In Situ Switching of Quantum Interference in Molecular Tunneling Junctions

Conventional electronics are made up of bulk materials where the components of the system are carved out from bulk (top-down). Molecular electronics takes a different approach where electronic systems are built up atom by atom in a chemistry lab (bottom-up). The majority of molecular electronics research investigates charge transport through metal-molecule-metal tunneling junctions comprising either self-assembled monolayers (SAMs) or single-molecules between two metal electrodes:

Self-assembled monolayer and a single-molecule junction

Single-molecule junctions are relatively easy to model compared to large-area SAMs. In this publication, excellent qualitative agreement has been shown between large-area experiments and Non-Equilibrium Green’s Function + Density Functional Theory (NEGF-DFT) calculations with BAND by Saurabh Soni. The authors describe the control of tunneling current in molecular junctions by switching one of the two parallel intramolecular pathways. A linearly‐conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of overall junction:

Protonation affects the conductance of the molecular junction.

This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; classical Kirchhoff’s circuit rules do not apply, meaning that the total electrical current going through the junction is unequal to the sum of electrical currents going through individual pathways. This concept is proven by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions, supported by state-of-the-art NEGF/BAND calculations. Converting a carbonyl into a planar carbocation allows selective switching between the parallel pathways. Future research could focus on systems that are more readily protonated to improve the parallel pathway switching and making it viable for large-area molecular tunneling junctions.

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Soni, S.; Ye, G.; Zheng, J.; Zhang, Y.; Asyuda, A.; Zharnikov, M.; Hong, W.; Chiechi, R. C.; Understanding Role of Parallel Pathways via In Situ Switching of Quantum Interference in Molecular Tunneling Junctions, Angew. Chem. Int. Ed. ASAP (2020).

Key concepts