Understanding collision cascades in molecular solids (original) (raw)
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Applied Surface Science, 2004
In order to further illuminate the sputtering process, the bombardment of Au {1 0 0} by 20 keV Ar is investigated using molecular dynamics (MD) simulations. The MD results are compared to experimental observations discussed by M.W. Thompson in his recent review of the atomic collision cascade process [Vacuum 66 (2) (2002) 99]. In his review, Thompson explains characteristics of experimental time-of-flight (ToF) and polar distributions using ejection mechanisms. Using mechanisms deduced from the MD results the ToF distributions are divided at 70 ms with atoms sputtered by direct recoil at shorter ToF and atoms sputtered by focused collision sequences at longer ToF. Surface lens assisted focusing arises from impacts along crystal symmetry lines (slice impact points) and results in a peak at surface normal in the polar distribution. These conclusions help to clarify the experimental observations made by Thompson and contribute to the overall description of sputtering.
Molecular Dynamics Simulation of Chemical Sputtering
2006
ABSTRACT We study chemical sputtering by D and D2(υ) at deuterated amorphous carbon surface. The dynamics of the surface characteristics is a function of the initial surface and the cumulative effect of the projectiles. We study evolution of the spectrum of the sputtered particles, hydrocarbons, in search for the steady ``state'' sputtering yield. The comparison of the steady yields of atoms vs. molecular projectiles in various vibrationally excited states enables interpretation of the ORNL experimental results.
Applied Surface Science, 2004
In order to further illuminate the sputtering process, the bombardment of Au {1 0 0} by 20 keV Ar is investigated using molecular dynamics (MD) simulations. The MD results are compared to experimental observations discussed by M.W. Thompson in his recent review of the atomic collision cascade process [Vacuum 66 (2) (2002) 99]. In his review, Thompson explains characteristics of experimental time-of-flight (ToF) and polar distributions using ejection mechanisms. Using mechanisms deduced from the MD results the ToF distributions are divided at 70 ms with atoms sputtered by direct recoil at shorter ToF and atoms sputtered by focused collision sequences at longer ToF. Surface lens assisted focusing arises from impacts along crystal symmetry lines (slice impact points) and results in a peak at surface normal in the polar distribution. These conclusions help to clarify the experimental observations made by Thompson and contribute to the overall description of sputtering.
Langmuir, 1995
The high-energy particle bombardment of a molecular film adsorbed upon a metal substrate has been investigated via molecular dynamics computer simulations with an empirical many-body potential energy function constructed for studying reactive dynamics. The specific system modeled is the bombardment of an ethylidyne (C2H3) overlayer adsorbed on Pt(ll1) by a 500-eV Ar atom beam. Approximately 80% of the ejected hydrocarbon species originate from a single C2H3 adsorbate, while the others result from reactions between two C2H3 adsorbates. A study of the internal energies of all of the ejected hydrocarbon aggregates reveals that those originating from a single C2H3 adsorbate are generally stable to any further fragmentation or rearrangement. Examples of common ejection mechanisms for species which originate from a single adsorbate, such as CH3, C2H3, or HCCH, and those which originate from more than one adsorbate, such as CH4, are given. (14) The correction to the screening factor in the Molihre is given by eq 6 of O'Connor, D. J.; MacDonald, R. J. Radiat. E f . 1977, 34, 247. The complete Molihre equation is also given herein. (15) (a) Foiles, S. M.; Baskes, M. I.; Daw, M. S. Phys. Rev. B 1986, 33,7983. (b) Daw, M. S.; Baskes, M. I. Phys. Rev. B 1984,29,6443. (c) Daw, M. S.; Baskes, M. I. Phys. Rev. Lett. 1983, 50, 1285. 0743-7463/95/2411-1220$09.00/0 0 1995 American Chemical Society 96,8538. Chang, X; Perry, M.; Peploski, J.; Thompson, D. L.; Raff, L. M. Mowrey,R.C.;Mintmire, J. W.;Robertson, D. H.; White, C. T. Mater. Res. SOC. SympProc. 1991,206,687. Dunlap, B. I.; Brenner, D. W.; Mintmire, J. W.; Mowrey, R. C.; White, C. T. J. Phys. Chem. 1991,95,5763. Robertson, D. H.; Brenner, D. W.; Mintmire, J. W. Phys. Rev. B 1992,45,12592. Lyons, M.; Dunlap, B. 1.; Brenner, D. W.; Robertson, D. H.; Mowrey, R. C.; Mintmire, J. W.; White, C. T.
Molecular dynamics simulation of sputtering from a cylindrical track: EAM versus pair potentials
Nuclear Instruments and …, 2005
Molecular dynamics simulations implementing the thermal spike model for sputtering by energetic particle bombardment are performed for a gold target represented with a many-body embedded atom method (EAM) potential. A linear dependence of sputtering yield on the effective energy deposition is observed over a broad range of sufficiently high excitation energies, suggesting that the conclusions of earlier simulations performed with pair potentials have a general character and are not sensitive to the choice of interatomic potential. At the same time, significant differences in cluster ejection are observed between the simulations performed with EAM and pair potentials. Clusters constitute a much larger fraction of the total yield in the EAM simulations, which is related to the environmental dependence of the interatomic interaction in metals that is correctly reproduced by EAM potential. An apparent disagreement between the analytical thermal spike model and its implementation in MD simulations cannot be attributed to the choice of interatomic potential but reflects a difference in the ejection mechanisms. Thermally activated evaporation from the surface is assumed in the analytical thermal spike model, whereas prompt ejection from a relatively deep part of the excited region and fast non-diffusive cooling of the spike region takes place in MD simulations.
Collision Cascade and Sputtering Process in a Polymer
The Journal of Physical Chemistry B, 2001
The particle induced fragmentation and sputtering of a ∼7.5 kilodalton organic sample is modeled using molecular dynamics (MD) simulations. The model system consists of a polystyrene coil containing 61 styrene repeat units adsorbed on Ag(111). It is bombarded by 500 eV Ar projectiles. To obtain a realistic picture of the dynamics for an organic material, we used the new adaptative AIREBO potential developed by Stuart, Tutein, and Harrison, which includes long-range van der Waals forces in the reactive potential created by Brenner (REBO) for hydrocarbon systems. Significant differences between the results obtained with and without the long-ranged interaction are identified. The development of the collision cascade in the organic medium is analyzed in detail using collision trees and movies of the results from the simulation. In addition to fast atomic collision processes, we show the existence of long-lived vibrational excitations and demonstrate their importance for the emission of kilodalton chain segments. Recombined and rearranged fragments are emitted, but their contribution to the mass spectrum is insignificant beyond 40 Da. Delayed emission via vibration-induced bond scission is also observed. Finally, we compare the MD results with new ToF-SIMS measurements performed in the context of this study.
Amorphization of silicon induced by nanodroplet impact: A molecular dynamics study J. Appl. Phys. 112, 054302 (2012); 10.1063/1.4748177 The molecular dynamics simulation of ion-induced ripple growth Self-sputtering of silver by mono-and polyatomic projectiles: A molecular dynamics investigation The impact of electrosprayed nanodroplets on ceramics at several km/s alters the atomic order of the target, causing sputtering, surface amorphization and cratering. The molecular mass of the projectile is known to have a strong effect on the impact phenomenology, and this article aims to rationalize this dependency using molecular dynamics. To achieve this goal, the article models the impact of four projectiles with molecular masses between 45 and 391 amu, and identical diameters and kinetic energies, 10 nm and 63 keV, striking a silicon target. In agreement with experiments, the simulations show that the number of sputtered atoms strongly increases with molecular mass. This is due to the increasing intensity of collision cascades with molecular mass: when the fixed kinetic energy of the projectile is distributed among fewer, more massive molecules, their collisions with the target produce knock-on atoms with higher energies, which in turn generate more energetic and larger numbers of secondary and tertiary knock-on atoms. The more energetic collision cascades intensify both knock-on sputtering and, upon thermalization, thermal sputtering. Besides enhancing sputtering, heavier molecules also increase the fraction of the projectile's energy that is transferred to the target, as well as the fraction of this energy that is dissipated. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
Molecular dynamic simulations of the sputtering of multilayer organic systems
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003
Sputtering of organic overlayers has been modeled using molecular dynamics computer simulations. The investigated systems are composed of benzene molecules condensed into one, two and three layers on an Ag{1 1 1} surface. The formed organic overlayers were bombarded with 4 keV Ar projectiles at normal incidence. The development of the collision cascade in the organic overlayer was investigated. The sputtering yield, mass, internal and kinetic energy distributions of ejected particles have been analyzed as a function of the thickness of the organic layer. The results show that all emission characteristics are sensitive to the variation of layer thickness. Although most of the ejected intact benzene molecules originate from the topmost layer, the emission of particles located initially in second and third layers is significant. The analysis indicates that the metallic substrate plays a dominant role in the ejection of intact organic molecules.