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

• Published in: Advanced electronic materials Vol. 6; no. 7
• Main Authors: Nguyen, Quyen, Tefashe, Ushula, Martin, Pascal, Della Rocca, Maria Luisa, Lafolet, Frederic, Lafarge, Philippe, McCreery, Richard L., Lacroix, Jean‐Christophe
• Format: Journal Article
• Language: English
• Published: Wiley 01.07.2020
• Subjects: Chemical Sciences; diazonium electroreduction; electron transport; electronic materials; metal‐centered molecular junctions; molecular electronics
• ISSN: 2199-160X; 2199-160X
• DOI: 10.1002/aelm.201901416

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. Cobalt terpyridine oligomers are compared with π‐conjugated and ruthenium‐centered layers in a molecular junction. A wide range of layer thickness is investigated and attenuation plots are obtained. A strong molecular signature on charge transport is found, and transitions between various dominant transport mechanisms and, for some systems, highly efficient long‐range transport with attenuation factors as small as 0.17 nm−1 are observed.