Molecular dynamics study of fullerite cross-linking under keV C 60 and Arn cluster bombardment

Molecular dynamics computer simulations are used to elucidate the cross-linking processes induced by 0.6-50 keV C60 and Arn cluster bombardment in a C60 fullerite solid sample. The obtained results indicate the presence of a "chemical effect" when C 60 projectile is used. Namely, the bombarding C60 delivers additional, highly reactive, radicals which interact with the atoms of the fullerite sample, increasing the efficiency of the cross-linking process. The omission of those interactions in the analysis makes the C60 very similar to the case of the Ar18 bombardment. For Arn cluster bombardment, the initial energy per atom in the projectile is the parameter which has the predominant influence on the cross-linking process. Furthermore, a relationship between the energy thresholds for fragmentation of the target molecules and cross-linking initiation and the size of the Ar clusters is observed. Both of these thresholds decrease with increasing size of the projectile. © 2013 American Chemical Society

Cluster impacts in organics: microscopic models and universal sputtering curves

This article reviews the results of molecular dynamics simulations of kilo-electron-volt projectile (Ga, Au 1-400, Bi 1-5,C60,Ar60-2000 and organic clusters) impacts with two model hydrocarbon targets, an amorphous solid made of kilodalton molecules and a crystal of polyethylene with infinite chain length. The universal dependences of the sputtered mass on the projectile energy, when scaled by the projectile mass, with their characteristic transition around 1eV/amu (or ~14km/s), are analyzed and explained by the physics of the interaction. We also comment on comparisons with recent experimental sputtering data

Re-print of "Sputtering of polymers by keV clusters: Microscopic views of the molecular dynamics"

This article reviews the results of molecular dynamics simulations of cluster sputtering of hydrocarbon polymers obtained in the last few years and expand them with unpublished data of Ar cluster bombardment. The targets are molecular solids of linear hydrocarbons, polyethylene and polystyrene, including a polyethylene substrate decorated with adsorbed globular macromolecules. The projectiles are (hydro)carbon and Arn clusters, from small to massive, as well as Bin and Au400 clusters. The study focuses on the dynamics of cratering and sputtering, using a coarse-grained representation of the samples, on the study of molecular fragmentation, crosslinking and freeHformation, using a fully atomistic model, and on the conditions of desorption of macromolecules by massive clusters. The results explain the similarities and differences between several cluster types and sizes and, to a large extent, the sputtering yields of Arn clusters and their ‘universal’ dependence on the scaled cluster energy, as observed in the experiments. They also demonstrate the reduction of sample fragmentation and crosslinking when going to larger clusters and the incidence angle dependence of intact macromolecule emission. Recent experimental validations obtained in our laboratory are also introduced and comparisons with data obtained by other groups are discussed in order to present a more complete picture of the physics of cluster bombardment of organic solids and polymers

Molecular Dynamics Simulations of Hydrocarbon Film Growth from Acetylene Monomers and Radicals: Effect of Substrate Temperature

In an attempt to rationalize the mechanisms occurring during plasma polymerization of acetylene, classical molecular dynamics computer simulations investigating the deposition and reaction of a mixed gas of acetylene molecules and radicals on the Ag(111) substrate were performed for a wide range of substrate temperatures. Prior to that, this article establishes a methodology for film deposition and identifies the appropriate potentials for hydrocarbons by comparison with electronic calculations using density functional theory. On the basis of this preliminary study, simulations of film growth are carried out at different temperatures using the reactive empirical bond order potential. Our results show that the rates of formation of new C–C and C–H bonds are higher at the beginning of the film growth when the substrate is still exposed than when it is covered with polymeric chains, and these initial reaction rates are proportional to temperature. The analysis of the hybridization of carbon atoms in the films shows that the substrate temperature increase leads to the formation of coatings containing more carbon atoms in the sp2 and sp3 configurations and less in the sp configuration with sp2 becoming dominant at high temperatures. We establish a polymerization–connectivity formalism that describes the structural transformation of the film during the deposition on the basis of each atom hybridization and bonding. Within this formalism, the evolution of the polymerization and the connection character of the polymers is observed and discussed

Molecular dynamics study of polystyrene bond-breaking and crosslinking under C60 and Arn cluster bombardment

Molecular dynamics computer simulations are used to elucidate the bond-breaking and crosslinking processes induced by 2.5 keV C60 and Arn cluster bombardment in an amorphous sec-butyl-terminated polystyrene sample. The obtained results indicate that replacement of C60 by Ar18 or Ar60 projectiles leads to the decrease of the number of broken bonds and, hence, to the decrease of formation of new intra- and intermolecular (crosslinking) bonds. When the number of atoms in the Arn cluster is increased from 60 to 250 or more, the total number of broken bonds and the total number of newly created bonds reach a zero value. Additional comparison to the case of a fullerite crystal reveals that the change of material properties leads to almost 7.5-fold reduction of the efficiency of the crosslinking process. © 2013 Elsevier B.V. All rights reserved

Single impacts of keV fullerene ions on free standing graphene: Emission of ions and electrons from confined volume

We present the first data from individual C60 impacting one to four layer graphene at 25 and 50 keV. Negative secondary ions and electrons emitted in transmission were recorded separately from each impact. The yields for Cn - clusters are above 10% for n ≤ 4, they oscillate with electron affinities and decrease exponentially with n. The result can be explained with the aid of MD simulation as a post-collision process where sufficient vibrational energy is accumulated around the rim of the impact hole for sputtering of carbon clusters. The ionization probability can be estimated by comparing experimental yields of Cn - with those of Cn 0 from MD simulation, where it increases exponentially with n. The ionization probability can be approximated with ejecta from a thermally excited (3700 K) rim damped by cluster fragmentation and electron detachment. The experimental electron probability distributions are Poisson-like. On average, three electrons of thermal energies are emitted per impact. The thermal excitation model invoked for Cn - emission can also explain the emission of electrons. The interaction of C60 with graphene is fundamentally different from impacts on 3D targets. A key characteristic is the high degree of ionization of the ejecta