Quantitative Analysis of Molecular Conductance in the Tunneling Regime
Par C. Daniel Frisbie – (Distinguished McKnight University Professor, Chemical Engineering & Materials Science, University of Minnesota
Friday, May 23 at 10h30 a.m., IEMN amphitheater
Abstract: This talk will describe measurements and quantitative analysis of tunneling conductance through molecules connected between metal electrodes. Experiments employ conducting probe atomic force microscopy (CP-AFM) to make electrical contacts to self-assembled monolayers (SAMs) on various metals. Quantitative analysis of the junction current-voltage (I-V) characteristics is greatly aided by an analytical model that allows extraction of the molecular orbital (e.g., HOMO or LUMO) offset from the Fermi level, ε, and the metal-orbital coupling, Γ. These two parameters and a voltage shift factor γ define the junction density of states and the basic tunneling transmission function. We show that the single level model applies extremely well to common molecular junctions in which the mechanism is single step tunneling. Using this analytical formalism and the CP-AFM platform, we address a number of different conductance problems in molecular electronics including: (i) the impact of molecular structure and metal work function; (ii) the identification of the orbital facilitating conduction; (iii) the role of metal-orbital coupling; (iv) Fermi and HOMO level pinning; (v) the causes of current rectification; (vi) the effects of image potentials; (vii) the general applicability and limits of the single level model; (viii) accurate I-V simulations and universal behavior; (ix) mixed molecular junctions and the role of intermolecular interactions; (x) the causes of recently discovered conductance-proton chemical shift correlations. The talk will focus on a subset of these issues. The overall aim of this work is to enhance fundamental understanding of molecular conductance and simultaneously to elevate molecular electronics as a quantitative sub-discipline in chemistry, physics, and materials science.
