Page 149 - Carbon Nanotubes

Vibrational modes of carbon nanotubes 139
cm-I. The feature at 1574 cm-' is strongly down-
shifted relative to the 1582 cm-' mode observed in
HOPG, possibly a result of curvature and closure of
the tube wall. These authors also observe reasonably
sharp second order Raman bands at 2687 cm-' and
2455 cm-',
Other Raman studies of cathode core material
grown by the same method, and also shown by TEM
to contain nested nanotubes as well as carbon nano-
particles, have reported slightly different results
(Figs. 7, 8). Chandrabhas et al. [24] report a first-order
Raman spectrum, Fig. 7, curve (b), for the cathode
core material similar to that of polycrystalline graph-
ite, Fig. 7, curve (a), with a strong, disorder-broadened 0
Ln
N
band at 1583 cm-', and a weaker, D-band at 1353 m
cm-' . For comparison, the Raman spectrum for the
outer shell material from the cathode, Fig. 7, curve (c),
is also shown. The spectrum for the outer shell exhibits
the character of a disordered sp2 carbon (Le., carbon Raman Shift (ern-')
black or glassy carbon, c.f. Figs. 2d and 2e). Addi-
tionally, weak Raman features were observed at very Fig. 8. Second-order Raman spectra of (a) graphite, (b) in-
low frequencies, 49 cm-' and 58 cm-', which are up- ner core material containing nested nanotubes, (c) outer shell
shifted, respectively, by 7 cm-' and 16 cm-' from of cathode (after ref. [24]).
the E;:) shear mode observed in graphite at 42 cm-'
(Fig. Id). The authors attributed this upshifting to
defects in the tubule walls, such as inclusion of pent- Kastner et al. [25] also reported Raman spectra of
agons and heptagons. However, two shear modes are cathode core material containing nested tubules. The
consistent with the cylindrical symmetry, as the pla- spectral features were all identified with tubules, in-
nar E;:) shear modes should split into a rotary and a cluding weak D-band scattering for which the laser ex-
telescope mode, as shown schematically in Fig. 9. The citation frequency dependence was studied. The
second-order Raman spectrum of Chandrabhas et al., authors attribute some of the D-band scattering to cur-
Fig. 8, curve (b), shows a strong line at 2709 cm-' vature in the tube walls. As discussed above, Bacsa
downshifted and narrower than it's counterpart in et al. [26] reported recently the results of Raman stud-
polycrystalline graphite at 2716 cm-', Fig. 8, curve ies on oxidatively purified tubes. Their spectrum is
(a). Thus, although the first-order mode (1583 cm-I) similar to that of Hiura et al. [23], in that it shows very
in the core material is broader than in graphite, indi- weak D-band scattering. Values for the frequencies of
cating some disorder in the tubule wall, the 2709 all the first- and second-order Raman features re-
cm-' feature is actually narrower than its graphitic ported for these nested tubule studies are also collected
counterpart, suggesting a reduction in the phonon dis- in Table 1.
persion in tubules relative to that in graphite.
4.3 Small diameter single-wall nanotubes
Recently, Bethune et al. [22] reported that single-
wall carbon nanotubes with diameters approaching the
diameter of a C6,, fullerene (7 A) are produced when
cobalt is added to the dc arc plasma, as observed in
TEM. Concurrently, Iijima et al. [21] described a sim-
ilar route incorporating iron, methane, and argon in
the dc arc plasma. These single-wall tubule samples
provided the prospect of observing experimentally the
many intriguing properties predicted theoretically for
small-diameter carbon nanotubes.
Holden et al. [27] reported the first Raman results
on nanotubes produced from a Co-catalyzed carbon
arc. Thread-like material removed from the chamber
was encapsulated in a Pyrex ampoule in -500 Torr of
__ . . He gas for Raman scattering measurements. Sharp
Raman Shift (cm-') first-order lines were observed at 1566 and 1592 cm-'
and second-order lines at 2681 and 3180 cm-', but
Fig. 7. First-order Raman spectra of (a) graphite, (b) inner
core material containing nested nanotubes, (c) outer shell of only when cobalt waspresent in the core of the anode.
carbonaceous cathode deposit (after ref. [24]). These sharp lines had not been observed previously in