We studied the effect of gamma-ray on the physical and chemical properties of pentacene thin film using synchrotron X-ray diffraction (XRD) and near edge X-ray absorption fine structure (NEXAFS), and Scanning Electron Microscopy (SEM). Vacuum evaporated pentacene thin films were irradiated at different total doses. XRD measurement showed the reduced intensity of thin film peak in accordance with the irradiation dose. At higher doses than 500 Gy the bulk phase slightly increased, which suggests the transformation from the thin film phase into the bulk phase. Both XRD and SEM results confirm the slight reduction of the grain size after irradiation. This may be due to the molecular disordering. Annealing treatment at 80℃ for 4 h induced the reordering of misoriented pentacene crystals in thin film phase and bulk phase. Performing the total electron yield NEXAFS, we found small peaks of oxygen K-edge after irradiation that suggests the existence of some radical oxidation groups. However, no carbon bond breakage was found in pentacene after irradiation. Later resistivity measurements showed that the resistance decreases at dose up to 400 Gy, but it increased at dose higher than 400 Gy. This observation is believed to be related to the grain size change and can be explained basing on the doping effect in the crystal. Our findings suggest that pentacene can be exploited as reversible radiation dosimeter. Gamma-ray drives the crystallinity from being well ordered to be disorder.

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