Molecular Dynamics Simulation of Fullerene Cluster Ion Impact
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ABSTRACT In order to interpret the projection range and to reveal the mechanism of damage formation by cluster ion impact, molecular dynamics simulations of a fullerene carbon cluster (C60 ) impacting on diamond (001) surfaces were performed. When the kinetic energy of C60 is as low as 200eV/atom, C60 implants into the substrate deeper than a monomer ion with the same energy per atom because of the clearing-way effect. The kinetic energy of the cluster disperses isotropically because of the multiple-collision effect, and then a large hemispherical damage region is formed. When the energy of the cluster is as high as 2keV/atom, the cluster dissociates in the substrate, and then cascade damage is formed like in a case of a monomer ion impact. The projection range of incident atoms becomes similar to that of the monomer with the same energy per atom. However, the number of displacements of C60 is larger than the summation of 60 monomer carbons. The displacement yield of fullerene is 4 to 7 times higher than that of monomer carbon. This result agrees with the measurement of the displacements made on sapphire substrates with C60 and C2 irradiation. INTRODUCTION A cluster consists of tens to several thousands of atoms, and cluster ion irradiation shows different non-linear effects from that of monomer ion irradiation. Damage formation, high yield sputtering and lateral sputtering by cluster ion irradiation have been previously observed [1-41. When a cluster impacts on a solid surface with its high density of particles, the collision zone acquires higher energy and a larger number of collisions occur between the cluster and surface atoms compared to a monomer impact. Molecular dynamics (MD) simulation is one of the effective methods which is widely used for studying the impact processes of various cluster ions on solid surfaces. As in a former study [5], we have performed the MD simulation of Ar cluster impacting on a Si (001) surface, and have explained a typical cluster impact process. Multiple collisions between cluster and surface atoms transform the kinetic energy of the cluster to the target's surface layers isotropically. Following the impact, crater-like damage is formed and some atoms are sputtered in a direction lateral to the surface. Ar gas clusters obtained in our experiment range from 100 to 3000 atoms, and have a broad size distribution. It is desired to use much smaller and well-selected cluster ion beams to do more precise analysis of the irradiation effect of cluster ion. 81 Mat. Res. Soc. Symp. Proc. Vol. 504 ©1998 Materials Research Society
Fullerene (C6,), which is a stable large carbon molecule, enables us to obtain a wellselected cluster ion beam, and is useful to investigate the non-linear irradiation effect of cluster ions by experimental methods [6-8]. In this work we have performed MD simulation of C60 impacting on a diamond (001) surface and investigated the range of incident energy in which C60 shows nonlinear properties in the penetration depth and in stopping power [9]. It has also been su
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