Sputtering polymers with buckminsterfullerene projectiles: A Coarse-Grain molecular dynamics study

Buckminsterfullerene projectiles have demonstrated their outstanding capabilities for the secondary ion mass spectrometric (SIMS) analysis of bulk organic films. In this contribution, we focus on modeling the mechanisms of energy transfer and sputtering induced by kiloelectronvolt C-60 projectiles in molecular solids and polymers, which are important from the viewpoint of applications and have not been theoretically studied yet. The chosen methodology relies on molecular dynamics (MD) simulations, with a coarse-grained representation of the samples that allows us to dynamically describe over sufficient time intervals the large ensembles of molecules required to properly confine the action induced by 1-10 keV fullerenes in organic targets. For 5 keV bombardment, the simulations explain the transfer of the projectile energy in the topmost layers of the surface, accompanied by the formation of a similar to 100 angstrom wide, similar to 50 angstrom deep hemispherical crater and the emission of molecules and fragments from the top 30 A of the surface. Using polyethylene samples with molecular weights ranging from 0.3 kDa up to 14 kDa, the secondary effects of chain length and entanglement on the crater size, sputtering yield, fragmentation, and intact molecule emission are investigated in detail. For instance, it is shown that, in order to be emitted intact, a molecule must be initially confined in the annular region of the forming crater that surrounds the similar to 30 angstrom wide energized core where most bond-scissions occur. In contrast, molecules that intersect this track core break upon impact while molecules that extend beyond the size of the crater end up forming the rim or dangling in the vacuum when most of the energy is dissipated. The evolution of the crater size, the sputtered mass and the number of intact molecules with increasing projectile energies (1-10 keV) are reported and connections with experiments are proposed.