Effects of electron irradiation on graphene drums

Using a scanning electron microscope, we irradiate graphene drums with electrons at an energy of 20 keV and a dosage of about 1.58 x 10(17 )electrons/cm(2). The drums consist of graphene exfoliated in ambient air over holes having a diameter of 4.6 mu m and etched into an SiO2 substrate. After irradiation, we observe that the drum's suspended monolayer (ML) region has a ratio of the Raman D peak height, I-D, to the Raman G peak height, I-G, as high as 6.3. In contrast, the supported ML on the SiO2 substrate has an I-D/I-G ratio of 0.49. Previous studies have shown that graphene drums containing air can leak in a vacuum at a low rate. We attribute the high I-D/I-G ratio of the suspended ML to the air that may be in the drums. We propose that the air produces much adsorbed water on the ML, resulting in a high average defect density during irradiation. We present Raman maps of the full-width-at-half maximum, position, and height of the G, 2D, D, and D' peaks before and after irradiation and maps of I-D/I-G and I-D/I-D'. We anneal the drums at temperatures from 50 to 215 degrees C and find that I-D/I-G significantly reduces to 0.42. The annealing data are analyzed using an Arrhenius plot. We also find that I-D/I-D' depends on annealing temperature and has values >= 8, in the range expected for sp(3) defects, for I-D/I-G <= 3.9. This irradiation method may help achieve high average defect densities in ML graphene, imparting novel and potentially valuable properties.