  Single-walled carbon nanotubes (SWNTs) are typically long (>100 nm) and have
been well established as novel quasi one-dimensional systems with interesting
electrical, mechanical, and optical properties. Here, quasi zero-dimensional
SWNTs with finite lengths down to the molecular scale (7.5 nm in average) were
obtained by length separation using a density gradient ultracentrifugation
method. Different sedimentation rates of nanotubes with different lengths in a
density gradient were taken advantage of to sort SWNTs according to length.
Optical experiments on the SWNT fractions revealed that the UV-vis-NIR
absorption and photoluminescence peaks of the ultrashort SWNTs blue-shift up to
30 meV compared to long nanotubes, owing to quantum confinement effects along
the length of ultrashort SWNTs. These nanotube capsules essentially correspond
to SWNT quantum dots.
