Understanding resonant charge transport through weakly coupled single-molecule junctions

James O Thomas,Jan A Mol,G Andrew D Briggs,Jakub K Sowa,Erik M Gauger,Kyle Willick,Jonathan Baugh,Harry L Anderson,Bart Limburg

doi:10.1038/s41467-019-12625-4

Abstract

Off-resonant charge transport through molecular junctions has been extensively studied since the advent of single-molecule electronics and is now well understood within the framework of the non-interacting Landauer approach. Conversely, gaining a qualitative and quantitative understanding of the resonant transport regime has proven more elusive. Here, we study resonant charge transport through graphene-based zinc-porphyrin junctions. We experimentally demonstrate an inadequacy of non-interacting Landauer theory as well as the conventional single-mode Franck–Condon model. Instead, we model overall charge transport as a sequence of non-adiabatic electron transfers, with rates depending on both outer and inner-sphere vibrational interactions. We show that the transport properties of our molecular junctions are determined by a combination of electron–electron and electron-vibrational coupling, and are sensitive to interactions with the wider local environment. Furthermore, we assess the importance of nuclear tunnelling and examine the suitability of semi-classical Marcus theory as a description of charge transport in molecular devices.

Highlights

Off-resonant charge transport through molecular junctions has been extensively studied since the advent of single-molecule electronics and is well understood within the framework of the non-interacting Landauer approach
In contrast to redox molecular junctions[3], the current that flows through the molecular junction during resonant transport in a weakly coupled junction is a result of these sequential electron transfers to and from the molecule
Zinc porphyrin molecules, functionalised with anchor groups that have been designed to bind to the graphene electrodes via π–π stacking and van der Waals interactions[23] (Fig. 1b), are deposited from solution. 3,5Bis(trihexysilyl)phenyl aryl groups increase the solubility of the porphyrin and prevent aggregation, we do not expect them to directly contribute to the charge transport, as DFT calculations indicate that during the oxidation/reduction of the molecular species the additional charge density is localised on the porphyrin ring and the pyrene anchor groups

Summary

Introduction

Off-resonant charge transport through molecular junctions has been extensively studied since the advent of single-molecule electronics and is well understood within the framework of the non-interacting Landauer approach. In contrast to redox molecular junctions[3] (in which the charging/discharging of the molecule has no direct contribution to the current), the current that flows through the molecular junction during resonant transport in a weakly coupled junction is a result of these sequential electron transfers to (i.e., a reduction process) and from (i.e. an oxidation process) the molecule. As both the electron occupancy and the equilibrium geometry of the molecule and its local environment change upon an electron transfer event, the electron–electron and electron-vibration interactions can no longer be ignored[4]. When only a single spin-degenerate level is involved in transport the number of possible transitions is accounted for by setting Ω to 0 for the N/N+1 transition or 1 for the N−1/N transition, as discussed in Supplementary Note 2 and in detail elsewhere[9]

Methods

Results

Conclusion