Zn nanoparticles irradiated with swift heavy ions at low fluences: Optically-detected shape elongation induced by nonoverlapping ion tracks

Abstract

Elongation of metal nanoparticles (NPs) embedded in silica (SiO 2 ) induced by swift heavy-ion (SHI) irradiation, from spheres to spheroids, has been evaluated mainly by transmission electron microscopy (TEM) at high fluences, where tens to thousands of ion tracks were overlapped each other. It is important to clarify whether the high fluences, i.e., track overlaps, are essential for the elongation. In this study the elongation of metal NPs was evaluated at low fluences by linearly polarized optical absorption spectroscopy. Zn NPs embedded in silica were irradiated with 200-MeV Xe 14+ ions with an incident angle of 45• . The fluence ranged from 1.0 × 10 11 to 5.0 × 10 13 Xe/cm 2 , which corresponds to the track coverage ratio (CR) of 0.050 to 25 by ion tracks. A small but certain dichroism was observed down to 5.0 × 10 11 Xe/cm 2 (CR = 0.25). The comparison with numerical simulation suggested that the elongation of Zn NPs was induced by nonoverlapping ion tracks. After further irradiation each NP experienced multiple SHI impacts, which resulted in further elongation. TEM observation showed the elongated NPs whose aspect ratio (AR) ranged from 1.2 to 1.7 at 5.0 × 10 13 Xe/cm 2 . Under almost the same irradiation conditions, Co NPs with the same initial mean radius showed more prominent elongation with AR of ∼4 at the same fluence, while the melting point (m.p.) of Co is much higher than that of Zn. Less efficient elongation of Zn NPs while lower m.p. is discussed.

Shape elongation of embedded Zn nanoparticles induced by swift heavy ion irradiation: A SAXS study

“…This phenomenon has been receiving much attention until now, because this is a promising method to fabricate aligned nano‐rods (NRs) or elongated NPs from spherical NPs embedded in solid matrix. In fact, anisotropic properties of the NRs or elongated NPs formed by this method have been confirmed in anisotropic magnetization , optical linear dichroism , birefringence , and anisotropic optical nonlinearity .…”

“…According to the simple black‐painting ion‐track model , the coverage ratio CR by the tracks is described as where r and Φ denote the track radius and the fluence, respectively. Since the track radius in silica irradiated with 200 MeV Xe was estimated to 4.5 nm , overlaps of the tracks are not negligible at Φ c ∼ 2( πr 2 ) −1 ∼ 4 × 10 12 ions/cm 2 or higher. At fluences much higher than 4 × 10 12 ions/cm 2 , the sample is homogeneously covered by tracks, and the SAXS signal from the tracks should disappear since the SAXS detects the density fluctuation.…”

“…The corresponding TEM images for the unirradiated state and at the fluence of 5 × 10 13 ions/cm 2 are shown in Ref. . However, since the images are cross‐sectional ones and since the NPs are partially superimposed with each other, quantitative determination of the mean IP distance from the TEM images was not easy.…”

“…It was better to be described by the concept of the minimum size for elongation . If the initial sizes are enough large, the elongation is induced without the threshold fluence, i.e., a single impact of SHI induces the elongation of a NP, which was confirmed by the optical linear dichroism (OLD) spectroscopy .…”

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.…”