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

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.