Redox-controlled conductance of polyoxometalate molecular junctions

TL;DR

This study demonstrates reversible redox control of conductance in polyoxometalate molecular junctions, revealing how photoreduction alters electron transport properties and providing insights into their potential for nanoelectronic applications.

Contribution

It introduces a method for in situ photoreduction of polyoxometalate junctions and analyzes their electron transport behavior using machine learning, highlighting their switchable conductance properties.

Findings

Photoreduction increases conductance by ca. 10 times.

Electron transport is mainly controlled by LUMO at 0.6-0.7 eV.

A fraction of junctions are already reduced before photoreduction.

Abstract

We demonstrate the reversible in situ photoreduction of molecular junctions of phosphomolybdate [PMo12O40]3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo12O40]4- molecular junction is increased by ca. 10, this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6 - 0.7 eV above the Fermi energy with ca. 25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (ca. 18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions are systematically featuring slightly asymmetric current - voltage behaviors, which is ascribed to electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrode and by a closely located single unoccupied molecular orbital (SUMO) at ca. 0.3 eV above the SOMO with a weak electronic coupling to the electrodes (ca. 50% of the dataset) or at ca. 0.4 eV but with a better electrode coupling (ca. 50% of the dataset). These results shed lights to the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.