FT-ICR Reaction Experiments and Molecular Dynamics Simulations of Precursor Clusters for SWNTs

Abstract

The formation mechanism of single-walled carbon nanotubes (SWNTs) is studied with cluster beam experiments and molecular dynamics simulations. A FT-ICR mass spectrometer directly connected to the laser-vaporization cluster beam source was employed to study the metal-carbon binary clusters generated by the laser-vaporization of Ni/Co loaded carbon materials used for the laser-furnace production of SWNTs. Enhanced production of C ⁺₆₀ , C ⁺₇₀ and larger even-numbered pure carbon clusters in the size range up to 200 carbon atoms were observed for positive cluster ions. In clear contrast to the pure graphite, negative cluster ions up to about C ⁻₂₀₀ with even numbers of carbon atoms were detected. In addition, small signals of NiC ⁻_n , CoC ⁻_n and NiCoC ⁻_n are observed. The chemical reaction experiments of these clusters with NO strongly suggested that metal atoms were outside of the carbon cage. Eventually, some larger metal-carbon binary clusters with about 13 to 15 Co atoms were also observed. Complementary to cluster beam experiments, the growth process of metal-carbon clusters from completely random mixtures of the vapor phase was simulated by the classical molecular dynamics method. A Ni atom on the face of the random cage prohibited the complete closure and anneal of the cage structure. Collisions of such imperfect random-cage clusters lead to the elongated caged structure, which can be regarded as an imperfect SWNT.