Effects on graphene of electron irradiation at 25 keV and dosages up to 10¹⁸ electrons/cm²

We report on the properties of defects in exfoliated graphene on SiO2 produced by electron irradiation at 25 keV and dosages from 4.96 × 1015 to 9.56 × 1017 electrons/cm2. With increasing dosage, graphene exhibits the two-stage amorphization trajectory reported for the Ar ion bombardment of graphene. Initially, the ratio of the D-peak height, ID, to the G peak height, IG, increases as new defects are formed. In the second stage, ID/ IG decreases as defects cover most of the sample. In the second stage, we find that the full width at half maximum of the Raman 2 D, D, and G peaks increases by 3, 3, and 6 cm−1, respectively. These values are less by factors of about 10, 5, and 10, respectively, than those reported for amorphous graphene produced by Ar ion bombardment. We find that ID/ IG monotonically decreases in the second stage as the annealing temperature increases from 80 to 220 °C. Assuming that ID/ IG is proportional to the defect density, we find an activation energy for defect healing, Ea = 0.48 eV, which is significantly less than Ea = 0.95 eV reported for vacancies and closer to Ea = 0.29 and 0.58 eV reported for hydrogen and hydroxyl group adsorbates, respectively. We propose that, in the second stage, graphene does not become amorphous, and the defects responsible for the D peak are adsorbates.