Molecular Rectification and Conductance Switching in Carbon-Based Molecular Junctions by Structural Rearrangement Accompanying Electron Injection

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 +3 V 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 0.1-1 e -/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.

Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n-Alkanethiolates

“…We recognize that such a plot tends to convert a range of data into linear relationships but point out that the greatest deviation (ref 6 by McCreery et al in Figure 5) differs in calculated and observed values by only a factor of 8. The fact that the azo compound ( Figure S5 in the SI) described by McCreery et al 6 does not follow the expected trend ( Figure 5) for the "spherical" approximation but does for the "extended" approximation is easily rationalized: this group could be trans-extended, and therefore ordered, at the surface of the SAM. Correspondingly, the compound reported by Ashwell et al 10 is composed of the bulky terminal group and the thin alkyl backbone ( Figure S5 in the SI) and plausibly disordered.…”

Covalent modification of graphitic carbon substrates by non-electrochemical methods

“…Graphitic carbons have many attractive characteristics, including ease-ofhandling, low-cost, high mechanical stability and good electrical conductivity. The material is readily available in a number of forms including high surface area felts and cloths suitable as supports for combinatorial chemistry [1,2], carbon powders useful for chromatography [3], glassy carbon (GC) [4], a convenient material for planar electrodes and very smooth films of pyrolysed photoresist [5] suitable for applications in molecular electronics [6,7,8]. Another advantage of graphitic carbon is that the surface can be easily modified by methods which give very stable molecular coatings, attached by strong C-C [9] or C-N covalent bonds.…”

Molecular Transport Junctions: An Introduction

“…A comparison has been made of transport through junctions containing single molecules with transport through those based on molecular adlayer films; once again, the results highlight the crucial role that contacts play in molecular junction transport. In particular, a substantial reduction in contact resistance has been demonstrated using covalent interactions at carbon electrodes; 17 less effective mixing occurs through Lewis interactions at metal/thiol or isocyano links.…”