Substrate preparation techniques for direct investigation by TEM of single wall carbon nanotubes grown by chemical vapor deposition

We have investigated and evaluated different TEM sample preparation techniques for studying carbon single-walled nanotube (C-SWNT) nucleation and growth, issued from CVD processes when the catalyst is supported on a substrate. This kind of study requires means to observe individual and isolated tubes. It implies using synthesis conditions able to produce only a low density of tubes and to thin the substrate to electron transparency, to observe the nanotubes and the catalytic particles from which they have grown in their native state. We have tested two approaches, depending if the substrate is thinned after or before the synthesis. The low tube density requirement led us to exclude all the techniques where the substrate is thinned to electron transparency after the synthesis. We have shown, that, with this last approach, all TEM preparation techniques dramatically suffer from a lack of control of thin areas with respect to the location of the tubes, which is unknown. However we have demonstrated that the suitable approach is to perform synthesis directly on transparent substrates presenting several holes. We have tested the capabilities and the potentialities of these supports for studying the size distribution and composition of the catalytic particles, the nucleation mode, the diameter and helicity of the tubes. These results are very promising and represent an important step for performing specific nanoscale TEM analyses necessary for the study of the growth mechanism of nanotubes on substrates.