Molecular Signature and Activationless Transport in Cobalt-Terpyridine-Based Molecular Junctions

Cobalt terpyridine oligomers are compared with pi-conjugated and ruthenium-centered layers in molecular junctions (MJs) with identical contacts. A wide range of layer thickness is investigated, and attenuation plots are obtained. Strong dependence of charge transport on molecular layers is found with a variation of four orders of magnitude of current density ( J) for different molecules and d = 7 nm. For a Ru(bpy)(3) complex and bis-thienylbenzene MJs, the attenuation plot shows two different regions corresponding to two different dominant transport mechanisms. On the contrary Co(tpy)(2) and viologen-based MJs show no transition thickness in the attenuation plot, indicating a possible change of mechanism with film thickness, and very low attenuation factors (beta of 0.17 and 0.25 nm(-1) from 2 to 14 nm, respectively). These beta values indicate highly efficient long-range transport. This is attributed to the fact that the energy levels of the frontier orbital involved in transport are between, and thus almost in resonance with, the Fermi levels of the electrodes. Temperature-dependence measurements suggest that field ionization followed by multistep hopping and redox events can occur above 100 K, while the activationless region at low T indicates incoherent tunneling between redox sites with reorganization concerted with charge transfer.