Hierarchical Electronic Transport in Giant Mixed-Valence {Mo154} Rings Revealed in Anhydrous Single-Molecule-Layer Networks

Abstract

Polyoxometalates (POMs) are redox-active metal–oxo clusters of central interest in molecular and materials chemistry. The giant mixed-valence {Mo154} ring, built from 14 repeating subunits, is a prototypical system expected to exhibit rich electronic functionality; however, its transport has often been obscured by water-mediated proton conduction. Here, we fabricate anhydrous networks consisting predominantly of a single-molecule layer of {Mo154} rings by removing crystalline water in vacuum and investigate their transport properties using nanoscale electrical measurements. This approach enables direct observation of predominantly electronic conduction within the cluster network. The current–voltage characteristics exhibit clear nonlinearity and thermal activation, and their temperature- and electric-field-dependent responses are consistently described by a one-dimensional variable-range hopping model incorporating an effective temperature with temperature-dependent localization length and Mott parameters. The extracted parameters indicate a marked reduction of the localization length upon cooling, implying enhanced carrier confinement and weakened electronic coupling between subunits. These results establish a hierarchical transport picture in which conduction evolves from localized motion within the ring architecture to inter-ring hopping once water-mediated pathways are effectively eliminated.