  By employing the first-principles calculations, we investigate electronic
properties of a novel carbon nanostructure called a carbon nanobud, in which a
$C_{60}$ molecule covalently attaches or embeds in an armchair carbon nanotube.
We find that the carbon nanobud exhibits either semiconducting or metallic
behavior, depending on the size of the nanotube, as well as the combination
mode. Moreover, with respect to the case of the corresponding pristine
nanotubes, some new electronic states appear at 0.3-0.8 eV above the Fermi
level for the carbon nanobuds with the attaching mode, which agrees well with
the experimental reports. In addition, the vibrational properties of the carbon
nanobuds are explored. The characteristic Raman active modes for both $C_{60}$
and the corresponding pristine nanotube present in Raman spectra of the carbon
nanobuds with attaching modes, consistent with the observations of a recent
experiment. In contrast, such situation does not appear for the case of the
carbon nanobud with the embedding mode. This indicates that the synthesized
carbon nanobuds are probably of the attaching configuration rather than the
embedding configuration.
