Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Local heating induced by single MeV C60 ion impacts
Introduction
When a swift heavy ion impinges on a solid surface the ion lose its kinetic energy mainly via electronic excitations. The energy of the excited electrons is transferred to the lattice and the temperature rises around the impact position in a short time period of ∼10−11 s [1], [2], [3]. Recently, we have demonstrated that such local heating can be traced by observing desorption of gold nanoparticles from the surface upon ion impact [4]. According to the MD simulations, the gold nanoparticles are desorbed when the temperature surpasses their melting temperature [5]. Thus, the nanoparticle-cleared region around the ion impact position, which can be observed by transmission electron microscopy (TEM) after ion irradiation, corresponds to the region where temperature surpassed the melting temperature of gold nanoparticles. The observed local heating of amorphous silicon oxide (a-SiO2) and amorphous silicon nitride (a-SiN) films (thickness 20–30 nm) induced by 420 MeV Au ions was well explained by the inelastic thermal spike (i-TS) model [4]. It was also found that the local heating at the exit surface is enhanced compared to the entrance surface. This was ascribed to the effect of δ-rays (high-energy secondary electrons) produced by the Au ion. The δ-rays carry away the deposited energy and do not contribute to heating the place of production but do contribute in the deeper region. As a result, the deposited energy at the entrance surface is smaller compared to the exit surface. We have also evaluated local heating when 1.1 MeV C603+ ions impinged on a-SiN films [6]. The observed results indicated enhancement of local heating at the exit surface compared to the entrance surface. In this case, the enhancement cannot be ascribed to the effect of δ-rays because the 1.1 MeV C603+ ion is too slow to produce δ-rays. The origin of the observed enhancement was suggested to be the broadening of the spatial distribution of constituent carbon ions after traveling through a-SiN films, although no quantitative estimation was given.
In the present paper, we perform more comprehensive study on the local heating of a-SiN and a-SiO2 films induced by MeV C603+ ions with emphasis on the difference between the entrance and exit surfaces. The observed behavior is somewhat complicated. The local heating at the exit surface sometimes higher and sometimes lower than the entrance surface depending on the ion energy and film thickness. This complicated behavior can be well explained by the unified thermal spike (u-TS) model taking account of the spatial distribution of the constituent carbon ions at the exit surface.
Section snippets
Experimental
Self-supporting a-SiN films (thickness 20 and 30 nm, nominal density 3 g/cm3) and a-SiO2 films (thickness 20 nm, nominal density 2.2 g/cm3) were purchased from Silson Ltd, and Structure Probe Inc., respectively. The composition of the a-SiN film was determined to be Si0.47±0.02N0.53±0.02 using high-resolution Rutherford backscattering spectrometry [7]. Gold and platinum nanoparticles were deposited on the a-SiN and a-SiO2 films by vacuum evaporation. The prepared Au- and Pt-deposited films were
Desorption of nanoparticles
Fig. 1(a) shows an example of the TEM bright field images of the Pt-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 9.6 × 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 0.73 nm and a width of
Conclusion
The local heating of a-SiN and a-SiO2 films upon impact of single MeV C60 ion was studied both experimentally and theoretically. The temperature of the entrance and exit surfaces were traced by observing desorption of gold and platinum nanoparticles deposited on the surface. The observed local heating at the exit surface is enhanced compared to the entrance surface in some cases while opposite results were observed in other cases depending on the film thickness and the incident energy of the C60
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 Number 26246025).
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