Potential energy threshold for nano-hillock formation by impact of slow highly charged ions on a CaF2(1Â 1Â 1) surface

We investigate the formation of nano-sized hillocks on the (1 1 1) 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 (∼GeV) heavy ions.