Metal-based thin-film electrodes

For technical purposes (as well as in the laboratory) RuOz and Ru based thin film electrodes are prepared by thermal decomposition techniques. Chlorides or other salts of the respective metals are dissolved in an aqueous or alcoholic solution, painted onto a valve metal substrate, dried and fired in the presence of air or oxygen. In order to achieve reasonable thicknesses the procedure has to be applied repetitively with a final firing for usually 1 hour at temperatures of around 450°C. A survey of the various processes can be found in Trasatti s book [44], For such thermal decomposition processes it is dangerous to assume that the bulk composition of the final sample is the same as the composition of the starting products. This is especially true for the surface composition. The knowledge of these parameters, however, is of vital importance for a better understanding of the electrochemical performance including stability of the electrode material. 

In addition to thin-film electrodes, compact diamond single crystals grown at high temperature and high pressure have become the object of electrochemical study in recent years. These so-called HTHP crystals can be obtained by crystallization from a carbon solution in a metal melt (e.g., based on the Ni-Fe-Mn system) at /arranges that correspond to the conditions of thermodynamic stability of diamond. These crystals can be also doped with boron in the course of growth. 

Recent studies have been performed on alternative electrode materials. Nano-sonic has developed low modulus, highly conducting thin film electrodes by molecular level self-assembly processing methods capable of maintaining conductivity up to strains of 100% [217, 218]. Recent developments have enabled the reduction of the modulus to less than 1 MPa and an increase in the strain to rupture to 1000% [219]. A version of the material is commercially available under the name Metal Rubber . Delille et al. have developed novel compliant electrodes based on a platinum salt reduction [220]. The platinum salt is dispersed into a host elastomer and immersed in a reducing agent. A maximum conductivity of 1 S cm was observed and conductivity was maintained for strains up to 40%. 

Fig. 14 Main characteristics of (a) carbon-based electrodes and (b) metallic thin-film electrodes for DEAs (Rosset and Shea 2013)...

TABLE 2.1 Variations of photopotentials, photocurrents, quantum yield on a current of thin film electrodes based on the azasubstituted metal complexes of tetrabenzo-porphyrin depending on the degree of substitution and the type of central atom... 

Among the carbon-based electrodes, used to replace the mercury electrode, modification of the electrode surface to improve its performance was made by the addition of carbon nanotubes, magnetic or metallic nanoparticles, thin films, etc. The literature reports the use of biosensors incorporating tyrosinase (polyphenol oxidase) for the detection of phenols in aqueous and in organic medium. 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

Metal oxide semiconductor field-effect transistors (MOSFETs) are field effect transistors with a thin film of silicon dioxide between the gate electrode and the semiconductor. The charge on the silicon dioxide controls the size of the depletion zone in the polype semiconductor. MOSFETs are easier to mass produce and are used in integrated circuits and microprocessors for computers and in amplifiers for cassette players. Traditionally, transistors have been silicon based but a recent development is field-effect transistors based on organic materials. 

Preorganisation of redox anion sensors on electrode surfaces is a promising new technique for electrochemical anion sensing. Self-assembled monolayers or thin polymer films of metal-based receptors have the potential to generate an amplified response to anion binding akin to the dendritic effect. 

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