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Vaporlike phase of amorphous Si O2 is not a prerequisite for the core/shell ion tracks or ion shaping

Amekura, H. ; Kluth, P. ; Mota-Santiago, P. LU ; Sahlberg, I. ; Jantunen, V. ; Leino, A. A. ; Vazquez, H. ; Nordlund, K. ; Djurabekova, F. and Okubo, N. , et al. (2018) In Physical Review Materials 2(9).
Abstract

When a swift heavy ion (SHI) penetrates amorphous SiO2, 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 SiO2 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... (More)

When a swift heavy ion (SHI) penetrates amorphous SiO2, 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 SiO2 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 (Se) irradiations (>10 keV/nm), the deviations between them are evident at low-Se irradiation (3-5 keV/nm). Even though the iTS calculations exclude the vaporization of SiO2 at the low Se, 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 Se∼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 (Se∼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.

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type
Contribution to journal
publication status
published
subject
in
Physical Review Materials
volume
2
issue
9
article number
096001
publisher
American Physical Society
external identifiers
  • scopus:85059641928
ISSN
2475-9953
DOI
10.1103/PhysRevMaterials.2.096001
language
English
LU publication?
no
additional info
Publisher Copyright: © 2018 American Physical Society.
id
d1a43946-c27d-45a5-955f-981755e48d70
date added to LUP
2023-04-05 16:21:26
date last changed
2023-04-21 16:13:59
@article{d1a43946-c27d-45a5-955f-981755e48d70,
  abstract     = {{<p>When a swift heavy ion (SHI) penetrates amorphous SiO<sub>2</sub>, 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<sub>2</sub> 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<sub>e</sub>) irradiations (&gt;10 keV/nm), the deviations between them are evident at low-S<sub>e</sub> irradiation (3-5 keV/nm). Even though the iTS calculations exclude the vaporization of SiO<sub>2</sub> at the low S<sub>e</sub>, 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<sub>e</sub>∼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<sub>e</sub>∼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.</p>}},
  author       = {{Amekura, H. and Kluth, P. and Mota-Santiago, P. and Sahlberg, I. and Jantunen, V. and Leino, A. A. and Vazquez, H. and Nordlund, K. and Djurabekova, F. and Okubo, N. and Ishikawa, N.}},
  issn         = {{2475-9953}},
  language     = {{eng}},
  month        = {{09}},
  number       = {{9}},
  publisher    = {{American Physical Society}},
  series       = {{Physical Review Materials}},
  title        = {{Vaporlike phase of amorphous Si O<sub>2</sub> is not a prerequisite for the core/shell ion tracks or ion shaping}},
  url          = {{http://dx.doi.org/10.1103/PhysRevMaterials.2.096001}},
  doi          = {{10.1103/PhysRevMaterials.2.096001}},
  volume       = {{2}},
  year         = {{2018}},
}