arachidate layers is known (Kato, Ohshima, Suzuki, Thin Solid Films, 178, 37-45). In this procedure, insoluble monolayers are compressed on the air/water interface, subjected to a heat treatment and then applied to a support. This is supposed to lead to virtually defectfree calcium arachidate LB films. - 3 The heat treatment of layers composed of various metals has been described many times in the literature. In this process, thin films composed of metals or inorganic compounds (for example metals of rare earths, gallium arsenides) are applied to a support, for example by vapor deposition, and then subjected to a heat treatment, resulting in the formation of a variety of intermetallic phases or other formations. The temperatures applied in these processes are between 200 and 700°C. (J. Vac. Sci, Technol. A, 7 (5), 3016-3022; Mater. Res. Soc. Symp.

Proc. 144, 526-530, CA 112 (14): 129936 v; J. Less-Common Met. 151, 263-9; CA H 111 (12): 105999 w) .

However, these processes do not ensure the preparation of layers having a defined molecular structure. Therefore, there was still the object of finding a process which enables metal films having a defined film thickness in the order of magnitude of molecules to be prepared selectively, in which the metal, depending on the particular application, can be present in oxide or atomic form on the support. The present process achieves this object.

It is based on the finding that in multilayers composed of salts of organic acids the organic component can be selectively desorbed from the individual layers by a heat treatment, as a result of which the inorganic component remains on the support.

The process according to the invention makes it possible to produce inorganic microstructures composed of Langmuir-Blodgett films by transferring one or more monomolecular layers of salts of organic acids to a support while maintaining their order and then desorbing the organic component by heating, as a result of which the metal ion remains on the support surface. The adjacent layers applied to the support can, as desired, be composed of identical or different salts.

The process according to the invention comprises in principle three steps: - 4 a) construction of a layered structure b) heat treatment of the multilayer c) optional aftertreatment of the multilayer.

The construction of a layered structure comprising 5 several monomolecular layers can be effected by the Langmuir-Blodgett method in a very simple manner. Tightly packed mono- or multilayers can be prepared by spreading salts of organic acids on a water surface or else by spreading organic acids on a subphase in which metal ions are dissolved (Thin Solid Films, 146 (1987) L15-L17), compression of this layer, followed by transfer to a support. The organic component of the salt can be, for example, an arachidic acid, stearic acid, palmitic acid or other fatty acid radical, or another amphiphilic acid, for example a charged phospholipid. Inorganic components which can be used are in general all metals, preferably all transition metals, in particular iron, cobalt, cadmium, chromium, copper, nickel, manganese, platinum, silver, gold and rhodium, but also sodium, potassium, calcium, strontium and barium. Furthermore the starting materials used can also be polymer salts. The layers suitable for the process according to the invention can be prepared by adsorption processes or by the LangmuirBlodgett method. The term Langmuir-Blodgett films is in general understood in this context to mean thin, ideally monomolecular, layers or multilayers of defined structure.

The layered structures should be such that the distribution of metal ions and organic components alternates along the surface normal. To this end, a monolayer of an organic acid is spread on a water surface. The aqueous subphase has a pH of more than 5, preferably in the range from 5 to 9, as a result of which a proton dissociates from the organic acid and the metal ions are transferred to the subphase by addition of an inorganic salt (for example chloride, bromide, iodide, nitrate, and the like). When a divalent metal is incorporated, its - 5 concentration in the subphase is preferably 10'* to 10'3 M and in the case of trivalent or higher valent ions the concentration can be many times less, while in the case of monovalent metals it should be more than 10'*, prefer5 ably more than 10'2 M. The salt can be transferred by the LB method to a support which can be made, for example, of glass or silicon, multilayers in which the layers comprising aliphatic chains are intercalated by a metal layer being produced by repeated immersion and with10 drawal. By changing the subphase between the individual dipping processes, it is also possible to construct intercalating layers comprising different metal ions. In this manner, the process according to the invention makes it possible to produce microstructures composed of different metals or metal ions.

The multilayer can be melted or sublimed by subsequent heat treatment. Selection of a suitable temperature allows desorption of the organic component (at 100 to 350°C), while the inorganic component remains on the support surface. The temperatures to be used in each case are dependent on the corresponding ion and counterion.

The following two mechanisms of thermodesorption of multilayers have been disclosed to date: -a) When multilayers are heated to 100-160°C, first droplets are formed. When the temperature is further increased, the surface breaks up more and more with further formation of droplets, the organic component is desorbed and what remains are clusters of the corresponding metal or the corresponding metal ions.

(Example 1 ) -b) When the multilayer is heated, the layer does not break up and no formation of droplets takes place. The organic component is desorbed and the metal or the metal ions remain on the support as a coherent layer. (Example 1 ).

Thus, depending on the selection of the inorganic and organic components, the process according to the invention makes it possible to produce microstructures of different design. Using the example of desorption of iron stearate, it has been shown that plate-like solids linked to one another and containing iron(III) which differ from those formed by thermodesorption of cadmium arachidate can be formed.

The diameter of the cadmium clusters is usually in the range from 0.5 to 2 pm at a spacing of 3 to 5 pm (Example 2). The clusters formed are in this case not linked to one another.

The distribution of the microstructures can be controlled by adjusting preparation parameters such as desorption kinetics, nucleation or layer thickness.

The thermodesorption temperatures used in each case depend on the organic and inorganic components used. The bond between metal and acid radical must not be too weak nor too strong in order to ensure desorption at sufficiently low temperatures and thus the microstructure formation of the metal ions. The advantage of the process according to the invention is in particular that relatively mild temperatures are used. The microstructures themselves which predominantly contain the metal component but hardly any carbon are only desorbed at temperatures above 350°C or even higher, depending on the metal ion used.

By treating the layers thus formed with hot hydrogen (T = 500*C), it is possible to change the oxidation states of the metal ions. Using the example of iron stearate, it could be shown by Mofibauer spectroscopy that after thermodesorption of the stearate radical first iron(III) ions are present which are reduced by the hydrogen treatment, so that afterwards the iron atoms are also present in the oxidation state 0. By varying the treatment time and temperature, it is possible to control the reduction ratio. (Working Example 3).

The process according to the invention makes it possible to construct inorganic microstructures at relatively low temperatures, which have a high degree of order and can additionally be structured laterally by means of photolithography. Possible aspects of economic interest are in particular in the area of applications in optics, data storage, catalyst technology but also in magnetometry.

Working examples Example 1: XPS (X-ray-induced photoelectron spectroscopy) measurements on transferred LB films before and after thermodesorption provided information on the composition of the chemical elements of the layers.

The measurements were carried out on a cadmium arachidate (Figure la) and an iron(III) stearate (Figure lb) multilayer. Coating was carried out by the conventional LB method; the support used was hydrophilic silicon having a natural silicon dioxide layer of about 20-50 A.

The cadmium arachidate film comprising 7 monolayers was applied after spreading arachidic acid onto Millipore water together with 10'3 M CdCl2 at a pH of 7 (6 mg of NaHCO3/1 of water) at 20°C, followed by compression at a constant pressure of 30 mN/m. The iron(III) stearate multilayer (17 monolayers) was formed from stearic acid on a subphase containing 4xl0‘5 M FeCl3, pH 5.5, 40°C and under a pressure of 30 mN/m. In both cases, the rate of application was 10 mm/min.

The XPS measurements before thermodesorption (Figure la, b) showed with both samples that the organic component (high C signal) and the inorganic metal ion (high cadmium or iron intensity) had been transferred to the substrate.

The measurements after thermodesorption (Figure la, b, the samples were heated to 250°C for 30 minutes) show that the organic component is almost completely desorbed (considerably lower C intensity), the metal ion however remains on the support (distinct Cd, Fe signals). In the sample containing cadmium, the support signal (Si) is clearly visible after desorption (Figure la) . This can only be explained by the fact that large portions of the support are uncovered and the cadmium ions are present on the support in the form of clusters which are not linked together. In the iron sample, the support signal is hardly visible, since the silicon support is covered by a coherent iron layer (probably in the form of iron oxide), making it impossible to detect any electrons having the binding energy of silicon.

Example 2: The melting of a cadmium arachidate layer (7 monolayers on silicon, preparation conditions as in Example 1) was monitored under a Nomarski microscope. The breakup of the layer and the subsequent droplet formation accompanied by desorption could be observed. At a heating rate of 0.7 K/s, followed by cooling to room temperature, the bright cadmium clusters (Figure 2) having a diameter of 0.5 to 2 pm can be seen.

In the iron stearate samples neither droplet nor cluster formation was observed.

Example 3: In order to determine the oxidation state of the iron ions, the thermodesorbed iron multilayers were investigated by conversion electron MoBbauer spectroscopy. Figure 3 shows such a measurement. The sample (15 monolayers) was prepared as described in Example 1, - 9 except that up to 95 % of enriched Fe57(III) chloride was used. Desorption was carried out at 510°C under a hydrogen atmosphere (1 hour). This gave a proportion of 36 % of Fe3+, 24 % of Fe2+ ions and 40 % of Fe° (metallic iron).

Figure 1: XPS spectra (intensity plotted versus bonding energy) of a silicon support, originally coated with a) 7 monolayers of cadmium arachidate, b) 17 monolayers of iron(III) stearate before (25°C) and after thermodesorption (heated at 250eC for 30 minutes and then cooled to room temperature).

Figure 2: Photograph under a Nomarski microscope of a film comprising 7 monolayers of cadmium arachidate on silicon, heated at a rate of 0.7 K/s up to 350°C and then cooled to room temperature.

Figure 3: Conversion electron MoBbauer spectrum of a multilayer originally comprising 15 monolayers of iron(III) stear20 ate, followed by thermodesorption (1 hour) at 510®C under a hydrogen atmosphere.

Claims (10)

What is claimed is:

1. A process for the preparation of inorganic microstructures, from Langmuir-Blodgett films, which comprises transferring one or more monomolecular 5 layers of salts of organic acids to a support, while maintaining their order, adjacent layers being made up of identical or different salts, then thermodesorbing the organic component by heating, the inorganic component remaining on the support. 10

2. The process as claimed in claim 1, wherein the organic component of the salt is a fatty acid radical or a phospholipid acid radical.

3. The process as claimed in claim 1, wherein the inorganic component of the salt is a transition 15 metal ion, preferably iron, cobalt, cadmium, chromium, copper, nickel, manganese, platinum, silver, gold or rhodium.

4. The process as claimed in claim 1, wherein the inorganic component of the salt is an alkali metal 20 ion or an alkaline earth metal ion.

5. The process as claimed in claim 1, wherein the aqueous subphase has a pH of greater than 5, in particular in the range from 5 to 9.

6. The process as claimed in claim 1, wherein the metal 25 ion concentration of the subphase is more than 10'* M, preferably more than 10' 3 M, in the case of divalent metals, and more than 10* M, preferably more than 10' 2 M, in the case of monovalent metals.

7. The process as claimed in claim 1, wherein the 30 organic component is desorbed at temperatures from 100 to 350°C. •Ε 912355 - 11

8. The process as claimed in claim 1, wherein the inorganic layer is partially or completely reduced after thermodesorption of the organic component. - 12

9. A process as claimed in claim 1, substantially as hereinbefore described and exemplified.

10. Inorganic micro-structures whenever prepared by a process claimed in a preceding claim.