studies on aligned multi-walled carbon nanotubes show a strong dependence of
the graphite-like G-band and disorder induced D-band on the polarization geometry [91].
Pressure-induced phase transformations under static and dynamic loading between the many allotropes of carbon like diamond, graphite, C 60 and C 70 and
their polymeric and amorphous forms are of academic and practical importance.
Pressure-effects on SWNT bundles have been probed by Raman spectroscopy up
to a maximum pressure of 25.9 GPa (1 GPa ¼ 10
9 N m
À2 ) in a diamond anvil cell
[92, 93]. The spectra arising from the radial and tangential modes at 0.1 GPa are
similar to those reported earlier at atmospheric pressure [89]. The two dominant
radial bands in the spectrum of the sample recorded at 0.1 GPa were at 172 and
182 cm
À1 . For an isolated SWNT, the calculated frequencies of the radial mode
o R [cm
À1 ] for a tube of diameter d[nm] fit to o R ¼ 223:75=d, irrespective of the
nature of the tube [94]. This gives o R ¼ 164 cm
À1 for the (10,10) tube and 183
cm
À1 for the (9,9) tube. The inclusion of van der Waals interaction between the
(9,9) tubes shifts the radial mode frequency from 171.8 cm
À1 (for an isolated tube)
to 186.2 cm
À1 . This blue shift of 14.4 cm
À1 is due to intertube interaction, and is
independent of the tube diameter [95]. Accordingly, the empirical relation for the
diameter dependence of the radial mode frequency in a SWNT bundle is given by,
o R ¼ 14:4 þ 209:9=d, which retains the 1=d dependence of o R and reproduces
o R ¼ 186:2 cm
À1 for the (9,9) tube [96]. The tangential modes are assigned in
terms of the irreducible representations of D nh ðD nd Þ for even n (odd n), with 1531
cm
À1 as E 1g , 1553 and 1568 cm
À1 as E 2g , 1594 cm
À1 with unresolved doublet
A 1g þ E 1g and 1606 cm
À1 with E 2g symmetry [97, 98]. The intensities of the radial
modes fall rapidly with increasing pressure, and were not discernible beyond 2.6
GPa, but the features are reversible. The intensities of the tangential modes also
decrease with pressure. The modes at o T ¼ 1568 cm
À1 and 1594 cm
À1 show softening between @10–16 GPa, beyond 16 GPa, the band position increases with
pressure. Remarkably, when the pressure is reduced from the highest pressure of
25.9 GPa, the peak positions follow the same trend as in the increasing pressure
run. Studies of SWNTs under high pressure confirm the potential of these materials as the strongest ever carbon nanofibers and also their remarkable resilience
[96, 99].
8.2.3
Mechanism of Formation
Several growth models are proposed for the carbon nanotubes prepared by the pyrolysis of hydrocarbons on metal surfaces. Baker and Harris [100] suggested a fourstep mechanism. In the first step, the hydrocarbon decomposes on the metal surface to release hydrogen and carbon, which dissolves in the particle. The second
step involves the diffusion of the carbon through the metal particle and its precipitation on the rear face to form the body of the filament. The supply of carbon onto
the front face is faster than the diffusion through the bulk, causing an accumulation of carbon on the front face, which must be removed to prevent the physical
8 Nanotubes and Nanowires
222
the graphite-like G-band and disorder induced D-band on the polarization geometry [91].
Pressure-induced phase transformations under static and dynamic loading between the many allotropes of carbon like diamond, graphite, C 60 and C 70 and
their polymeric and amorphous forms are of academic and practical importance.
Pressure-effects on SWNT bundles have been probed by Raman spectroscopy up
to a maximum pressure of 25.9 GPa (1 GPa ¼ 10
9 N m
À2 ) in a diamond anvil cell
[92, 93]. The spectra arising from the radial and tangential modes at 0.1 GPa are
similar to those reported earlier at atmospheric pressure [89]. The two dominant
radial bands in the spectrum of the sample recorded at 0.1 GPa were at 172 and
182 cm
À1 . For an isolated SWNT, the calculated frequencies of the radial mode
o R [cm
À1 ] for a tube of diameter d[nm] fit to o R ¼ 223:75=d, irrespective of the
nature of the tube [94]. This gives o R ¼ 164 cm
À1 for the (10,10) tube and 183
cm
À1 for the (9,9) tube. The inclusion of van der Waals interaction between the
(9,9) tubes shifts the radial mode frequency from 171.8 cm
À1 (for an isolated tube)
to 186.2 cm
À1 . This blue shift of 14.4 cm
À1 is due to intertube interaction, and is
independent of the tube diameter [95]. Accordingly, the empirical relation for the
diameter dependence of the radial mode frequency in a SWNT bundle is given by,
o R ¼ 14:4 þ 209:9=d, which retains the 1=d dependence of o R and reproduces
o R ¼ 186:2 cm
À1 for the (9,9) tube [96]. The tangential modes are assigned in
terms of the irreducible representations of D nh ðD nd Þ for even n (odd n), with 1531
cm
À1 as E 1g , 1553 and 1568 cm
À1 as E 2g , 1594 cm
À1 with unresolved doublet
A 1g þ E 1g and 1606 cm
À1 with E 2g symmetry [97, 98]. The intensities of the radial
modes fall rapidly with increasing pressure, and were not discernible beyond 2.6
GPa, but the features are reversible. The intensities of the tangential modes also
decrease with pressure. The modes at o T ¼ 1568 cm
À1 and 1594 cm
À1 show softening between @10–16 GPa, beyond 16 GPa, the band position increases with
pressure. Remarkably, when the pressure is reduced from the highest pressure of
25.9 GPa, the peak positions follow the same trend as in the increasing pressure
run. Studies of SWNTs under high pressure confirm the potential of these materials as the strongest ever carbon nanofibers and also their remarkable resilience
[96, 99].
8.2.3
Mechanism of Formation
Several growth models are proposed for the carbon nanotubes prepared by the pyrolysis of hydrocarbons on metal surfaces. Baker and Harris [100] suggested a fourstep mechanism. In the first step, the hydrocarbon decomposes on the metal surface to release hydrogen and carbon, which dissolves in the particle. The second
step involves the diffusion of the carbon through the metal particle and its precipitation on the rear face to form the body of the filament. The supply of carbon onto
the front face is faster than the diffusion through the bulk, causing an accumulation of carbon on the front face, which must be removed to prevent the physical
8 Nanotubes and Nanowires
222