Molecular Rectification and Conductance Switching in Carbon-Based Molecular Junctions by Structural Rearrangement Accompanying Electron Injection Richard McCreery,* Jon Dieringer, Ali Osman Solak, ² Brian Snyder, Aletha M. Nowak, William R. McGovern, and Stacy DuVall Contribution from the Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 Received May 19, 2003; E-mail: mccreery.2@osu.edu Abstract: Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3V was 0.71 A/cm 2 , or about 10 5 e - /s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1e - /molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude. Introduction The electronic properties of several types of molecular junctions have attracted significant interest, because they are fundamental to the growing area of molecular electronics. A metal/molecule/metal junction combines properties of traditional metallic conductors with those of single molecules or groups of molecules, thus raising the prospect of introducing molecular properties into electronic circuits. 1-14 The mechanism of electron transport across molecular junctions has been a very active topic of research in part because it involves concepts from electro- chemistry, solid-state physics, and long-range electron transfer phenomena in chemical and biochemical systems. 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