Potential energy threshold for nano-hillock formation by impact of slow highly charged ions on a CaF2(111) surface

We investigate the formation of nano-sized hillocks on the (111) surface of CaF2 single crystals by impact of slow highly charged ions. Atomic force microscopy reveals a surprisingly sharp and well-defined threshold of potential energy carried into the collision of about 14 keV for hillock formation. Estimates of the energy density deposited suggest that the threshold is linked to a solid-liquid phase transition (melting) on the nanoscale. With increasing potential energy, both the basal diameter and the height of the hillocks increase. The present results reveal a remarkable similarity between the present predominantly potential energy driven process and track formation by the thermal spike of swift (similar to GeV) heavy ions. (c) 2007 Elsevier B.V. All rights reserved.