Mechanisms of swift-heavy-ion-assisted shaping of Au nanospheres into wires and their subsequent “Ostwald ripening”

At the IBMM conference 2004 (Monterey), Arjen Vredenberg has shown for the first time that Au nanospheres embedded in a SiO₂ layer change their shape significantly under swift heavy ion irradiation. At fluences of a few 10¹⁴cm^-2, Au spheres of 15 nm diameter transform into rods. They become even a few hundert nm long wires at higher fluences. Several Au nanospheres are consumed for the formation of such long wires. The ion efficiency of shaping increases linearly with the electronic energy transfer to the SiO2 matrix, starting at a threshold of about 6 keV/nm. Because optical and magnetic properties of metal nanoparticles depend strongly on shape anisotropy, ion beam shaping may have potential for tuning nanomaterials in fields like photonics or magnetic data storage. Here we present models and atomistic computer simulations on ion beam shaping. Ion beam shaping occurs if the ion track temperature exceeds the SiO₂ melting temperature, which was concluded from calculations of radial temperature-time profiles of ion tracks. Using the solubility (several ppm) and the high diffusivity of Au in molten silica, the “ripening” of the wires can be attributed to anisotrope, spatio-temporal limited Au diffusion in ion tracks, as it was proven by atomistic simulations. Thermocapillarity has been identified as a huge force elongating Au nanoparticles in SiO₂ at the sub-picosecond timescale. Kinetic Monte Carlo simulations reproduce the experimental shape evolution. Preliminary molecular dynamics studies introduce the correct kinetics and explain the low fluence necessary for ion beam shaping.