Phase diagram for nanostructuring CaF$_2$ surfaces by slow highly charged ions

TL;DR

This study develops a phase diagram for nanostructuring CaF$_2$ surfaces by slow highly charged ions, revealing thresholds for defect creation and surface modifications through experiments and simulations.

Contribution

It introduces a unifying phase diagram for surface modifications caused by slow highly charged ions, supported by experimental evidence and molecular dynamics simulations.

Findings

Identification of a second threshold for defect creation.

Surface modifications depend on both potential and kinetic energies.

Defect aggregates form in CaF$_2$ top layers above certain energy thresholds.

Abstract

Impacts of individual slow highly charged ions on alkaline earth halide and alkali halide surfaces create nano-scale surface modifications. For different materials and impact energies a wide variety of topographic alterations have been observed, ranging from regularly shaped pits to nano-hillocks. We present experimental evidence for a second threshold for defect creation supported by simulations involving the initial electronic heating and subsequent molecular dynamics. From our findings a unifying phase diagram underlying these diverse observations can be derived. By chemically etching of CaF$_2$ samples after irradiation with slow highly charged ions both above and below the potential energy threshold for hillock formation another threshold exists above which triangular pits are observed after etching. This threshold depends on both the potential and kinetic energies of the incident ion. Simulations indicate that this second threshold is associated with the formation of defect aggregates in the topmost layers of CaF$_2$.