Formation Process of Empty and Metal-containing Fullerene Molecular Dynamics and Ft-icr Studies

The formation mechanism of empty and metal-containing fullerene was studied through MD (molecular dynamics) simulations and FT-ICR (Fourier transform ion cyclotron resonance) mass spectroscopy of laser vaporized carbon cluster. Multi-body classical potential functions for metal-carbon and metal-metal interactions were constructed based on DFT (density functional theory) calculations of various forms of small clusters MCn and Mn (M = La, Sc, Ni). Using the modified Brenner potential for carbon-carbon interaction, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in gas phase was simulated under the controlled temperature condition at 3000K. The difference of clustering process of La@Cn, Sc@Cn and NiCn were compared with empty fullerene formation simulation. FT-ICR mass spectrometer directly connected to the laser vaporization cluster beam source was implemented in order to experimentally study the clustering process. The increase of cluster nozzle pressure roughly corresponded to the later stage of the molecular dynamics simulation. The FT-ICR mass spectra of metal-carbon composite clusters were compared for various sample * To whom correspondence should be addressed materials used for arc-discharge generation of metal-containing fullerene and SWNT (single-wall carbon nanotube); La, Y, Sc, Gd, Ce, Ca, and Ni-Y. Positive La-C, Y-C, Sc-C, Gd-C, Ce-C binary clusters commonly showed strong MC2n + signal in the range of 36 < 2n with intense magic numbers at MC44 , MC50 + and MC60 . It was speculated that the even-numbered clusters corresponded to the annealed random caged clusters observed in the MD simulation. INTRODUCTION After the discovery of C60 by Kroto et al. , macroscopic amount of empty fullerene, metallofullerenes, higher fullerenes and carbon nanotubes were successively produced and isolated. Recently, the high quality generation of SWNTs has demonstrated new possibilities of applications of this material. Even though the generation of fullerene is now possible by the arc-discharge technique or the laser-oven technique, the formation mechanism of such spherical molecules is not clear. We have performed MD simulations of the clustering process of carbon atoms to investigate the fullerene formation mechanism, and the temperature dependence of the cluster structures was observed. In addition, we have demonstrated the formation of perfect C60 structure by giving sufficient collisionfree annealing time, and examined the time and temperature scale of the annealing process through the reaction rate of the network transformations. Based on these results, a new formation model of empty fullerene including the temperature effect was proposed. In spite of possible interesting applications of metallofullerene, it is still difficult to obtain macroscopic amount of sample because of extremely low yield of generation. In order to find the optimum generation condition, it is inevitable to understand the formation mechanism. According to experimental studies, the metal atoms such as La, Y, Sc, Ca and lanthanide can be enclosed inside the carbon cage, and the preferred carbon cage structure depends on the metal. On the other hand, Ni, Co, or Fe, which are not experimentally assigned to be encapsulated in the fullerene cage, are required to generate the SWNTs. Here, the role of these metal atoms on the carbon cluster growth process is still unknown. In this paper, the formation process of metallofullerene is studied by using the MD simulations and FT-ICR mass spectrometry of metal-carbon binary clusters generated by the laser-vaporization supersonic-expansion cluster beam source. MOLECULAR DYNAMICS SIMULATION The principal technique of the classical MD simulation is the same as our previous simulations of pure carbon system using the modified Brenner potential. In addition, we have constructed the potential function between metalcarbon and metal-metal potential function, based on the calculations of the binding energy and charge state of various forms of small clusters MCn and Mn (M: La, Sc, Ni). Here the density functional theory (DFT) based on the Becke’s three-parameter exchange functional with the Lee-Yang-Parr correlation (B3LYP) was applied with the effective core potentials derived from the LANL2DZ basis from Gaussian 94. The multi-body potentials between metal-carbon potential functions were constructed as functions of carbon coordinate number of a metal atom. The total potential energy was expressed as the sum of binding energy Eb as follows. C A R b V V V E + + = (1) { } ) ( 2 exp 1 ) ( e ij e ij R R r S S D r f V − − − = β (2) { } ) ( / 2 exp 1 ) ( * e ij e ij ij A R r S S S D B r f V − − − ⋅ − = β (3) ij ij C r c c e r f V M C