Conductance of a phenylene-vinylene molecular wire: Contact gap and tilt angle dependence

Abstract

Charge transport through a molecular junction comprising an oligomer of p-phenylene-vinylene between gold contacts has been investigated using density-functional theory and the nonequilibrium Green's function method. The influence of the contact gap geometry on the transport has been studied for elongated and contracted gaps, as well as various molecular conformations. The calculated current-voltage characteristics show an unusual increase in the low bias conductance with the contact separation. In contrast, for compressed junctions the conductance displays only a very weak dependence on both the separation and related molecular conformation. However, if the contraction of the gap between the electrodes is accommodated by tilting the molecule, the conductance will increase with the tilting angle, in line with experimental observations. It is demonstrated that the effect of tilting on transport can be interpreted in a similar way to the case of the stretching the junction with a molecule in an upright position. The lowest conductance was observed for the equilibrium gap geometry. With the dominant transport contribution arising from the system of the frontier junction orbitals, all the predicted increases in the conductance arise simply from the better band alignment between relevant frontier orbitals at the nonequilibrium geometries at the expense of weaker coupling with the contacts.

From fused aromatics to graphene-like nanoribbons: The effects of multiple terminal groups, length and symmetric pathways on charge transport

“…4 The important differences are the use of both the Au(001) and Au(111) surfaces for the metal contacts and the local density approximation (LDA) 6 in the present study versus only the conventional Au(111) surface and the generalized gradient approximation (GGA) considered previously. The addition of the (001)-oriented gold substrate has been dictated by the aim of investigating the effects of the symmetric transport pathways on the overall conductance across the junction.…”

“…The range of the atomic orbitals was evaluated from an energy increase of 1 mRy arising due to the spherical confinement, as opposed to 7 mRy in the previous GGA-based study. 4 The smaller energy shift was chosen so as to reduce the effects of confinement, which serve to reinforce the tendency of LDA to overbind, thereby yielding an improved gold lattice constant and optimized bond lengths with respect to the experiment. A split-valence scheme was employed to generate a single-ζ basis set for the 6s and 5d states of Au, and a double-ζ basis set for the 1s state of H, 2s and 2p states of C and F, and 3s and 3p states of S, with a single-ζ shell of polarization functions for all five elements.…”

“…Conjugated aromatic molecules, such as polycyclic hydrocarbons, [1][2][3] and oligomers, such as phenylene-vinylene, 4 represent some of the most frequently investigated molecular wires. Despite the large body of conductivity studies found in the literature, a class of wires with transport properties suitable for molecular circuit usage has not yet been identified.…”

Anomalous length dependence of the conductance of graphene nanoribbons with zigzag edges

“…5 in Ref. 31), in this case do not arise from the frontier molecular orbital, highest occupied (HOMO) and lowest unoccupied (LUMO), levels. In fact, the gap between them exhibits a rather flat continuum of conducting channels, a feature familiar to the related studies of transport by GNRs with dual thiol terminal groups.…”

“…Interestingly, for [ highly non-planar, due to a torsional twist caused by the steric repulsion between H atoms, the π type LUMO resonance exhibits a familiar resilience against the structural distortion. 31 In summary, both the zGNR series with phenanthroline terminal groups exhibit very poor conductance and rapid attenuation with length. Despite the highly conjugated C backbone of the zGNRs and inherent multiple transport pathways at the interface with metal contacts, provided by the dual pyridine rings, the weak coupling of the sp 2 hybridized N atoms with gold makes 1,10-phenanthroline unsuitable for use in molecular circuitry.…”