  We have studied nickel, gold, and ferritin coatings on catalytically grown
multiwall carbon nanotubes, as well as the generation of secondary nanotubes by
resubmitting the decorated nanotubes to the chemical vapor deposition process.
Nickel layers sputtered on nanotubes show a stronger interaction with the
nanotube walls than gold coatings. At ambient temperature this results in a
metal film that is more homogeneous for Ni than for Au. Surface mass transport
at elevated temperatures leads to a transformation of the coating to nanoscale
clusters on the nanotube surface. The resulting Au clusters are sphere-like
with a very small contact area with the nanotube whereas the Ni clusters are
stretched along the tube axis and have a large contact area. Secondary
nanotubes were established by growing nanotubes directly on the walls of
primary nanotubes. Thin Ni layers or ferritin served as catalyst. We compared
the field emission properties of samples with and without secondary nanotubes.
The presence of secondary nanotubes enhances the field emission substantially.
