A Study of Precursor Clusters of Single-walled Carbon Nanotubes (swnts) by Ft-icr Mass Spectrometer

Single-walled carbon nanotubes (SWNTs) [1] is now recognized as the most fascinating material of so-called nano-technology in this century. Despite the expectation of future applications, it is still difficult to obtain large amount of pure SWNTs, because the generation process is still not clear. It is well known that transition metals such as La, Y and Sc can be encapsulated inside the fullerene cage. On the other hand, transition metals such as Ni, Co or Fe are required to generate SWNTs. And the diameter distribution of SWNTs depends on the catalyst metal. However, the role of metal atom in the growth process of SWNTs is not yet known. In this paper, by using FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometer [2], we studied laser-vaporized clusters from Ni/Co, Ni/Y and Rh/Pd (these metals were typical catalyst metals for SWNTs) loaded graphite samples. The FT-ICR mass spectrometer used in this study was based on the design in Smalley’s group at Rice University [2]. Metal/carbon binary clusters were produced by laser vaporization in cluster beam source. The cluster beam was directly injected to the magnetic field through a skimmer with the opening diameter of 2 mm and a deceleration tube. After deceleration, cluster ions were trapped by electric field in the ICR cell. And cluster ions were excited for the detection of the mass distribution. Depending on metal species, generated cluster distributions were drastically different. In case of a Ni/Co doped carbon material, the peak of mass distribution appeared at relatively high mass range such as equivalent to C150. On the other hand, for a Rh/Pd doped carbon material, the observed mass distribution of clusters were limited only up to C70. This result can be clearly compared with the facts that SWNTs from Ni/Co doped sample has larger diameter such as 1.2 nm, in contrast to about 0.8 nm from Rh/Pd doped sample. Diameters of cage structured C150 and C70 are roughly 1.1 nm and 0.8 nm, respectively. Hence, it is speculated that the size of precursor clusters may determine the diameter of SWNTs. We propose that metal atoms affect the growth of clusters, and as a result, the final SWNTs diameter distribution is determined by the size of these precursor clusters. 20 40 60 80 100 120 140 20 40 60 80 100 120 140 Number of Carbon Atoms In te ns ity (a rb itr ar y) (b) Ni/Co/C(0.6%/0.6%) (c) Rh/Pd/C(1.0%/1.0%) (a)Pure C Number of Carbon Atoms Figure A-1 FT-ICR mass spectra of as injected negative clusters from pure graphite and metal-doped graphite samples.