curvature to the otherwise flat graphene sheet made of hexagons. Thus, nanotubes
with pentagons and heptagons will have unusual curvatures and shapes. Bent
nanotubes arising from the presence of pentagons and heptagons on opposite
sides of the tube have been observed [66]. Based on force field calculations, Tersoff
and Ruoff [67] suggest that the nanotubes will form cylindrical bundles in a crystal
and large tubes will be hexagonal to maximize the van der Waals contact between
the tubes. Simulation studies indicate radial p-orbital character and large pyramidalization angles at the sites of local deformation [68].
An interesting observation with SWNTs is the presence of rings in the electron
microscopic images [69]. The rings are formed during the ultrasound treatment in
an acidic medium, followed by settling of nanotube dispersions on the substrate.
Similar ring morphologies are observed in atomic force microscope (AFM) and
SEM studies of catalytically produced multi-walled nanotubes [70a]. Huang et al.
[70b] have observed ‘‘crop circles’’ of aligned nanotubes in a direction normal to
the substrate surface, on pyrolysing Fe(II)phthalocyanine. Yet another important
nanostructure discovered recently is the presence of encapsulated fullerenes inside
SWNTs, as observed by high resolution TEM [71, 72]. Fullerenes were observed
inside the nanotubes after annealing laser-synthesized SWNTs. We present a typical TEM image showing the presence of C 60 in SWNTs in Figure 8.6 (inset). Analysis of the size distribution of the encapsulated fullerenes inside arc-produced
SWNTs shows that there are indeed significant quantities of fullerenes in the size
range C 36 aC 120 in the nanotube capillaries [73].
X-ray diffraction (XRD) measurements have been employed to characterize carbon nanotubes [74, 75]. The XRD patterns of nanotubes show only the (hk0) and
(001) reflections but no general (hkl) reflections. This is the case in turbostratically
modified graphites [76]. Warren [77] has suggested special methods for the analysis of the (hk0) reflections and such studies support the electron microscopy data
in showing that structural correlations exist along the direction perpendicular to
the carbon nanotube axis as well as within each individual tube, but not in any
combination of these. The correlation lengths obtained from the analysis of the
XRD patterns are in the same regime as that from microscopy.
Ebbesen et al. [78] find that the nanotube bundles are self-similar in the sense
that large cylindrical bundles comprise smaller ones and the smaller ones are
made up of nanotubes and so on. STM has been used to probe the electronic
structure of carbon nanotubes deposited on various substrates [81–83]. STM has
also been used to probe sp
3 defect structures, closure of the tips and pentagoninduced changes in the electronic structure in carbon nanotubes [84]. Venema
et al. [85] have obtained atomically resolved STM images of SWNTs, wherein the
chirality of the nanotubes is unambiguously determined, which in turn influences
the electronic property of the nanotubes. Raman spectroscopy has provided important insights into the structure of nanotubes. Jishi et al. [86] calculated the
Raman-active phonon modes using a zone-folding method for a 2D graphene
sheet, and demonstrated that there are 15 allowed Raman modes for each diameter
of the tube. The frequency of the allowed mode depends on the tube diameter and
the chiral angle, the number of modes being independent of the diameter. Hiura
8 Nanotubes and Nanowires
220
with pentagons and heptagons will have unusual curvatures and shapes. Bent
nanotubes arising from the presence of pentagons and heptagons on opposite
sides of the tube have been observed [66]. Based on force field calculations, Tersoff
and Ruoff [67] suggest that the nanotubes will form cylindrical bundles in a crystal
and large tubes will be hexagonal to maximize the van der Waals contact between
the tubes. Simulation studies indicate radial p-orbital character and large pyramidalization angles at the sites of local deformation [68].
An interesting observation with SWNTs is the presence of rings in the electron
microscopic images [69]. The rings are formed during the ultrasound treatment in
an acidic medium, followed by settling of nanotube dispersions on the substrate.
Similar ring morphologies are observed in atomic force microscope (AFM) and
SEM studies of catalytically produced multi-walled nanotubes [70a]. Huang et al.
[70b] have observed ‘‘crop circles’’ of aligned nanotubes in a direction normal to
the substrate surface, on pyrolysing Fe(II)phthalocyanine. Yet another important
nanostructure discovered recently is the presence of encapsulated fullerenes inside
SWNTs, as observed by high resolution TEM [71, 72]. Fullerenes were observed
inside the nanotubes after annealing laser-synthesized SWNTs. We present a typical TEM image showing the presence of C 60 in SWNTs in Figure 8.6 (inset). Analysis of the size distribution of the encapsulated fullerenes inside arc-produced
SWNTs shows that there are indeed significant quantities of fullerenes in the size
range C 36 aC 120 in the nanotube capillaries [73].
X-ray diffraction (XRD) measurements have been employed to characterize carbon nanotubes [74, 75]. The XRD patterns of nanotubes show only the (hk0) and
(001) reflections but no general (hkl) reflections. This is the case in turbostratically
modified graphites [76]. Warren [77] has suggested special methods for the analysis of the (hk0) reflections and such studies support the electron microscopy data
in showing that structural correlations exist along the direction perpendicular to
the carbon nanotube axis as well as within each individual tube, but not in any
combination of these. The correlation lengths obtained from the analysis of the
XRD patterns are in the same regime as that from microscopy.
Ebbesen et al. [78] find that the nanotube bundles are self-similar in the sense
that large cylindrical bundles comprise smaller ones and the smaller ones are
made up of nanotubes and so on. STM has been used to probe the electronic
structure of carbon nanotubes deposited on various substrates [81–83]. STM has
also been used to probe sp
3 defect structures, closure of the tips and pentagoninduced changes in the electronic structure in carbon nanotubes [84]. Venema
et al. [85] have obtained atomically resolved STM images of SWNTs, wherein the
chirality of the nanotubes is unambiguously determined, which in turn influences
the electronic property of the nanotubes. Raman spectroscopy has provided important insights into the structure of nanotubes. Jishi et al. [86] calculated the
Raman-active phonon modes using a zone-folding method for a 2D graphene
sheet, and demonstrated that there are 15 allowed Raman modes for each diameter
of the tube. The frequency of the allowed mode depends on the tube diameter and
the chiral angle, the number of modes being independent of the diameter. Hiura
8 Nanotubes and Nanowires
220