Charging induced asymmetry in molecular conductors

TL;DR

This paper explains the asymmetry in I-V characteristics of symmetric molecules as arising from contact coupling and charging effects, using a self-consistent NEGF model that matches experimental data.

Contribution

It introduces a self-consistent NEGF-based model capturing contact asymmetry and charging effects to explain I-V asymmetries in molecular conductors.

Findings

Asymmetry depends on contact coupling and charge effects.

Model matches experimental I-V curves in shape and magnitude.

Direction of asymmetry varies with HOMO or LUMO conduction.

Abstract

We investigate the origin of asymmetry in various measured current-voltage (I-V) characteristics of molecules with no inherent spatial asymmetry, with particular focus on a recent break junction measurement. We argue that such asymmetry arises due to unequal coupling with the contacts and a consequent difference in charging effects, which can only be captured in a self-consistent model for molecular conduction. The direction of the asymmetry depends on the sign of the majority carriers in the molecule. For conduction through highest occupied molecular orbitals (i.e. HOMO or p-type conduction), the current is smaller for positive voltage on the stronger contact, while for conduction through lowest unoccupied molecular orbitals (i.e. LUMO or n-type conduction), the sense of the asymmetry is reversed. Within an extended Huckel description of the molecular chemistry and the contact microstructure (with two adjustable parameters, the position of the Fermi energy and the sulphur-gold bond length), an appropriate description of Poisson's equation, and a self-consistently coupled non-equilibrium Green's function (NEGF) description of transport, we achieve good agreement between theoretical and experimental I-V characteristics, both in shape as well as overall magnitude.