Molecular-dynamics simulations of hillocks induced by highly-charged Arq+, Xeq+ ions impact on HOPG surface

The hillocks on highly oriented pyrolytic graphite (HOPG) surface induced by highly charged Arq+, Xeq+ ions are studied by using molecular-dynamics (MD) simulations. And a hybrid potential created by combining the ReaxFF potential with the repulsive ZBL potential is used to describe the interatomic interactions. The effects of incident highly charged ion (HCI)'s kinetic energy and the energy gain due to the interaction of HCI with its own image on the formation of the hillocks are considered in the present simulations. Our results show that both potential and kinetic energy of HCI may affect the hillock size. However, the potential energy of HCI increases dramatically with charge state, which is more important than kinetic energy in the formation of the hillock in extremely high charge states. And it is found that both the height and width of the hillock agree well with experimental data. In addition, the bond breaking and bond formation during the formation of the hillock are also investigated, and the results show that there are many σ bonds breaking and interlayer bonds formation in one layer or between two layers during this process. Furthermore, most of the interlayer bonds in HOPG surface induced by HCI impact are sp2 bond, although some interlayer sp3 bonds are also observed in the present work.