Carbon nanoclusters, produced from an arc-discharge between graphite electrodes under helium atmosphere , show a rich variety of shapes that reflect a strict discipline of the growth and may ultimately delineate the growth mechanism (Seraphin et al., 1994a). High-Resolution Transmission Electron Microscopy (HRTEM) imaging gave valuable structural information in a two-dimensional projection of the interior of the carbon nanotube, and Scanning Tunneling Microscopy (STM) revealed its surface structure. Scanning Electron Microscopy (SEM) outlined the tube distribution in the cathodic slag, without giving details of the morphology and growth phenomenon due to resolution limits of the conventional technique. We report here on Field-Emission SEM (FESEM) characterization of nanoclustersstraight multiwalled tubes as well as strings of spherical beadsthat due to its higher resolution provides fine details of the growth phenomena in addition to the three-dimensional depth of the image, and the benefits of a much simpler sample preparation for the electron microscopy analyses.
The carbon nanotube samples were produced by an arc-discharge apparatus, in which pure graphite rods were employed as the electrodes. Ni-graphite composite anode was used to produce the carbon nanobeads (Seraphin et al., 1994 b). The arc-discharge was operated at a current density of 220 A/cm2 and a 27 V potential difference between the electrodes under a helium atmosphere of 500 tom. The cathodic slag was removed from the surface of the cathode, and cleaved apart for SEM analysis. For TEM analysis, the sample was prepared by grinding the slag into powder, sonicating the powder in acetone, and dropping the suspension on a holey carbon copper grid. The characterization of the sample was carried out by Hitachi H-8100 TEM and Hitachi S-4500 FESEM.
A two-dimensional HRTEM projection of the internal multi-walled structure of a nanotube, shown in Fig. , reveals that the outer diameter of the tube is reduced during growth. In contrast to the TEM image, a FESEM image of a similar structure with a thicker outer diameter shows that growth steps reducing the tube diameter as well as polygonal surface structures are clearly resolved (Fig. ). The SEM image shows contrast differences in thicker parts overlapping the thinner ones. Fig. l(a) A two-dimensional HRTEM projection of the internal multi-walled structure of a nanotube which reduces its diameter during growth. Fig. I(b) A FESEM image of a similar structure. Note the contrast change (arrows) along the thinner tube due to the secondary electron emission behind it, as well as the polygonal cross section of the tube.