Page 164 - Carbon Nanotubes

Nanoparticles and filled nanocapsules 155

Fig. 2. A model of growth processes for (a) a hollow nanoparticle and, (b) a nanotube; curved lines depicted
around the tube tip show schematically equal potential surfaces.

their planes parallel to the external layer. The flat whole particle is greater for the stuffed nanocapsules
planes of the particle consist of nets of six-member than that for hollow nanoparticles. While the inner
rings, while five-member rings may be located at the space within a hollow nanoparticle is only - 1070 of the
corners of the polyhedra. The closed structure contain- whole volume of the particle, that for a filled nano-
ing pentagonal rings diminishes dangling bonds and capsule is 10 to 80% of the whole volume.
lowers the total energy of a particle. Because the density The lanthanides (from La to Lu) and yttrium form
of highly graphitized carbon (= 2.2 g/cm3) is higher isomorphous dicarbides with a structure of the CaCz
than that of amorphous carbon (1.3-1.5 g/cm3), a type (body-centered tetragonal). These lanthanide
pore will be left inevitably in the center of a particle carbides are known to have conduction electrons (one
after graphitization. In fact, the corresponding cavi-
ties are observed in the centers of nanoparticles.

4. FILLED NANOCAPSULES

4.1 Rare earths
4.1.1 Structure and morphology. Most of the
rare-earth elements were encapsulated in multilayered
graphitic cages, being in the form of single-domain
carbides. The carbides encapsulated were in the phase
of RC2 (R stands for rare-earth elements) except for
Sc, for which Sc3C,[2O] was encapsulated[21].
A high-resolution TEM image of a nanocapsule en-
caging a single-domain YC2 crystallite is shown in
Fig. 3. In the outer shell, (002) fringes of graphitic lay-
ers with 0.34 nm spacing are observed and, in the core
crystallite, lattice fringes with 0.307-nm spacing due
to (002) planes of YC2 are observed. The YC2 nano-
crystal partially fills the inner space of the nanocap-
sule, leaving a cavity inside. No intermediate phase
was observed between the core crystallite and the gra-
phitic shell. The external shapes of nanocapsules were
polyhedral, like the nanoparticles discussed above,
while the volume ratio of the inner space (including Fig. 3. TEM image of a YC, crystallite encapsulated in a
the volume of a core crystallite and a cavity) to the nanocapsule.