Electric Field-induced Charge Transport in Redox-active Molecular Junctions

Abstract: The formation of well-defined three-dimensional (3D) redox-active molecular nanostructures at the electrode surfaces may open additional routes to achieve higher conductance in molecular junctions (MJs). We report here experimental and theoretical charge transport analysis on electroactive ruthenium(II)-tri(phenanthroline) [Ru(Phen)3]-based molecular junctions covalently grown on patterned ITO electrode. Thicknesses of the molecular layers are varied between 4 to 13 nm, thanks to the potential-driven electrochemical technique to achieve it. A thin layer of Al was deposited on top contact over ITO/ Ru(Phen)3 to fabricate large-area solid-state molecular junctions with a stacking configuration of ITO/[Ru(Phen)3]4nm, 10nm, 13nm/Al. The electrified molecular junctions show LUMO-mediated electron-driven resonant charge conduction with attenuation in conductance as a function of the length of Ru(Phen)3 layers (\b{eta} = 0.48 to 0.60 nm-1). Molecular junctions consisting of 4 nm Ru(Phen)3 layers follow quantum tunneling, while the thicker junctions (10, and 13 nm) follow Poole-Frenkel and electric-field induced charge conduction. Considering the energy level of frontier molecular orbitals, Fermi energy of ITO, and Al contact, a mechanism of symmetric current-voltage features with respect to the bias-polarity is predicted. The present work describes a simple, controllable, low-cost, and versatile approach to fabricating 3D molecular assembly for mimicking conventional electronic functions.