Electronic conductivity Future directions

As discussed in this entry, a number of novel materials and composites have been proposed as potential anodes for direct hydrocarbon solid oxide fuel cells. While many are promising, a commercially viable solution has not yet been found. The discusskm in this entry is deliberately framed arotmd the cxmcepts of ionic and electronic conductivity, electrocatalysis, and stability. It is essential for future researchers to address all of these topics when discussing new materials. The schematic in Fig. 3.4 represents both the complexity of the problem and the simpUcity that could potentially be achieved if a material meeting all of these requirements can be found. 

Pig. 10.13 Future directions of hot electron studies, including (a) development of hybrid nanoparticle-nanodiode systems, and (b) in situ surface characterization. The cartoon depicts the conductive atomic force microscopy experiments on Au/liOj nanostructures under exothermic catalytic reactions or photon irradiation... 

Composites fabricated with fixed catalyst VGCF can be designed with fibers oriented in preferred directions to produce desired combinations of thermal conductivity and coefficient of thermal expansion. While such composites are not likely to be cost-competitive with metals in the near future, the ability to design for thermal conductivity in preferred directions, combined with lower density and lower coefficient of thermal expansion, could warrant the use of such VGCF composites in less price sensitive applications, such as electronics for aerospace vehicles. 

The prospective applications ofmolecular assemblies seem so wide that their limits are difficult to set. The sizes of electronic devices in the computer industry are close to their lower limits. One simply cannot fit many more electronic elements into a cell since the walls between the elements in the cell would become too thin to insulate them effectively. Thus further miniaturization of today s devices will soon be virtually impossible. Therefore, another approach from bottom up was proposed. It consists in the creation of electronic devices of the size of a single molecule or of a well-defined molecular aggregate. This is an enormous technological task and only the first steps in this direction have been taken. In the future, organic compounds and supramolecular complexes will serve as conductors, as well as semi- and superconductors, since they can be easily obtained with sufficient, controllable purity and their properties can be fine tuned by minor adjustments of their structures. For instance, the charge-transfer complex of tetrathiafulvalene 21 with tetramethylquinodimethane 22 exhibits room- temperature conductivity [30] close to that of metals. Therefore it could be called an organic metal. Several systems which could serve as molecular devices have been proposed. One example of such a system which can also act as a sensor consists of a basic solution of phenolophthalein dye 10b with P-cyciodextrin 11. The purple solution of the dye not only loses its colour upon the complexation but the colour comes back when the solution is heated [31]. 

Finally, it should be noted that the recent development of so-called third generation biosensors to achieve direct electron transfer from redox enzyme, oxidoreductase to the electrode without mediators, but through a series of enzyme cofactors or conductive polymers to transfer electrons from the enzyme redox center to the electrode surface [161-164]. This concept with the current technology for preparing miniature sensors with nanotechnology is of great interest to many researchers trying to develop practical sensors in clinical, environmental and industrial analysis. Whether with mediators or without, research for optimum sensor development for various purposes will be intensive in the future. 

Overall there are considerable difficulties in analysing conductive polymers and much doubt about the relationship between structure and properties. Research is currently directed at improving this situation. No applications yet exist for these materials but they seem to have considerable potential for use in a variety of electronic applications in the future and novel technology-based on these materials is widely predicted. 

Future challenging aspects involve the design of molecular cables in order to directly connect the active site of an enzyme with the electrode surface. For this purpose, either the overall electron-transfer distance can be subdivided by integration of redox relays into the monolayer (Fig. 12b) or parts of the nonconducting alkyl spacer may be replaced by conducting oligomers (Fig. 12c). 

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