Electron Transfer through a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide

Electron transfer through a Langmuir-Blodgett monolayer of hexadecylquinolinium tricyanoquinodimethanide (C 16 H 33 Q-3CNQ) sandwiched between metal electrodes is studied carefully. Either current rectification (rectification ratio as high as ∼20) for positive bias applied on the metal overlayer or symmetric current voltage curves were observed for samples with the highest resistivity. More leaky devices (resistivity of about a decade lower) show rectification (with a smaller ratio ≤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 structure, 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 asymmetry in the metal/LB monolayer/metal structure but are also related to the strong asymmetry of the donor-π-bridge-acceptor C 16 H 33 Q-3CNQ molecules.