A pure carbon quantum dot can be designed by introducing pentagon-hept
agon defects in a carbon nanotube. This system has completely confined
electronic states induced by the presence of topological defects. By
varying the distance between the pentagon-heptagon pairs, the energy d
ifference, spatial confinement, and number of discrete levels can be m
odified, thus opening a way for carbon-nanotube-based engineering of z
ero-dimensional systems.