Dominant impact of Ion velocity on defect formation in suspended graphene

Suspended (“free-standing”) graphene samples were irradiated with noble gas ions at varying energies, while maintaining a constant ion velocity. The resulting defect formation was analyzed using Raman spectroscopy. This process is attributed to the combined effects of nuclear and electronic mechanisms. While the efficiency coefficient (yield) is determined based on calculations for the nuclear mechanism, experimental results reveal that the defect concentration remains consistent for ions of different masses but identical velocities. This observation is interpreted as evidence of the electronic mechanism's contribution to defect formation, where the energy transferred to the graphene lattice primarily depends on the ion's velocity through the lattice rather than its mass. The results of the study show that increasing ion velocity leads to larger defect structures, providing a controllable approach for tuning defect size in graphene. Furthermore, at low defect densities, vacancy-type point defects are predominant; however, as defect concentration rises exceeds a specific threshold, the formation of elongated, edge-type defects becomes evident, likely due to the merging of vacancies. This ultimately leads to the creation of nanopores within the graphene lattice. Raman scattering line widths associated with defects were found to be consistent across different ions at the same velocity. However, higher ion velocities led to larger defects and increased line broadening, attributable to shorter phonon lifetimes caused by intensified scattering from larger defects. Additionally, when two phonons are generated within the same scattering event, their lifetimes are further reduced due to mutual interactions. Collectively, these results demonstrate that defect characteristics in graphene can be finely tuned by adjusting the velocity of irradiating ions, offering practical potential for applications requiring precise control of graphene defect structures.