Electrical rectification by a monolayer of hexadecylquinolinium tricyanoquinodimethanide measured between macroscopic gold electrodes

Unimolecular rectification was detected between oxide-free Au. electrodes for a Langmuir-Blodgett (LB) monolayer of the zwitterionic D+-pi -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 x 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 An electrode.