In-depth investigation into defect-induced Raman lines in irradiated graphene

Identifying the type of structural defects and determining their concentration is crucial for effective defect engineering strategies since they govern various physical, chemical, and optoelectronic properties of graphene. Here, we study the effects of Ga ion irradiation on freestanding monolayer graphene, specifically focusing on the behavior of three defect-induced Raman lines (D, D' and (D+ D')). By employing a modified approach of the local activation model, we determine the key defect parameters of each line and show their dependence on different vibrational configurations of the iTO and iLO phonons emitted during scattering. The redshift of the lines and the broadening of their width, observed with an increase in the concentration of radiation defects over N_d ≈ 10¹³cm⁻², are explained by the tensile stress of the graphene film and a decrease in the phonon lifetime, respectively. The resulting intensity ratio I(D)/I(D') of 9.7 in samples with different defect densities is consistent with theoretical predictions for vacancy-like defects with a predominance of double vacancies. Our findings provide valuable insights into the underlying mechanisms driving defect-induced Raman scattering in graphene, thereby contributing to an enhanced understanding of defect engineering for diverse applications, such as next-generation electronics, sensing devices, and energy storage systems.