Electrical Rectification by a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Measured between Macroscopic Gold Electrodes

Abstract

Unimolecular rectification was detected between oxide-free Au electrodes for a Langmuir−Blodgett (LB) monolayer of the zwitterionic D + -π-A - molecule hexadecylquinolinium tricyanoquinodimethanide, C 16 H 33 Q−3CNQ. The top gold pad was deposited by a process that cools the metal vapor before deposition. The maximum rectification ratio is 27.5 at 2.2 V (the average rectification ratio is 7.55). The currents are as large as 9.04 × 10 4 electrons molecule -1 s -1 . The result reinforces previous work with oxide-bearing Al electrodes, but the currents with Au electrodes are larger by 3−5 orders of magnitude. Rectification was also seen for a nine-monolayer Z-type LB film between similar Au electrodes but not for a monolayer of arachidic acid. The direction of enhanced electron current is from the negatively charged dicyanomethylene end of C 16 H 33 Q−3CNQ to the quinolinium ring, as predicted by the Aviram−Ratner analysis. However, there is a good fit to the behavior expected for quantum conduction dominated by a single molecular level. The best-fit energy for this level is 1.31 ± 0.25 eV above the Fermi level of the Au electrode.