Charge transport through image charged stabilized states in a single molecule single electron transistor device

TL;DR

This paper presents a theoretical model explaining a new charge transport mechanism in single-molecule transistors, where electrons tunnel into localized image charge states, allowing access to multiple redox states within a narrow energy window.

Contribution

It introduces a Hubbard model-based theory for image charge stabilized states in molecular transistors, highlighting the geometric and chemical conditions for this mechanism's effectiveness.

Findings

The model fits experimental data with reasonable parameters.

Effective charge transport requires terminal benzene rings to be near electrodes.

Chemisorption can significantly hinder the image charge effect.

Abstract

The present paper gives an elaborate theoretical description of a new molecular charge transport mechanism applying to a single molecule trapped between two macroscopic electrodes in a solid state device. It is shown by a Hubbard type model of the electronic and electrostatic interactions, that the close proximity of metal electrodes may allow electrons to tunnel from the electrode directly into a very localized image charge stabilized states on the molecule. Due to this mechanism, an exceptionally large number of redox states may be visited within an energy scale which would normally not allow the molecular HOMO-LUMO gap to be transversed. With a reasonable set of parameters, a good fit to recent experimental values may be obtained. The theoretical model is furthermore used to search for the physical boundaries of this effect, and it is found that a rather narrow geometrical space is available for the new mechanism to be effective: In the specific case of oligophenylenevinylene molecules recently explored in such devices several atoms in the terminal benzene rings need to be at van der Waal's distance to the electrode in order for the mechanism to be effective. The model predicts, that chemisorption of the terminal benzene rings too gold electrodes will impede the image charge effect very significantly because the molecule is pushed away from the electrode by the covalent thiol-gold bond.