Ab initio study of charge transport through single oxygen molecules in atomic aluminum contacts

We present ab initio calculations of transport properties of atomic-sized aluminum contacts in the presence of oxygen. The experimental situation is modeled by considering a single oxygen atom (O) or one of the molecules O2 and O3 bridging the gap between electrodes forming ideal, atomically sharp pyramids. The transport characteristics are computed for these geometries with increasing distances between the leads, simulating the opening of a break junction. To facilitate comparison with experiments further, the vibrational modes of the oxygen connected to the electrodes are studied. It is found that in the contact regime the change of transport properties due to the presence of oxygen is strong and should be detectable in experiments. All three types of oxygen exhibit a comparable behavior in their vibrational frequencies and conductances, which are well below the conductance of pure aluminum atomic contacts. The conductance decreases for an increasing number of oxygen atoms. In the tunneling regime the conductance decays exponentially with distance and the decay length depends on whether or not oxygen is present in the junction. This fact may provide a way to identify the presence of a gas molecule in metallic atomic contacts.Comment: 8 pages, 9 figures; added appendi

Highly conductive molecular junctions based on direct binding of benzene to platinum electrodes

Highly conductive molecular junctions were formed by direct binding of benzene molecules between two Pt electrodes. Measurements of conductance, isotopic shift in inelastic spectroscopy and shot noise compared with calculations provide indications for a stable molecular junction where the benzene molecule is preserved intact and bonded to the Pt leads via carbon atoms. The junction has a conductance comparable to that for metallic atomic junctions (around 0.1-1 Go), where the conductance and the number of transmission channels are controlled by the molecule's orientation at different inter-electrode distances.Comment: 4 pages, 4 figure

Cluster-based density-functional approach to quantum transport through molecular and atomic contacts

We present a cluster-based density-functional approach to model charge transport through molecular and atomic contacts. The electronic structure of the contacts is determined in the framework of density functional theory, and the parameters needed to describe transport are extracted from finite clusters. A similar procedure, restricted to nearest-neighbor interactions in the electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)]. Here, we show how to systematically improve the description of the electrodes by extracting bulk parameters from sufficiently large metal clusters. In this way we avoid problems arising from the use of nonorthogonal basis functions. For demonstration we apply our method to electron transport through Au contacts with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure

N-silylamine junctions for molecular wires to gold: The effect of binding atom hybridization on the electronic transmission

In molecular conduction studies, amine groups are often used as linkers between molecules and gold surfaces. We demonstrate using density functional theory (DFT) that N-silyl substitution of 1,4-benzenediamine (BDA) produces a highly polarizable junction with a conductance and electrostatic profile that depends strongly on the local hybridization. In BDA itself, a forced change in hybridization from sp3 to sp2 is predicted to increase the conductivity by an order of magnitude. N-silyl substitution is shown to make this effect accessible while maintaining a 5-fold conductance ratio and sufficient binding strength to allow junctions to spontaneously assemble. Hence N-silylamines are predicted to form useful, externally controllable molecular junctions. © 2009 American Chemical Society

Accurate and computationally efficient third-nearest-neighbor tight-binding model for large graphene fragments

Owing to the large sizes involved, most calculations of the electronic properties of graphene and its fragments involve empirical tight-binding models restricted to nearest-neighbor interactions only. Such approaches fail to predict key electronic and magnetic properties, however, and rely on assumed geometries. While alternative approaches based on density-functional theory are much more successful in predicting properties, they are often computationally prohibitive to apply. We introduce a simple third-nearest-neighbor π -only tight-binding approach that maintains the computational efficiency of the empirical method while achieving the accuracy of the density-functional methods to which it is parametrized. It yields both nuclear geometries and electronic structures of graphene fragments, providing an efficient and accurate replacement for traditional tight-binding models of graphene. © 2010 The American Physical Society

The conduction properties of α,ω-diaminoalkanes and hydrazine bridging gold electrodes

The conduction properties of α,ω-diaminoalkanes and hydrazine bridging gold electrodes are investigated using density functional theory in combination with Green's function techniques and group theory. We show that more than 99% of the current at low bias is carried by one channel of A′ symmetry whose form is invariant to both the alkane chain length and realistic changes of the injection energy. The total conductance decreases exponentially with a decay constant of 0.73 Å-1 with increasing chain length while the calculated gold-amine contact resistance is ca. 150 kΩ. © 2008 Elsevier B.V. All rights reserved

Interference-induced electron- and hole-conduction asymmetry

Principles established by Shephard and Paddon-Row for optimizing and controlling intramolecular electron transport through the modulation of interfering pathways are employed to design new molecules for steady-state conduction experiments aimed at manifesting electron-hole conduction asymmetry in a unique way. First, a review of the basic principles is presented through application to a pertinent model system in which a molecule containing donor and acceptor terminal linking groups with an internal multiple-pathway bridge is used to span two metal electrodes. Different interference patterns are produced depending on whether the through-molecule coupling pathways are symmetric or antisymmetric with respect to a topological bisecting plane, giving rise to asymmetric electron and hole conductances at the tight-binding (Hückel) level; this process is also described from a complementary molecular-orbital viewpoint. Subsequently, a new molecular system based on organic polyradicals is designed to allow such asymmetry to be realized in single-molecule conduction experiments. These polyradicals are analyzed using analogous simple models, density-functional theory (DFT) calculations of steady-state transmission, and intermediate neglect of differential overlap (INDO) calculations of intramolecular connectivity, verifying that polyradicals at low temperatures should show experimentally measureable electron-hole conduction asymmetry. A key feature of this system is that the polyradicals form a narrow partially occupied band of orbitals that lie within and well separated from the HOMO and LUMO orbitals of the surrounding molecular scaffold, allowing for holes and electrons to be transported through the same molecular band. © 2011 Springer-Verlag