Arnaud Denis Delcorte oral presentation (B&N-Tue1-2-3)
Relationships between crater and sputtered material characteristics in large gas cluster sputtering of polymers: Results from MD simulations
Université catholique de Louvain - Institute of Condensed Matter and Nanosciences, 1 Place Louis Pasteur Box L4.01.10, 1348 Louvain-la-Neuve, Belgium
The sputtering induced by argon clusters in polymers has been predicted using molecular dynamics (MD) simulations, with a coarse-grained representation of the target (A. Delcorte, M. Debongnie, J. Phys. Chem. 2015, 119, 25868). In particular, both the calculated and the experimental “universal” dependences of the scaled sputtering yield (Y/n or Y/m) as a function of the scaled energy (E/n or E/m) show an extended linear region at high scaled energy, preceded by a non-linear increase. The agreement between the model and the experiments involving similar mass molecules for 45° incidence (Fig.1a) provides us with sufficient confidence to further analyze the microscopic details of the interactions. Here, we focus on the relationships between the sputtered volume, the origin of the emission of fragments and intact molecules and the crater sizes and shapes as a function of scaled energy, upon Ar and CH4 cluster bombardment of an amorphous solid made of 1.4 kilodalton polymers (CH3-(CH2)97-CH3). The craters were satisfactorily modelled by semi-ellipsoids. First, our results show that the ratio of the sputter yield volume Yv over the crater volume Cv is constant at high scaled energy (~⅓) but decreases in a non-linear fashion at low scaled energy, so that large impact craters are still formed under 0.025 eV/amu (Fig. 1b) but with almost no ejection, only material displacement in the surface. Second, while the crater depths are essentially constant for a given total cluster energy (range: 2.5-15 keV), irrespective of the cluster size, the crater diameters increase with the cluster size. Third, while the sputtered material originates from the top half of the crater at high scaled energy, the ejection becomes gradually confined to surface molecules at low energy (Fig. 1c), suggesting that larger, low energy clusters might provide more surface sensitivity for analysis, in addition to softer emission. The variance with the results obtained for CH4 cluster bombardment will be discussed. Finally, the analysis of the relationships between crater and sputtering will be extended to ultrathin layers (2-15 nm) on a rigid substrate.