Structural and morphological investigation of the development of electrical conductivity in ion-irradiated thin films of an organic material

Thin films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) develop low electrical resistivity (10−3 Ω cm) after irradiation with 2-MeV Ar+ ions. Electron microscopy and diffraction show that vacuum-deposited films of this material consist, prior to ion-irradiation, of discrete crystalline grains (∼20–50 nm diameter) in which the molecules are disposed closely parallel to the substrate (average inclination ∼10°–15°). Upon irradiation with up to ∼1014 Ar+/cm2, the grains become progressively more defect-ridden and eventually amorphous. At that stage, the resistivity begins to decrease by ∼12 orders of magnitude at doses between ∼1014 and ∼5×1016 Ar+/cm2, while the intergranular boundaries become diffuse and the grains begin to merge. The temperature dependence of resistivity in this regime is as exp(const/T1/2), which is consistent with our morphological and structural results as it implies carrier hopping between conducting islands embedded in a nonconducting matrix. At the highest ion doses (5×1016 Ar+/cm2) the grains become connected into a rather uniform and featureless network akin to amorphous carbon, and the resistivity reaches its lowest value and becomes independent of temperature. The amorphous-carbon-like character of the highly irradiated material is evidenced not only by its diffraction pattern but also by its crystallization with a graphitic-type structure during annealing to 1200 °C.