Graphene as a Promising Electrode for Low-Current Attenuation in Nonsymmetric Molecular Junctions.

Qian Zhang,Li Yang,Richard J Nichols,César González,Cezhou Zhao,Yannick J Dappe,Congyi Wang,Shuhui Tao,Longlong Liu

doi:10.1021/acs.nanolett.6b03180

Abstract

We have measured the single-molecule conductance of 1,n-alkanedithiol molecular bridges (n = 4, 6, 8, 10, 12) on a graphene substrate using scanning tunneling microscopy (STM)-formed electrical junctions. The conductance values of this homologous series ranged from 2.3 nS (n = 12) to 53 nS (n = 4), with a decay constant βn of 0.40 per methylene (-CH2) group. This result is explained by a combination of density functional theory (DFT) and Keldysh-Green function calculations. The obtained decay, which is much lower than the one obtained for symmetric gold junctions, is related to the weak coupling at the molecule-graphene interface and the electronic structure of graphene. As a consequence, we show that using graphene nonsymmetric junctions and appropriate anchoring groups may lead to a much-lower decay constant and more-conductive molecular junctions at longer lengths.

Highlights

A lthough technologically relevant molecular electronic devices still seem a long way off, the ability to measure the electrical properties of single molecules can be achieved with a variety of techniques that were not available at the genesis of the field.[1]
There is an increasing realization that new singlemolecule electrical junction functionality can be achieved through the use of nonmetallic electrodes, with contacts such as indium−tin oxide (ITO),[12−15] carbon-based materials, and even novel two-dimensional (2D) graphene being considered.[16−18] Kim et al have formed graphite−molecules− Au molecular junctions by the use of the scanning tunneling microscopy (STM)-BJ technique and measured the conductance of amine-terminated oligophenyl compounds.[19]
The combination of these factors leads to a strong reduction of the electronic length decay value, which is found to be about half of the value obtained for symmetric gold junctions

Summary

Nano Letters

Carbon based materials have the potential to be valuable alternative electrode materials for molecular electronics in the generation of nanostructured devices. The effect of graphene is mainly to decouple the molecule from the second electrode and to favor a stronger charge transfer at the S−Au interface, relocating the HOMO level near the Fermi level In this case, the effect is less due to the electronic properties of graphene than to the weak coupling associated with van der Waals interactions. Theoretical computations of the junction conductance values were performed to investigate the electrical properties as a function of molecular length These results show that the decay is related to the junction electronic structure, the nonsymmetric contact, and the weak coupling at the molecule−graphene interface, leading to a stronger charge transfer at the gold electrode−molecule interface.

Author Contributions

■ REFERENCES