Swift Heavy Ion Beam Shaping of Au and Ge Nanoparticles in SiO2

Lasers can process materials at spatiotemporal µm and ps scales. Here it will be shown that swift heavy ions can be used for materials processing at even shorter length and time scales. Swift-heavy-ion-induced deformation of spherical Au and Ge nanoclusters (NCs) embedded in SiO2 was studied experimentally and theoretically. Ge NC shaping is size dependent under irradiation with 38 MeV iodine ions and with 89 and 185 MeV gold ions. Large NCs don’t deform, smaller ones become discus-shaped, and very small ones show Ge loss at their equator. Small Au NCs deform into rods and wires, and, rather exotic, at critical NC size Au wires are squeezed out of the poles of the Au spheres.
Modeling and atomistic computer simulations identified the main driving forces: (i) The materials dependent electronic stopping power, (ii) the volume change upon melting, (iii) the asymmetric hydrodynamic flow due to stress field hysteresis, as well as (iv) far-from-equilibrium steady-state solubilities and strongly anisotropic diffusion. The latter one leads to “Ostwald ripening” of deformed NCs. The NC size distributions, shapes and anisotropies can be tailored by appropriate tuning of the driving forces. Our model describes the ion-induced shape evolution of different elements for different ion species, energies and fluences even quantitatively, where only one fit parameter describes all experiments. It is based on classical thermodynamics and hydrodynamics only. An even stronger proof is the shape change of nanospheres of critical size under swift heavy ion irradiation. For such particles, exclusively central ion impacts induce shaping, where Au is squeezed out of the poles. Using an unimodal size distributions and changing the ion impact angle during irradiation, tailoring of very exotic nanoparticle shapes become feasible.