Electron transfer through a monolayer of hexadecylquinolinium tricyanoquinodimethanide

Electron transfer through a Langmuir-Blodgett monolayer of hexadecylquinoli nium tricyanoquinodimethanide (C(16)H(33)Q-3CNQ) sandwiched between metal e lectrodes is studied carefully. Either current rectification (rectification ratio as high as similar to 20) for positive bias applied on the metal ove rlayer or symmetric current voltage curves were observed for samples with t he highest resistivity. More leaky devices (resistivity of about a decade l ower) show rectification (with a smaller ratio less than or equal to 5) for a negative bias on the top electrode. These results are analyzed regarding various mechanisms: (1) Aviram and Ratner proposal for a molecular diode, (2) geometrical asymmetry of the metal/Langmuir-Blodgett monolayer/metal st ructure, and (3) a polarization charge density effect. This study leads us to confirm that the observed electron-transfer properties through the C(16) H(33)Q-3CNQ LB monolayers are not only due to possible geometrical asymmetr y in the metal/LB monolayer/metal structure but are also related to the str ong asymmetry of the donor-pi-bridge-acceptor C(16)H(33)Q-3CNQ molecules.