Page 63 - Carbon Nanotubes

Carbon nanotubes with single-layer walls 53
P

Fig. 3a. Small-diameter tubes are often bent and curled.

4. THE METAL PARTICLES ical and are wrapped with layers of graphene sheet with
no gaps. The low-temperature phases, a-Fe and a-hcp
The encapsulated ferromagnetic particles produced Co, form abundantly, whereas the high-temperature
by this process may eventually be of technological in- phases, P-Fe and 0-fcc Co, comprise less than 10% of
terest, for example, in the field of magnetic storage the metal particle. In contrast, the metal particles found
media. Some work characterizing the magnetic prop- in the soot on the chamber wall contain mostly high-
erties of the encapsulated Co particles produced by temperature phases, such as Fe,C[2], fcc-C0[3,47], and
arc co-evaporation with carbon has been recently re- fcc-Ni[l6], and not all of the particles are wrapped in
ported[46]. The phase and composition of the metal- graphitic layers. These findings show that as the par-
containing particles may also provide information on ticles move away from the arc their temperature is
the growth conditions in the reactor. The temporal and rapidly quenched. The relatively fast time scale for re-
spatial profiles of temperature, metal and carbon den- action that this implies may be crucial for the growth
sities, and reaction rates all affect the growth of both of single-layer carbon nanotubes and, in particular, it
these particles and the nanotubes. The composition of may preclude the growth of additional layers of car-
the metal-containing particles in the soot deposited at bon on the single-layer tubules.
regions away from the electrode is not the same as for The presence of sulfur is found to enhance the for-
those found in the cathode deposit. For iron group mation of graphitic carbon shells around cobalt-
metals, pure metallic particle as well as cementite phase containing particles, so that cobalt or cobalt carbide
(Fe,C, Co3C, and Ni,C) exist in the outer surface of particles encapsulated in graphitic polyhedra are found
the cathode deposit[ 161. These particles appear spher- throughout the soot along with the single-layer nano-