Paper NS+NC-WeM3
Quantitative Evaluation of Carbon Nano Tubes by the Scanning Atom Probe

Wednesday, October 22, 2008, 8:40 am, Room 311

Quality of carbon nanotubes (CNT) strongly depends on the amount of impurities such as hydrogen, oxygen and others. However, few efforts have been paid for the quantitative evaluation of them. In order to evaluate the CNT at the atomic level point of view we mass analyzed various CNTs such as single walled CNT (SWCNT), double walled CNT (DWCNT) and multi walled CNT (MWCNT) utilizing the unique capability of the scanning atom probe (SAP).¹ In this mass analysis carbon atoms of the CNTs are field evaporated as positive ions of single atoms and clusters of few atoms. The mass to charge ratios of these ions are obtained by measuring their flight times from a specimen surface to an ion detector. One of the analyzed SWCNTs is grown by the high CO pressure process (Hipco) and other is synthesized by a direct-current arc-discharge method in He gas with Fe/Ni/S catalysis. The DWCNTs are commercially supplied. The MWCNTs are directly formed on a substrate of Ni-Cr-Fe alloy by thermal CVD at 650° using a mixture of acetylene, hydrogen and argon gases. CNTs are deposited on a W tip by silver paste. The amount of impurities is evaluated by counting the number of cluster ions forming mass peaks. For example, the numbers of C and H atoms forming the mass peak of 150 CH₃ clusters ions are 150 and 450, respectively. The number of C, H, O and other atoms are counted for all major mass peaks. The numbers of H, O and Na atoms in a Hipco SWCNT 3.7 %, 2.6 % and 0.1 % of the total number of detected atoms, respectively. Similarly, the amounts of the impurities in the CVD SWCNTs are 9 % of H, 0.7 % of O and 0.19 % of Na. The commercial DWCNTs contain fairly large amount of impurities with 20 % of H and 1.9 % of O. The MWCNT also contains a significant amount of impurities, 27 % of H, 4 % of O and 0.4 % of Na. Even if specimens are prepared by the CNT formed by an identical preparation process, most specimens show a different mass spectrum with different amount of impurities. However, many Hipco SWCNT specimens exhibit a similar mass spectrum. The binding between C atoms forming CNTs also evaluated. The mass spectrum with a large number of C²⁺ and C⁺ indicates that the C-C bonds of the CNT is strong and uniform. The relation between the binding state and the impurity content of the analyzed CNT will be discussed.

¹ O. Nishikawa, Y. Ohtani, K. Maeda, M. Watanabe and K. Tanaka: Mater. Char., 44, 29 (2000).