Impact of Anchoring Groups on Ballistic Transport: Single Molecule vs Monolayer Junctions.

Veronika Obersteiner,David A Egger,Egbert Zojer

doi:10.1021/acs.jpcc.5b06110

Abstract

Tuning the transport properties of molecular junctions by chemically modifying the molecular structure is one of the key challenges for advancing the field of molecular electronics. In the present contribution, we investigate current–voltage characteristics of differently linked metal–molecule–metal systems that comprise either a single molecule or a molecular assembly. This is achieved by employing density functional theory in conjunction with a Green’s function approach. We show that the conductance of a molecular system with a specific anchoring group is fundamentally different depending on whether a single molecule or a continuous monolayer forms the junction. This is a consequence of collective electrostatic effects that arise from dipolar elements contained in the monolayer and from interfacial charge rearrangements. As a consequence of these collective effects, the “ideal” choice for an anchoring group is clearly different for monolayer and single molecule devices. A particularly striking effect is observed for pyridine-docked systems. These are subject to Fermi-level pinning at high molecular packing densities, causing an abrupt increase of the junction current already at small voltages.

Highlights

Electronic devices in which individual molecules or a molecular assembly are used as semiconducting components constitute a promising approach for ultimate miniaturization.[1−3] One of the key challenges in realizing such “molecular electronics” is a microscopic understanding of charge transport through metal− molecule−metal systems
This is a consequence of collective electrostatic effects that arise from dipolar elements contained in the monolayer and from interfacial charge rearrangements
An efficient way of tuning the transport properties of molecular devices is exploiting the enormous versatility of organic chemistry that is mainly achieved by chemical substitutions within the molecular backbone[4] and via specific side groups. Another commonly used “molecular design” approach is to control charge transport in molecular junctions by changing the anchoring group linking the molecule and the metal.[5−7] This, on the one hand, offers the possibility to tune the properties of the individual molecules, i.e., the ionization potential (IP) and electron affinity (EA)

Summary

Introduction

Electronic devices in which individual molecules or a molecular assembly are used as semiconducting components constitute a promising approach for ultimate miniaturization.[1−3] One of the key challenges in realizing such “molecular electronics” is a microscopic understanding of charge transport through metal− molecule−metal systems. The properties of thiolate-bonded molecular junctions were, shown to be quite sensitive to the binding geometry.[11] the S−Au bond seems to have some disadvantage compared to, e.g., the Se bond to coinage metals.[12] the structural details of the S−Au bond are strongly disputed in the literature,[13−15] suggesting that a coexistence of several different geometries might be especially relevant for thiol-Au bonded junctions causing a wide spread of experimentally measured conductances.[16−18] alternative anchoring groups have been studied extensively both theoretically and experimentally. In the coherent transport regime, this determines the tunneling barrier for charge carriers and is of key importance in molecular electronics.[33,34] Chemical trends of conductance and junction stability were investigated by Hong et al.[10] for tolane molecules attached to gold via different

Results

Discussion

Conclusion