Thickness-Dependent Charge Transport in Three Dimensional Ru(II)– Tris(phenanthroline)-Based Molecular Assemblies
Ritu Gupta, S. S. Bhandari, Savaş Kaya, Konstantin P. Katin, Prakash Chandra Mondal
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, E a ∼ 43 meV) Poole–Frenkel charge conduction, showing a clear “molecular signature” in the nanometric junctions.