Elongation mechanism of the ion shaping of embedded gold nanoparticles under swift heavy ion irradiation

Abstract

The elongation process under swift heavy ion irradiation (74 MeV Kr ions) of gold NPs, with a diameter in the range 10-30 nm, and embedded in a silica matrix has been investigated by combining experiment and simulation techniques: three-dimensional thermal spike (3DTS), molecular dynamics (MD) and a phenomenological simulation code specially developed for this study. 3DTS simulations evidence the formation of a track in the host matrix and the melting of the NP after the passage of the impinging ion. MD simulations demonstrate that melted NPs have enough time to expand after each ion impact. Our phenomenological simulation relies on the expansion of the melted NP, which flows in the track in silica, followed by its recrystallization upon cooling. Finally, the elongation of the spherical NP into a cylindrical one, with a length proportional to its initial size and a width close to the diameter of the track, is the result of the superposition of the independent effects of each expansion/recrystallization process occurring for each ion impact. In agreement with experiment, the simulation shows the gradual elongation of spherical NPs in the ion-beam direction until their widths saturate in the steady state and reach the value of the track diameter. Moreover, the simulations indicate that the expansion of the gold NP is incomplete at each ion impact.

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

“…An attempt to bridge this limitation was carried out by a new theoretical study, proposed to follow the complete shaping process for a wider range of Au NP sizes using a phenomenological simulation code based on the expansion/recrystallization mechanism observed by MD simulations [39]. As a result, it was concluded that the NP cannot completely expand into the matrix during the ion shaping process and should be limited by the time the track formation and the expansion process, suggesting that the matrix strongly resists the expansion of the Au NP afterwards.…”

Insights into nanoparticle shape transformation by energetic ions using atomistic simulations

“…A small elongation increment occurs after each impact. In a scenario supported by MD simulations 8,28 , the molten and expanding material from the NPs pushes to the underdense, molten track on top and beneath the nanoparticle on impact. Silica remains solid elsewhere when the ion trajectory intersects the NP from the center region.…”

“…Atomistic simulations performed by several groups have shown that NPs change shape on impacts 8,27,28,34 . On the other hand, the mere reproduction of the effect in the simulation does not unambiguously identify the underlying mechanism, and the conclusions drawn from MD simulations appear contradictory.…”

“…27 attribute the elongation to the formation of a shock wave, extremely high temperatures of the NP (9000K), and associated interfacial pressures. On the other hand, other authors 8,28,34 contribute the driving force to be the volume expansion due to melting and heating that occurs in considerably lower temperatures (2000K). Using first-principles MD computations, authors in Ref.…”

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.…”