Critical Roles of Metal-Molecule Contacts in Electron Transport Through Molecular-wire Junctions

TL;DR

This paper investigates how different metal-molecule contact structures in molecular junctions influence electron transport, revealing that contact geometry critically affects conductivity and potential device functionalities.

Contribution

It provides a detailed analysis of how various Au-S contact configurations alter electron transmission, highlighting the importance of contact structure in molecular electronics.

Findings

Broad transmission peaks in flat Au(111) contacts lead to high conductivity.

Narrow peaks in top or island contacts cause large variability in conductance.

Au vacancy adsorption results in sharp spectral features and potential switch behavior.

Abstract

We study the variation of electron transmission through Au-S-benzene-S-Au junctions and related systems as a function of the structure of the Au:S contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly coordinated in-hollow and bridge positions are connected with broad transmission peaks around the Fermi level, due to a broad range of transmission angles from transverse motion, resulting in high conductivity and weak dependence on geometrical variations. In contrast, for (unstable) S-adsorption on top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission peaks narrow up due to suppression of large transmission angles. Such more one-dimensional situations may describe more common types of contacts and junctions, resulting in large variations in conductivity and sensitivity to bonding sites, tilting and gating. In particular, if S is adsorbed in an Au vacancy, sharp spectral features appear near the Fermi level due to essential changes of the level structure and hybridization in the contacts, admitting order-of-magnitude variations of the conductivity. Possibly such a system, can it be fabricated, will show extremely strong non-linear effects and might work as uni- or bi-directional voltage-controlled 2-terminal switches and non-linear mixing elements. Finally, density-functional-theory (DFT) based transport calculations seem relevant, being capable of describing a wide range of transmission peak structures and conductivities. Prediction and interpretation of experimental results probably require more precise modeling of realistic experimental situations.