Page 150 - Carbon Nanotubes

140 P. C. EKLUND ef at.
because they were not present after boiling the thread-
like material in toluene. The overall strength and rel-
ative intensities of the sharp peaks at 1566, 1592,2681,
and 3180 cm-' remained the same, implying that
these features were not related to fullerenes or other
toluene soluble impurities, such as polyaromatic hy-
drocarbons. The significant strength of the 1592 cm-'
line suggests that a resonant Raman scattering process
may be involved[27]. Importantly, l0(w) shows no
evidence for any of the sharp first- or second-order
features and is very similar to that of Fig. 2d (disor-
dered carbon black). Noting that these disordered sp2
carbons likely contribute to both I, (a) and Ic0 (a),
Holden et al. [27] compute the "difference spectrum";
&iff( w) = Ic0 ( W) - do (a), which is shown in Fig. lob.
Fig. 9. Schematic view of cylindrical shear modes for a
nested tubule: telescope mode (wT) and rotary mode (aR). &iff( W) was constructed to emphasize contributions
from new carbonaceous rnaterial(s) (e.g., carbon nano-
tubes), which form only when Co is present in the
carbonaceous materials and were assigned to single- plasma. This difference spectrum has a fairly flat base-
wall carbon nanotubes. A representative spectrum, line with sharp first-order lines at 1566 and 1592 m-'.
Zco(w), is shown in Fig. loa for Co-cafalyzed, arc- The inset shows a Lorentzian lineshape fit to the first-
derived carbons (solid line) over the frequency range order spectrum. Sharp second-order features at 2681
300-3300 cm-'. This sample also contained a large and 3180 cm-' are also observed.
fraction of other sp2 carbonaceous material, so a Hiura et aZ.[23] observed two Raman lines in
subtraction scheme was devised to remove the spec- their spectrum of nested carbon nanotubes at 1574
tral contributions from these carbons. The dashed line (FWHM = 23 cm-') and at 2687 cm-'. It is interest-
in the figure represents the spectrum lo(a) obtained ing to note that their first-order peak at 1574 cm-'
from thread-like carbon removed from the chamber lies between, and is more than twice as broad, as ei-
when cobalt was notpresent in the carbon anode. All ther of the two first-order lines in laiff(u) identi-
other sample preparation conditions were identical to fied[27] with single-wall nanotubes. These two
those used to prepare the Co-catalyzed carbons. lo(w) observations may be consistent if an inhomogeneous
was scaled by a factor 01 = 0.85 to superimpose with broadening mechanism, originating from a distribu-
Ico(o) in the region near 1590 cm-'. tion of tubule diameters and chiralities is active. Also,
Prominent in both first-order Raman spectra Fig. 10a the second-order feature of Hiura et al. [23] at 2687
is the broad D-band centered at 1341 cm-'. Two cm-' is slightly broader than, and upshifted from,
second-order features, one at 2681 cm-' = 2(1341 the second-order feature at 2681 cm-' in I&ff(w). It
cm-') and 3180 the other at cm-' = 2(1592 cm-') should also be noted that the second-order features in
are apparent in the Co-catalyzed carbon. Weak fea- &(w) are downshifted significantly relative to other
tures near 1460 cm-' were identified with fullerenes sp2 carbons (see Table I).

h ..........
Y Co absent
'S 3-
d - Copresent
s
Y 2 2t
.i
.@ 2-
2
Y
........................ >..L
0 -
I
I I I I I I C1 I I I
500 1000 1500 2000 2500 3000 500 1000 1500 2000 2500 3000
Raman Shift (cm-') Raman Shift (cm-')
Fig. 10. (a) Raman spectra (T = 300 K) of arc-derived carbons from a dc arc: cobalt was absent (dotted
line) and cobalt was present (solid line) in the carbon anode, (b) the difference spectrum calculated from
(a), emphasizing the contribution from Co-catalyzed nanotubes, the inset to (b) depicts a Lorentzian fit
to the first-order spectrum (after ref. 1271).