  By sequential feeding of catalyst materials, it is revealed that the active
growth sites are at the bottom of the carbon nanotubes (CNTs), and that
catalyst particles are constantly encapsulated into nanotubes from the bottom.
This gives a better insight into the mechanism of CNT formation and on ways to
control the growth process. CNTs encapsulated with different materials should
enable the study of their electronic or magnetic properties, with potential
applications as building blocks for nanoelectronics and as fillers in
composites for electromagenetic shielding.
