Shaping of nanometals by swift heavy ions

Metal nanorods and nanowires have great potential in a wide range of fields, because of their tunable (by shape and size) optical and magnetic properties. We present and discuss an unique way for producing perfectly parallel nanorods and -wires that are embedded in a solid. Starting from monodisperse spherical Au nanocolloids (15-50 nm diameter) in a silica matrix we show that the colloids are shaped controllably into rods -- and at later stages -- wires under heavy-ion irradiation above a threshold energy loss of ~6keV/nm. The threshold coincides with the formation of a continuous molten ion track, as follows from ion track temperature calculations. Nanowires of ~10nm diameter with lengths up to several hundred nanometer form and align parallel to the ion beam in the fluence regime of 10¹⁴-10¹⁵ ions/cm². Based on experiments with different colloid sizes and concentrations we conclude that all Au nanospheres initially elongate into nanorods, with their long axis along the ion track. Above a critical fluence (e.g. 1x10¹⁴/cm² for 54 MeV Ag) individual nanorods disintegrate while others continue to grow by uptake of the atoms of the disintegrated particles. This novel type of ripening process is corroborated by 3D kinetic Monte Carlo simulations.
Based on atomistic simulations we suggest that the lengthening is a result of thermocapillarity, which drives material of a gold nanosphere that is touched by an ion track from the hot equator to the colder pole regions. Temperature gradients of several billions Kelvin per cm can be reached, conditions that are similar to femtosecond laser processing of Au layers [1] where frozen nanojets have been observed.
[1] F. Korte, J. Koch, and B.N. Chichkov, Appl.Phys. A 79 (2004) 879-881