UNUSUAL SEMIMETALLIC BEHAVIOR OF CARBONIZED ION-IMPLANTED POLYMERS

We report a comprehensive charge transport study of ion-implanted rigi d rod and ladder polymers p-phenylenebenzobisoxazoIe, p-pbenylenebenzo bisthiazole, and benzimidazobenzophenanthroline. The three pristine ma terials are strong and stable polymers with a room temperature conduct ivity sigma(RT)similar to 10(-12) Slcm. After high dosage ion implanta tion using Kr+, a carbonized and conducting layer forms on the surface of the film samples with sigma(RT) > 10(2) S/cm. The experimental res ults suggest that this carbonized layer is semimetallic with unusual p roperties. The observed de conductivity follows sigma(T)=sigma(0)+Delt a(sigma)(T), where Delta sigma(T) is weakly temperature dependent and interpreted within the model of weak localization and electron-electro n interaction effects. The model reveals that the interaction effect i s three dimensional for the experimental temperature range (3-300 K), whereas the weak localization effect undergoes a dimensional crossover at similar to 60 K from three to two-dimensions with decreasing tempe rature. The magnetoconductance, thermoelectric power, and microwave di electric constant results are all in agreement with this semimetallic model. In addition, all these results consistently point to an enhance d interaction effect at low temperatures due to the reduced dimensiona lity of the localization effect. It is concluded that a sp(2) rich and three-dimensional interconnected carbon network reformed upon ion imp lantation of the densely packed pristine polymers is responsible for t he semimetallic behavior.