Double-walled carbon nanotubes (DWCNTs) are a special tube species intermediate between single-walled carbon nanotubes and multi-walled carbon nanotubes. They attract recently particular attention, since they exhibit several properties which are of scientific and technological relevance. DWCNTs can be grown either directly in a CVD process or from precursor routes. In the latter case either peapods as a starting material or a catalytic process inside the tubes can be used.
We report on detailed Raman, x-ray scattering, HRTEM, and photoemission studies performed for the precursor routes. For the peapod as a starting material an analysis with 13C labelled fullerenes the growth process is found to be local which means immediately on the cage. Stone-Wales transformations play a crucial role. They lead to an intermediate phase between the fullerenes and the final inner-shell tube. For the catalytic process ferrocene is used as a starting material. Eventually iron carbide is the active catalyst. Tube growth can proceed for temperatures as low as 600 K. There is evidence for charge transfer between the catalyst and the outer-shell tubes. The final DWCNT product is different for the two precursor routes even for equal starting material. Some characteristic differences to the CVD grown tubes will be elucidated.