Electronic structure/function relationships in metal nanowires : components for molecular electronics

• Main Author: Georgiev, Vihar Petkov
• Other Authors: John, McGrady
• Published: University of Oxford 2011
• Subjects: 621.3; Computational chemistry : electron transport
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543479

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. The miniaturisation 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. Recent studies suggested that extended metal atom chains (EMAC) may have many potential applications in molecular electronics, but it is clear that this potential can only be realised if we establish a link between the fundamental electronic properties of these systems and the transport of electrons. For this reason the ultimate goal of this thesis is to relate the electronic structure of extended metal chains to their electron transport properties. We address the problem using non-equilibrium Green’s function, in conjugation with density functional theory. In the results sections of this thesis we present calculations on tricobalt, trichromium and trinickel chains. Our data suggested that in the trimetal chains, the dominant electron transport channel is the σ manifold, while the π systems establish the contact with the electrodes. The implication of this is that even when the highly polarized π channels are strongly rehybridised by the applied electric field, current flow is not affected. In the trichromium systems we find that the distortion of the chain away from the symmetric equilibrium structure does not perturb the current flow but rather enhances it. Our rather counter intuitive conclusion is therefore that ‘broken wires’ (highly unsymmetric) are more efficient conductors than their symmetric counterparts. We have performed calculation on longer penta- and heptacobalt structures chains to establish the extent to which longer structures attenuate the conductance. Our calculations show significant oscillations of the conductance due to development of a one-dimensional band structure about the Fermi level. The evolution of the electron transport properties in cobalt chains with different length is a complex one, but it is clear that narrowing the band gap in longer chains makes it increasingly likely that the Fermi level will be in resonance with one or more of the orbitals of the extended metal atom chain.