Ricardo Meurer Papaleo

Crater formation by single ions in the electronic stopping regime: Comparison of molecular dynamics simulations with experiments on organic films

An incident fast ion in the electronic stopping regime produces a track of excitations which can lead to particle ejection and cratering. Molecular Dynamics simulations of the evolution of the deposited energy were used to study the resulting crater morphology as a function of the excitation density in a cylindrical track for large angle of incidence with respect to the surface normal. Surprisingly, the overall behavior is shown to be similar to that seen in the experimental data for crater formation in polymers. However, the simulations give greater insight into the cratering process. The threshold for crater formation occurs when the excitation density approaches the cohesive energy density, and a crater rim is formed at about six times that energy density. The crater length scales roughly as the square root of the electronic stopping power, and the crater width and depth seem to saturate for the largest energy densities considered here. The number of ejected particles, the sputtering yield, is shown to be much smaller than simple estimates based on crater size unless the full crater morphology is considered. Therefore, crater size can not easily be used to estimate the sputtering yield.

Probing glass transition of PMMA thin films at the nanometer scale with single ion bombardment and scanning force microscopy

Abstract Poly(methyl methacrylate) (PMMA) thin films maintained at temperatures from −196 up to 150 °C are bombarded at grazing angles by 20 MeV Au ions at very low fluences. Immediately after the irradiation, the samples are cooled down at two distinct rates. Scanning force microscopy (SFM) images of the bombarded targets after cooling reveal nanometer-sized craters and/or raised regions (hillocks) around the point of each ion impact. The size of the craters is independent of the temperature for −196 ° C ° C , but for higher temperatures, crater dimensions increase steeply and no hillock is observed behind the crater. The temperature above which the hillocks disappear is sensitive to the cooling rate and for rapidly cooled targets it is near the tabulated value for the glass transition temperature, Tg, of the polymer. Since the variation of the morphology of ion-induced defects occurs over a narrow temperature range and depends on the cooling rate of the targets, single ion bombardment coupled to SFM may be used as an alternative method to estimate Tg and relaxation rates of nanodeformations in thin polymer films. The post-irradiation thermal annealing of the surface tracks produced at room temperature is also discussed.

Surface tracks on polymers: A means to probe material properties at the nanometer scale?

Abstract The morphology and the dimensions of nanometer-sized craters and crater rims induced by individual MeV heavy ions impacting polymer surfaces have been systematically investigated by means of scanning force microscopy. We demonstrate that the morphology of the surface tracks varies systematically with temperature, molecular weight ( M w ) and tacticity of polymer thin films. Such parameters influence chain mobility and the characteristic relaxation times of the polymer, which appear to be key factors determining the final shape and size of observed holes and hillocks. Because of the correlation of the impact features with the thermal and rheological properties of polymers, the problem can be inverted and the surface track morphology may be used to identify physical parameters of the targets. This procedure has been applied to model polymer films. It was possible to identify a critical target temperature above which a sharp change in crater morphology is observed in the scanning force microscopy images. We propose this temperature to be the glass transition temperature. Also, crater size decreases steeply with increasing molecular weight up to a critical M w above which crater dimensions change very little. The transition region is around M c ′ , the critical molecular weight at which entanglement coupling of macromolecular chains becomes important.

Cratering by MeV–GeV ions as a function of angle of incidence

Abstract We report on a systematic scanning force microscope study of crater formation induced by swift heavy ions as a function of angle of incidence. PMMA films were bombarded with 197Au (20 MeV), 209Bi (2320 MeV) and 238U (2640 MeV) ions at angles θ varying from 0° to 84° to the surface normal. In all cases, the length of the craters as well as rim height and length scale with (cosθ)−1. Crater width showed a much weaker (cosθ)−0.3 dependence. Similar angular dependences were observed for the different ion species and energies used. The experimental data is compared to molecular dynamics simulations of crater formation in a model solid. The simulations show a (cosθ)−1 dependence for the crater length, but no dependence for the crater width, unless a wide initial track of excitation is used.

Thermal properties of ion beam irradiated engineering thermoplastics

Abstract Thermal properties and recrystallization ability of commercial grade poly(para-phenylene sulphide) (PPS) and poly(ethylene terephthalate) (PET) thin foils, bombarded with 380 keV He+ ions, were studied by Differential Scanning Calorimetry (DSC) and X-ray Diffractometry (XRD). The following phenomena are observed on bombarded targets: a) decrease of the enthalpy of fusion (ΔHf ) and recrystallization (ΔHrc ) and b) decrease of the onset of the melting and recrystallization temperature. This behaviour indicates amorphisation of crystalline phases, increase of imperfections in the remaining crystallites, and loss of the recrystallization ability of the amorphous zones upon repeated thermal cycles. The amorphisation fluence, for which no crystalline regions were detected by DSC and XRD, is 2 × 1014 He+ / cm2 for PPS and 1014 He+ / cm2 for PET. However, the irradiated targets lose their ability to recrystallize at much lower fluences, around 4 × 1013 He+/cm2 for PPS and 2 × 1013 He+/cm2 for PET. The loss of crystallization ability is attributed to the presence of reactive defects on the bombarded targets which may generate crosslinks (and chain scissions) during the heating stage of the DSC run.