Evidence of structural changes in ion-irradiated graphene independent of the incident ions mass

Raman spectra were measured in various adjacent groups of monolayer graphene samples: "supported" by a SiO2 substrate or suspended over pre-made pits ("free-standing" monolayer). Different groups of samples were irradiated with protons, or with helium ions (He+), with energies of 0.7 MeV and 2.8 MeV respectively, specially selected to provide the same velocity of incident ions. It is shown that in "free-standing" samples, the positions of all Raman lines are shifted towards low frequencies ("redshift"), which is the same for irradiation with protons or He+. This coincidence is considered as a manifestation of an electronic mechanism for changing the structure of graphene, which depends on the charge and velocity of the incident ion, but not on its mass. It was proposed that the redshift is due to tensile deformation that occurs in a free-standing film after accumulation of defects. Deformation of the irradiated film was observed in AFM measurements. In "supported" samples, the redshift is much weaker, and can be explained by the adhesion of the film to the substrate, which prevents its deformation.