Vaporlike phase of amorphous SiO2 is not a prerequisite for the core/shell ion tracks or ion shaping

Abstract

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.

C60 ions of 1 MeV are slow but elongate nanoparticles like swift heavy ions of hundreds MeV

“…evaluated amorphous SiO 2 by small angle X-ray scattering (SAXS) and reported the total track diameters of ~10 nm for low velocity and ~8 nm for high velocity, respectively 28 . It should be noted that tracks in amorphous SiO 2 have core/shell structures 15 , which is different from the hard-cylinder-type of crystalline SiO 2 30 .…”

“…Although the mechanism of NP shape elongation induced by SHIs is yet to be identified, a majority of researchers agree with the importance of the ion-track formation in matrix materials 4–15 . However, the tracks cannot be directly observed by TEM in amorphous matrices 28 , except in a very thin self-standing film 29 .…”

“…Modification of materials using swift heavy ions (SHIs), i.e., high energy ions with stopping power in materials that are primarily dominated by electronic forces, has recently received an increasing amount of attention 1 . The examples include the anisotropic deformation of amorphous metals/glasses 2,3 , shape elongation of metal nanoparticles (NPs) embedded in materials 4–15 , and etched ion-track engineering 16 . When NPs embedded in SiO 2 are irradiated with SHIs, the NPs are elongated toward the direction parallel to the SHI beam.…”

Elongation of metallic nanoparticles at the interface of silicon dioxide and silicon nitride

“…This process leads to the formation of a local temperature spike at the metal/dielectric boundary where very high temperatures can be reached. This may explain the low energy threshold observed for ion shaping of ~4 keV/nm in a-SiO2 [48]. As a consequence, the ion-shaping process can take place if the anisotropic thermal profile around the metal/dielectric boundary surpasses the melting temperature of the surrounding medium, and the efficiency is linked to the duration of this process, which is affected by the thermal conductivity of the surrounding medium.…”

Plasmonic Nanoparticles in Dielectrics Synthesized by Ion Beams: Optical Properties and Photonic Applications

“…[ 74 ] To date, there are two major mechanisms proposed for the elongation: a) synergy effects by ion hammering and NP melting, [ 75–82 ] and b) ion shaping by ion tracks. [ 83–88 ] Ion hammering was discovered by Klaumunzer in 1983 that the NP dimensions grow (shrink) perpendicular (parallel) to the ion beam direction, which has been well understood by effective‐flow temperature approach. [ 89 ] In 2003, D'Orleans et al observed the ion shaping that spherical NPs become prolate with major axis oriented parallel to the incident direction.…”