Electron transport through extended metal atom chains

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The dramatic expansion of the electronics industry over the past 40 years has been based on the progressive reduction in size of the silicon-based semiconductor components of integrated circuits. Indeed, Moore anticipated this trend in his much-quoted 1965 paper, where he predicted an approximate 2-fold increase in the performance of such circuits every 12 months ('Moore's law'). The minituarisation of semi-conductor circuits cannot, however, continue indefinitely, and we are rapidly approaching the stage where quantum effects will prevent further dramatic improvements in computer performance using existing technology. As a result, the field of molecular electronics, which seeks to identify and develop much smaller molecular analogues of the transistors that make up integrated circuits, has expanded rapidly over the past few years.At a simplistic level, the key property of any component of an electronic circuit is its ability to control the flow of electric current. As a result, most work has focussed on conjugated carbon-based materials, where the delocalised pi system provides an obvious electron transport pathway. Very recently, however, an alternative strategy based on extended chains of metal atoms surrounded by an insulating sheath of organic material has been developed independently by two experimental groups (Peng, Taipei and Cotton, Texas). The superficial resemblance between these Extended Metal Atom Chains (EMACs) and macroscopic wires is very appealing, but, at a more subtle level, the inate flexibility of the metal-metal bonds that define the core offers a range of possibilities for controlling electron transport.The initial inspiration for this proposal came from our exploration of the electronic structure of a rather simple EMAC, containing a metal core of just three cobalt atoms. The remarkable structural chemistry of this species had been a long-standing mystery: the molecule could apparently exist in at least two distinct forms, whose structures were markedly dependent on temperature. Using density functional theory, we showed that the unique properties of this system arise through a bi-directional redistribution of electron density within the molecule: electrons move in one direction through the sigma framework and in the other through the manifold of delta orbitals in response to subtle changes in temperature. This discovery prompted us to ask a very simple question: if this bi-directional flow of electrons occurs in response to such subtle environmental changes , what might happen if the system is placed under an applied voltage, as it would be in an integrated circuit? The most exciting possibility is that the molecule might act as a molecular rectifier, allowing current to flow more easily in one direction (through the sigma framework, for example) than the other. Rectification of current in junction diodes was perhaps the single most important discovery that led to the development of the transistor in the 1950's, and the discovery of molecular analogues is likely to have a similar impact in the next generation computer architecture. Aviram and Ratner set out the basic features of a molecular rectifier (based on conjugated aromatics) as early as 1974, but the possibility that metal chains may act in a similar way, albeit through a completely different mechanism, has not yet been considered. We aim to use our detailed understanding of the electronic structure of the isolated EMACs as a platform to explore how they behave when placed under an applied voltage. Ultimately, we hope to use the knowledge gained from our theoretical study to construct a rational framework for the future design of components for molecular electronics.