Nano-hillock formation on CaF₂due to individual slow Au-cluster impacts

We present a direct way to generate hillock-like nanostructures on CaF₂(111) ionic crystals by kinetic energy deposition upon Au-cluster irradiation. In the past, the formation of similar nanostructures has been observed for both slow highly charged ions and swift heavy ions. However, in these cases, potential energy deposition of highly charged ions or the electronic energy loss of fast heavy ions, respectively, first leads to strong electronic excitation of the target material before the excitation energy is transferred to the lattice by efficient electron-phonon coupling. We now show that the kinetic energy deposited by slow single Au-clusters directly in the lattice of CaF₂(111) leads to the production of nano-hillocks very similar to those found with slow highly charged and swift heavy ions, with heights between 1 and 2 nm. Our results are in good agreement with previous cluster irradiation studies regarding energy deposition and hence nano-structuring of surfaces, and we present Au-cluster irradiation as novel tool to fine-tune nanostructure formation.