Ion beam shaping of nanometals: process modeling and atomistic simulations of extreme conditions

Recently, a novel type of ion-beam induced deformation of metal nanoobjects has been found. Under heavy ion irradiation, Au nanospheres in a silica matrix first elongate into rods. At higher fluences they combine into nanowires that continue to grow during irradiation. Such anisotropically shaped metal nanoparticles may have great potential in a wide range of fields. For example, nanorods exhibit a split plasmon resonance, with one of the bands shifting into the infrared. Arrays of such particles have a great potential as nanophotonic guides in the (infra)red, an important telecom wavelength regime, but outside the range of plasmon resonances of spherical metal particles. Here, we present a model and atomistic computer simulations of this ion beam shaping. The experimental lower threshold for ion beam shaping of 6 keV/nm ion energy deposition along the ion track coincides with the theoretically required energy for melting of SiO₂ in the ion track of a few nanometer diameter. Heating occurs on a timescale of a few tens of fs. Thus, temperature gradients of several billion Kelvin per cm can be reached. The Au nanosphere experiences such an extreme environment if it is touched by an ion track. These extreme conditions are similar to femtosecond laser processing of Au layers where frozen nanojets have been observed. Laser-induced nanojet formation and ion beam shaping have obviously the same driving force: Based on atomistic simulations we prove that thermocapillarity drives material of a Au nanosphere, which is touched by an ion track, from the hot equator to the colder pole regions(Thermocapillarity is the driving force for the wellknown Marangoni effect). Additionally, the transiently extreme high temperature in the molten SiO₂ ion track dissolves Au, which diffuses fast and precipitates during cooling into tiny Au clusters. Superposition of tracks leads to a highly anisotropic “track diffusion” transport of Au from short nanorods to longer ones, which can be considered as a special case of Ostwald ripening.