Effect of polyatomic ion structure on thin-film growth: Experiments and molecular dynamics simulations

The experiments described here examine 25-100 eV CF⁺₃ and C₃F⁺₅ ion modification of a polystyrene (PS) surface, as analyzed by x-ray photoelectron spectroscopy. The molecular dynamics computer simulations probe the structurally and chemically similar reactions of 20-100 eV CH⁺₃ and C₃H⁺₅ with PS. CF⁺₃ and C₃F⁺₃ each form a distribution of different fluorocarbon (FC) functional groups on PS in amounts dependent upon the incident ion energy, structure, and fluence. Both ions deposit mostly intact upon the surface at 25 eV, although they also undergo some crosslinking upon deposition. Fragmentation of the two ions increases as the ion energies are increased to 50 eV. Both ions show increases in total fluorine and fluorinated carbon content when changing the ion energy from 25 to 50 eV. The simulations predict that CH⁺₃ and C₃H⁺₅ behave in a similar fashion to their FC analogs, remaining mostly intact and either embedding or scattering from the surface without reacting at 20 eV. At 50 and 100 eV, the simulations predict fragmentation most or all of the time. The simulations also show that the chemical products of the collisions depend significantly on the structure of the incident isomer. The simulations further illustrate how the maximum penetration depth of ion fragments depends on ionic structure, incident energy, and the identity of the penetrating fragment. These ion-surface results are discussed in terms of their possible role in plasmas.