Unusual semimetallic behavior of carbonized ion-implanted polymers

Abstract

We report a comprehensive charge transport study of ion-implanted rigid rod and ladder polymers $p$-phenylenebenzobisoxazole, $p$-phenylenebenzobisthiazole, and benzimidazobenzophenanthroline. The three pristine materials are strong and stable polymers with a room temperature conductivity ${\ensuremath{\sigma}}_{\mathrm{RT}}\ensuremath{\sim}{10}^{\ensuremath{-}12}$ S/cm. After high dosage ion implantation using ${\mathrm{Kr}}^{+},$ a carbonized and conducting layer forms on the surface of the film samples with ${\ensuremath{\sigma}}_{\mathrm{RT}}>{10}^{2}$ S/cm. The experimental results suggest that this carbonized layer is semimetallic with unusual properties. The observed dc conductivity follows $\ensuremath{\sigma}(T)={\ensuremath{\sigma}}_{0}+\ensuremath{\Delta}\ensuremath{\sigma}(T),$ where $\ensuremath{\Delta}\ensuremath{\sigma}(T)$ is weakly temperature dependent and interpreted within the model of weak localization and electron-electron interaction effects. The model reveals that the interaction effect is three dimensional for the experimental temperature range (3--300 K), whereas the weak localization effect undergoes a dimensional crossover at \ensuremath{\sim}60 K from three to two-dimensions with decreasing temperature. The magnetoconductance, thermoelectric power, and microwave dielectric constant results are all in agreement with this semimetallic model. In addition, all these results consistently point to an enhanced interaction effect at low temperatures due to the reduced dimensionality of the localization effect. It is concluded that a ${\mathrm{sp}}^{2}$ rich and three-dimensional interconnected carbon network reformed upon ion implantation of the densely packed pristine polymers is responsible for the semimetallic behavior.