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

Abstract

Cobalt terpyridine oligomers are compared with π‐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 (β of 0.17 and 0.25 nm−1 from 2 to 14 nm, respectively). These β 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.

Functional Redox‐Active Molecular Tunnel Junctions

“…As mentioned in Sections 2.2 and 2.4, normally the values of β for aliphatic or conjugated molecules are relatively high mainly due to large values of δ E ME . In contrast, redox‐active molecules have the E HOMO or E LUMO close in energy to E F and, consequently, low δ E ME and small β [23a,b] (Eqs. 1 and 2).…”

Bias‐Polarity‐Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox‐Active Molecular Junction

“…In this regime, the current also depends on d but with significantly smaller β than that of coherent tunneling. [ 5 , 8 ] Figure 1b graphically illustrates the Marcus parabolas for charge transfer (or hopping) for an exothermic redox process where the black/green parabola depicts the potential energy curves of the donor–acceptor pair consisting of neutral molecule (here BTTF)/charged molecule (here BTTF ·+ ). [ 6 , 9 ] The E a is defined by the crossing between the two parabolas, representing the energy that must be overcome to enable charge hopping, Δ E 0 is the energy difference between neutral and charged states, and λ is the molecular reorganization energy.…”

Single-Molecule Junctions with Highly Improved Stability

“…12,13,14 It is possible to record I(V) curves at various film thicknesses and temperatures, to obtain an attenuation plot (slope β) vs. junction length and to study the dominant transport mechanism. 10,[15][16][17] Production yield can be high and, when robust multifunctional layers based on diazonium electroreduction are used, the top contact electrode can be deposited by direct evapora-tion. Since large-area MJs are stable, many important results, leading to a better understanding of transport mechanisms, have been reported.…”