Sputtering of nanospheres - a computer simulation study

The sputtering of spherical objects – such as clusters, nanoparticles or aerosol particles – being exposed to energetic ion irradiation has been studied using both Monte Carlo simulation in the binary collisions approximation (MC) and molecular dynamics simulation (MD). 20 keV Ar impact on a-Si has been chosen as a typical example. With a denoting the mean depth of energy deposition in a planar target, inverse scaled cluster radii a/R ranging from 0 to 20 have been investigated, both addressing the impact parameter dependence and the average sputtering yield. For large radii (a/R < 0.2) sphere sputtering follows closely the sputtering of planar targets, if the variation of the incidence angle on the sphere surface is taken into account. For smaller radii, the yield increases due to the influence of sphere curvature. For a/R > 1, pronounced forward sputtering leads to a maximum in the sputter yield. In the limit of small radii, sputter emission becomes largely isotropic, but decreases in yield since the projectile energy is only partly deposited in the sphere and the surface area shrinks. However, for all spheres studied, the average sputter yield is larger than for the planar surface. Within the uncertainties of the modelling, there is an excellent agreement between the planar surface sputtering yields as obtained by MC and MD. For small spheres, however, the MD yields are significantly larger which is attributed to the influence of collisional spikes.