title: A Single-Level Tunnel Model to Account for Electrical Transport through Single Molecule- and Self-Assembled Monolayer-based Junctions.
abstract: We present a theoretical analysis aimed at understanding electrical conduction in molecular tunnel junctions. We focus on discussing the validity of coherent versus incoherent theoretical formulations for single-level tunneling to explain experimental results obtained under a wide range of experimental conditions, including measurements in individual molecules connecting the leads of electromigrated single-electron transistors and junctions of self-assembled monolayers (SAM) of molecules sandwiched between two macroscopic contacts. We show that the restriction of transport through a single level in solid state junctions (no solvent) makes coherent and incoherent tunneling formalisms indistinguishable when only one level participates in transport. Similar to Marcus relaxation processes in wet electrochemistry, the thermal broadening of the Fermi distribution describing the electronic occupation energies in the electrodes accounts for the exponential dependence of the tunneling current on temperature. We demonstrate that a single-level tunnel model satisfactorily explains experimental results obtained in three different molecular junctions (both single-molecule and SAM-based) formed by ferrocene-based molecules. Among other things, we use the model to map the electrostatic potential profile in EGaIn-based SAM junctions in which the ferrocene unit is placed at different positions within the molecule, and we find that electrical screening gives rise to a strongly non-linear profile across the junction.
authors.affiliation: Department of Physics, University of Central Florida, Orlando, Florida 32816 - USA.
authors.forename: Alvar R
authors.initials: AR
authors.lastname: Garrigues
authors.affiliation: Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore.
authors.forename: Li
authors.initials: L
authors.lastname: Yuan
authors.affiliation: Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore.
authors.forename: Lejia
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of Physics, University of Central Florida, Orlando, Florida 32816 - USA.
authors.forename: Eduardo R
authors.initials: ER
authors.lastname: Mucciolo
authors.affiliation: Department of Physics and Energy, University of Limerick, Ireland.|Materials and Surface Science Institute, University of Limerick, Ireland.
authors.forename: Damien
authors.initials: D
authors.lastname: Thompon
authors.affiliation: Department of Physics, University of Central Florida, Orlando, Florida 32816 - USA.
authors.forename: Enrique
authors.initials: E
authors.lastname: Del Barco
authors.affiliation: Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore.|Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546 Singapore.
authors.forename: Christian A
authors.initials: CA
authors.lastname: Nijhuis