Potential energy threshold for nano-hillock formation by impact of slow highly charged ions on a CaF$_2$(111) surface

TL;DR

This study identifies a sharp potential energy threshold of about 14 keV for nano-hillock formation on CaF$_2$(111) surfaces caused by slow highly charged ions, linking it to nanoscale melting.

Contribution

It demonstrates a clear potential energy threshold for nano-hillock formation and draws parallels with thermal spike mechanisms in swift heavy ion tracks.

Findings

Threshold potential energy for hillock formation is about 14 keV.

Hillock size increases with higher potential energy.

Process resembles thermal spike-induced track formation.

Abstract

We investigate the formation of nano-sized hillocks on the (111) surface of CaF$_2$ 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 ($\sim$ GeV) heavy ions.