Orbital Control of Long-Range Transport in Conjugated and Metal- Centered Molecular Electronic Junctions Ushula M. Tefashe, † Quyen Van Nguyen, ‡ Amin Morteza Najarian, † Frederic Lafolet, ‡ Jean-Christophe Lacroix,* ,‡ and Richard L. McCreery* ,† † Department of Chemistry, University of Alberta, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada ‡ Université Paris Diderot, Sorbonne Paris Cité , ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France * S Supporting Information ABSTRACT: Large area molecular junctions consisting of covalently bonded molecular layers between conducting carbon electrodes were compared for Co and Ru complexes as well as nitroazobenzene and anthraquinone to investigate the effect of molecular structures and orbital energies on electronic behavior. A wide range of molecular layer thickness (d) from 1.5-28 nm was examined and three distinct transport regimes in attenuation plots of current density (J) vs thickness were revealed. For d < 5 nm, the four molecular structures had comparable current densities and thickness dependence despite significant differences in orbital energies, consistent with coherent tunneling and strong electronic coupling between the molecules and contacts. For d > 12 nm, transport depends on the electric field rather than bias, with the slope of ln J vs d near-zero when plotted at a constant electric field. At low temperature (T < 150 K), transport is nearly activationless and likely occurs by sequential tunneling and/or field-induced ionization. For d =5-10 nm, transport correlates with the energy gap between the highest occupied and lowest unoccupied molecular orbitals, and ln J is linear with the square root of the bias or electric field. Such linearity occurs for all three transport regimes and is consistent with the energy barrier lowering by the applied electric field. The results clearly indicate a strong dependence of charge transport on molecular orbital energies provided d > 5 nm, with a variation of 7 orders of magnitude of J for different molecules and d = 10 nm. The results provide insights into charge transport mechanisms as well as a basis for rational design of molecular electronic devices. ■ INTRODUCTION A long-standing and fundamental question in the field of molecular electronics is the relationship between the molecular structure and electronic behavior of “molecular junctions (MJs)” consisting of single molecules or ensembles of many molecules oriented between two conducting contacts. When the transport distance, d, is in the range of 1-30 nm between the contacts, the charge transport mechanism may differ fundamentally from those in “organic electronic” devices, where d usually exceeds 50 nm. 1-4 The rational design of molecular devices with useful electronic behavior should depend on molecular structures, and presumably on the molecular orbital energies and their interactions with the contacts and/or adjacent molecules. Coherent quantum mechanical tunneling is generally accepted as the dominant mode of transport when d < 2 nm for aliphatic molecules and d < 5 nm for conjugated systems, with the tunneling barrier determined by the energy of occupied or unoccupied molecular orbitals relative to the electrode Fermi level. However, strong electronic coupling between aromatic molecules and conducting contacts can diminish the effects of orbital energies, leading to minor effects on transport from >2 eV variation in highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy when d < 5 nm. 5,6 The exponential dependence of tunneling on d is often manifested by linear plots of ln J vs d, where J is the current density at a given bias, with a slope equaling the attenuation coefficient β with units of nm -1 . Many investigators have reported β =2-3 nm -1 for π-conjugated structures across different junction designs (e.g., single molecule and large area MJs) as well as departures from linearity of ln J vs d attributed to a change in the mechanism to various incoherent, “hopping” mechanisms 7-9 or to resonant tunneling. 10,11 Several examples of major departures from the 2-3 nm -1 range include porphyrins (β < 0.06 nm -1 ), 10,11 organometallics ( β < 0.03 nm -1 ), 12, 13 and viologen oligomers. 14,15 Molecular junctions containing metal centers have been shown to conduct over large distances exceeding 40 nm, with the possible involvement of redox reactions underlying transport. 11-13 Received: October 12, 2018 Revised: November 26, 2018 Published: November 28, 2018 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C 2018, 122, 29028-29038 © 2018 American Chemical Society 29028 DOI: 10.1021/acs.jpcc.8b09978 J. Phys. Chem. C 2018, 122, 29028-29038 Downloaded via UNIV OF ALBERTA on February 1, 2019 at 17:02:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.