title: Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior.
abstract: The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third -CH2- was replaced with O or S, that is, following the general formula HS(CH2CH2X)nCH2CH2SH, where X = CH2, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH2< O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, Rc, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset εh(i.e., Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current-voltage (I-V) data. The εhvalues were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in Rcto the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental I-V curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics.
authors.affiliation: Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Zuoti
authors.identifier: 0000-0002-1828-0122
authors.initials: Z
authors.lastname: Xie
authors.affiliation: Theoretische Chemie, Universität Heidelberg , INF 229, D-69120 Heidelberg, Germany.|National Institute of Lasers, Plasma and Radiation Physics, Institute of Space Science , POB MG-23, RO 077125 Bucharest, Romania.
authors.forename: Ioan
authors.initials: I
authors.lastname: Bâldea
authors.affiliation: Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Stuart
authors.initials: S
authors.lastname: Oram
authors.affiliation: Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Smith
authors.affiliation: Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
authors.forename: C Daniel
authors.initials: CD
authors.lastname: Frisbie
keywords: electronic coupling; heteroatom substitution; molecular tunnel junctions; quantum chemical calculations; single-level model; transition voltage; universal behavior out of equilibrium