When a swift heavy ion (SHI) penetrates amorphous SiO 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 SiO 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 (S e) irradiations (>10 keV/nm), the deviations between them are evident at lowS e irradiation (3-5 keV/nm). Even though the iTS calculations exclude the vaporization of SiO 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 ∼ 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 ∼ 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.

The mechanism of the shape elongation of metal nanoparticles (NPs) in amorphous silica, which is induced under swift heavy ion irradiation, is discussed. Since the discovery of this phenomenon, several mechanisms were proposed and debated. Now, only two major mechanisms have survived: (i) the synergy model between the ion hammering and the transient melting of NPs by the inelastic thermal spike, and (ii) the thermal pressure and flow model. Here, we discuss that three experimental results are inconsistent with (i). The latter is supported by two-temperature molecular dynamics simulations (TT-MD), which simulate not only the atomic motions but also the local electron temperatures. While a remarkable correlation was observed between the temporal evolution of the silica density around the ion trajectory and that of the aspect ratio of the NP later than ~1 ps after the ion impact, no correlation was observed earlier than ~1 ps. Since the silica has a much higher electronlattice (e-L) coupling than the metal NP, the lattice temperature quickly increases up to remarkably high values, which results in quick and large expansion and recovery in silica. By contrast, metal NPs have low e-L coupling, which results in slow temperature change. The NP remains in a solid state in the period where silica experiences the quick expansion, and only melts and deforms when the silica is already in the recovery stage. The large difference of the temperature evolution between silica and metal NPs is the origin of the shape elongation.

Highly energetic ions have been previously used to modify the shape of metal nanoparticles embedded in an insulating matrix. In this work, we demonstrate that under suitable conditions, energetic ions can be used not only for shape modification but also for manipulation of nanorod orientation. This observation is made by imaging the same nanorod before and after swift heavy ion irradiation using a transmission electron microscope. Atomistic simulations reveal a complex mechanism of nanorod re-orientation by an incremental change in its shape from a rod to a spheroid and further back into a rod aligned with the beam.