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 (${\mathrm{SiO}}_{2}$) 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 ${\mathrm{Xe}}^{14+}$ ions with an incident angle of 45\ifmmode^\circ\else\textdegree\fi{}. The fluence ranged from $1.0\ifmmode\times\else\texttimes\fi{}{10}^{11}$ to $5.0\ifmmode\times\else\texttimes\fi{}{10}^{13}$ Xe/${\mathrm{cm}}^{2}$, 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 $5.0\ifmmode\times\else\texttimes\fi{}{10}^{11}$ Xe/${\mathrm{cm}}^{2}$ (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 $5.0\ifmmode\times\else\texttimes\fi{}{10}^{13}$ Xe/${\mathrm{cm}}^{2}$. Under almost the same irradiation conditions, Co NPs with the same initial mean radius showed more prominent elongation with AR of \ensuremath{\sim}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.