When a swift heavy ion (SHI) penetrates amorphous $\mathrm{Si}{\mathrm{O}}_2$, a core/shell (C/S) ion track is formed, which consists of a lower-density core and a higher-density shell. According to the conventional inelastic thermal spike (iTS) model represented by a pair of coupled heat equations, the C/S tracks are believed to form via “vaporization” and melting of the $\mathrm{Si}{\mathrm{O}}_2$ induced by SHI (V-M model). However, the model does not describe what the vaporization in confined ion-track geometry with a condensed matter density is. Here we reexamine this hypothesis. While the total and core radii of the C/S tracks determined by small angle x-ray scattering are in good agreement with the vaporization and melting radii calculated from the conventional iTS model under high electronic stopping power $\left(S_e\right)$ irradiations (>10 keV/nm), the deviations between them are evident at low-$S_e$ irradiation (3–5 keV/nm). Even though the iTS calculations exclude the vaporization of $\mathrm{Si}{\mathrm{O}}_2$ at the low $S_e$, both the formation of the C/S tracks and the ion shaping of nanoparticles (NPs) are experimentally confirmed, indicating the inconsistency with the V-M model. Molecular dynamics (MD) simulations based on the two-temperature model, which is an atomic-level modeling extension of the conventional iTS, clarified that the “vaporlike” phase exists at $S_e\sim 5$ keV/nm or higher as a nonequilibrium phase where atoms have higher kinetic energies than the vaporization energy, but are confined at a nearly condensed matter density. Simultaneously, the simulations indicate that the vaporization is not induced under 50-MeV Si irradiation $(S_e\sim 3$ keV/nm), but the C/S tracks and the ion shaping of nanoparticles are nevertheless induced. Even though the final density variations in the C/S tracks are very small at the low stopping power values (both in the simulations and experiments), the MD simulations show that the ion shaping can be explained by flow of liquid metal from the NP into the transient low-density phase of the track core during the first ∼10 ps after the ion impact. The ion shaping correlates with the recovery process of the silica matrix after emitting a pressure wave. Thus, the vaporization is not a prerequisite for the C/S tracks and the ion shaping.

• Received 22 May 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.096001