Evaluation of local temperature around the impact points of fast ions

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Abstract

Gold and platinum nanoparticles of few-nm size were deposited on amorphous silicon nitride (a-SiN) films. These samples were irradiated with 1.1 MeV C603+ ions to a fluence of ∼5 × 1010 ions/cm2 and observed using transmission electron microscopy (TEM). The ion tracks were clearly seen as bright spots and the metal nanoparticles disappeared from a neighboring region (5–10 nm) around each ion track. The platinum-nanoparticle-cleared region is slightly smaller than that of gold nanoparticles. This trend can be reproduced by the u-TS calculations assuming that the nanoparticles are desorbed when the local temperature surpasses the melting point of nanoparticles as was predicted by molecular dynamics simulations (Anders et al., 2009). This indicates that the temperature distribution in a nanometer region can be evaluated by observing the desorption of nanoparticles of different metals having different melting temperatures.

Introduction

When swift heavy ions penetrate a solid the ions excite solid electrons. The energy of the excited electrons is gradually transferred to the lattice and eventually cylindrical damage regions, so-called ion tracks, may be created when the electronic energy loss is larger than a material dependent threshold value [1], [2]. There are several models proposed for the mechanism of the track formation [2], [3], [4], [5], [6]. Among these models, the inelastic thermal spike (i-TS) model seems most plausible because it explains the electronic energy loss threshold for the track formation and the evolution of the track radius with the electronic energy loss [2], [6]. In the thermal spike model, ion tracks are assumed to be formed if the atomic temperature surpasses a material dependent threshold energy per atom. This threshold energy per atom has been considered as the energy to reach the melting temperature plus the latent heat for solid to liquid phase transition. The heating occurs in a highly localized area of nanometer size and continues only for a short time period of ∼1011 s. Such a fast and localized heating cannot be measured using conventional techniques.

Recently, we have observed desorption of gold nanoparticles from the surfaces of amorphous SiO2 (a-SiO2) and amorphous silicon nitride (a-SiN) upon ion impact [7], [8] using transmission electron microscopy (TEM). The TEM observation showed that the gold nanoparticles were disappeared from the neighboring area around the ion impact position. Considering the result of the molecular dynamics (MD) simulations, which showed that the gold nanoparticles desorb in several ps from the surface when the nanoparticles are heated beyond the melting temperature [9], the observed result suggests that the temperature surpassed the melting point of gold in the nanoparticle-cleared region. Actually, the i-TS calculation showed that the temperature of the substrate surpasses the melting point of gold in the nanoparticle-cleared region both for a-SiO2 and a-SiN [8]. This result suggests that detailed temperature distribution can be evaluated by observing the desorption of nanoparticles having different melting points. In the present paper, we observed desorption of gold and platinum nanoparticles from a-SiN films upon bombardment of 1.1 MeV C60 ions. The observed results is compared with the theoretical calculation based on the i-TS model including the effect of nuclear energy loss.

Section snippets

Experimental

Self-supporting a-SiN films (thickness 30 nm) with a nominal density of 3 g/cm3 were purchased from Silson Ltd. The composition of the a-SiN film was determined to be Si0.49±0.02N0.51±0.02 using high-resolution Rutherford backscattering spectrometry [10], which is Si rich compared to the stoichiometric Si3N4 by several at.%. A small amount of gold or platinum was vapor deposited on the a-SiN films at room temperature. For the gold deposition, different amounts of gold were deposited to prepare

Desorption of nanoparticles

Fig. 1(a) shows an example of the TEM bright field images of the platinum-deposited a-SiN film observed before irradiation. There are many platinum nanoparticles formed by the vapor deposition. The areal density, N, of these nanoparticles was measured to be 8.1 × 1012 particles/cm2. The size distribution of these nanoparticles was derived from the observed TEM images and shown by solid circles in Fig. 2. The distribution shows a Gaussian-like well-defined peak with a peak radius of 0.8 nm and a

Conclusion

The desorption of gold and platinum nanoparticles from a-SiN films upon irradiation with 1.1 MeV C60 ions was observed using TEM. It was found that the nanoparticles were desorbed around the ion impact position and the gold-nanoparticle-cleared region is larger than the platinum-nanoparticle-cleared region. According to the MD simulations, it is expected that the temperature surpassed the melting temperature of the nanoparticles in the nanoparticle-cleared region. In order to confirm this

Acknowledgements

This work was performed under the shared use program of JAEA facilities. The authors are grateful to the crews of the 400-kV ion implanter at JAEA/Takasaki for irradiation of C60 ions. This work was partly supported by JSPS KAKENHI Grant (Grant Numbers 24651114 and 26246025).

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Cited by (1)

  • Local heating induced by single MeV C<inf>60</inf> ion impacts

    2017, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
    Citation Excerpt :

    A small reduction (16% reduction) of the threshold energy compared to the melting energy reflects the size effect on the melting temperature. Assuming that the same reduction rate is applicable to the Pt nanoparticles, the threshold energy for Pt-nanoparticle desorption is estimated to 0.64 eV/atom [9]. Thus the present result shows that the temperature rises up to 0.64 eV/atom at a distance 5.5 and 6.1 nm from the ion impact position on the entrance and exit surfaces of the 20-nm a-SiN film, respectively.

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