The electrical properties of metalhyphen;sandwiched Langmuirndash;Blodgett multilayers and monolayers of a redoxhyphen;active organic molecular compound

摘要

Both ac and dc conduction processes through thin Langmuirndash;Blodgett (LB) films of the dodecyloxyphenylurethane of 2hyphen;bromohyphen;5 (2rsquo;hyphen;hydroxyethoxy) tetracyanoquinodimethan (DDOPhyphen;Chyphen;TCNQ, a redoxhyphen;active molecule) sandwiched between dissimilar metal (Pt and Mg) electrodes have been studied. The dc conduction changed from a linearI/Vcharacteristic for the lowest applied voltages (plusmn;20 mV) to a symmetric nonlinear characteristic obeying a lnthinsp;Iprop;V1/4dependence for voltages up to plusmn;1.5 V, in ashyphen;prepared samples. For larger positive voltages, a large increase in current was observed with the dependence changing to a lnthinsp;Iprop;V3law, a dependence not reported previously for metal/LB film/metal systems. For increased negative voltages, the lnthinsp;Iprop;V1/4was again observed. For low applied ac fields, the conductance was found to follow a ohgr;nlaw with a value ofnclose 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 DDOPhyphen;Chyphen;BHTCNQ layer film. For the monolayer device, hysteresis is observed in the positive bias, and both ashyphen;prepared and annealed samples exhibit lnthinsp;Iprop;V3behavior. 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 threehyphen;dimensional arrays for both monolayer and multilayer metal/LB film/Mg structures are presented. The weight of existing experimental results does not support the Aviramndash;Ratner lsquo;lsquo;molecular rectifierrsquo;rsquo; concept for presently known Pt/LB monolayer/Mg devices. thinsp;