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 × 10¹⁷ electrons/cm². The drums consist of graphene exfoliated in ambient air over holes having a diameter of 4.6 μm 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 °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 sp³ 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. [ABSTRACT FROM AUTHOR]