title: Charge Transport in 4 nm Molecular Wires with Interrupted Conjugation: Combined Experimental and Computational Evidence for Thermally Assisted Polaron Tunneling.
abstract: We report the synthesis, transport measurements, and electronic structure of conjugation-broken oligophenyleneimine (CB-OPI 6) molecular wires with lengths of ∼4 nm. The wires were grown from Au surfaces using stepwise aryl imine condensation reactions between 1,4-diaminobenzene and terephthalaldehyde (1,4-benzenedicarbaldehyde). Saturated spacers (conjugation breakers) were introduced into the molecular backbone by replacing the aromatic diamine with trans-1,4-diaminocyclohexane at specific steps during the growth processes. FT-IR and ellipsometry were used to follow the imination reactions on Au surfaces. Surface coverages (∼4 molecules/nm(2)) and electronic structures of the wires were determined by cyclic voltammetry and UV-vis spectroscopy, respectively. The current-voltage (I-V) characteristics of the wires were acquired using conducting probe atomic force microscopy (CP-AFM) in which an Au-coated AFM probe was brought into contact with the wires to form metal-molecule-metal junctions with contact areas of ∼50 nm(2). The low bias resistance increased with the number of saturated spacers, but was not sensitive to the position of the spacer within the wire. Temperature dependent measurements of resistance were consistent with a localized charge (polaron) hopping mechanism in all of the wires. Activation energies were in the range of 0.18-0.26 eV (4.2-6.0 kcal/mol) with the highest belonging to the fully conjugated OPI 6 wire and the lowest to the CB3,5-OPI 6 wire (the wire with two saturated spacers). For the two other wires with a single conjugation breaker, CB3-OPI 6 and CB5-OPI 6, activation energies of 0.20 eV (4.6 kcal/mol) and 0.21 eV (4.8 kcal/mol) were found, respectively. Computational studies using density functional theory confirmed the polaronic nature of charge carriers but predicted that the semiclassical activation energy of hopping should be higher for CB-OPI molecular wires than for the OPI 6 wire. To reconcile the experimental and computational results, we propose that the transport mechanism is thermally assisted polaron tunneling in the case of CB-OPI wires, which is consistent with their increased resistance.
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Davood
authors.initials: D
authors.lastname: Taherinia
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Smith
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Soumen
authors.initials: S
authors.lastname: Ghosh
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Samuel O
authors.initials: SO
authors.lastname: Odoh
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Luke
authors.initials: L
authors.lastname: Balhorn
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Laura
authors.initials: L
authors.lastname: Gagliardi
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Cramer
authors.affiliation: Department of Chemistry, ‡Department of Chemical Engineering and Materials Science, and §Chemical Theory Center, and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: C Daniel
authors.initials: CD
authors.lastname: Frisbie
keywords: CP-AFM; DFT; activation energy; charge transport; molecular junctions; polaron hopping; thermally assisted polaron tunneling