The electrical properties of metal-sandwiched Langmuir–Blodgett multilayers and monolayers of a redox-active organic molecular compound

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Abstract
Both ac and dc conduction processes through thin Langmuir–Blodgett (LB) films of the dodecyloxyphenylurethane of 2‐bromo‐5 (2’‐hydroxyethoxy) tetracyanoquinodimethan (DDOP‐C‐TCNQ, a redox‐active molecule) sandwiched between dissimilar metal (Pt and Mg) electrodes have been studied. The dc conduction changed from a linear I/V characteristic for the lowest applied voltages (±20 mV) to a symmetric nonlinear characteristic obeying a ln I∝V1/4 dependence for voltages up to ±1.5 V, in as‐prepared samples. For larger positive voltages, a large increase in current was observed with the dependence changing to a ln I∝V3 law, a dependence not reported previously for metal/LB film/metal systems. For increased negative voltages, the ln I∝V1/4 was again observed. For low applied ac fields, the conductance was found to follow a ωn law with a value of n close to 0.8. The effect of heat annealing the samples was also studied with significant differences in the observed changes in conductance between bilayer and monolayer structures. Emphasis was placed on understanding the conduction process through the single DDOP‐C‐BHTCNQ layer film. For the monolayer device, hysteresis is observed in the positive bias, and both as‐prepared and annealed samples exhibit ln I∝V3 behavior. Such behavior was not observed for the corresponding negative voltages, indicating rectification across a distance approximated by the length of an individual molecule. Possible models of the three‐dimensional arrays for both monolayer and multilayer metal/LB film/Mg structures are presented. The weight of existing experimental results does not support the Aviram–Ratner ‘‘molecular rectifier’’ concept for presently known Pt/LB monolayer/Mg devices.