Nanostructures formation on metal surfaces by an impact of slow highly charged ions: Theoretical model

A cohesive energy model for the nanostructure formation by the impact of slow highly charged ions (charge q, 1, velocity v <= 0 . 4 a.u.) on a metal surface is formulated. The first step of the given model calculates the neutralization energy and deposited kinetic energy. We use the time-symmetrized quantum approach (two-state vector model) and the micro-staircase model in the analysis of the cascade neutralization above the surface. We consider the elastic collisions below the surface using the charge dependent ion-target atom interactions. Based on the critical velocities defined within the model we discuss the shape of the formed nanostructures; the model predicts the appearance of the nanohillocks for the velocities lower than the critical one and the nanocraters for higher velocities. In the second step of the model, we assume that the total energy deposition results in an alteration of the cohesive energy of the active volume of the solid. In the case of hillock formation, the neutralization, as a dominant process, reduces the strength of the metallic bonds; the rearrangement of atoms leads to the rise of the volume above the surface. In the process of crater formation the atomic collisions below the surface play the most significant role. The strength of the metallic bonds inside the crater volume approaches zero and a number of atoms are ejected from the surface. The proposed mechanisms enable us to predict a simple relation between the initial and final cohesive energies dependent on the metallic crystal structures and to obtain a general expression for the nanostructure diameters. According to the model, we get a nontrivial q and v dependence of the diameters for both nanostructure types. The results are in good agreement with the experimental data obtained for hillocks and craters formed on the titanium and gold targets (nanolayers) by the impact of Xe q + ions.