The fabrication of highly anisotropic metallic nanoparticles embedded in transparent dielectrics is of particular interest due to their linear and nonlinear optical properties. Controlling the size, shape and orientation provides a means of tuning the optical response. Swift heavy-ion irradiation of metallic nanoparticles (MNPs) embedded in dielectric materials has been demonstrated as an efficient method to induced controlled anisotropy. This process has been studied predominantly using amorphous silica as the host matrix, where key parameters, such as the existence of an energy threshold for elongation, the need of an ion track with an under-dense core, and MNPs with size comparable to the ion track dimensions, have been identified. Yet, more work is required to extend the current understanding of the process. Studying the process in silicon nitride, which possess interesting thermophysical properties and is compatible with Si-based device fabrication has not been previously performed and will yield significant new insights into the physical mechanism operational in the ion shaping process.

In this talk, I show a systematic study of the ion beam shaping of Au NPs using swift heavy-ion irradiation while embedded in amorphous silicon dioxide and silicon nitride. This includes a comprehensive study of the ion track formation process in silica and silicon nitride, using small angle X-ray scattering, Infrared spectroscopy and Molecular Dynamics (MD) simulations. The combined analysis revealed a mean track morphology resembling an under-dense core surrounded by an over-dense shell with a smooth transition for both materials, yet the track size and density changes are significantly different. As a consequence, the ion beam shaping process is significantly less efficient in amorphous silicon nitride due to the short-lived thermal spike, which enables the use of silicon nitride as a diffusion barrier constraining the ion shaping process to the silica layers (Fig. 1). Three-dimensional calculations of the inelastic thermal spike suggest the difference in thermophysical properties is the cause of this behavior. The system is characterized by transmission electron microscopy (TEM) and high angular annular dark field (HAADF) in STEM mode. The results not only represent a step towards plasmonic device fabrication but also provide deep insights into the ion beam shaping process.