Theoretical Calculations of Electrical Properties of Nanoscale Systems Under the Influence of Electric Fields and Currents

Summary

Recently, extensive experimental attempts to clarify the properties of various nanomolecular systems have been performed. Although many interesting results have already been reported and more are expected in the near future, interpretation of the results of such experiments is not necessarily straightforward, because of the strong interaction between the nanomolecular systems and the experimental probe. In this situation, reliable theoretical calculations are useful for interpreting experimental results, exploring novel properties of nanomolecular systems, and deriving guiding principles for designing nanomolecular systems so that they will be optimal for given purposes.

On the other hand, nanomolecular systems present a challenge to theoretical calculations, because conventional methods, which are powerful for examining the properties of bulk crystals and isolated molecules, are often insufficient for nanomolecular systems. First, it is often desirable to consider semi-infinite electrodes explicitly instead of representing them by finite clusters or thin slabs. Second, external fields often play an essential role in measurements of properties of nanomolecular systems, but the conventional methods have serious difficulties in treating their effects.

In this chapter, keeping the above in mind, we review recent theoretical approaches to examining the properties of nanomolecular systems, focusing on those which take account of semi-infinite electrodes and external electric fields explicitly. In particular, we discuss the electrical properties of systems under the influence of electric fields and currents, because these properties are very important in designing molecular electronic devices. After summarizing the problems of the conventional methods in Sect. 10.1, methods developed to examine the electrical properties of nanoscale systems are reviewed in Sect. 10.2. Recent applications of such methods to simple nanoscale systems without organic molecules are then described in Sect. 10.3. To examine such systems is useful, because some of the essential features of the properties of nanomolecular systems are also seen in them. Next, recent progress in theoretical analysis of the electrical properties of nanomolecular systems is reviewed in Sect. 10.4, followed by some concluding remarks.