Asymmetric Effect on the Length Dependence of Oligo(Phenylene ethynylene)-Based Molecular Junctions

Abstract

It is well known that the electrical conductance of molecular junctions in the tunneling regime varies exponentially with the length of the molecular backbone. This behavior is strongly influenced by anchoring groups, which connect the molecular backbone to the electrodes and locate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) resonances with respect to the Fermi level. Nevertheless, most of the studies have been performed on symmetric junctions, namely, using the same electrodes and anchoring groups at both sides of the junctions. Only recently, there have been some reports detailing the influence of introducing asymmetry into single-molecule junctions, using different contacts or different anchoring groups at either end of the molecular bridge. These studies have revealed that such junction asymmetry strongly impacts electrical characteristics. In this study, Au and graphene electrodes were used to provide asymmetry to a single-molecule junction. The conductance and length dependence of amine and methyl sulfide-terminated oligo­(phenylene ethynylene) have been determined experimentally and theoretically. The impact of introducing this asymmetry has been quantified by comparing the conductance and β values of oligo­(phenylene ethynylene) (OPE)-based molecules within Au/Au electrodes and Au/graphene junctions, respectively. Our results show that the introduction of a graphene electrode leads to lower conductance values and attenuation factors, similar to what has been previously observed in alkane chains. This is attributed to a shift of the electronic molecular levels toward the Fermi level, mainly driven by acetylene groups linking adjacent phenyl groups.

Destructive quantum interference in meta-oligo(phenyleneethynylene) molecular wires with gold–graphene heterojunctions

“…Moving to the gold|molecule|graphene heterojunctions formed from PN and MN, remarkably similar transmission curves are obtained, a consequence of the similar mechanism of electrode–molecule coupling through the amine group to both gold and graphene. 38 These results are therefore consistent with the experimentally observed single-molecule conductances (Table 1).…”

“…The anticipated effects of the chemical structure of the molecular backbones on molecular conductance regardless of the electrode materials used in the construction of the junction are clear (Table 1). Thus, whilst the linearly conjugated para-series (PN, 38,70,71 PS, 38 PC 73 ) display CQI and relatively high single-molecule conductance, the emergence of DQI features in the transmission functions at the Fermi levels results in a 1-2 order of magnitude decrease in the single-molecule conductance of the meta-structured analogues (MN, MS, MC). 13,20,77 For any given pair of electrodes, changes in the anchor group result in a small variation in the most probable conductance.…”

“…In order to support the experimental measurements, density functional theory (DFT) calculations have been performed using the Fireball code, 54 following the same procedure as detailed elsewhere for oligophenylene ethynylene-based molecular junctions. 38 This approach is based on a self-consistent version of the Harris-Foulkes LDA functional, 55 instead of the traditional Kohn-Sham functional. The potential is calculated by approximating the total charge by a superposition of spherical charges around each atom.…”

“…35 Beyond wire-like conductance, graphite|molecule|Au junctions featuring α,ω-diamino oligo(phenylene)s exhibit rectifying properties due to asymmetric coupling to the two electrodes, 36 whilst gold|molecule|graphene heterojunctions induce a reduction of the attenuation factors ( β ) of symmetrically functionalised alkanes and oligo(phenylene ethynylene)-based SMJs. 37,38…”

“…35 Beyond wirelike conductance, graphite|molecule|Au junctions featuring α,ω-diamino oligo(phenylene)s exhibit rectifying properties due to asymmetric coupling to the two electrodes, 36 whilst gold|mole-cule|graphene heterojunctions induce a reduction of the attenuation factors (β) of symmetrically functionalised alkanes and oligo(phenylene ethynylene)-based SMJs. 37,38 In this work, the electrical properties of para-and meta-connected oligo( phenylene ethynylene) (OPE) molecules contacted within symmetric (gold|molecule|gold) SMJs and asymmetric gold|molecule|graphene single molecule heterojunctions by one of three different anchoring groups (amine, methyl sulfide and ethynyl) have been determined. Whilst as noted above, the anchoring group and electrode material have been used to tune the relative energies of the electrode Fermi level and the DQI induced dip in T (E) in symmetric SMJs, here comparison of the respective single molecule conductances from gold|molecule| gold and gold|molecule|graphene SMJs allows exploration of these effects in asymmetric junctions.…”