Raman spectroscopy study of low energy He+ ion irradiation effect in graphene transferred onto SiO2

We studied the low energy (5.4 keV) He⁺ ion irradiation effect on the properties of chemical vapor deposited graphene transferred onto SiO₂/Si substrate. For a small dose density of 4 × 1012 He⁺/cm2, both the G and 2D bands in the Raman spectra exhibit blue-shift and the intensity ratio of the 2D and G peaks decreases. The intensity ratio of the D and G peaks increases a lot as the dose density is larger than 1.2 × 10¹³ He⁺/cm². It suggests that before the formation of large amount of defects, low energy He⁺ ion irradiation may induce charge-transfer doping in graphene due to the presence of polymer residues. Broad spatial distributions of the band shift and the intensity ratio indicate inhomogeneity of doping level in graphene.