Paper NS+MS-TuA10
Study of Characteristic Fragmentation of Nano Carbon by the Scanning Atom Probe

Tuesday, October 16, 2007, 4:40 pm, Room 616

Since the characteristic fragmentation of a material is closely related with the binding state between the atoms forming the material, multiwall carbon nanotubes (MWCNT), graphite nanofibers (GNF) and ultrapure graphite are studied by field evaporating these specimens and by mass analyzing the fragmented cluster ions with the scanning atom probe. Two kinds of MWCNT were analyzed: commercially available and laboratory fabricated MWCNTs. GNF is grown on a 304 stainless steel tip by thermal CVD. Purity of the analyzed graphite is 99.9999%. The mass spectra of both MWCNTs are quite similar, particularly mass to charge ratio M/n from 0 to 100. However, the commercial MWCNT exhibits many unidentifiable small mass peaks throughout the mass range up to a few thousands. The most significant feature is the large mass peak at M/n = 340 which could be C₂₈H₄. The proposed structure of this cluster is a squarely arranged 8 hexagonal rings. This structure is suitable to form a tube. The mass spectrum of GNF is quite different from that of MWCNT and shows the highest mass peak at M/n = 278, C₂₃H₂. The proposed structure of this cluster is the triangularly arranged six hexagonal cells. Two corner carbon atoms of the triangle are hydrogen-terminated and third corner carbon atom is bound with an extra carbon atom. Two dimensional extension of the fragments shows a hexagonal ring formed by 6 hydrogen atoms terminating the carbon bonds and 6 extra carbon atoms forming a hexagon. The hydrogen hexagonal ring is quite similar to that of a kekulene molecule. The graphite exhibits two completely different mass spectra. One is quite similar to that of MWCNT showing the characteristic large mass peaks. The other closely resembles those of silicon and diamond. The number of detected ions decreases with mass. Thus, the largest mass peak is C²⁺ and then C⁺. The clusters formed by the odd number of carbon atoms are more abundant than those of even number. Most clusters are doubly charged. This implies that the binding between carbon atoms in this analyzed section is strong and uniform and that the graphite has two phases: diamond and graphite. Although only few H⁺ ions are detected from MWCNTs and GNF, most fragments contain more than 1 hydrogen atom. On the other hand few ions detected from the diamond-like graphite are bound with hydrogen.