Au and Ge Nanoparticle Shaping by Swift Heavy Ion Irradiation

The driving forces of nanomaterials processing by swift heavy ions as identified by our studies are (i) the mate-rials dependent electronic stopping power, (ii) the vol-ume change upon melting, (iii) the asymmetric hydro-dynamic flow due to stress field hysteresis, as well as (iv) far-from-equilibrium steady-state solubilities and strongly anisotropic diffusion coefficients. Size distributions, shapes and anisotropies of nanoparticles can be tailored by appropriate tuning of these driving forces. The evolution of Au and Ge nanospheres under swift heavy ion irradiation was studied experimentally and by atomistic computer simulations. Ge nanospheres of different sizes embedded in SiO2 show different response to I7+ ion irradiation at 38 MeV. Spheres below a critical size become discus-shaped, very small ones show Ge loss at their equator. Computer simula-tions based on a model which includes the driving forces listed above describe the Ge shaping and the Au shaping, where Au nanospheres of 15 nm diame-ter elongate to rods. Our model describes the ion-induced shape evolution of different elements for different ion species, energies and fluences quantitatively, where only one fit parameter describes all experiments. This is a strong evidence that our model based on classical thermodynamics and hydrodynamics describes the shaping mechanism appropriate. Using critical-size nanospheres with an unimodal size distributions and changing the ion impact angle during irradiation, tailoring of very exotic nanoparticle shapes become feasible.