Catalysts to Control the Growth and Structure of Single-Wall Carbon Nanotubes

In previous reports, we compared the catalytic efficiencies of various metals in producing single wall nanotubes (SWNT) by arc discharge and Nd:YAG laser ablation, and discussed what causes the different catalytic activities. We suggested that the crystallographic orientation of catalysts might influence the growth of SWNTs and mentioned that it might also control the chirality of SWNTs. It is known that a metal film deposited on a surface with a defined crystallographic face takes a unique orientation. We also reported that Ni deposited on different faces of sapphires took different orientations and had different graphitization effects on carbon. These results suggest that SWNT growth on catalysts whose crystallographic orientation is controlled by a well defined surface would enable us to study how the catalysts influence the structures of SWNTs. In our first trial of this, we grew SWNTs on Fe-coated sapphire by chemical vapor deposition and found that the yield and diameter distribution depended on the crystallographic face of the sapphire. Further investigation indicated that strong Fe-sapphire interaction resulted in Fe-Al alloy formation, and the sapphire-surface damages were considerably increased by carbon. Degree of these structure changes depended on the crystallographic face and surface states of sapphires. It is likely that these structure changes also occur when another metals and substrate materials were used for SWNT growth. Studies on the carbon-metal-substrate interaction would enable to prepare the SWNT-growth catalysts suitable to grow SWNTs with well-defined structures.