Carbonization of polyimide by swift heavy ion irradiations: Effects of stopping power and velocity

We have studied the carbonization of polyimide Kapton-H (pyromellitic dianhydride-oxydialinine, PMDA-ODA) thin films under heavy ion irradiations in the electronic slowing down regime. Irradiations were performed with 650-MeV Ni, 843-MeV Xe, 2.6-GeV Xe, 1.1-GeV Ta, and 707-MeV Pb ions that were transmitted through the polymer films with the electronic stopping power ranging between 3.5 and 17 keV nm-1. Room-temperature ac/dc electrical conductivity measurements, together with ESR, UV-visible optical absorption, and micro-Raman spectroscopy were used to characterize the irradiated films. These new data are compared to our previous results obtained with heavy ion irradiations in the 1-MeV uma-1 energy range. At the largest fluences near 1013 cm-2 the resulting carbonaceous material is found to be 8-9 orders of magnitude less conducting than with the slower ions. Such large dc-conductivity deviations are confirmed by the spin-lattice relaxation times deduced from the ESR line saturation measurements that are two orders of magnitude longer with the swift ions than with the slower ions. The evolution of the Raman spectra also shows that the appearance of a conjugated sp2-bonded structure is delayed with respect to the slow ion case, and is probably incomplete in the observed fluence range. The yield of paramagnetic centers is found to decrease when the ion energy increases. This is interpreted by an ion-velocity effect on damage due to a dilution of the deposited energy in the ion tracks. We think that this also induces a smaller sp2 cluster density impeding charge transport in the carbonaceous material