Layered metal phosphonate thin films

The problems of reversibility and selectivity were addressed simultaneously by growing copper phosphonate thin films by the sequential adsorption method. This method requires a tnsphosphonic acid (adsorption stq> 3 in Figure S), which bridges between metal-oxygen layers. Sequential adsorption of Cu and I gave a layer repeat distance of 13.4 A by ellipso-... 

In certain cases, self-assembly methods can be employed to prepare multilayered thin films analogous to LB films. Typically, once the surface has been primed with a molecular adhesion layer, subsequent layers are assembled in a layer-by-layer fashion where the end group of the previously deposited layer directs the assembly of the next layer. Strong electrostatic or covalent interactions between the layers serve to stabilize the assemblies. The most notable examples of self-assembled multilayered films are those based upon metal phosphonates [21]. Although these multilayers are structurally analogous to LB films, their thermal and solvent stability makes them potentially more useful in many applications, including electron-transfer studies. 

Recently developed techniques for the fabrication of thin-film metal phosphates and phosphonates provide another method for the preparation of layered solids at surfaces. Layers of precisely controlled thickness can be built up by alternate immersion of a suitably pretreated surface in aqueous solutions of a soluble phosphate or phosphonate followed by an appropriate metal salt. This leads to the sequential build-up of thin metal containing films at the surface [72, 214] (see figure 6.18). The method is quite flexible and can be used to build up mixed microporous films on the surface which show molecular sieving properties [215, 216]. This building up approach looks very attractive for the systematic development of thin, selective films. 

Figure 5. Procedure for layer-by-layer growth of divalent metal phosphonate films on gold surfaces. The sample is washed with ethanol between adsorption steps. Repetition of steps (2) and (3) gives multilayer thin films.

Phosphonate salte of tetravalent, trivalent, and divalent metal ions contain strong ionic-covalent bonds within the metal-oxygen sheets that determine the details of their lamellar structures. Tbe divalent metal (Zn2+ and Cu2+) compounds can be made nanoporous by various techniques, and subsequent intercalation by small molecules such as ammonia, amines, and other small molecules forms the basis for size- and shiqw-selective piezoelectric sensors. Several techniques have been developed for depositing these materials as thin Hlms on quartz crystal microbalance (QCM) devices. The most successful of these, in terms of eliminating interferences and speed of device response, involves layer-by-layer growth of films through adsorption of their components from non-aqueous solutions. 

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