Shape elongation of Zn nanoparticles in silica irradiated with swift heavy ions of different species and energies: scaling law and some insights on the elongation mechanism.

Zinc nanoparticles (NPs) embedded in silica were irradiated with swift heavy ions (SHIs) of seven different combinations of species and energies. The shape elongation induced by the irradiations was evaluated by optical linear dichroism (OLD) spectroscopy, which is a sensitive tool for determining the change in the mean aspect ratio (AR) of NPs. Although the mean AR change indicated a linear fluence dependence in the low- and medium-fluence regions, it indicated a nonlinear dependence in the high-fluence region. The data reveal that the elongation efficiency of Zn is correlated with the electronic stopping power 'Se in silica' and is not correlated with either the 'Se in Zn' or the nuclear stopping power. The elongation efficiency plotted as a function of the 'Se in silica' revealed a linear relationship, with a threshold value of ∼2 keV nm(-1), which is the same dependence exhibited by the ion-track formation in silica. The log-log plot showed that the elongation efficiency increased linearly with Se above a critical value of ∼3 keV nm(-1) and steeply decreased with Se to the power of 5 below the critical Se. The steep decrease can be ascribed to the discontinuous nature of the ion tracks, which is expected at Se ∼ 2-4 keV nm(-1) in silica. The fluence Φ dependences of AR - 1 under various irradiations are well-normalized with the electronic energy deposition of SHIs, i.e., the product of Se and Φ, with a Se greater than the same critical value of ∼3 keV nm(-1). The normalized data above the critical value fell on a linear relation, AR(Φ) - 1 ∝ SeΦ, for SeΦ < 2 keV nm(-3) and a sublinear relation, AR(Φ) - 1 ∝ (SeΦ)(1/2) for SeΦ > 2 keV nm(-3). On the basis of these experimental results, we discuss some insights into the elongation mechanism.