In-Situ Sample Rotation as a Tool to Understand Chemical Vapor Deposition Growth of Long Aligned Carbon Nanotubes

A new tool for studying the process of carbon nanotube chemical vapor deposition (CVD) synthesis is described. By rotating the substrate in situ during the CVD process, the orientation of floating nanotubes with respect to the substrate is changed by interaction with the gas stream. Nanotubes lying on the surface of the substrate, however, will maintain their relative orientation. As a result different nanotube alignment angles are observed. By defining a time window through multiple rotation steps it is possible to study carbon nanotube behavior during CVD growth in a time-resolved manner. As an example, the settling process (i.e., the sinking of the nanotube to the substrate) is investigated. The analysis of forces acting on a floating nanotube shows that a vertical gas stream due to thermal buoyancy over the sample can keep long nanotubes floating for extended times. A stochastic process, indicated by a constant settling rate over time, forces the nanotube to make contact with the substrate, and this process is attributed to flow induced instability. Additional information on the floating and settling process are revealed from our study. The settling velocity could be inferred from curved nanotubes. The clearance between a floating nanotube and the substrate was found to be several hunded micrometers.