Gateway state-mediated, long-range tunnelling in molecular wires

Abstract

a If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of highconductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae . It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety ( phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to E F through coupling to the bridge moiety.This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.

Advances and challenges in single-molecule electron transport

“…Only in this limit is the conductance truly exponential in the length, and the gap exponent β is a property of the band structure of the long wire, i.e., independent of the contact arrangement. In their experimental work, Sangtarash et al (2018) observed in alkane wires with an extra aromatic center unit an approximately exponential decay GðNÞ ≈ e −β 0 N with an effective exponent β 0 that is considerably smaller than β. Sangtarash et al explained their observation by invoking in-gap (gateway) states. From our perspective one would expect the effective exponent β 0 to characterize a preasymptotic regime that crosses over into a steeper decay at larger N.…”

Theory of electron, phonon and spin transport in nanoscale quantum devices

“…where ε 1 and ε 2 are the energy levels coupled to each other by V d . If ε 1 (ε 2 ) is weakly bonded to the left (right) lead, the transmission coefficient T (E) could be obtained form equation 65 as [32,33]:…”

Charge transport at a molecular GaAs nanoscale junction

“…The X[Ph]X series has already been characterised in Au-molecule-Au junctions, and an unusually low value of b was experimentally determined. 21,22 This phenomenon was later ascribed to the presence of two orbitals located at the metal-sulfur interface of the molecular junction, which act as charge transport "gateways" that reduce the effect of molecular length on the overall conductance. 22 The effect was only observed in covalently-bonded molecular wires, where the thiol proton is lost upon chemisorption at the Au electrodes, with the formation of strong Au-S bonds.…”

“…21,22 This phenomenon was later ascribed to the presence of two orbitals located at the metal-sulfur interface of the molecular junction, which act as charge transport "gateways" that reduce the effect of molecular length on the overall conductance. 22 The effect was only observed in covalently-bonded molecular wires, where the thiol proton is lost upon chemisorption at the Au electrodes, with the formation of strong Au-S bonds. In nGaAs-X[Ph]X-metal junctions, b was found again to be in good accordance with the value obtained employing the same molecular wires in Au-molecule-Au junctions, thus suggesting that the same "gateway" states are present at the semiconductor-molecule interface.…”