Scanning Electron Microscopy

Scanning electron microscopy is to a certain extent a limited tool to characterize SWNTs, regardless of the high magnifications that can be achieved with SEM instruments. The main problem with the application of SEM to SWNT characterization analysis is that it cannot differentiate between SWNTs and MWNTs. This is mostly due to the tendency of SWNTs to adhere strongly to each other, forming bundles or ropes of 5-20 nm in diameter. In contrast to TEM, SEM cannot resolve the internal structure of these SWNT bundles.

Nevertheless, SEM can yield valuable information regarding the purity of a sample as well as an insight on the degree of aggregation of raw and purified SWNT materials. For instance Figure 4 illustrates how SEM can be used to answer substantive questions about the quality of SWNT samples [45]. Figure 4a shows a SEM micrograph of raw SWNT material produced by laser ablation. The presence of other forms of carbon besides SWNT and the occurrence of a large amount of catalysts particles is apparent. After a gas phase purification step using a gas mixture of H2 and Cl2, Figure 4b shows an improvement in the degree of dispersion as well as in purity since the image predominately shows nanotube ropes. Finally the bright metal particles observed in Figure 4b (cobalt and nickel catalyst), visible after the first gas treatment, can be removed by a subsequent liquid phase treatment in HCl. The final product obtained after this sequential purification process appears to be high-quality single-wall nanotube material.

Figure 4. (a) SEM micrograph of raw SWNT material obtained by laser ablation. (b) The same material in (a) after a gas phase purification treatment and (c) the same material in (b) after a liquid phase purification treatment in HCl. Reprinted with permission from [45], J. L. Zimmerman et al., Chem. Mater. 12, 1361 (2000). © 2000, American Chemical Society.

Figure 4. (a) SEM micrograph of raw SWNT material obtained by laser ablation. (b) The same material in (a) after a gas phase purification treatment and (c) the same material in (b) after a liquid phase purification treatment in HCl. Reprinted with permission from [45], J. L. Zimmerman et al., Chem. Mater. 12, 1361 (2000). © 2000, American Chemical Society.

0 0

Post a comment