Page 147 - Carbon Nanotubes

Vibrational modes of carbon nanotubes 137
I I 1 I I I I I ,
(32.12) IRI I 31.0
(28,16) 11111 I 30.4
n
(24,9) 111 I I 23.3
I
II I ni 22.8 ~
0
Y
II I I II Ill1 15.5
II I I U II I 15.2 -2
n
I I1 I I II I111 1.12
/Ill I I1 11111 1.55
I I I I I L - L
0 400 800 1200 1600
Frequency (cm-l)
(a)

I I I I I I I I
I I1 I 31.0
111 I 30.4
23.3 0s
n
IR I n
I II I I 22.8
h
Q)
u
I I1 II 15.5 i
I II II 15.2 .z
n
I II II I II 7.72
I I1 II 111 1.55

I I I I I I I I
400 800 1200 1600
Frequency (cm-I)
(b)
Fig. 4. Diameter dependence of the first order (a) IR-active and, (b) Raman-active mode frequencies for
“chiral” nanotubes.

4.1 Synthesis and purification as discussed in section 2. It is very desirable, therefore,
Nested carbon nanotubes, consisting of closed con- to remove as much of these impurity carbon phases as
centric, cylindrical tubes were first observed by Iijima possible. Successful purification schemes that exploit
by TEM[37]. Later TEM studies[38] showed that the the greater oxidation resistance of carbon nanotubes
tubule ends were capped by the inclusion of pentagons have been investigated [40-421. Thermogravimetric
and that the tube walls were separated by -3.4 A. A analyses reveal weight loss rate maxima at 420”C,
dc carbon-arc discharge technique for large-scale syn- 585°C) and 645°C associated with oxidation (in air)
thesis of nested nanotubes was subsequently reported of fullerenes, amorphous carbon soot, and graphite,
[39]. In this technique, a dc arc is struck between two respectively, to form volatile CO and/or COz. Nano-
graphite electrodes under an inert helium atmosphere, tubes and onion-like nanoparticles were found to lose
as is done in fullerene generation. Carbon vaporized weight rapidly at higher temperatures around 695°C.
from the anode condenses on the cathode to form a Evidently, the concentration of these other forms of
hard, glassy outer core of fused carbon and a soft, carbon can be lowered by oxidation. However, the
black inner core containing a high concentration of abundant carbon nanoparticles, which are expected to
nanotubes and nanoparticles. Each nanotube typically have a Raman spectrum similar to that shown in Figs. Id
contains between 10 and 100 concentric tubes that are or IC are more difficult to remove in this way. Never-
grouped in “microbundles” oriented axially within the theless, Ebbesen et al. [43] found that, by heating core
core[l4]. material to 700°C in air until more than 99% of the
These nested nanotubes may be harvested from the starting material had been removed by oxidation, the
core by grinding and sonication; nevertheless, substan- remaining material consisted solely of open-ended,
tial fractions of other types of carbon remain, all of nested nanotubes. The oxidation was found to initiate
which are capable of producing strong Raman bands at the reactive end caps and progress toward the cen-