NANO LETTERS
Stability of Carbon Nanoonion C20@C60@C240: Molecular Dynamics Simulations
2002 Vol. 2, No. 3 215-217
S¸ akir Erkoc¸ * Department of Physics, Middle East Technical UniVersity, 06531 Ankara, Turkey Received October 25, 2001; Revised Manuscript Received December 12, 2001
ABSTRACT The structural stability of carbon nanoonion C20@C60@C240 has been investigated by performing molecular dynamics computer simulations. Calculations have been realized by using an empirical many-body potential energy function for carbon. It has been found that carbon nanoonion is not so resistive against heat treatment, nor is it as strong as isolated single carbon nanoballs. Although single nanoballs resist heat treatment up to 4300 K, nanoonion disintegrates after 2600 K.
1. Introduction. Graphite is the most stable form of carbon at ambient conditions.1 The concentric arrangement of spherical fullerenes possesses extreme robustness, which led to the hypothesis that the quasi-spherical onion-like graphitic particles are the most stable form of carbon particles.2 A great deal of interest has accumulated on this contradiction between planar configuration of macroscopic graphite with sp2 type hybridization and the apparent spherical shape in a nanometric system.3 Calculations of the structure of giant fullerenes are not able to give a clear-cut answer.4-7 Thus, there is an unresolved question of minimal energy configuration of carbon clusters (onion-like particle: spherical or polyhedral) and the size where the transition is between these closed surface particles and the macroscopic planar graphite. The onion-like graphite particles may display a wide range of structures, going from polyhedral to nearly spherical. 1 The electric arc is the easiest and most frequently used technique to produce onion-like particles.1 However, the formation mechanism of fullerenes and related structures is not well understood. In the case of carbon melting experiments8 and electric arc,9,10 it has been suggested that the onion-like particles are generated by the graphitization of a liquid carbon drop. The growth of graphitic layers is supposed to begin at the surface and progress toward the center. A similar mechanism has been suggested by Saito et al.11 They consider a certain carbon volume on the electrode surface, which possesses a high degree of structural fluidity due to the He ion bombardment. In the present theoretical study, heat energy is pumped into a C20@C60@C240 composite system in order to get some information that may shed some light onto the formation of
fullerenes. Note that the understanding of the formation mechanism and energetics involved would allow the development of efficient production methods. In this work the structural stability of carbon nanoonion C20@C60@C240 has been investigated by performing molecular dynamics (MD) simulations using an empirical manybody potential energy function (PEF). The smallest possible cage structure of carbon, C20, the most stable cage structure of carbon at room temperature, C60, and relatively larger ball structure, C240, have been considered to form the onion-like structure C20@C60@C240. Particular attention has been paid to the stability of carbon nanoonion against heat treatment. 2. The PEF and the MD Simulations. The empirical many-body potential energy function developed for carbon12 is used in the calculations. This PEF accurately describes many properties of diamond crystal as well as the properties of the individual basal planes of graphite.13 The total interaction energy of a system of particles is taken to be the sum of total two-body and total three-body contributions Φ ) φ2 + φ 3
Total two-body and three-body energies are expressed, respectively, as N
φ2 ) A
U(1) ∑ ij i
