Unconventional Current Scaling and Edge Effects for Charge Transport through Molecular Clusters.

Veronika Obersteiner,Nick Papior,Egbert Zojer,Georg Huhs

doi:10.1021/acs.nanolett.7b03066

Abstract

Metal–molecule–metal junctions are the key components of molecular electronics circuits. Gaining a microscopic understanding of their conducting properties is central to advancing the field. In the present contribution, we highlight the fundamental differences between single-molecule and ensemble junctions focusing on the fundamentals of transport through molecular clusters. In this way, we elucidate the collective behavior of parallel molecular wires, bridging the gap between single molecule and large-area monolayer electronics, where even in the latter case transport is usually dominated by finite-size islands. On the basis of first-principles charge-transport simulations, we explain why the scaling of the conductivity of a junction has to be distinctly nonlinear in the number of molecules it contains. Moreover, transport through molecular clusters is found to be highly inhomogeneous with pronounced edge effects determined by molecules in locally different electrostatic environments. These effects are most pronounced for comparably small clusters, but electrostatic considerations show that they prevail also for more extended systems.

Highlights

Molecular electronics aims at realizing electronic devices by contacting nanoscale assemblies of molecules between metallic electrodes.[1,2] A key goal is to meet the increasing technical demands of miniaturization
We elucidate the collective behavior of parallel molecular wires, bridging the gap between single molecule and large-area monolayer electronics, where even in the latter case transport is usually dominated by finite-size islands
The field of molecular electronics can be divided into two branches, namely single molecule electronics, where junctions ideally consist of an individual molecule[3,4] and molecular ensemble electronics[5,6] comprising junctions with large numbers of molecules or even extended monolayers with a quasi-infinite number of molecules in parallel

Summary

Nano Letters

Experiments, contact is intentionally made to comparably small clusters of organic molecules, for example, when growing them on metal nanoparticles.[24]. Compared to the pyridine-docked systems, one observes two differences: the shifts between single molecule and monolayer are smaller, which is a consequence of the smaller interfacial dipoles;[26] due to the stronger quantum-mechanical coupling between molecular and metal states, the transmission peaks are significantly broadened, which makes a reliable determination of the positions of individual transmission features difficult, if not impossible. It complicates the identification of edge effects. Details to the applied DFT-NEGF methodology (computational workflow, numerical setup, and computational costs); calculated (zero-bias) transmission functions for all systems; (zero-bias) conductance for the pyridine-linked junctions; details to the electrostatic model; a description of the tight binding model and the cluster calculations used to distinguish between the impacts of quantum-mechanical inter-molecular coupling and electrostatic edge effects (PDF)

Author Contributions

■ ACKNOWLEDGMENTS

■ REFERENCES