Thickness-Dependent Charge Transport in Three Dimensional Ru(II)− Tris(phenanthroline)-Based Molecular Assemblies

Abstract

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)−tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4−16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current−voltage (j−V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm−1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO− molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10−16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole−Frenkel charge conduction, showing a clear “molecular signature” in the nanometric junctions.