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STEM and HREM analysis of vertically aligned carbon nanotubes

Structural observations: A typical VACNT grown at 900°C on a substrate covered with ~20 nm diameter Ni-Fe catalyst particles are several microns in length and have an external diameter of around 20 nm. The tip of one such nanotube, decorated with a Ni-Fe particle, is shown in Figure la. Electron diffraction pattern analysis shows that the bulk interior of the catalyst particle is an FCC single crystal that is usually oriented such that the <110> direction is parallel to the nanotube axis. The crown of the particle, which is exposed to the C2H2:NH3 reaction mixture is seen to consist of a 2.5 nm thick “skin” exhibiting weaker contrast than the interior of the particle. HREM observations of this layer (Figure lb), shows it to be semicrystalline. Lattice fringe periodicities of about 2 nm, which are close to those expected from the {111} planes of the Ni-Fe alloy, are commonly found in this crown region. Midway along the length of this catalyst particle, there is a definite transition region (Figure lc) where the capping layer contrast changes to that of 3.2 A spaced graphene planes. These cylindrical structures of graphene planes, which also have a total thickness of about 2.5 nm, subsequently become the walls of the extruded nanotube, which is created in the wake of the moving catalyst particle during growth. The nanotube itself is multiwalled, typically comprising of 5 to 8 graphene planes, and is quite disordered in nature. A characteristic of these VACNTs is that periodically along their length, bundles of graphene planes are imaged that can run perpendicular to the tube axis, forming the cross-strut of a bamboo-type structure (Figure Id). This structure, sometimes termed “stacked-cone” has been observed previously for microwave plasma enhanced CVD grown material (Okai et al. 2000). Again the strut thickness is about 6 graphene layers and occurs with a periodicity of about 60 to 80 nm along the axial length of the tube. A final point worthy of mention is that there is fuzzy amorphous speckle contrast at the rear of the nanoparticle (Figure la) suggesting that the tubes may not be entirely hollow.